Rudders

By Paul Williams.
Article posted 24th February 2009.

The rudder is a control surface that steers and stabilises the boat. There are many different sizes and shapes of rudder, but they all do the same basic thing: when rotated from the neutral point, a rudder generates drag on one side of the boat, causing that side to slow and the boat to turn in the desired direction.

Like most things in FE boats, rudders are the subject of much discussion and debate. When all's said and done, the most important factor in choosing a rudder for a powerboat is drag. All rudders cause drag, both when steering the boat and in the "neutral" position. Obviously, therefore, we should select the rudder profile and size that gives the best compromise between effective steering, straightline stability and drag.

On a high speed model, the rudder will be stabilising the boat, keeping it tracking straight. Some very high speed boats need two rudders to keep them running true.

Types Of Rudder

Rudders for surface drive FE boats can be broadly divided into flat section and wedge section. The theory behind wedge profile rudders is that, above a certain speed, they cause less drag than a flat section rudder. Theory and practice, however, do not necessarily always agree. Many fast boats have been built with flat rudders. Indeed, one of the great pioneers of FE boats, Ed Hughey, built, raced, won and set records with boats using flat profile rudders. So, do not discount flat rudders. They are cheap, strong and effective, and can weigh less than a fancy CNC machined alloy rudder.

Wedge rudder theory. The wedge shape maintains water pressure down the face of the blade. The rudder trails an air-filled cavity behind it, with the water stream merging far behind the blade.

Wedge rudder theory states that a flat plate or aerofoil section rudder blade will cavitate above a certain speed. In other words, a low-pressure vapour-filled void will form on the blade, in the same way a submerged drive propeller cavitates. This vapour-filled void reduces or destroys rudder effectiveness, as part of the rudder blade is now running in a near-vacuum instead of water.

Submerged aerofoil section rudders at high speed generate a cavitation void at the thickest part of the blade, extending along the blade.

Surface piercing flat plate rudders cavitate or ventilate as the flow along the blade face breaks away and becomes turbulent.

A wedge rudder section maintains water pressure down the face of the blade, and because a wedge rudder is surface piercing, an air-filled cavity is allowed to form behind the rudder blade. At high enough speeds, the air cavity extends to the depth of the rudder. Water flowing off each face of the rudder merges at some distance behind the rudder, moving drag effects aft of the rudder blade.

At very high speeds, the air cavity behind a wedge rudder can close too quickly if the wedge angle is too narrow, leading to the completely counter-intuitive situation of a thicker wedge actually having less drag than a thinner wedge.

Etti aluminium wedge rudder assembly.

Wedge rudders do have one advantage which a plate rudder cannot match. If a wedge rudder is a thick enough section, it can have a water pickup for the cooling system. This obviates the need for pickups set through the hull or placed in the propeller thrust cone, and is a very useful and neat solution to getting a source of cooling water into the boat.

Rudder Selection

The only way to know for certain which size and shape of rudder is best for your boat is to time your boat on a fixed course. For circuit racing, whichever size and shape rudder gives the lowest lap times or the highest number of laps in the required race time is the one to choose. We couldn't give a stuff about ultimate top speed for racing. The important thing is the lap as a whole. Having a rudder that gives very high top speed but is hopeless in the turns is obviously a bad choice. The converse is true for Speed Records or "SAWS" boats - turning ability is totally secondary to ultimate top speed.

Handling Problems

Monohulls which bank in a turn alter the angle at which the rudder is presented to the waterline. Typically this results in the rudder generating lift which makes the back of the boat wobble around and may cause the boat to spin out.

The effects of rudder angle relative to the planing waterline.

Most designs of rudder allow you to alter the running angle in order to fine tune the boat's cornering. Usually, you will probably want the rudder angled so that it has a neutral effect, ie., it neither lifts the back of the boat nor pulls it down. However, you may find that tinkering with the rudder angle can dramatically improve the cornering ability of your boat, and usually a tiny alteration can have a very large effect.

Rudder arms made from aluminium and stainless steel. The ball link end is secured to the tiller arm with an M2 stainless cap head bolt.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

This article may not be reproduced wholly or in part without the written permission of the author and www.fastelectrics.net. If you would like to use this article or the accompanying pictures/diagrams please email articles@fastelectrics.net.

Last modified: 08th July 2010 @ 09:06