Will an improper trim cause a motor to overheat?

I'm still new at this water craft stuff and I'm wondering about setting the trim and whether I have been doing in correctly.

I've set the motor parallel to the back of the boat which is a small 10ft aluminium boat and I usually have a 5hp or smaller motor on it but I'm trying out a heavier 9.9 Sears Ted Williams/Eska on it.

If I go at an even low speed my impeller pumps nice and I can go for a bit before it starts warming up and eventually stalls. Went I go at a quicker speed the back of the boat tilts down a bit and the motor heats up fast and koncks out. I don't have any trouble with my other motors.I replaced the powerhead and impeller over the winter and the motor runs well in the backyard barrel but out in the open sea I'm wondering what is up.

I'm not a skinny guy and I usually put some weight in the front of the boat to sort of level things off when I'm alone, but the back still goes down abit.

Would adjusting the trim cause the motor to run better and not overheat?
Is the motor too big for the boat?


I await your experienced opinions. Thank you.

Re: Will an improper trim cause a motor to overheat?

I seriously doubt that is the problem. If anything you are mashing the back of the boat and motor down deeper into the water. Overheat problems come when it isn't deep enough.

Stop running that thing till it siezes. That'll ruin it. Find the problem and fix it.

Re: Will an improper trim cause a motor to overheat?

How does the new motor sit on the boat? The antiventilation plate should be flush with the bottom of the boat. It sounds like yours is above it and you are starving the water pump once you get going.

Check the motor on the boat. The "fin" on the lower unit should be even with the bottom of the boat. If it is above it, you may have a short shaft motor on a long shaft transom. Might just be that simple.

Re: Will an improper trim cause a motor to overheat?

I originally replaced the powerhead because the old one had a crack from overheating, I presumed. I didn't notice it when I bought the motor used last summer.

I replaced all the gaskets because some were blocking the cooling chambers and there was water coming out between the mounting plates. Its snug now.

There was nothing coming out of the impeller at all usually so that was replaced. Supposedly the cooling function changes from the impeller to some other internal process at some point. But after awhile lately the water spray gets steamy. When the engine cools for 5 minutes its ok.

I'm at a loss at what else could be causing the overheating. I use the proper mixture of fuel 1:50 as the manual says, so I haven't a clue as to what is causing it.

I realize these repairs are a learning process but what else would make it overheat?

Re: Will an improper trim cause a motor to overheat?

redfury said:

How does the new motor sit on the boat? The antiventilation plate should be flush with the bottom of the boat. It sounds like yours is above it and you are starving the water pump once you get going.

Check the motor on the boat. The "fin" on the lower unit should be even with the bottom of the boat. If it is above it, you may have a short shaft motor on a long shaft transom. Might just be that simple.

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I don't think that its a short shaft on a high transom. The motor is short shaft.

The transom is 15 inches I believe. I think the motor flexes it when it gets going. Should I reinforce the transom with some plywood???

I'm going out again tomorrow I'll check things better. It appears to me that it needs to be angled a bit better with a 2 by 4 or something. I dunno.

Re: Will an improper trim cause a motor to overheat?

I believe most 10 ft aluminum boats are rated no more than 6hp.I'm sure the transom is not strong enough for a 9.9.You also may have liability/insurance issues running over the rated hp.Your cooling problem is with the motor.
Be sure the intake is open they are forward just under the antivent plate. Plate is just above the prop.Check impeller, gaskets, water passages,sealing surfaces.Anti vent plate should be approximately even with the bottom of the boat being lower isn't a problem for cooling just not as efficient higher(not likely on that boat)could be a problem.
Do to the heavy driver trimming in(prop closer to the boat) may be necessary
trimming in helps to plane and get the bow down.
Your over heat is a water supply problem running faster generates more heat.If you have the slightest feeling it is over heating shut it down.
If you tested it in a barrel and the water wasn't up to the bottom of the mount clamp you may have burned the impeller.If the old impeller broke up pieces may be plugging the passages.You may want to give some thought about not investing any more money in it.Part it out on ebay and put the money toward an Evinrude/Johnson 6hp. Perfect for the boat and routine parts are available right here at iboats or your local NAPA.

Re: Will an improper trim cause a motor to overheat?

steelespike said:

Part it out on ebay and put the money toward an Evinrude/Johnson 6hp.

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You read my mind.