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Steering wander after fitting bigger prop

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gkm24

Regular Contributor
I just fitted a bigger prop to my 18ft sports/cruiser as it used to be kept on a river which had a speed limit of 8 Kt.
I now find that while operating it at low speed(entering or leaving the Marina), it seems to suffer a lot of torque steer.
I did notice that the anode above the prop does not have the trim tab fitted to it.
Do you think that fitting the tabbed anode would cure the problem?
Thanks as usual for the help.
 
Steering "wander" or torque steer.... which is it?
Steering wander is the tendency of a boat, at low i.e., "displacement" speeds, to oscillate back and forth around one's desired normally straight course.

torque steer is the tendency of a boat, usually at higher speeds, to have a constant bias or pull to only one side.

Two different problems with different causes....and yes, a given boat can exhibit both problems.

A missing anode trim tab will cause torque steer,
 
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I would add,

with the boat out of the water. grab the outdrive and move it from side to side without moving the steering cable.

is there a lot of side to side movement? If so the gimbal ring may be worn at the steering pin or possibly the two screws may need to be tightened slightly.

Virtually all mercruiser Alpha powered boats at low speed "wander" from side to side. The higher the drive is trimmed up the worse it will be.
 
Thanks for your replies. it certainly was worse with the trim up. To be honest, I did not see whether it oscillated back and forth as I was in a narrow part of the marina and corrected it continuously. I will check to see what happens if uncorrected and I will check the drive for play. I don't recall it having any play. Before this prop was fitted it did not wander at all. Anyway, all advice taken on board and will re-test.
 
gkm24 said:
Thanks for your replies. it certainly was worse with the trim up. To be honest, I did not see whether it oscillated back and forth as I was in a narrow part of the marina and corrected it continuously. I will check to see what happens if uncorrected and I will check the drive for play. I don't recall it having any play. Before this prop was fitted it did not wander at all. Anyway, all advice taken on board and will re-test.
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All props have a forward AND a sideways force vector on the water as they turn... depending on trim angle and hull design this will result in some back and forth wander at low speeds. The higher the prop pitch, the greater the sideways vector force. Once boat starts moving fast enough the "keel effect" ( a function of the hull design and in particular the deadrise of the hull) will stabilize the boat's forward motion. Tiz the nature of the beast.

The boat, at low speed, will tend to drift off course due to the sideways torque... You compensate and turn the boat back... inevitably you over correct somewhat and... repeat...
 
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I checked the steering for play and all is good, so no problem there. I know what you mean by over correcting and was taking this into account but could not "damp out" the wandering. Anyway, the boat now seems to have a transom plate seal leaking so it will be a little while before it goes back into the water (Longer if its a bad transom) and then I will re-test it. I think the original prop was a 15 pitch and the new one is a 21 pitch. I feel sure that it will sort itself as you say, when the speed gets up.
Thanks for the help.
 
gkm24 said:
I checked the steering for play and all is good, so no problem there. I know what you mean by over correcting and was taking this into account but could not "damp out" the wandering. Anyway, the boat now seems to have a transom plate seal leaking so it will be a little while before it goes back into the water (Longer if its a bad transom) and then I will re-test it. I think the original prop was a 15 pitch and the new one is a 21 pitch. I feel sure that it will sort itself as you say, when the speed gets up.
Thanks for the help.
Click to expand...

you went from a 15 to a 21???? It is very unusual to go from a 15 to a 21 successfully..... unless your engine grossly over revved at WOT with the 15. Check the specified WOT RPM range for your engine. On MERCS usually printed on the decal on the flame arrestor. At WOT with a "normal" load in calm conditions, your engine should rev at WOT to within that RPM range.... no higher... no lower ( ESPECIALLY no lower!)

BTW.... Volvo single prop drives also exhibit low speed wander under the same conditions... 'tis the nature of non duoprop sterndrives I suspect. after a while you get used to it...
 
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Thanks for the reply. I know it was a bit radical to go from a 15 to a 21 prop but the boat was only used on a river with a 4 Kt speed limit and revved out when I took it to sea.
I will test the WOT setting to see where I should be.
 
gkm24 said:
Thanks for the reply. I know it was a bit radical to go from a 15 to a 21 prop but the boat was only used on a river with a 4 Kt speed limit and revved out when I took it to sea.
I will test the WOT setting to see where I should be.
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4kts!!!! That's below my speed @ idle!... and when I had a 21P prop on my boat, idle speed was 5.5kts!
 
Does the boat have a distinct keel? Flat bottom boats are more susceptible to wind and current altering the course especially at slow speeds. Just a thought.
 
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