Rudders Part One
front of the blade, without crossing it. Te centre of pressure is not so easy—
Reader Bill Courington asked, “What makes some tillers feel like a direct con- nection between arm and boat, and oth- ers feel like the rudder turns some mush which turns the boat?” I am going to split this over two issues to cover it adequately. Tere are a few factors that affect what
you feel on the tiller and how responsive the boat is to the tiller. Tese factors in- teract with each other, so it’s not possible to pin down the reason for a mushy helm on a particular boat without sailing it and experimenting with changes to find improvements. If your rudder is mushy, start by check-
ing all movable connections in the system and adjust-out all sloppiness that can allow the rudder blade to move when the tiller is not moving or cause delay between moving the tiller and the blade following that tiller movement. This includes the tiller extension universal joint, the pintles, and the attachments of the blade to the rudder stock. If you turn the tiller one degree, the blade should turn the same amount. Any movement of the blade that is not due to input from the tiller will allow the blade to follow eddies in the water and feel a bit mushy. Te more slop between arm and blade, the more mush. If your boat has an outboard engine in
a well in front of the rudder and you are unhappy with the helm, try sailing with the engine removed from the well. Te engine leg and propeller dirty the water flow and you will feel the difference on the tiller. A mushy feel can also come from too
much rudder balance. Te balance is the relationship between the centre of pres- sure exerted on the blade by the water and the axis on which the rudder pivots when you move the tiller. Te axis is easy to establish; it is a line drawn through the rudder shaſt or the centres of the pintles, extended down the rudder blade. A rud- der with no balance will have the axis in
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for most rudders it is nowhere near to the geometric centre of the side view of the blade (the plan shape). If you drag the rudder sidewards through the water then the centre of pressure will be pretty much at the geometric centre of the plan shape. As soon as there is water flow from the leading edge toward the trailing edge, the centre of pressure moves toward the leading edge and becomes much more difficult to pin down. It depends on the foil section used, the plan shape and aspect ratio of the blade, and what is in front of the blade. Tat centre also moves around a bit from turbulence in the water flow. Te aim is to have the axis safely in front of the centre of pressure, so that the centre of pressure can never move forward of the axis. If the centre of pressure moves forward
of the axis then it pushes on the front of the blade and tries to turn the rudder around, i.e. to reverse it. This is very disconcerting and can yank the tiller out of your hand. This is an over-balanced rudder and can be dangerous. At the other extreme is a rudder with
little or no balance, where the centre of pressure is a long way aſt of the axis. Tis can make for heavy weather helm that needs a long tiller or a powerful mechan- ical steering mechanism for comfortable steering. Most dinghies have no rudder balance but have high aspect rudders, which reduce helm loading, and relatively long tillers. A helm that feels best has enough
weather helm to turn the boat into the wind if the tiller is not held but it does not pull heavily on your arm and become tiring. If that weather helm is lacking or too light then the feel may be mushy because a light weather helm helps us to feel when the boat is “in the groove” and sailing well. It gives us feedback so that the boat can tell us that it is happy and sailing at its best. I aim at about 15% of the area forward
of the pivot axis, 85% behind the axis. Increase the balance area to 20% and the helm may start to feel mushy and at 25% it is likely to be over-balanced. Te diagram above shows the Didi 38 rudder, which has 16.5% balance. The dashed line is the rudder axis. Those percentages are basic values for a moderate rudder in clean water
flow, i.e. it is not behind a full skeg or keel. If it is a high-aspect dinghy blade connected to a long tiller then you will likely have better helm feel with zero balance. Tiller length has a large effect on the feel of the helm because it is the lever that transmits the rudder load to your hand. Doubling tiller length will halve the weather helm load felt by your hand. Te best helm that I have experienced
was on my own Didi 38 Black Cat. She has very light helm in light conditions, gradually increasing in heavier conditions until it tells you clearly that you need to reef down in strong conditions. And if that warning is ignored, the rudder stalls, the helm goes light and the boat heads up until the crew reduces power in the main to take load off the rudder. I used the feel of her helm to guide me for trimming the boat for best speed. Moving weight forward brings her bow down and increases helm, as does heeling her to leeward. Moving weight aſt and to windward reduces the load on her helm. Tis meshed very well with her need to be bow-down for best speed in light con- ditions and stern-down for best speed in heavy conditions. On my Paper Jet I sail singlehanded. In
light conditions I helm from far forward in the cockpit. In strong conditions I am aſt and hiked out to windward or on trapeze. My position affects the steering characteristics of the boat. Most boats are affected by fore/aſt trim and heel angle, felt through the helm. Experiment on your boat by moving crew around the so that you can feel what it does. In Part 2 of this article I will discuss foil shape and a few other issues. SCA•
SMALL CRAFT ADVISOR
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