Crabbing and what causes it..........?

[Liberator*21]

I have an issue with the boat "CRABBING", i.e. to go straight the motor is turned or steered off center. It only does this with certain props. Lower unit is a S/Master 1.62R on a 300x, 15" Atlas Hyd J/Plate boat is a Liberator 21', prop shaft is 2 1/2" above pad, prop is a 30P Drag 4. I'm assuming its torque steer from the prop is the issue ?????????????

[97 TWISTER]

Motor to high

[D.B.S]

Motor to high

simple fix.

[Liberator*21]

Can it be prop related as well ? Reason I ask is it doesn't seem to do it with my O/Shore Cut Chopper II ???????

[Ray Neudecker]

Many four blades have a smaller diameter than three blades and therefor may need to be run slightly lower.

[Liberator*21]

The Drag 4 and the O/Shore Cut Chopper both run at the same height. Only 1/4" difference in the diameters.

[Stitch King]

High rake props like the drag 4 have what is called the paddle wheel effect. This causes the boat to crab as you describe. You may want to drop it down 1/4". Also which skeg does your sporty have?

[Liberator*21]

Ken,
Straight skeg, no factory torque tab.

[Mr. Demeanor]

My boat crabs more with cleavers than ET's. I think everyone above is correct in that all of those things can effect it. Blade style, diameter, number of blades, engine height, skeg style, etc.

I actually made this video to see exactly what my lower unit was doing. I set the camera up with the engine centered and the tip of the bullet centered on the center of camera lens.
You can see when I am really going the tip of the bullet is slightly to the port side telling I probably have a little too much torque tab in it.
About 3:45 into the video you can see it.

[97 TWISTER]

1/4" can make all the difference in the world. I think 2.5" sounds pretty high even with that much set back. I would plug your numbers into a prop slip calculator and see how much your slipping. I think 8 percent would be a good base for a 21 liberator??? IMO 1.5" above the pad is plenty high.

[Stitch King]

1.5" on the Lib is to low. It's a recipe for liftoff. Don't do it.

Gary, When I had that boat I ran it 15" back, 2.25" up and with a Drag 4 30 or 32. The Sporty I had on it had a tab added on. You may want to consider one.

[Mr. Demeanor]

1/4" can make all the difference in the world. I think 2.5" sounds pretty high even with that much set back. I would plug your numbers into a prop slip calculator and see how much your slipping. I think 8 percent would be a good base for a 21 liberator??? IMO 1.5" above the pad is plenty high.

My bests speeds are at about 11% right now. Dont get caught up on slip numbers. Speed is what counts as long as the boat remains stable. I raised mine until the back end got spooky loose with a cleaver. Switched to an ET and went up another 5/8" before it felt loose again and picked up a few more MPH.

I made the mistake of raising my motor until speed stopped climbing. I should have continued raising it until speed dropped or handling suffered. I now know that I was at a height where another 1/4" or so wasnt effecting anything for some reason. Once i got beyond that, speed started climbing again quite a bit. I guess there was a kind of a grey area for my setup I had to get past.

[Mr. Demeanor]

Gary, you could try molding a little tab out of PC7 epoxy right on the skeg. The stuff is pretty thick like putty and if you laid the lower unit on its side I think it would work great. I stuffed the intake of my motor with it and the motor was mounted. Keep checking it and shape it as it cures with a little laquer thinner on your fingers. Dont go so big like the TH Marine tabs. Look at a stock sporty and make it just like that. You can always add to it or grind it off and not have a skeg that has welding on it.

[97 TWISTER]

I agree you can't hang your hat on the prop slip calculator but its a good reference. If you show some crazy number in excess of 15 percent then you know you have major issue. I'm no liberator expert nor have I ever ran that much set back on any thing but I would think with the right prop and set up you should be able get below 10 percent jmo.

[pyro]

It's crabbing because you don't have a tab. Straight skegs should only be used with twins. The surfacing prop's thrust is not straight and symmetrical, so it needs to be driven on an angle to make the boat go straight. This also puts tremendous stress on the skeg, mid, bracket, and steering components. The torque tab corrects all of this. A tab fashioned from epoxy as described will solve the problem without potentially weakening the skeg like a welded tab can.