25 Merc pulls boat right-trim tab?

[fishbait14]

I'm new to this site and basically boating. I just bought a 14' open boat w/25hp Merc tiller motor. I took it out yesterday for the first time and @ WOT noticed the boat listing a bit to the right and the motor pulling to the left causing the boat to want to go right. At one point at WOT, the boat started going into a tight right turn with no imput from me. I backed off the throttle and kept it at half throttle from then on. I was rattled a bit, at one point I thought the boat was going to swamp in the unintentional right turn.

I searched the threads here and came across one with the same Merc and problem I have experienced. A member named "roscoe" had supplied a schematic drawing of the motor (below) suggesting that the trim tab (#39) might be the prob. I looked at my motor...The bolt holding it on was loose (I could wobble the trim tab with my finger) and the trim tab was almost unrecognizable (pic below). This might be hilarious but...could that be the problem?

 

[Deano1234]

Re: 25 Merc pulls boat right-trim tab?

Ya it sounds like you are exp. whats called "prop walk" it happens when you have a larger motor on a light boat so ya, you do have to have the trim on and adjusted properly it compensates the rotation of the prop

 

[jebeebe]

Re: 25 Merc pulls boat right-trim tab?

If it is pulling to the left , turn the tab to the left. Pulling to the right turn the tab to the right.

 

[fishbait14]

Re: 25 Merc pulls boat right-trim tab?

Thanks for you imput. The trim tab, when it was on, was not secured. The bolt was not tightened down so it just wobbled. Should I stsrt out with the new trim tab centered and adjust after that?

 

[diaric]

Re: 25 Merc pulls boat right-trim tab?

yes, start centered and go from there. adjust in little increments towards your pull if nessesary. also be aware of what there is for uncentered weight in the boat. mine has the battery on the right side and steering on right side, so if i don't have a passenger on the left I have to center my weight or it will take off.

 

[fishbait14]

Re: 25 Merc pulls boat right-trim tab?

If the tiller is pulling to port, I rotate the trim to port (clockwise, looking down at the prop)?

 

[diaric]

Re: 25 Merc pulls boat right-trim tab?

best way to explain. if the boat is trying to goto the right, move the tab slightly to the right to counteract it, and vice versa.

 

[fishbait14]

Re: 25 Merc pulls boat right-trim tab?

thanks for the help. I'm off to pick up a new trim tab.

 

[dr_gallup]

Re: 25 Merc pulls boat right-trim tab?

I've got a similar problem although I've got 45 years of driving tiller steer outboards. I bought a used 25 hp Merc of indeterminate age and put it on a 14' steel boat. Even with the trim tab adjusted full right the boat still wants to turn hard right at full throttle. I've looked at the mounting and the lower unit and everything looks straight. I tighten the friction drag on the steering pivot as much as possible & still be able to turn freely. Is it possible the trim tab has been replaced with an undersized part? It looks newer than the rest of the lower unit. Are there different size tabs available?

 

[KIGuy52]

Re: 25 Merc pulls boat right-trim tab?

In reality, the trim tab adjustment will make very little difference on a 25. Mine broke off years ago and I've never replaced it (I'm on fresh water). When I mounted my motor on the new boat, it pulled to one side like crazy. I took it out on the water and moved the motor left and right on the transom slightly until I got it just right. After a few tries, it stopped pulling. I marked the clamp position with a Sharpie to make sure I get it in the same position every time. Works pretty good now.