Balanced rudder in Classic 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.
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 planning 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.
By Paul Taylor