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12
April 2019
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Model 10
DF65 A+
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Weather Helm Discussion
These musings are meant to provide some background to the set-up of the DF65 winch and rudder
using the FlySky system.
When racing, ideally, in steady-state conditions, we want the boat to track in a straight line
particularly to windward with slight weather helm. If there is too much weather helm the boat will
round-up and go into irons.
There are a number of measures theoretically possible to achieve balance:
1.
Constant and skilful rudder movement will provide straight line tracking however rudder
movement is slow as the rudder when used provides a greater projected area. It acts a bit
like a brake.
2.
Mast rake by adjusting forestay and backstay tension. By this adjustment the sail’s
combined Centre of Effort (CE) can be moved fore and aft relative to the foil’s Centre of
Lateral Resistance (CLR). By this we are essentially controlling the couple between these
two forces. There is plenty of reading on the internet and elsewhere about this and wont be
repeated here. Generally a small amount of weather helm is safer and it is desirable to
provide a bit of feedback to the helmsman otherwise the steering will feel dead. The latter
does not apply to RC yachting.
There is no known, easy way of setting up optimal mast rake except by trial-and-error. The
recommended measurement from a reference point on the upper mast to a point on the deck
is about all there is. This method is a guide only and not very accurate. See
3.
Changing the CLR by lifting the centreboard/keel (impossible with DF65)
4.
Reduce the power of mainsail by easing main sheet (more of this later), flattening foot or
ease boom vang to soften leech (ie. spill more wind from back of sail)
5.
Reef main
6.
Increase power of jib by moving its CE forward and thus moving the entire rig CE forward
In an RC yacht a bit of weather helm is desirable, not for feel, but for another reason. When
moving through the water the foils provide lift. This is analogous to the wing (foil) on an
aircraft, particularly a glider. The downward gravitational force of the glider is balanced or
counter-acted by the lift provided by the wing. One approach with DF65s might be to use
the Trim tab referred to earlier and this might be a quick fix on one tack however it may
have to be removed or re-applied on the other tack. There is no known way that an RC
yacht can be provided with an offset rudder to provide the required minimal amount of
rudder on both tacks.
