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[AlanP]

I have been given a fast electric speed boat, the boat pulls to the right when flat out, it is fitted with trim tabs.

My question is which one do I adjust to counteract the prop walk

[boatmadman]

I'm not sure adjusting trim tabs will compensate for prop walk, Alan.

I may be wrong, but I thought the prop had to be offset to overcome that issue.

Ian

[triumphjon]

ian , i was thinking the same thoughts , trim tabs wont compensate prop walk , could you not djust it using the trim on the rudder ?

[ids987]

The solution will depend largely on what kind of hull it is, and also the drive setup.
Deep vees - by nature, are inherently prone to this.
If it's surface drive (?), then probably the first thing to check is prop sharpness. I would say that the two most common causes of veering right - in a surface drive deep vee, are torque roll, and prop walk. A blunt prop will tend to cause, and/or exacerbate both of those. If the main cause is torque roll - which you will normally see, because the boat will list right - then start to follow the list, then lowering the right hand trim tab should help - depending on the position of the trim tabs. If they are too far in (ie close to the centre/keel line of the boat), they may not be able to do much to prevent listing. Tabs close to centre are usually to push the transom up and the nose down; tabs further out are for attitude in turns, and can help with listing. Using rudder trim is not normally the best answer - in my experience at least. If you manage to trim out the flat out veering (in itself not always easy, because it is not always consistent), it will tend to go left when it's not running flat out. The drag from the rudder deflection will slow the boat down, reduce battery duration too, and may make it difficult to turn right at reduced speed (depending how much trim is required) - unless you have a lot of rudder movement.

[AlanP]

Many thanks for the replies, armed with all that information I will see what I can do.


Thanks again


Alan

[ids987]

Hi Alan,

I didn't want to throw too many variables out at once, but a few more thoughts now. Once again, some of these are specific to surface drive.

1) Strut or stinger position. In general, lower down will cause more roll.
2) Shaft angle (surface or submerged) steeper downward angle (down towards the prop), will generally increase roll.
3) Prop type has a fairly big impact. Generally, lifting props will make things worse.
4) In transom mounted rudders, a rudder which is offset to the right, will help to counteract torque roll (due to lift from the rudder). The greater the offset, the more it will lift the right side (and the more it will impact ability to turn left). Size, shape, and rudder angle of attack also have a bearing.
5) As "boatmadman" said, driveline offset can also help - so that the lift from the prop/strut/stinger is slightly offset to the right; helping to counteract the torque. Not always easy to retro fit though.

Generally speaking, and fairly logically, the less of the boat is in the water, the more susceptible it becomes to torque roll. If the boat needs inner tabs to correct straight line ride attitude, that can make things worse. As the tabs push the transom up, and the nose down, you lose the supporting water at the transom end, and the bow becomes a pivot - tightening the turn.