4 minute read
1. Introduction and philosophy
If the steering had really jammed, it would more than likely be steering the boat round in circles. Quite often, autopilots can be switched from standby to auto by people without them noticing. Autopilot controls on sailing boats are often set into the cockpit coaming where crew sit and, shuffling about, they can hit the auto-on button by accident.
Worth remembering – most motorboat engines won’t start when they are in gear – it’s a safety feature… or if the kill cord isn’t attached. Most yacht engines will start when in gear.
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Never drive a boat with a kill cord attachment if the kill cord is not attached to your wrist or thigh. Carrying a spare kill cord will enable a passenger to start the engine and pick up the original driver who fell out of the boat and is now 30 or more metres behind, with the kill cord still attached to them!
p As it says…
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Stress-free engine maintenance
Introduction and philosophy 13
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Gearboxes and drives
The type of vessel determines the best propulsion system for it.
Shaft and propeller
The most basic and traditional type for an engine is a propeller on a shaft. It is a simple design using a basic gearbox with forward and reverse and the gearbox is normally bolted on to the back of the engine, aft of the engine. Attached to that is a coupling, which then attaches to the shaft. The shaft then goes through the stern tube set in the hull of the boat and some sort of seal is used, either a stuffing box, lip seal or face seal, to allow the shaft to turn in the tube and yet prevent water from entering the boat.
The propeller will be fitted on to the end of the shaft. There may be a keyway in the shaft and the propeller to prevent it slipping on the shaft. A keyway is a slot cut in the shaft and the propeller into which a rectangle of metal plate slots and is held in place by the propeller once fitted.
Having a keyway also reduces the chance of the nut on the end of the propeller coming loose as there is much less movement of the propeller as it moves from forward to reverse gear than if there was no keyway.
The shaft itself may be held in position by the addition of either a cutlass bearing, which is fitted into the hull at the exit point of the shaft, or it may also be suspended on what we call a ‘P’ bracket, which supports the shaft and houses a cutlass bearing within it.
A ‘P’ bracket allows you to have a longer shaft. This is important when it comes to getting the propeller closer Gearshift solenoids Drive shaft coupling Drive shaft
p Shaft drive and propeller.
Volvo Penta TAMD 63-P gearbox
Dipstick to check oil
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Stress-free engine maintenance
Keyway in the propeller
p Propeller showing keyway. P-bracket (shaft removed)
p P-bracket and cutlass bearing inside. Cutlass bearing in P-bracket
to the rudder, which allows you to throw more water off and so do not need a large rudder to deflect the rudder and thus make the rudder more responsive sufficient water to be able to give them steerage. under engine. • Sailing boats heel a great deal more and need a large
You will find gearbox and shaft propeller systems rudder to deflect enough water when the angle of on both sailing boats and motorboats, but a note about the rudder is no longer perpendicular, especially as steerage: they are also travelling relatively slowly through the • The rudder on a sailing boat becomes less efficient the more it heels. water by comparison with most motorboats.Motorboats generally go faster than sailing boats
To overcome this, designers have introduced twin rudders, but what you gain on the swings you lose on the roundabouts as this can make close-quarter handling trickier. Generally you need a bit more speed than with a single rudder to get a flow of water over the rudders that are angled away from the perpendicular to get the best out of them.
q Twin rudders on a sailing boat.
When perpendicular, the rudder is very efficient at deflecting water left or right to provide steerage. As the boat heels, the rudder starts deflecting water not so much left and right as partially up and down and so becomes less efficient.
As a consequence of having small rudders, motorboats do not derive much steerage from the rudders in slow speed close-quarters manoeuvres or when going astern. If a motorboat has twin engines, then it is these working individually or in opposition that will be used when close-quarter manoeuvring, or of course bow and stern thrusters, which you will also find on some sailing boats.
