After having a look at my (bent) rudder, I did some research and came up with this interesting article.
Balanced rudder and Flying Fifteen sailing.
The classic fleet of Flying
Fifteens in Cardiff Bay is an excellent example of diversity; the range of
boats varies from Shepherds, Copland, Windebank, Wyche and Coppock all designs
that employ a balanced rudder. Whilst we need to recognise that there is a
considerable range of crew ability, it also has to be recognised that despite
the boats all being Flying Fifteens they are quite different in performance.
In fact, despite there
being some excellent classic tuning guides available, each boat builder needs
to be treated as a separate ‘mini class’.
Issues such as mast and
keel position, shroud separation and length, jib track position and mast
ramming make enormous differences to performance and pointing ability.
There is, however one often
forgotten feature of the Flying fifteen that has a really significant effect on
performance and enhances the sailing experience his is, of course, the balanced rudder.
When Uffa Fox designed the fifteen he did not have the benefit of hydro dynamics to establish best practice for foils. Nevertheless the Flying fifteen its balanced rudder together with its keel as a combination work pretty well together.
When Uffa Fox designed the fifteen he did not have the benefit of hydro dynamics to establish best practice for foils. Nevertheless the Flying fifteen its balanced rudder together with its keel as a combination work pretty well together.
A modern design of rudder
probably would not come up with what we currently use.
The shape of the balanced
rudder together with its angle of installation means that as it turns and so
changes the direction of the boat. It experiences other forces that can affect
the speed of the boat.
These combined forces are
the feedback that is felt through the tiller and it’s extension and affects the
‘feel’ of the boat.
The article hopes to be
able to explain what these forces do, give the theory of how a balanced rudder
works and how we can change them it will also show some examples.
Influence of the balanced rudder on the boat
If everything is square, in
that by that the post is vertical and the boat is level, the only effect of
turning the rudder is that it changes the direction of the boat.
If the rudder is turned
when the boat is heeled, say to windward then pushing the tiller away, (heading
up) will lift the transom up and drive the bow down.
Still heeled to windward
and pulling the tiller toward you (bearing off) the rudder acts as a lowering
plane and drives the transom deeper into the water.
This is one of the many
reasons why boats go faster when they are upright. Most modern dinghy designs
have a vertical rudderpost.
So altering the angle of
the rudder post simply by heeling the boat has an effect, if the angle is
altered further by moving the top of the post forward, it is not surprising
that the boat will experience additional forces, these are not always
beneficial.
With the flying fifteen,
not only is the post not vertical, but also certainly within the classic fleet
the angle of the post and its position in the boat are inconsistent.
A survey of our dinghy park
reveals the position of the post exiting the hull varies by as much as 50mm out
of a sample of 9 classics.
Add to that the position
that the post enters the balanced rudder varies by about 30mm and the angle
that the post passes through the boat and the overall rudder performance can
vary by a considerable amount.
A cursory analysis
indicates that the overall position of the rudder fore and aft in the boat can
be different by 50mm. When you consider moving the mast step 10mm can transform
the feel of a boat it reveals how significant rudder optimisation can be.
So can we do anything about
the weather helm that remains even after optimising keel and mast position?
Well one option is to
consider getting an optimised or balanced rudder made specifically for the
boat.
What is a balanced rudder?
A rudder has a shape
similar to a wing, when water flows over the rudder that is straight it passes
evenly on both sides and the pressure on both sides is even.
When the rudder is turned,
the water strikes it with a greater force on one side, and a lesser force on
the other side.
The rudder moves towards
the lower pressure and takes the stern with it, so turning the boat.
An unbalanced rudder is one
that has its entire surface behind the post. Typically a balanced rudder
has about 20% perhaps up to 40% in front of the post.
It is described as a
balanced rudder because at some point the pressure behind will counterbalance
the pressure in front of the post. When the rudder is balanced the feel of the
rudder will be light. A compromise is built into the rudder to give the rudder
some feel or feedback to sail with.
Balanced rudder today
When Uffa Fox designed the
Flying Fifteen he was designing a planing keelboat, the keel was basically in
place to offer ballast in the water and support on land. The sideways
resistance (CLR) offered by the keel is actually quite small compared to the
sail area. So by utilising a balanced rudder, Uffa Fox was able to increase the
lateral resistance of the boat, clever move!
By tweaking or optimising
the balance we can reduce the pull felt on the rudder that we describe as
weather helm making the boat much more pleasant and less tiring to sail.
The balanced rudder for
classic fifteens is a customised build, if you are considering a build its
worth talking to the builder first, he will need to know the angle that the
post tube makes with the bottom of the boat, the original builder might be
information that will help him as well.