6
12
April 2019
The only way an RC yacht can move in a straight is by the accurate alignment of CLR and CE.
However any particular setting is only good for a particular set of wind and wave conditions. And
may only be good for a particular tack.
In a wind gust these balanced forces become out-of-balance. The wind force on the sails increases
with the square of the wind velocity but the counteracting force from velocity of water over the foils
does not increase because in the instant the gust hits the boat speed has not increased and the boat
rounds up.
The second impact of a gust is to heel the boat. The underwater shape of the hull changes from one
that is symmetrical (when mast is vertical) to one where the line of least resistance is for the hull to
move to weather. So to counter the boat rounding-up, rudder is used to force the bow down. But as
the hull heals the rudder has less effective area in the direction of travel and more and more rudder
has to be used. Any rudder movement slows the boat because of Newtons First law and the greater
the rudder movement the greater the impact on speed. In short, any rudder movement is slow.
The other means of countering the effect of a gust is to ease the sheets for the duration of the gust
and a way of doing this is by a
Throttle curve
set-up using Switch B – Winch Control. It is
acknowledged that the sheets can be eased with slight winch movement at any time – SWB just
might just make it easier and more consistent.
Also in a gust the apparent wind moves aft and as we have suggested, the impact on heeling-over
and rounding-up can be minimised by easing sheets and increasing velocity. However this is not
the only possible response. Theoretically one could remain close-hauled and benefit from this
velocity lift by sailing a slightly higher course. In a dinghy this is often possible by hiking harder
and thus keeping the hull flat.
The recommended response to a wind gust is to slightly ease sheets although this needs to be
contested.
No recommendation is made regarding taking the velocity lift and I would welcome feedback for
further dissemination
Rudder Use Discussion
If during the starting sequence or when racing in a variable wind, and the boat is above close-hauled
and either stationery or moving slowly, like a dinghy, it is possible that by sculling, she may be
brought down to a close-hauled course. This does not violate RRS 42 Propulsion. See RRS 42.3
(d). That is, if the hull lies above close-hauled on the Port tack, by moving the rudder stick to the
left and releasing so that the spring loading moves it to the centre neutral position, the scull may
force the bow down to close-hauled.
This suggests the rudder
End points
setting may have to be greater than that required for normal
steering.
It is essential to have a physical mid rudder reference point on the underside of hull so that before
launching one can determine whether the rudder is physically aligned fore and aft. To establish this
neutral rudder reference point, remove rudder and turn the hull upside down. Carefully align a
straight edge between centre of keel and centre of rudder post opening. Using a fine permanent
marker, draw a short line aft of the rudder trailing edge position. This will be a reference for a
neutral rudder.
Assuming a 35 degrees rudder throw say, and using a protractor or 30/60 set square, mark the
position of the trailing edge of rudder when helm is hard over - both tacks.
If there is any rudder slop, insert a short length of rigging line into the rudder post hole and trim
excess later. Replace rudder and secure after carefully aligning with neutral helm marker. Ensure
rudder moves freely.