UK-Cherub Class

Get Your Heart Racing

Setup a T-foil Rudder

In the past few years the T-foil rudder has enjoyed increasing popularity in the UK-Cherub class. This page aims to look at the different systems that have been used on various boats and the advantages/disadvantages of each.

Basic Principles

The basic principle of the T-foil as used on the Cherub and other classes such as the International 14 is to use the lift from the foil to reduce the displacement of the boat, this helps to promote planing. As the lift is produced by the rudder at the stern this will tend to lift the stern and push down on the bow. Moving the crew weight back in the boat will counteract this returning the hull to the correct trim. The movement of crew weight is effective when going upwind, however down wind it is not possible to get far enough back to keep the transom on the water and the bow up. To prevent undesirable bow-down trim (and possibly an unstoppable pitch pole) when going around the windward mark you need to reduce the lift from the foil. The easiest way to do this is to reduce the angle of attack of the foil. Typically the range of angles required are from plus 7 to minus 2 degrees. The boats t-foil system will need to be able to cope with adjustment across this range without any adverse effects on the boat.

The system should be easy to adjust while sailing as having the t-foil in the wrong configuration will slow you down upwind and could lead to a pitchpole downwind. Many systems use a control line lead to the sidedecks for adjustment and are used in a slightly binary fashion: On upwind and Off downwind. Having the adjustment setup so that twisting the tiller extension controls the angle of attack has the advantage immediate adjustment. The ability to do this is of particular importance when using the foil down wind as with a bit of foil on the acceleration from a gust can lead to a serious nosedive. However if the loads on the system are high then to get a low torque on the tiller extension a powerful gearbox is required meaning that many turns of the tiller are required for the full range of adjustment, making the adjustment at windward mark rather slow.

System Requirements

  • Strong enough to take the loads from the rudder and T-foil.
  • Adjustable to give a range of angle attack from approx +7 to -2 degrees.
  • Allow full range of motion with limted play or distortion of the system.
  • The feel of the helm should not change too much over the range of motion.
  • Easy to make both small and large adjustments in angle of attack.
  • Light.
  • Low Drag.

Names of Things

Stock Typically of the dagger type where because of the blades of the T-foil you have to insert the rudder in from the bottom

Pintle The Vertical pin that the rudder rotates around as you steer.

Gudgeon The holes both on the rudder and boat that the pintle fits through

T-Foil The horizontal lifting foil parts of the rudder.

Systems in use

Rotating Pintle

The basic idea of the rotating pintle is that the entire stock including pintle rotates to change the angle of attack. The pintle stays at the same angle to the rudder throughout the range of motion so that the feel of the helm does not change very much. All the adjustment is done on the boat so any weight is measured with the boat giving a slight decrease in overall sailing weight. One disadvantage is that as the adjustment system is on the boat it works best using a control line lead out to the sides, it is tricky to set the system up with a tiller twist grip.

The push rod system on AquaMarina and Primal Scream allows the pintle to rock back and forward around a fixed point at the bottom, but with the top able to slide forward and backward giving the motion. A rod with a block on it pushes against the pintle to increase the angle of attack. In light winds some elastic can be used to pull the pintle forward again.

While the system is simple it has limited range of movement limited by the gap between the pintle and the leading edge of the rudder. Also the bottom gudgeon for both the rudder and boat needs careful design to avoid wear as it is highly loaded yet needs to cope with movement of the rudder in two planes. While it would be possible to route the control system up to the tiller twist grip with this system there are problems with the large loads involved and steering affecting the adjustment.


Rotating Gantry

With the Rotating gantry system as used on Atum Bom the entire gantry rotates. It is held on two bearings on the back beam of the boat and is controlled using a purchase from the deck to the gantry that prevents the gantry from rotating to decrease the angle of attack, it is held up using some strong elastic. This system has the advantage that there is no real restriction on the range of possible movement, all the adjustment system is part of the boat so is included in the measured weight It may be difficult to eliminate slop in the system and if you get wear in the bearings they would require a major rebuild to repair. Contrary to what may be thought, there are no serious consequences of a failure in the control string: The unrestricted t foil adopts an angle of about -3 degrees and flies along like an upside-down kite, only floating up when the boat stops. Although the main control is through a string system to a cleat in the boat, this system has also been setup with a twist grip tiller, because of the positioning of the block there is a slight change in angle of attack when steering. In addition because of the high load in the control line a large purchase is required to make the twist grip work this means that it takes many turns of the tiller extension to get a significant change in angle of attack.

Part BuiltRotating Gantry from a distanceRotating Gantry closeup

Rocking Plate

An alternative approach to the rotating Gantry is the rocking plate system as used on Ronin. This uses a plate with gudgions on that rotates around a bolt in a carefully profiled gap in the gantry. This system can have a larger range of motion than the push rod system and because the pintle is fixed with respect to both the rudder and the rocking plate it is easier to make the gudgions tight and wear resistant. To keep slop to a minimum the gap that the plate sits in must be a close fit and needs careful design to ensure that grit or sand does not cause jamming.

Rocking Plate GantryRocking Plate Gantry Adjustment

Rotating Stock

With the rotating stock as used on amongst others Subtle Knife, Cherub Dæmon the pintle is fixed with respect to the boat and all adjustment is kept within the stock, this has the advantage that no modifications need to be made to the boat when retrofitting. In addition it works well with the twist grip tiller extension. To provide the angle of attack adjustment the pintle is allowed to slide back and forward in a slot in the top of the stock extending into the tiller. The movement is controlled by twisting the tiller with the direction changed through a gearbox and the mechanical advantage gained through a worm drive. Care needs to be taken to ensure that through the range of motion the interface between the bottom gudjions remain wear free at all angles of attack. One disadvantage of this system is that as the angle between the pintle and the rudder changes so does the balancing or feel of the helm. This will typically lead to a light helm upwind and heavy downwind. The range of this motion needs to be carefully setup so that the feel of the helm does not become either too heavy or overbalanced and too light.

Whole StockClose up of slot in tillerStock from sideStock and Gantry

cardinal_sin-20070112g.jpgcardinal_sin-20070112h.jpgcardinal_sin-20070112i.jpg Alternate stock design

Rotating Foil

With the Rotating Foil system as used on the majority of Beiker Int 14's and }} the blades of the T-foil itself rotate with respect to the rudder this allows the t-foil to work without changing the positioning or feel of the rudder. There is not much force required to move the blades and all the adjustment is contained within the rudder so this system is ideal for using with a twist grip and no modifications need to be made to the boat to fit. The disadvantage of this system is that to give enough strength in the axle the blades often need to be slightly thicker than those directly attached to the rudder. In addition to keep slop to a minimum the tolerance between the axle and bearing in the rudder need to be tight with many “International 14's using titanium axles.

Flexible Flap

The Flexible flap is the system favoured by the International Moths and was used by Shiny Beast at the Nationals in 2005. The Flexible Flap system has a fixed T-Foil with the back third able to rotate. The flap can either be attached using a piano type hinge or a section of silicone rubber or rubbery epoxy impregnated Kevlar. As the Flap moves this changes the profile of the t-foil changing the amount of lift generated. This is effective for small angles of flap but at large angles the sharp change in shape at the hinge point becomes quite high drag. Again because the system is entirely held within the rudder it is easy to setup with a twist grip tiller.

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