Getting the most from your Rudder - by Ben
Morris
Return to Sailing Hints
page to examine other Hints
Too often the rudder is overused and results in the yacht
slowing or a loss of control of the yacht or both. How does this
happen? We need to look at the nature of the rudder and how it
operates. Like the sails and the fin, the rudder is an airfoil and
obeys the rules that apply to all airfoils only in this case the foil is
operating in water. Not surprisingly this makes the rudder much more
effective as an airfoil than say the sails operating in air as the density
(actually viscosity) of water is much greater than air which means the area
of the rudder can be much smaller than the sails yet still generate quite
large forces. The other readily observed factor is
that the rudder become more effective as the speed of the yacht increases.
These ideas are encapsulated in a concept called the Reynolds Number which
defines the nature of the flow of fluids. The flow can be laminar
where the fluid flows smoothly past an obstruction resulting in low drag or
the flow can become turbulent resulting in high drag. Most real
situations in sailing produce some turbulent flow so its a matter of
reducing this turbulence and thus drag.
So what determines the degree of turbulence and hence drag in
the rudder?
The shape and size of the rudder. The size effect is
easier - the larger the rudder the more the drag, The shape is more
complex but can be simplified to it's thickness and the ratio of width to
length (aspect ratio). In general a thin rudder moves less water out
of the way and will produce less turbulence (starts further back along the
rudder) and thus less drag while a long narrow rudder should also produce
less turbulence and drag than a short wide one (turbulence starts the a
similar distance back from the front so less rudder in turbulence if it is
narrow). The cross-section shape is also important and one like a wing
with a smooth rounded leading edge and a nice sharp trailing edge will be
better than a flat plate especially if the leading edge is not rounded or
the trailing edge is not sharp. In addition a smooth fair surface will
produce less turbulence/drag than a rough bumpy one.
So if all that we had to was to reduce drag we would all have
long narrow this rudders with a good airfoil section with rounded leading
edge and sharp trailing edge. Have you seen ones like this? and they
are fine BUT.........
A rudder however has to do more than just move through the
water with a minimum of drag. It also has to turn the yacht to go
about, gybe or supply a force to overcome weather helm. Under these
conditions the rudder is now moving at some angle to the flow of water and
producing a force by changing the direction of the water flow off the
trailing edge. The rudder will the be held at some angle to the flow
of water over it.
This introduces an addition force (lift) at right angles to
the rudder which can be resolved into two component - one at right angles to
the yacht trying to turn the stern of the yacht and another acting backwards
slowing the yacht. Providing the angle of attack (angle between rudder
and incoming water flow) is small say less than 10° then the major force is
the turning force while the slowing force is small. However, if the
angle of attack becomes much larger then two things happen. One is the
the slowing force become increasingly larger but even more significant is
that the water flowing over the leading edge now has to turn through a large
angle in order to remain smooth. Eventually the flow will become very
turbulent not flow smoothly. This causes the sideways force to
drastically reduce while the slowing force dramatically increases. So
not only will the yacht not turn but it will act as if a brake has been
applied! So often I see this happen near a mark or near the reeds when
the natural instinct is to apply more rudder to get out of a sticky
situation such as missing the buoy or not sailing into the reeds.
To make matters worse thin section narrow rudders stall at
much lower angles of attack than thicker section wider ones. So
that beaut low drag thin narrow rudder is great when going straight but can
cause real issues when using it to turn as it stall so easily losing grip on
the water and creating significant drag.
What can be done then? The answer lies in the way the
rudder is used and that is determined by the way the rudder stick on your
transmitter is used. When using a rudder you need to keep the angle of
attack between the direction of the water flow and the rudder blade to less
than 10°. As the yacht starts to turn the stern of the yacht moves
sideways so the water flow is now going across the hull allowing a greater
angle of steering to be applied. In other words the rudder is moved
gently to one side and should only reach the maximum provided the hull is
already turning fast. You will soon come to realise if your doing it
too quickly get out there and practise turns many times.
What happens if you did turn your rudder too quickly?
What do you do? You should notice a braking effect on the yacht and a
lack of turning. Immediately you need to realize the rudder has
stalled. You need to very quickly get the water flow reattached to the
rudder and the only way to do that is to let go of the rudder stick so that
the water now flows over it smoothly then start turning the rudder gently
again and this time things should work. This action is
counter-intuitive as the mind says as you are heading into the mark or the
reeds 'turn the rudder more!!!'. You have to have the courage to
straighten the rudder first to allow the rudder to work then begin your turn
again.
There are other ways to assist this issue and one of them
involves having what's called 'differential' programmed into the
transmitter. See a whole section on radio selection and use in another
Hint page to come soon.
Where should the rudder be pivoted? The centre of lift
of an airfoils lies somewhere between 25-30% of the chord length back from
the leading edge. The pivot position or rudder post then should be
placed a little in front of the say at 15% - 20% of the chord. Where
the rudder is not a rectangular section the placing the rudder post at 15% -
20% of the average chord will do. The reason for having the pivot
position in front of the centre of lift is that the force generated by the
rudder will always be behind the rudder post which will attempt to
straighten the rudder back to the centreline of the yacht. Having the
rudder post in this position will mean the force required to move (or hold)
the rudder is less. The rudder servo will supply the force necessary
to keep it in position. If the rudder post is situated closer to the
leading edge then the rudder servo will need to generate a greater force to
keep it turned. This causes additional current drain on the system and
will hasten the wear on the servo bearings.
Why not place it further back then? If the rudder post
is too far back and lies behind the centre of lift of the rudder airfoil
then the force acting on the rudder will attempt to turn it further.
While the servo will attempt to hold it there, any slop in the drive system
will allow it to move that little bit further accentuating the turning
action or producing oversteer. The skipper will need to correct in the
reverse direction and oversteer will occur there. The result can be an
oscillation of the rudder causing additional drag while steering becomes
very erratic. If in doubt, err on the side of having the rudder post a
little more forward
Conclusions.
Stick to your thin rudder sections as they really help reduce
drag when going straight which is most of the time and learn to recognise
when the rudder shows signs of stalling and respond accordingly and lastly
make sure your yacht is balanced when beating to windward so that the rudder
doesn't have to generate lift all the time. See another new Hint page
to come soon on balancing your yacht
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