Design and construction of a windsurfer longboard by Malcolm Jones

Design and construction of a windsurfer longboard By Malcolm Jones

November 24, 2008 (Updated October 8, 2010)

Preface The following began as my working notes when designing a windsurfer I recently built. I’d do a bit of the design, leave it, come back to it and forget what I’d done and which design I was going with. Therefore these notes were written primary to remind me of what I did. Once I started building it I decided to continue the documentation just in case I was every crazy enough to do it again. The design was from scratch and this was my first attempt at building a windsurfer. I’d done small repairs before and built a canoe using a mould so had some prior fibreglassing experience but had not done any vacuum bagging. The notes are not a step-by-step how-to. More importantly they are based on a first time backyard board builders experience. I’m sure there are many things I did which could be improved or done entirely differently. There is not that much info on designing and building a windsurfer. However info on building surfboards, canoes, sailing boats is more plentifully and can be useful. The best reference that gives detailed step by step information for windsurfers was at the website http://www.ecboards.co.uk. It is titled Building a composite windsurfer and gave details on building a speed board. I found any info provided on the web to be invaluable hence I decided to put these notes out there for anyone else attempting a similar project. Before embarking on building a board you have to ask the question why? If it is to save money don’t do it. The material costs equated to roughly half the retail price of on equivalent board. However if you factor in labour there’s certainly no savings. Realistically it was equivalent to a month full time. I took 6 days off work and spent the best part of a month worth of weekends on it. It took far longer than I had anticipated. This is partly because it was my first time and I was learning as I went. I’d make small mistakes which then took time to fix. For me the motivation behind the project started with an old raceboard that was leaking. I’d previously fixed the centre board box and suspected it was leaking. So I took the drastic step of cutting a 1 × 0.1 m slot down the middle of the board and rebuilt the centre box. It turned out there was no water there, however there was water getting in the front. The board was already 18 kg and with more major repairs would be getting close to 20 kg, also it was 20 years old. Time for a new or better 2nd-hand board. Only problem is new boards of this kind are hard to get. Exocet and Starboard have started making them and Mistral appears to resurrecting the equipe. On the 2ndhand market a couple of boards appear every 12 months Australia wide. So with the centre board box built and some vacuum bagging skills acquired I decided to go all the way and build from scratch. I also liked the idea of designing and building myself. Would I do it again? Probably not. Overall I was happy with the result. While it’s obviously an amateur build it came out under weight and without any flaws that would effect performance. Basically this is my light wind “sailing” board, maybe I’ll give racing a go but it wasn’t build with that in mind. I’d tried wide-style early planers but found them a bit one-dimensional. My biggest sail is 8.5m2 which on a 85cm wide board gives me planing threshold of ∼ 8 knots minimum (i.e. realistically a “steady” wind averaging 10knots). But if it’s a summer’s day with an average 8 knots I’d be tempted to venture out and end up struggling in the lulls. So I experimented with an old raceboard and found I was having heaps more fun in the 8-12knots winds.

Contents

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5 5 5 6 6 6 6 7 12 13 14

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15 15 15 16 16 17

Mast track, fin-box and footstrap plugs 3.1 Mast track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Finbox and Footstrap plugs . . . . . . . . . . . . . . . . . . . . . . . . . .

20 20 21

Board Construction 4.1 Shaping . . . . . . . . . . . . . . 4.1.1 Templates . . . . . . . . 4.1.2 Core . . . . . . . . . . . 4.1.3 Rocker and deck . . . . 4.1.4 Vee . . . . . . . . . . . . 4.1.5 Planshape . . . . . . . . 4.1.6 Rails . . . . . . . . . . . 4.2 Laminating . . . . . . . . . . . . 4.2.1 Carbon/HDF to bottom 4.2.2 Carbon/HDF to deck . 4.2.3 Fitting the fittings . . . .

22 22 22 23 23 23 23 23 24 28 28 29

Board design 1.1 Maximum dimensions . . . 1.2 Rocker profile . . . . . . . . 1.3 Deck centre-line profile . . . 1.4 Vee, concaves, tail kick . . . 1.5 Planshape . . . . . . . . . . 1.6 Deck cross section and rails 1.7 Data files . . . . . . . . . . . 1.8 Design weight . . . . . . . . 1.9 Material costs . . . . . . . . 1.10 Time required to build . . . Centreboard 2.1 Design of centreboard . . . 2.1.1 Aerofoil section . . . 2.1.2 Planshape . . . . . . 2.2 Construction of centreboard 2.3 Centreboard Box . . . . . .

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The finished product 5.1 Photos from Inverloch 25/04/2010 . . . . . . . . . . . . . . . . . . . . . .

35 39

4.3

4.2.4 HDF to rails . . . 4.2.5 Outer lamination Finishing . . . . . . . . . 4.3.1 Filler coat . . . . 4.3.2 Gaskets . . . . . . 4.3.3 Painting . . . . . 4.3.4 Deck grip . . . .

A Tooling A.1 Compressor . . . . . . . A.2 Vacuum controller . . . . A.3 Hotwire cutter . . . . . . A.4 Hotwire voltage control

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B References and links

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44 44 44 46 46 49

Chapter

Board design 1.1

Maximum dimensions maximum length = l = 3800 mm maximum width = wm = 640 mm maximum thickness = tm = 180 mm

1.2

Rocker profile

The bottom rocker profile, ρ (in mm), is defined by the equation  x < 800  0, ρ( x ) = ( x − 800)4  , x > 800. 3 × 1011

(1.1)

where x (in mm) is the lengthwise coordinate and ρ is measured relative to the horizontal datum z = 0 where z is the vertical coordinate as defined in figure 1.1. Hence at the stern of the board (i.e. for x < 800 mm) the rocker is a true flat. Forward of x = 800 mm the rocker is defined by a quartic giving a maximum rocker value at the bow of the board of ρ = 270 mm. The quartic curve generates very little rocker initially, it’s only around x = 2000 m that the rocker starts to kick in. Actually the rocker profile given by (1.1) can be expressed in terms of two independent parameters: the maximum rocker, ρm , and the coordinate where the rocker begins, x f . Rewriting (1.1) in terms of these two parameters gives

ρ( x ) =

 0,       ρm

x < xf x − xf l − xf

where l is the maximum length.

!4 , x > xf

(1.2)

Design and construction of a windsurfer longboard

1.3

Deck centre-line profile

The deck profile was generated by sketching a spline, keeping in mind I didn’t want the trailing edge of the centreboard to protrude through the deck when it was in the retracted position. The centreboard has a maximum chord of 170 mm so the maximum thickness of the board was set to 180 mm.

1.4

Vee, concaves, tail kick

The reference board I used during the design was heavily concaved, figure 1.2. I’m not sure how one would accurately shape these contours. In any case most modern boards seem to have abandoned concaves. A simple vee should do the job of counteracting the slapping tendency of a flat hull, which concaves also serve to do. Concaves may assist is lateral resistance but probably add drag, there seems to be very little information on their effect. In the end I decided to go for a small amount of vee only; no tail kick or concaves. The actual vee is constant at 1 degree along the full length.

1.5

Planshape

Figure 1.3 shows the planshape of the bottom of the board. The rails are parallel for approximately the range x = 1700 to x = 2100 mm, where the width is the maximum value wm = 640 mm. The planshape was generated by a spline sketch and hence cannot be represented in equation form, although its not far off an ellipse.

1.6

Deck cross section and rails

Finally the deck and rail profiles are required to completely specify the geometry of the board. Raceboards have sharp rails which run almost the complete length of the board. This feature greatly simplifies the design since one cross sectional profile can be specified and simply scaled appropriately based on the thickness and width of the board at a given x coordinate. I played around with using quadratics, arcs and the like to define the cross section but in the end it was simpler just to sketch a spline that looked about right. Figure 1.4 shows the normalised cross section used for all locations. I’ll probably knock the corner off this profile near the nose of the board where sharp rails are more of a liability. Individual cross sectional profiles for the back half of the board are shown in figure 1.5 and for the front half of the board in figure 1.6. The area bounded by the individual cross sections can be calculated by numerically integrating the curves and hence the volume of the board is calculated from the summation V = A1 ∆x1 + A2 ∆x2 + A3 ∆x3 + . . . + An ∆xn =

∑ Ai ∆xi i

where Ai is the area of the ith cross section and ∆xi is the “thickness” of the cross section slice. For the data given above the volume turns out to be V = 252

litres. 6

Design and construction of a windsurfer longboard

The surface area of the board can be found by calculating the length of individual lines show in figures 1.5 and 1.6. For the deck and rails this length is given by integrating the upper curves using the formula s 2 Z w/2 dz S= 1+ dy. dy −w/2 If this is done for many x locations on the board then the surface area is given by the summation n

∑ Si ∆xi = 2.56 m2

deck and rails.

A similar calculation can by done for the bottom of the board but it is much simpler since the lines are straight and at a given x location S=

w cos θ

where θ is the vee angle, which is small, and hence S ≈ w so A=

Z 3800 0

w dx = 1.95 m2

bottom,

where w is the width of the board which is a function of x.

1.7

Data files

Plain text files containing the coordinates for the: • rocker • planshape • and deck

Ď = 270 8

tm = 180 t( x ) 1000

x 2000

l = 3800

3000 Ď ( x )

Figure 1.1: Centre-line profile

Design and construction of a windsurfer longboard

Figure 1.2: The board used as a rough guide for the design. The actual board I designed is quite different, the only dimension I matched was the maximum rocker (nose kick?). It’s good to have something to reference your design against just to ensure you don’t design something “off the scale”. 9

y (mm) w Stern 0

l = 3800 mm

-500

500 170

320

1000

1500 1375

535

2000

2500 2180

1580

Figure 1.3: Planshape

3000

3500

x (mm)

Design and construction of a windsurfer longboard

500

Design and construction of a windsurfer longboard

1 z t

0.5

-1

-0.5

0.5

y w/2

Figure 1.4: Spline defining the deck and rail, normalised curve. 200 x=100 x=250 x=500 x=750 x=1000 x=1500 x=1900 x=750 x=1000 x=1500 x=1900

150 z 100 50 0 -350

-250

-150

-50

150

250

350

Figure 1.5: Individual cross sectional profiles for the back half of the board. Note constant vee of 1o . Note in these figures I’m really plotting z − ρ, i.e. I’m not including the effect of the rocker which would shift each successive curve upwards as you move forward along the board. 200 x=2000 x=2500 x=3000 x=3250 x=3500 x=3600 x=3700 x=3750 x=3250

150 z 100 50 0 -350

-250

-150

-50

150

250

350

Figure 1.6: Individual cross sectional profiles for the front half of the board. Note constant vee of 1o . 11

Design and construction of a windsurfer longboard

1.8

Design weight

Summarising, the design volume and total surface area are Volume = 252 l

and Area = 4.5 m2 .

The construction will be carbon sandwich with the following layup: 200 gm−2 carbon fibre then high density foam (HDF) 80 gm−3 with a thickness of 5 mm then another carbon layer (200gm−2 ). Epoxy resin will be used and assuming a fibre to resin ratio of 40 : 60 (see FGI data) then each layer of carbon will require 300 gm−2 of resin. Hence an individual carbon lamination will weigh 500 gm−2 (note FGI quotes 480). The HDF will add 80 × 5 × 10−3 = 400 gm−2 . So HDF sandwiched between two layers of carbon the skin weight is 4.5 × (2 × 500 + 400) gm−2 which gives a total of carbon sandwich weight = 6.3 kg The core is low density polystyrene with a density of 14 kgm−3 which gives a core weight = 0.252 × 14 = 3.5 kg and hence the subtotal of board without fittings and paint is 9.8 kg. The other components are estimated to weigh: • centreboard case 1.2 kg (HDF, carbon construction) • Mast track 1.1 kg (0.55 kg RSX part +0.55 kg for reinforcements) • Fin Box 0.6 kg (Tuttle std. and reinforcements) • Footstrap plugs (×12) 0.4 kg • Fibre patches around fitting 0.5 kg (1 m2 ?) So total weight of fittings is 3.8 kg.1 For filling and painting the estimates are: Filler =0.2 kg Paint =0.5 kg

maximum estimate assuming 1 litres of paint

Hence, total weight = 14.3 kg excluding centreboard (which weighs 0.85 kg) footstraps & fin. Note an additional full lamination adds 10Y g where Y is the fibre weight is gm−2 , i.e. 130 gm−2 glass fibre adds a further 1.3 kg. For comparison the claimed weights of currently available production boards are: Starboard Phantom 13.9 kg, Exocet Warp 13.5 kg and Mistral One Design 15.5 kg.

1 will

lose about .35 kg when polystyrene is routed out

Design and construction of a windsurfer longboard Material Polystyrene core 1st layer of carbon 1st layer of resin HDF 2nd layer of carbon 2nd layer of resin Carbon patches Resin patches Filler Paint Footstrap plugs Mast track Fin Box Centre box

Density

Quantity

Weight kg

14 kg/m3 (est.) 200 g/m2

0.25 m3 4.5 m2 1.35 litre 0.0225 m3 4.5 m2 1.35 litre 1 m2 0.3 litre

3.5 0.9 1.35 1.8 0.9 1.35 0.2 0.3 0.2 0.5 0.4 1.1 0.6 1.2

80 kg/m3 200 g/m2 200 g/m2

1 litre 8 footstraps

Total

14.3 Table 1.1: Summary of design weight.

1.9

Material costs

Table 1.2 lists the actual costs of the materials that go into the board. If you were to build in glass only you could probably get it down to $1300. There is excess HDF, but 2 sheets isn’t quite enough. There is also the cost of consumables: • paint brushes, rollers • sandpaper • masking tape • gloves, masks • material for templates • measuring beakers I didn’t keep track of the cost of the consumables but it’s of the order $100. The other costs include: footstraps which for a the full 8 straps at $25 each would be $200 (luckily I had some old ones I could use) and a fin $150. One thing I hadn’t factored in was the uni/extension. I tried to adapt an existing uni by replacing the pin to suit the rsx track but it seems the pin is unique to the rsx track so I could not source a suitable pin. In the end I had to buy a new rsx mastfoot, which was incompatible with my extensions, therefore a new extension too, all up another $120. I have not included the cost of any tools I had to buy e.g. sur-form, set-square and any of the electronics associated with the vacuum pump and hotwire.

Design and construction of a windsurfer longboard Material Fin Box Mast Track Glass 130g/m2 × 4m Carbon fibre 200g/m2 12m ×1.27m HDF sheets 5mm ×2 HDF sheets 3mm ×1 polystyrene core Resin epoxy 5kg Hardner epoxy 1kg foot strap inserts paint filler vac bag, peel ply, breather

AUD 29.00 80.00 25.70 496.32 218.89 92.43 244.50 92.16 28.14 20.00 200.00 40.00 116.00 1,683.14

Table 1.2: Actual material costs in Australian dollars, August 2008. For comparison: Exocet Warp 380 raceboard $3500, Starboard formula $3150, Starboard Futura (freeride board) $2500 or Neil Pryde X9 - 490cm mast $1600. For the complete board also factor in footstraps and fin and possibly the uni/mast extension. These items take the cost to around $2000 which is more than a Exocet Kona. My point is you have to have a compelling reason to build it yourself other than cost.

1.10

Time required to build

This was the most challenging aspect and I would advise anyone thinking of building to realistically consider whether they have the time available to complete the project. For me the project changed from being enjoyable to a chore at about the 75% complete mark. When this happens you tend to start rushing and trying to take short cuts. I made the first hotwire cut on 29 September and applied the last coat of paint on 8 November. Prior to starting the board I’d made the centre board, centre board box, mast track box and finbox reinforcement. I worked on these bits and pieces over about 6 months on and off. Also factor in the time to make the templates. The following is roughly how much time was spent on each step. I estimate at least 100 hours of labour, maybe closer to 200 hours. Job Building the centreboard, fittings & templates Hotwire cutting and board shaping Laminating 80% done Final deck top lamination Filling the weave and sanding More surface prep. Painting & fitting gaskets More painting Finish painting

When Sporadically

Days 2-3 weekends

Weekend Week off work & weekend Weekend Weekend Week nights melb. cup 4 day long weekend Before and after work Weekend

2 full days 7 full days 1 full day 2 part days 1 day equivalent couple hrs.day 1 day equivalent 1 full day

Chapter

Centreboard 2.1

Design of centreboard

The physics of the flow around an aerofoil is not trivial. Motivated by the desire for efficient and fast flight a great deal of scientific and engineering research has been devoted to the problem. Methods for predicting the performance of aerofoils involve approximations which are quite accurate at low angles of attack, more challenging is predicting the stall characteristics. In designing the centreboard there are two main aspects to consider: 1. the aerofoil section which includes the maximum thickness of the aerofoil and point of maximum thickness; 2. the plan shape. Since the centreboard is a symmetric aerofoil, camber and twist are zero, and the parametric design space is somewhat reduced. A considerable effort can be devoted to the design process. However it is worth keeping in mind that hand shaping techniques will ultimately limit your ability to accurate produce it.

2.1.1

Aerofoil section

Based on wind tunnel experiments a large database of aerofoil characteristics was complied in the 1930â&#x20AC;&#x2122;s by NACA (National Advisory Committee of Aeronautics, now NASA). Figure 2.1 shows the what is referred to as the NACA0010 aerofoil section. The important parameter is the maximum thickness of this aerofoil which, in this case, is 10% of the chord length, c, (hence the 10 in NACA0010, 00 means no camber). Note the chord is the straight line joining the leading edge to the trailing edge. The numbers in figure 2.1 refer to selected x, y (mm) coordinates on the upper surface relative to the origin at the leading edge, where in this example the chord c = 170 mm, which is the chord length at the root of the aerofoil (i.e. at the point where the centreboard is against the hull). Left of the vertical red line is where I used the balsa wood moulding which just happened to fit the NACA0010 profile quite well. The maximum thickness of the aerofoil is at x = 51 mm where y = t/2 = 8.5 and hence t = 17 mm (i.e. 10% of c) where t is the thickness. 15

Design and construction of a windsurfer longboard NACA0010 profile c = 170 mm 20, 7

32, 8

51, 8.5

75, 8

94, 7

108,6

121,5

132,4

143,3

153,2 162,1

Figure 2.1: The NACA0010 aerofoil section, plotted for a chord length of 170 mm. This figure is to scale and can be used as a template if you print the page at 100% scale.

Figure 2.2: Contour plot of what we want to achieve, with hand tools and limited skill this is a challenge! As will be seen in the next section we will choose an elliptic planform so that the chord length progressively reduces towards the tip of the centreboard. This means we must continuously scale down the profile in figure 2.1. Figure 2.2 shows the surface contour of the centreboard required to achieve a NACA0010 section at all cross sections.

2.1.2

Planshape

Mathematical analysis of wings is difficult. However a classic result obtained in the 1930â&#x20AC;&#x2122;s is that for unswept wings an elliptical planform gives the minimum possible drag for a given value of lift (i.e. it achieves the best Lift/Drag ratio). So why not use the elliptical planform, if nothing else it looks better than a rectangular shaped centreboard.

2.2

Construction of centreboard

The centreboard is built using laminated cedar strips wrapped in a layer of carbon/glass fibre using epoxy as the resin. There are several useful articles on design and construction of centreboard on the Internet (see for example Philâ&#x20AC;&#x2122;s Foils & Composites, Moth Hardware: Phil Stevenson and Design and construction of centreboards and rudders, Paul Zander .) I used a rough-sawn cedar weather-board and ripped strips of around 35 mm wide. Since I was cutting from a weather-board I could rip pieces of different thickness and build up a coarse approximation to the final aerofoil section. The strips were glued with epoxy. For the leading edge I used a piece of balsa around 12 mm wide which was pre-shaped into an aerofoil leading edge (try a model aircraft shop). The centreboard is shaped using an electrical sander with coarse sandpaper. I only made a template of the root sectional profile. So at the end of this stage I had a rectan16

Design and construction of a windsurfer longboard

Figure 2.3: Cedar strips (approx. 35 mm wide) and rounded balsa lead leading edge.

Figure 2.4: After laminating a layer of carbon/glass. gular planform with a constant sectional profile. Next I cut the elliptic planform with a jigsaw. The aerofoil section needs to be progressively thinned down towards the tip (i.e. trying to keep the ratio l/c constant to achieve the contour of figure 2.2). To do this accurately you’d need several sectional profiles for several different locations. In the end I couldn’t be bothered with trying to accurately produce figure 2.2 and just thinned it out by eye. Figure 2.3 shows the centreboard at this point, low spots and imperfects have been filled (pink coloured epoxy filler). After cutting the required shape of the area that goes through the board it’s ready to be laminated. I used 200 gm−2 carbon fibre, double thickness from the handle to around 50 mm below the root. I ended up adding an extra layer of glass (130 gm−2 ) as I thought I’d used too much resin and wanted to soak it up. I did it all in one go and wrapped it in a vacuum bag and held it under vacuum overnight while it dried. The vacuum isn’t essential but does improve the result. A photo of the centreboard after removing from the vacuum bag is shown in figure 2.4, the excess fibre on the trailing edge is easily trimmed off with a knife and sanded smooth, don’t make it razor shape.

2.3

Centreboard Box

The centreboard is highly loaded when working to windward. Therefore it is necessary to reinforce the board at the location of the centreboard. I chose to build what I call a centreboard box which serves the same role as the fin box, that is it provides structural support for the lateral loading. The difference between a fin box and centreboard box, apart from size, is that the centreboard box must also provide a pivot point and a sufficient cavity for the centreboard to fit in when in the retracted position. The side 17

Design and construction of a windsurfer longboard

walls of the box are made of 25 mm thick polyurethane foam with 2 sheets of HDF laminate onto the surface (between each layer of foam is 130 gm−2 glass). To provide a pivot point a 5 mm thick piece of marine ply is used, with a channel cut into it. This piece is plywood is recessed into the top HDF layer, which also has a channel cut into it. Figure 2.5 shows the foam and plywood layup and figure 2.6 shows details of the pivot point prior to the final lamination. The final lamination is made up of a layer of 200gm−2 carbon and a layer of 130 gm−2 glass. 25 mm urethane foam 1000 mm 180 mm

5 mm PVC

5 mm plywood

Figure 2.5: Layup of sidewall panels for centreboard box.

Figure 2.6: Plywood reinforcement provides support for centreboard pivot. The width of the centreboard box cavity is ≈ 25 mm and chocks of foam 25 mm thick are glued in between the side wall panels. At the front of the box I used a triangular shaped piece of HDF and at the rear I used polyurethane foam shaped to follow the shape of the trailing edge of the centreboard, figure 2.7.These chocks of foam were first laminated with fibreglass before gluing them in between the side panels. After using the board a few times I discovered a small leak at the rear of the box between the central piece of foam and the sidewalls so I’d recommend also laying a layer of fibreglass over the join. By sanding or building up the strips you can adjust the amount of friction holding the centreboard.

Layers of HDF foam 25 mm

polyurethane foam

Figure 2.7: Foam panels placed in between the sidewall panels. Ideally the centreboard should be a firm fit in the box otherwise, when in the retracted position, it has a tendency to flop back down particularly when bouncing over 18

Design and construction of a windsurfer longboard

Figure 2.8: Centre board box, bottom view.

1 0 111 000 0 1 0 1 0 1 0 1 0 1

Deck

sidewall panel friction strip

pivot pin of centreboard

Figure 2.9: Friction strip which fits in the channel in the centreboard box. choppy waters. Unless you are able to achieve high tolerances when manufacturing centreboard box, it is difficult to achieve a tight fit without having the centreboard jam when it is rotating. It is better to leave some clearance in the centreboard box then pack the centreboard tight with, what I call, â&#x20AC;&#x153;friction stripsâ&#x20AC;?. These are simply strips about 5 mm thick which are placed in the rebatted channel (shown in figure 2.6) after the centreboard is in place. Initially I used plywood but found it wore out quickly so I ended up using sections from a sail batten built up to the required thickness with layers of carbon fibre and shown in figure 2.9. The strips are screwed down to the deck of the board. Make sure you place an adequate screw plug in the board. Make the strips long enough so that they are in contact with the centreboard pivot pin figure 2.9.

Chapter

Mast track, fin-box and footstrap plugs When placing fittings into the board it is important to reinforce the board around the fitting using a higher density foam than the polystyrene core. My approach was to build up a block of foam around each fitting. I used 25 mm polyurethane, 5 mm HDF and fibreglass to make sandwich panels that go around each part. These panels help spread the load and increase the top surface area of each part so there is more area for the deck or hull lamination to adhere to. I prepared all these parts before I started building the actual board. This gave me practise in vacuum bagging on a small scale.

3.1

Mast track

The part I had most difficulty getting was the sliding mast track. I was quoted $200 for a mistral one-design track or $80 for a RSX track. I opted for the RSX, the only drawback is that they donâ&#x20AC;&#x2122;t have as much travel as a traditional raceboard track, figure 3.1. Figure 3.2 shows the mast-track-box made out of high density foam. The actual track is screwed into the channel. I used footstrap plugs as the anchor points for the screws.

Figure 3.1: RSX mast track.

3.2

Finbox and Footstrap plugs

I used a standard tuttle finbox. You may wish to use a deep one instead but I believe formula style fins (or a fin > 50 cm) are unnecessary for longboards so a standard should do. Foam sandwich panels are packed around the finbox and footstrap plugs to provide reinforcement, figure 3.3. 20

Design and construction of a windsurfer longboard

Figure 3.2: Mast track box made of urethane/HDF/fibre sandwich.

Figure 3.3: Finbox and footstrap plug reinforcements made of urethane/HDF/fibre sandwich.

Chapter

Board Construction 4.1

Shaping

The board is shaped as much as possible using a hotwire cutter. Since the hotwire is a straight wire you cannot shaped doubly curved surfaces with it e.g. rails and deck, for these areas I shaped with sandpaper. The sequence of shaping is as follows: 1. hotwire cut rocker 2. hotwire cut vee 3. hotwire cut planshape 4. hotwire cut linear approximation of rail profile 5. hand sand deck and rail to required curve. The idea is to do as much shaping with a hotwire using templates to guide the cut. Even when it came to fairing in the deck and rails I used cardboard templates of the curve.

4.1.1

Templates

I used A3 paper to print out templates of the centre-line profile (i.e. rocker) and planshape. When creating these templates I subtracted the thickness of the HDF foam off the template outline so that the specifications given earlier are for the built board not the core. To plot the profiles in full scale I used the vector graphics language Asymptote. Since an A3 piece of paper is 594 × 420 mm I printed a section of the profile on each page then joined them all together. These are the pdf files containing the templates • core-centre-line.pdf • planshape-port.pdf • planshape-star.pdf

1 Note

the maximum half-width of the board is wider than an A3 sheet of paper so the planshape in these files is referenced to a line offset 25 mm from the centre-line.

Design and construction of a windsurfer longboard

4.1.2

Core

The core is made of polystyrene, I used two blocks measuring 2.0 × 0.6 × 0.3 m, figure 4.1. Polystyrene is available in two forms expanded or extruded. The lightest but weakest is expanded which comes in different densities, from around 12 − 14 kgm−3 upwards. First I glued them together using liquid nails, make sure the glue stays within the cutting path of the hotwire. Note I could have bought the block as one piece but did not have a vehicle to transport such a long block in, I wouldn’t risk it on a roof rack. Check the squareness of the block and take this into account when attaching the templates.

4.1.3

Rocker and deck

The rocker/deck centre-line profile is transferred to a two pieces of MDF (medium density fibre?). These templates are aligned and then screwed into the sides of the foam block. I did the deck cut first, then flipped the board and cut the rocker profile. Figure 4.2 shows the core after the rocker and deck centre-line profile has been cut.

4.1.4

Vee

The design has 1o of vee along the full length of the board. For a block 700 mm wide this equates to a drop off of 350 × tan(1o ) = 6 mm at the edge of the block. Therefore one rocker template is lowered by 6 mm. To provide a guide on the other side the opposite template is raised by 6 mm. Note when you do the 2nd vee cut you need to raise the opposite side 2 × 6 = 12 mm relative to its edge. To make sure the wire does not cross the centre line I placed a strip of masking tape along the board’s centre-line (see figure 4.2).

4.1.5

Planshape

I used cardboard for the planshape templates. These are taped and pinned to the top and bottom of the board again alignment is important. The hotwire cutter is then used to cut the planshape. Actually I did this cut by myself but even though the width of the cut is smaller than for the rocker and vee it’s still difficult to keep an eye on both templates and there is a tendency to lift off every so often. You end up with a hump which is easily sanded back but for a perfect cut 2 people are best. Figure 4.3 shows the core so far.

4.1.6

Rails

The curve of the deck and rails can be rough cut by making a series of hotwire cuts. Figure 4.4 shows how a series of 4 hotwire cuts achieves a profile close to the required curve. Actually in retrospect I would not attempt the fourth cut since it is too shallow. Use masking tape to provide the guide for the hotwire. Obviously the cross section profile varies along the length of the board so this technique is applied to a section at a time. Starting at the widest point of the board and working towards the tips. Always make sure the hotwire cuts are on or outside of the final profile, since you want to sand 23

Design and construction of a windsurfer longboard

Figure 4.1: 4 m of polystyrene. Was this such a good idea? down to the desired shape rather than filling back up. Actually in retrospect it would be better to have stuck the tape guides along the entire length of the board rather than doing a section at a time and do one long continuous hotwire cut rather than sections. In figure 4.5 I have made the first cut at the mid section of the board. Once the rough cutting is done it is all faired in by sanding. For the mid-sections of the board there is very little variation in the cross sectional profile, so one template covers most of the length here. Toward the nose and tail a greater number of templates are required. These templates are used to guide the fairing of the rail and deck which is done by hand with sandpaper. The exact cross section profiles that are required are shown in figures 1.4 and 1.5. Figure 4.7 shows the final shaped core, I still have a bit of fine shaping. Unfortunately there are a few low spots on the deck/rail area that still need attention, these occurred due to the section-by-section approach I took. If I was to do it again Iâ&#x20AC;&#x2122;d do continuous rail cuts. Fortunately the hull, which is the critical geometry, is the easiest to shape. So small deck imperfections are more cosmetic (note: larger imperfections will cause bridging of the HDF and sites for delimitation).

4.2

Laminating

The lamination schedule is as follows 1. laminate carbon/HDF (5 mm) to bottom 2. laminate carbon/HDF (5 mm) to deck 3. glue in all fittings 4. laminate glass/HDF (3 mm) to rails 5. laminate carbon to bottom 6. laminate carbon to deck and rails

Design and construction of a windsurfer longboard

Figure 4.2: Foam core after cutting the rocker, blue tape marks centre-line and assist in guiding the hotwire for the vee cut to follow.

Figure 4.3: After cutting the planshape

Design and construction of a windsurfer longboard

use masking tape to guide the cuts

Last cut

required profile

linear approx.

hotwire cuts polystyrene block

First cut

Figure 4.4: The rail and deck is rough shaped by making a series of straight cuts. The foam is then faired to the required profile using sandpaper and templates. The circles in the figure show where the tape edge is placed for each cut. To determine the position I printed full scale cross sections, drew on the cuts and then measured the location of these points.

Figure 4.5: After make the first hotwire cut for the mid-section rail.

Figure 4.6: Shaped mid-rails, instead work the entire rail in one go, not section by section. 26

Design and construction of a windsurfer longboard

Figure 4.7: The bulk of the shaping done, just have to remove imperfections.

Design and construction of a windsurfer longboard

5 mm HDF (Klegecell 80 gmâ&#x2C6;&#x2019;3 )

11111111111111 00000000000000 00000000000000 11111111111111 00000000000000 11111111111111 00000000000000 11111111111111 00000000000000 11111111111111

carbon fibre 200 gmâ&#x2C6;&#x2019;2

Polystyrene core

Figure 4.8: Layup used on both deck and hull, on the rails 3 mm HDF is used. Figure 4.8 shows the general layup used although extra reinforcement patches are used around all fittings. These are the make it or break it steps. Whereas in the shaping stage things may not quite turn out as planned it is hard to totally stuff it up, no so with the laminating. Epoxy has a working time of around 1 hr so while the laminating is not particularly hard you donâ&#x20AC;&#x2122;t have time to muck around and if things do go horribly wrong it may not be possible to recover. Hence preparation is the key, have everything ready to go and at hand before you start. I actually did a dry run putting the blank in the vacuum bag and sucking it down just to check I could actually get the board in the bag and to test the vacuum system at full scale. For all these steps I worked alone but I would recommend having a helper. There are several different brands of epoxy available, in the shops near me: FGI, West System, Epiglass. For the majority of the project I used FGI brand. However towards the end I started having problems with it fully curing especially when mixed with fillers so I switched to West System which seemed more reliable and is less viscous making it easier to wet out the fibre.

4.2.1

Carbon/HDF to bottom

Using the planshape template I cut the HDF for the bottom of the board. The HDF foam sheets are 2400 mm long so the bottom comprises 2 pieces of foam (i.e. a back piece and front piece). There are no complex curves for the foam to follow and hence this step is straight forward. I scraped a layer of thickened epoxy over the blank to assist in sealing it then began the laminating. There are several methods for wetting out the carbon. I chose to place the carbon on the HDF and wet out the side of the carbon that will be in direct contact with the polystyrene core. Then I flipped the carbon onto the core and continued wetting out the side that will be in direct contact HDF. I used 480 ml to wet the approximately 2 m2 of carbon used here. I placed peel ply and breather fabric over the HDF and then placed it in the vacuum bag, bottom down. The magnitude of the maximum vacuum used was 60 kPa and this tended to pull the nose rocker up higher than it should be so to counteract this the board (in the bag) was placed on top of the rocker off-cut (rocker bed) and then using straps it is pulled down onto the rocker bed. Once dry the HDF was sanded flush to the rail. Figure 4.9 shows the HDF laminated to the bottom of the board.

Design and construction of a windsurfer longboard

Figure 4.9: After laminating on the 5 mm HDF to the bottom. Actually this photo was taken after laminating the deck so carbon strands can be seen wrapping around the rail.

4.2.2

Carbon/HDF to deck

Originally the plan was to laminate the deck and rail HDF in one go. I used 5 mm on the deck and 3 mm on the rails. The deck piece is cut 40 mm narrower than the planshape then strips of 3 mm are used for the rails. This means there are lots of pieces of HDF to align and while cutting the pieces of HDF it became apparent that it would be very difficult to do the deck and rail in one go. So instead the deck piece is laminated by itself. The carbon was cut to be flush with the bottom of the board so after this step the rails already have a layer of carbon, figure 4.10. Therefore when laminating on the rails I used a lighter weight fibre glass. Under vacuum the release film tended to tuck under the edge of the HDF slightly so once cured I had to trim a couple of mmâ&#x20AC;&#x2122;s off the edge of the HDF.

4.2.3

Fitting the fittings

Not strictly a laminating step but this is where it occurs in the sequence. The centreboard box and fin box are through deck fittings so slots are cut through the board. It was hard to bring myself to cutting a 1 m long slot for the centre-board box so soon after laminating, it seems such a drastic step, figure 4.11. A channel for the mast track was routered as were the cavities for the footstraps and vent plug. Thickened epoxy is used to glue in these fittings. Note there are different types of micro-spheres used to thicken epoxy some suited for gluing (stronger/hard to sand) and some suited to fairing (lighter and easier to sand). I used pieces of 3 mm thick HDF foam to fill voids and reduce the amount of epoxy filler required. I also glued a stainless steel nut into the deck to use as an air vent.

Design and construction of a windsurfer longboard

Figure 4.10: After laminating the 5 mm HDF to the deck .

Figure 4.11: No magic trick, the saw really does go through the board.

Design and construction of a windsurfer longboard

Figure 4.12: All the pieces of HDF cut and ready to be vacuum bagged onto the rail.

Figure 4.13: All HDF foam on, fittings in, gaps filled and sanded ready for outer lamination.

4.2.4

HDF to rails

This was actually a very time consuming step. Three millimetre thick HDF was used on the rails. Since the rail is curved in both directions many small pieces of foam must be cut. Particularly near the tightly curved contours at the nose and tail. Figure 4.12 shows all the pieces cut and ready to be vacuum bagged onto the rail. To glue them on I used 130 gmâ&#x2C6;&#x2019;2 fibreglass. Lots of masking tape was required to keep them all positioned before the vacuum was applied. Rather than being too ambitious I did one rail at a time as I was a little concerned about keeping it all aligned. Once the epoxy has set any small gaps at the joins are filled and then sanded and faired in to the deck piece. Figure 4.13 shows the board ready for the outer laminations.

4.2.5

Outer lamination

The outer lamination is a single layer of 200 gmâ&#x2C6;&#x2019;2 carbon, except over the fittings where an extra layer is used. The size of these reinforcement patches is given in figure 4.14. The hull is laminated first, I allowed about 25 mm to wrap around the rail. However because of the width of the carbon roll the widest point of the board did not have any fibre wrapping around the edge. I found I needed 780 ml of resin for the 2.43 m2 of carbon which represents a fibre to resin ratio of approximately 40 : 60. The lamination was held under vacuum for 12 hours. Note before laminating the hull I used a router to rebate the centre board box to provide space for the gaskets to be glued on later, figure 4.13. The deck is slightly more difficult since you must make sure the carbon follows the curve of the rails with out puckering. The larger surface also means around 1 litre of 31

Design and construction of a windsurfer longboard

A = 0.69 m2 111111111111 000000000000 000000000000 111111111111 000000000000 111111111111 000000000000 111111111111 000000000000 111111111111

Deck

2300 Ă&#x2014; 300 mm A = 0.48 m2 111111111 000000000 000000000 111111111 000000000 111111111 000000000 111111111

Hull

1600 Ă&#x2014; 300 mm Total area of patches = 1.17 m2 weight = 0.585 kg Figure 4.14: Size of reinforcement patches.

Figure 4.15: Outer layer of carbon is on. resin is required. Working solo I only just go it all wetted out and bagged before the resin gelled (the temperature was 30 o C). I mixed the resin in two 500 ml batches only mixing the second batch when it was required.

4.3

Finishing

4.3.1

Filler coat

I used West System - microlight filler (407) mixed with epoxy to peanut butter consistency (similar colour to peanut butter as well). A very thin layer was then scrapped onto the carbon-fibre. The amount of resin used for the hull was 90 g and for the deck 120 g.

4.3.2

Gaskets

Before painting I fitted the centre board gaskets. Iâ&#x20AC;&#x2122;d rebated about 2mm to allow for the gaskets. I used sailcloth folded in half and glued with epoxy glue. The thickness of the gasket is well less than the 2mm so I faired it in slightly to the hull with strips of filler.

Design and construction of a windsurfer longboard

Figure 4.16: After a scraping of filler was applied to the deck.

4.3.3

Painting

The painting layup was 1. Epoxy primer (3 coats) 2. Two pack polyurethane undercoat (2 coats) 3. Two pack polyurethane overcoat (3 coats). All the painting was done with a roller and a brush. The filler is effective in filling the residual weave pattern. For filling finer imperfections I then applied an epoxy primer. A primer coat highlights small imperfections so I applied filler to these. The quality of the original shaping and laminating becomes apparent after a coat of paint is applied. There where some small bumps and lumps on the deck and I could pick the join in the two main pieces of HDF which showed as a slight valley. These imperfections are small and I decided to live with them. I think at this stage filling and re-sanding would consume a great deal of time for incremental gains. I decided to accept the imperfections of my shaping and laminating and move on. After priming I hand sanded with 240 grit sandpaper. Then two coats of undercoat followed by sanding all over with 400 grit sandpaper. Finally three coats of topcoat with a light sand (400 grit) in between each. Note, deck grip was also applied, see below. By the way I used International Paints they provide excellent info an painting using 2 pack polyurethane including a cdrom with movies demonstrating the surface preparation and painting techniques, really useful.

4.3.4

Deck grip

I taped a perimeter defining the edge of the deck grip. Using the same topcoat paint I mixed in grip particles and applied one coat. I mixed in way more grip particles than recommend on the tin, my past experience is polyurethane is inherently slippery and I wanted to make sure there was sufficient grip. The final coat (no particles) was then applied over the deck grip coat to prevent the particles being abraded off. 33

Design and construction of a windsurfer longboard

Figure 4.17: After painting with epoxy primer.

Figure 4.18: Deck grip was mixed into the paint and one coat applied. I find mixing it in gives a more even distribution than sprinkling on wet paint.

Chapter

The finished product The weight of the bare board without footstraps, fin or centreboard is 14.7 kg. So this is 400 g above my design target. This makes sense since I tended to use more resin than the 40 : 60 fibre to resin ratio, which is really the minimum resin require to wet out the fibre. The weight of the footstraps is 850 g.

Design and construction of a windsurfer longboard

5.1

Photos from Inverloch 25/04/2010

Here are some photos taken at Inverloch, Victoria in about 15 knots of wind and using an 8.5 m2 sail.

Design and construction of a windsurfer longboard

Appendix

Tooling A.1

Compressor

Probably the cheapest way to generate a vacuum is to use an old fridge compressor. Using the suction side of the compressor a vacuum of around 80 % of an atmosphere is possible. I bought a 2nd hand one from a whitegoods recycler. Iâ&#x20AC;&#x2122;ve since picked up one from the hard rubbish collection. Be extremely careful with wiring and ensure no live wires or live connectors are exposed, if in doubt place the whole thing in an insulated box. Figure A.1 shows two old fridge compressors, one of which is connected to a pressure sensor and relay switch.

A.2

Vacuum controller

You need some way to control the pressure in the vacuum bag. Just running the compressor continually will create too strong a vacuum which is likely to crush the core. The pressure in the vacuum system can be visually monitored with a gauge, I used a the vacuum gauge from a car tuning kit, figures A.2. Details of a circuit to accurately monitor and switch the compressor on and off according the vacuum level are given below. To control the pressure in the vacuum bag I purchased a differential pressure transducer ($35). I built a circuit that amplifies the (small) output voltage difference from the transducer and compares it to the required switch on/off voltages and then sets the output high (5V) to switch the relay on or low (0V) to switch off, figure A.6. Basically the circuit should switch off the relay when the vacuum is high and the amplified voltage is 5 volts (rough figure from memory). We want it to stay off and give the compressor a rest so the circuit is designed to switch back on at 4.5V level, thatâ&#x20AC;&#x2122;s the theory anyway. However I couldnâ&#x20AC;&#x2122;t get the switch off voltage to be lower than the switch on voltage (hysteresis effect). For some compressors this does not matter as they will not switch back on immediately after they switch off, they have some circuit which creates a delay before switching back on. However other compressors will toggle on-off-onoff if the controlling circuit does not have a hysteresis effect. In the end I used an old computer which had analog-digital and digital-analog channels. This way I bypassed the comparator (compare stage) of the circuit and fed the analog out signal of the circuit

Design and construction of a windsurfer longboard

Figure A.1: Fridge compressors, the one on the left is connected to a pressure sensor and relay switch.

Figure A.2: Pressure vacuum gauge used for tuning a car engine. The gauge reading is indicating 80 kPa below atmosphere, atmosphere pressure being around 100 kPa. to the PC. A program then samples this voltage and sends an output voltage back to the external trigger (e.g. 5V to switch the relay on). Figure A.3 shows the system is use. Note I have two compressors running one is quite powerful (must be out of a big fridge) the other one is out of a bar fridge and can just hold the vacuum (at about 30 kPa) against leaks. The reason for two is redundancy in case one fails at a critical moment. Note, reading analog voltage signals into a PC requires rather expensive hardware, reading a voltage using a sound card may be possible? Sending trigger signals out is no drama as this can be done via the parallel port. If you have no easy, cheap way to sample a pressure sensor then an alternative is to forget about the pressure sensor and just switch the compressor on and off at programmed times via the parallel port. The signal from the parallel port is used to send the on/off relay signal to the compressorâ&#x20AC;&#x2122;s relay switch. Determining the on-time off-time (duty cycle) for a given bagging operation would require some trial and error. Once the bag is sealed you would run the compressor until the desired vacuum is achieved then turn the compressor off and time how long it takes for the vacuum to drop back to the desired â&#x20AC;&#x153;switch-back-onâ&#x20AC;? level, then turn the compressor on and time how long it takes to pull the vacuum back 45

Design and construction of a windsurfer longboard

Figure A.3: Vacuum system in action. In this photo I’m bagging on the rail pieces. down. This establishes the duty cycle and these numbers are then fed to the PC to automate the process. Obviously this method relies on the leakage rate remaining stable during bagging.

A.3

Hotwire cutter

To find information on hotwire cutters search Radio Controlled aircraft forums. The hotwire cutter is rather simple; just a bow that allows you to tension a wire of required length. Figure A.4 show the 3 different length bows I used, the larger 2 are made from 6 mm plywood the small one is a hand jigsaw (make sure the frame is insulated from the wire). The wire will extend on heating and for the large bows you need to be able to retention, I used eye-bolts to do this. The wire I used had a resistance of ≈ 10 Ω/m. It seemed around 2 Amps must be passed through the wire to generate the required cutting temperature (below red-hot). So for a hotwire of 1 m a 24 V power supply rated to a minimum of 2.4 A is necessary (≈ 60 Watts). However it is useful to be able to control the voltage particularly if you want to use smaller length hotwires.

A.4

Hotwire voltage control

The circuit I used to control the voltage is called a “chopper circuit” and is generally used to control D.C. motors. The circuit takes the 24 V supply as an input and outputs a square wave from 0 → 24 V. The ratio of off time (0 V) to on time (24 V) is determined by an adjustable resistor (potentiometer). You could probably find a circuit design on the internet, I purchased a kit for around $30 which saves a lot of effort and they are easy to assemble. Note $30 is just for the controller you still need a D.C. power supply, I bought a 24 V one rated to 60 W.

Design and construction of a windsurfer longboard

Figure A.4: Hotwire bows of lengths: 800 mm, 400 mm and 150 mm.

Figure A.5: Variable power supply for hotwire cutter.

Invert

Sum

−

Amplify

−(−V1 + V2 )

LM324

−

Analog out

20K

LM324

K (V1 − V2 )

12V

Compare

220K

LM311

−

10K

Trigger

Relay

Switch

BC546

External trigger

Figure A.6: My attempt at a circuit to control the compressor, not perfect.

i.e. Subtraction

Pressure sensor 2355835

Vs = 9 V

Note: V1 > V2 and V1 ≈ Vs /2

−V1

−

Pressure sensor and switch circuit

Compressor

Design and construction of a windsurfer longboard

Appendix

References and links • Building a composite windsurfer www.ecboards.co.uk • Building Custom Sailboards and Surfboards, Sail & Surf Tech Guide-4-698, SP Systems Composite engineering Systems (the only brief reference to building a raceboard that I’ve found) • How to Build Your First Surfboard by Stephen Pirsch www.surfersteve.com • Swaylocks forum www.swaylocks.com • Vacuum Bagging Techniques, West System, Cat. No. 002150 • User Manual, West System, Cat. No. 002950 • ... need to add some more refs. here