DAVE HOLLOM ON DESIGN
Towards a faster 5.5 With
two newbuilds under way in
Cowes,
on the Isle of
explains the design process and how it all came about. to be ready to launch by late
I
n 2019, in time for the 2020 Worlds, I produced a prospectus for a proposed new 5.5 Metre. It contained a number of new avenues of possible progress in the design of 5.5 Metre boats and it was hoped that the Australian, David Hayter, who very successfully sails one of my International Fourteen dinghies, would perhaps be able to contact prospective owners during those Worlds in Australia. As it turned out Peter Morton otherwise known as Morty, a British owner new to the class and who was keen to have his own 5.5, decided to take matters further. It all started in February of 2020 with a visit to Cowes to discuss the project with Steve Quigley an Australian naval architect and trusted advisor of Morty. During the design study used in the prospectus I had produced a number of VPPs to test various areas within the allowed rule space and Steve seemed duly impressed that the VPP (velocity prediction program) that was closest to Morty’s existing boat, agreed extremely well with the sailing numbers of that boat. A few weeks later Morty decided to push the go button on the
24 • fivepointfive • APRIL 2021
May 2021.
Wight, Dave Hollom
The
boats are expected
project with me designing the boat and with Steve running CFD (computational fluid dynamics) tests on each design. To add to the strength of the team Tom Schnackenberg of Australia II and New Zealand America’s Cup fame, and a very close friend of Morty’s, was recruited to oversee the performance predictions and David Lenz, Ruairidh Scott and Sam Haines looked after sail design. The boat was to be built by Gavin Tappenden of Composite Craft in Cowes with whom I have had a long-term relationship that spans a fair few successful boats. Andrew Palfrey, aka Dog, looked after the rig and general control systems on the boat. testing My in house VPP, rather than using standard sail coefficients, derived either from wind tunnel tests or from full size tests using tank test data scaled to full size, to work out the sail forces, uses a very simple rig model based on well researched aerodynamic theory. Given the aspect ratio and a lift coefficient (Cl) it works out the drag coefficient (Cd) and from that information together with sail area, apparent windspeed and heel angle it works out all the rig forces. The program then works on the ‘what if’ system. It asks the question ‘what if, in a particular true windspeed, the boat sailed a particular course to the true wind at a certain speed’. It then works out, from that given information, the apparent wind speed and angle and then from the rig model, assuming a given sail lift coefficient (Cl), the sail forces, both heeling and driving force. It then calculates the yaw and heel angle required to resist that heeling force and then compares the aerodynamic forces at that heel angle with the hydrodynamic forces working on the hull at that speed and that heel and yaw angle. Unless by some fluke chance they balance, which is highly unlikely, the program will alter the speed and try again until, by a process of iteration, it finds a boat speed at which all the forces balance to within a certain fine tolerance. At that point it
