UK-Cherub Class

Get Your Heart Racing

Building a T- Foil Rudder for a cherub.

Most of the Ideas for this came from looking at the back of several International 14s and reading all the chat that has gone on about T foils since I first saw them in Beer on the back of Paul Bieker's 14. I have never studied hydrodynamics or any kind of boat building so all my sizes and construction methods are more “best guess” than calculated.
The basic idea of the T-Foil in the boats like the i14 and the Cherub is to lift the transom of the boat while going upwind allowing the crew to sit further back. So that their weight is taken on the more hydrodynamically efficient foil rather than just through displacing water. Or, while planing, from the flow over the lower surface of an incredibly low aspect ratio foil (The hull).

The problem is that when you are going down wind the last thing that you want to do is to lift the transom thereby pushing the nose down. To get around this the angle of attack of the foil must be adjustable so that you can neutralize the lift and even go negative for the bearaway and the downwind leg. Over the years the two main designs that have evolved are the Bieker system, where the horizontal foils are supported on a central axle and are made to move up and down using a rod down the center of the rudder, and the Fixed Foil system where, the whole rudder, stock and pintle is moved in order to change the angle of attack. I decided that the Fixed Foil system having less moving parts was going to be easier to construct in my garden shed so this is what I will describe. For a bit of comparison have a look at a Video showing the transom wake two Patterson 7's “AquaMarina” and “Little Red Number” sailing in light winds Aqua with a T-foil and LRN with a standard one.

There are three main parts to the T-Foil system as used on AquaMarina:

  • The Rudder with horizontal Foil
  • The stock
  • The control system

The Rudder

I decided to build a separate rudder for the T-Foil rather than modifying my current rudder so that if the system did not work or broke I could still go back to the original and should not lose too much sailing. This also meant that the control system on the back of the boat had to be backwardly compatible with the old rudder and stock.Rudder Before Assembly I started off by building a foam carbon rudder with a bit more carbon uni’s towards the trailing edge of the blade than normal to take the loads transferred up from the horizontal foil. See technical article on building a rudder blade for the general idea on how to build the blade. The basic T part of the foil was planned to be a symmetric NACA four digit section but ended up being a bit thicker and out of shape due to some build errors and not taking into account the thickness of the laminate properly. The first forizontal foil that I made was 1,000mm long with a max thickness of 10mm and a chord length of 100mm. Unfortunately it broke due to just being carbon and a foam core with no real crush resistance. Due to the loads the central foam crushed reducing the thickness and therefore the strength with the inevitable result. The second Horizontal foil is 900mm long (being the length of wood that I had.) with a chord length of 100mm and a max thickness of 13mm. It was built on a cedar core with a layer of 200g woven carbon followed by three layers of 200g uni’s tapered in from the center. The center layer was approx. 100mm next 300mm and last 700mm long it was finished off with a second layer of 200g carbon cloth and a 150g glass cloth all hand consolidated with a layer of peel ply.Rudder Assembled

I then cut a hole in my newly constructed rudder around two thirds of the way down slotted in the horizontal blade made sure it was all square added a bit of filler and carbon to hold the two bits together. Then it was just a matter of fairing the whole thing smooth.

The Stock

The stock is constructed in the same way as a normal dagger rudder stock the only differences being that you must be able to insert the rudder from the bottom as the T will stop you from inserting the rudder from the top. It will also need to be a bit stronger than normal as it is not only taking the normal rudder loads (bigger than you might think) but also the lifting loads from the T-Foil, which could be somewhere in the region of 60 to 80kg. Attachment for the pintle rod also needs to be further away from the rudder blade especially at the top to give room for the adjustment mechanism. You are looking to get a total of 9 Degrees of movement, from Plus 7 to Minus 2. So the actual range of movement at the top needed depends on the distance between the top and the bottom gugion but with a distance of 20cm a bit of trig gives the answer as just over 3cm. There also needs to be a way of locking the blade down so that it does not lift up with the lift from the foil I used a hole saw the same diameter as a piece of aluminum tubing to cut through both the rudder and the stock. With the tube tied to the rudder with a piece of elastic it is simple then to stick the tube through the hole when the rudder is down to lock it in place.

The control Mechanism

The basic idea behind the control system is to give you the scope to easily change the angle of attack of the horizontal foil without upsetting the feel of the helm too much. This is done by moving the axis of rotation of the rudder blade, the pintle rod, in relation to the boat, while keeping it parallel to the rudder. Effectively this is done by rotating the 8mm stainless steel rod which acts as the pintle about the bottom of the gantry and allowing the top of the pintle to slide backwards and forwards in a slot at the top of the gantry.The control of the system is done using a pusher rod which forces the top of the pintle rod back in the slot thus increasing the angle of attack of the T foil. The drag from the water and some elastic is used to pull the rudder forward again. The push rod is moved using a six to one purchase lead to a cleat on the front of the gantry. The bottom gudgeon was made by adding an extension to my existing gantry out of a bit of foam with a thick layer of carbon plate on the bottom this was then covered in carbon top and bottom to make a nice solid base. A hole was then drilled on the centerline for the rudder pin to pass through. This hole had to be filed to an oval shape to allow the pin to rock back and forward as the angle of attack is altered. In practice I found that after a few days sailing the carbon had warn away and the pin was now sloppy. Bolting a stainless steel plate to the bottom so the plate takes the sideways loads and the up loads are still taken by the carbon cured this.

The pusher rod is made from a stainless steel block welded to a bit of 8mm stainless rod. It fits into a short section of glass tube, which was made by wrapping the glass around the well waxed piece of 8mm rod. This tube is then mounted on the top of the gantry on the centerline in such a way that it will bear on the pintle rod. To make the top gudgeon I made up two carbon plates and placed two 8mm rods between them to hold them parallel then a bit of carbon over the end to hold the whole lot together. This was then offered up to the boat and with the pintle rod and the pusher rod in position holding the whole lot square and parallel the whole lot was wrapped in plenty of carbon. With the addition of a 6:1 purchase to pull the pusher rod back and a cleat fitted to a bracket that I made for the front of the gantry the control system was complete.

Using the T-Foil


Thanks to the wings the rudder had to be slotted in from the bottom of the stock. Take the boat out to thigh depth water then tip it over about 30 to 40 degrees. The rudder then slots in to the bottom and the boat can be brought upright with the tip of the rudder sticking about 10cm below the bottom of the boat. Once in deep enough water push the rudder down and put in the locking tube and the system is ready to go.

On the Water

Upwind the foil is used to stop the transom dragging and take some of the weight of the boat. In practice you set the foil to the angle of attack that you want and then use your weight to trim the boat to the same sort of attitude as it would have been if the foil was not there. The lift generated by the foil depends on both the angle of attack of the foil and the speed of the boat through the water. To keep the boat trimmed correctly it takes a bit more body movement than normal, as when the speed of the boat changes the lift generated by the foil changes and so you need to move your weight to counteract. In lighter winds I used less foil as I did not want the drag penalty, moving up to maximum as my crew moved on to the wire. In stronger winds when you are overpowered as the speed of the boat increases you need to reduce the angle of attack as the foil will now be generating more lift than is needed. In general the foil needs to be adjusted less than the kicker

Bearing away and downwind

It is vital to let off the foil before bearing away or a capsize is inevitable. Downwind the boat feels a bit smoother slightly less bouncy but there is not much difference from a standard foil. A few times we tried pulling the foil on a bit in light I am not sure whether it helped or not. In theory if it gets really mental it would be possible to set the foil slightly negative but I have not had the chance to test that yet.

Returning to shore.

With the locking tube removed it is straight forward to get the foil up till the blades touch the bottom of the stock this still leaves enough to steer by. Once out of the boat just tip it over a bit and the foil can be removed from the bottom of the stock.

But is it quicker?

I think that there will be a bit of a speed penalty down wind but hopefully the improved upwind speed will more than pay for it considering how much more time I think that with practice it will be quicker especially in the marginal tricky zone for cherubs. In ghosting conditions one would expect it to be very slightly slower. In mental conditions one might expect it to be v marginally slower, but more controllable so it's probably worth it.

tech/t-foil_rudder.txt · Last modified: 2013/06/25 15:55 (external edit)