Propeller Effects

By Paul Williams.
Article posted 24th January 2008.

The Laws of Physics, specifically those governing motion and friction, affect our model boats in ways which may not, at first, seem very obvious. A basic understanding of the physics of the dynamics of a model powerboat, especially those unwanted side-effects caused by propeller rotation, goes a long way when trying to troubleshoot a handling problem.

Newton's Laws of Motion

Propeller Effects.

The propeller doesn't just provide forward thrust. It has other, less desirable effects on the boat. Most props used in FE boats are "left-hand" rotation, ie., when viewed from behind the boat, the prop rotates anti-clockwise. The examples that follow assume a left-hand rotation prop.

Torque Roll.

Before we go any further, it might be useful to nail down what we mean by "torque". Wikipedia's definition is this:

Torque is the tendency of a force to rotate an object about an axis (or fulcrum or pivot).

Torque Roll is the reaction force created by the propeller. The rotating prop tries to roll the boat in the opposite direction to prop rotation. This means the boat will be rolled to the right, causing the boat to steer to the right.

Torque roll. The reaction force created by propeller rotation forces one side down into the water and lifts the other side, the result being an imbalance in the level of drag on each side of the boat, causing the boat to steer off course.

Prop Walk.

Prop walk is the "paddle-wheel" action of the propeller on the back of the boat. The propeller generates a small sideways thrust (due to water being sprayed to the side) that tends to steer the boat to right by pulling the transom to the left.

Prop walk. The paddle-wheel action of the propeller throws water sideways resulting in a sideways reaction force, again causing the boat to steer off course.

Cures

So, you have a boat that always steers to the right. What do you do to fix it? No, the answer is not "dial in 100% left trim on your radio" - all that does is slow your boat right down. There are other, better methods of straightening out a boat so that it tracks straight and true with a neutral rudder.

Offset Drive

If the driveline is offset to the right of the boat's centre of gravity, this will push the back of the boat to the right and steer the boat to the left ever so slightly, usually enough to counteract any prop walk and/or torque roll.

Offsetting the drive to the right of the boat's centreline will cancel out some or all of the steering effect of prop walk and torque roll.

Angled Drive

There may be an instance where using an offset drive simply isn't practical, for example on a deep vee monohull where the only place the drive can exit the boat is in the apex of the vee. In this situation, an alternative method of correcting torque roll and prop walk is to angle the drive so it points to the left, pushing the transom to the right, thus steering left.

Angling the drive to the left can be very effective at cancelling out torque roll and prop walk, especially on a monohull where the drive cannot be offset.

Weight Bias

Moving the boat's Centre of Gravity off the centreline and to the left can partially cure torque roll, at the expense of an unsightly list at rest. However, we're not scale modellers, we're Fast Electrics and what the boat looks like when it's not tearing up the lake is really not an issue for most of us.

Moving the CoG to the left can be very effective at cancelling out torque roll and prop walk.

With a monohull, cat or scale hydroplane, you have the luxury (usually) of plenty of space to move stuff around and experiment with the best location for the cell pack and speed controller. Outrigger hydroplanes are usually pretty cramped. With my own designs of 'rigger, I plan where everything will go on the initial drawing, and if I get it wrong then it means I have to start again, as I often don't leave enough space to move anything around once the hull is built.

You may find that a self-righting monohull will require a weight bias towards the flood chamber to make the self-righting work.

Contra-rotating Propellers

Two propellers, rotating in opposite directions, cancel out each other's torque and prop walking effects, giving a boat with neutral handling.

Contra-rotating propellers cancel out torque roll and prop walk.

There are disadvantages to a twin propeller setup. For a start, it's not really practical for smaller boats where space is limited, and it's expensive - you will need two drives, two motors and two speed controllers, plus the range of propellers available in right and left hand rotations is limited.

Twin propeller setups are very popular among those modellers lucky enough to play with big cats. Each drivetrain can be dropped into a sponson, which gets the weight as low in the boat as possible, which helps with stability at speed.

Torque Wedges

Yet another method of curing torque roll and prop walking is a device known as a torque wedge. This is a small wedge or ramp fixed to the hull bottom on the right hand side of the hull that generates a small amount of extra lift, rolling the boat to the left. On a stepped hull, you may need to add wedges to both the transom and the forward step.

Torque wedges can be an effective solution to propeller torque effects.

© 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