Proper motor height, replacing transom

[Nightsnorkeler]

I am new to this forum and this is my first time attempting to make a repair of this type. I have a 24' Harris Flotebote, approximately 40 years old, with a 2006 Mercury 25 HP 4-stroke (F25EL). It was mounted by the shop that sold us this new motor back in 2006 and the wood has deteriorated to the point where it needs to be replaced, and I've decided to tackle this job myself. I've already read many posts and spoken to boat builders, etc. so I'm pretty confidant about the job, I'm just not sure how high I can safely mount this motor. I will be using 3 sheets of 3/4" plywood glued and sealed with resin which I've been told is good as long as it is sealed well.

As you can see from the picture, the motor is currently tilting back due to the flex of the old wood which makes it sit lower in the water. We are on a very shallow lake, so I'd like to mount this as high as possible to avoid bottoming out in the sand. Speed is not really a concern as we only use this boat for cruising on our rather small body of water. The way it sits right now, the water line was right at the level of the top of the three plates on the shaft. Would it be safe/advisable/appropriate to make the transom higher so that the motor sits higher in the water, maybe at the middle plate or somewhere between them?

Thank you very much for any advice you can give, and please forgive me for not using proper boating terms, but I'm just a weekend boater with some time on his hands and not afraid to get them dirty!

 

[Bamaman1]

Re: Proper motor height, replacing transom

Take the measurements of the old plate, and make a new one the same size. Harris knows best.

But, go ahead and use marine plywood--not off the shelf Lowes/Home Depot plywood. And an epoxy sealant would seal up the wood so much better than any other finish. You can look up West Marine's products to guide you.

 

[Nightsnorkeler]

Re: Proper motor height, replacing transom

Thanks for the quick reply! The original motor on this boat was a 25 HP two stroke which was on a different transom. This transom wood was installed with the new motor, and the motor has always seemed to sit pretty low in the water even though this transom is higher than the old original one. That's why I'm trying to determine just where the water line should be on this motor.

I've already purchased 2-part epoxy resin, from West Marine no less, and will be using that to seal the wood after all holes have been drilled.

 

[5150abf]

Re: Proper motor height, replacing transom

Looks like the dealer used a 2x12, plywood will do a much better job once it is glued and clamped.

You can do whatever you want since you are making your own transom board just be aware that toons have some funny water flow and the engines need to be a bit lower to avoid cavitation but you can always cut it down easier than making taller.

 

[DECK SWABBER 58]

Re: Proper motor height, replacing transom

I just redid the transom on my pontoon with a 9.8 Tohatsu 4 stroke. The Tohatsu website has a great diagram showing how to determine whether you need a long or short shaft motor which would apply to any motor. I looked on the Mercury site but didn't see anything. Obviously you have the correct shaft length but the general rule of thumb is to have the cavitation plate about even with the keel of the boat, or in this case the bottom of the pontoon.

As far as going higher you would to experiment with the boat in the water. The only way to really do that would be to install a "jack plate" on your transom that would allow you to easily raise or lower the motor.

Hope this helps and welcome aboard. You are now a member of the best boating website in the whole world!

 

[Nightsnorkeler]

Re: Proper motor height, replacing transom

Thanks for all of the input everyone. From what I've gathered so far, it seems like the general consensus is that the cavitation plate should be at about the same level as the bottom of the pontoons. After looking at the current configuration (adjusting for the tilt due to the flexing wood transom), it appears that this motor was mounted with the cavitation plate about 2" higher than the bottom of the pontoons. So it seems like if anything I should be lowering the motor!

My problem is that we are on a very shallow and very small lake. We only use this boat for "cruising" around the perimeter, usually at about 3-7 knots, maybe 10 max if it's windy. So we are not looking to increase performance as so many are, we are just trying to keep the motor from bottoming out as much as possible. We have 20" pontoons, which sit almost halfway down in the water, and the bottom of the prop is 12" below the cavitation plate. This means that if we mount the motor as recommended, we would have to stay in water at least 22" deep at all times.

I think what I'll do for now is make the transom so that the cavitation plate sits about 4-5" higher than the bottom of the pontoons and see how it works. This way both the main and secondary water intake holes should stay submerged at all times and should allow us to go into shallows of about 18" without digging sand. IF this doesn't work out well, as stated in an earlier post, it is easier to shorten the transom than to raise it and I'll just trim it down and reseal the cut.

Unless of course anyone tells me that I'm crazy and should not stray from the "cavitation plate at bottom of pontoon" rule.

 

[bav/270w]

Re: Proper motor height, replacing transom

I don't believe you need to go that low. the water flow between the two tubes is much higher than the normal water line, even when your underway across the water. go to any boat dealer and look at the new ones and see how low they sit.

 

[bav/270w]

Re: Proper motor height, replacing transom

Sorry. Correction to previous wording. the waterline is the same on both sides of tubes, but rises when underway. regardless you only need to have the cavitation plate just below the waterline.

 

[Nightsnorkeler]

Re: Proper motor height, replacing transom

Well I went ahead and made the new transom about 4" higher than the old one. This, coupled with the fact that the motor is no longer sagging, means that the prop is about 5" higher than it was and I should be able to go cleanly into 18" water, which is about 1/4 of our lake. The cavitation plate is 4" above the bottom of the pontoons, but still 6" below the water line when standing unloaded. Next year we'll find out how she rides and I'll trim it down then if needed.

Thanks again for everyone's input!