new transom

joebob14 · May 25, 2013

Ok so its time to replace the transom in the old tri-hull. Ive found plenty of info here on how to do it and what I will need but I am left with a bit of a dilemma. I have a 90 hp merc that runs great and I refuse to get rid of it, the problem is its a long shaft and the boat is setup for a short shaft. Question is one of opinion so I thought this would be a good place for it. just wondering if it would be better to make the transom taller to fit the motor as it is getting replaced anyhow or rebuild it like it is and put a jack plate on it. I have no need for the jack plate other than to make the motor fit and though changing the height of the transom would be a little more work it would do away with the need for the splashwell making more room in the boat. What would be the stronger better way to go?

jigngrub · May 25, 2013

Re: new transom

There's no doing away with the splashwell, it's a structural member of your boat and adds strength to your transom and stern.

joebob14 · May 25, 2013

Re: new transom

Ok that makes sense so is there any problem with just putting a longshaft motor on a jack plate?

Bondo · May 25, 2013

Re: new transom

joebob14 said:
Ok that makes sense so is there any problem with just putting a longshaft motor on a jack plate?

Ayuh,.... While neither option is Ideal,... Either idea will work,...

Ya just can't do away with the structure of the splashwell in either case....

jestor68 · May 25, 2013

Re: new transom

My gut feeling is that an old tri hull with a 15 inch transom height is not rated for a 90 hp motor.

Since it's rotted out anyway, why don't you get a proper boat(with a 20 inch transom)to put your 90 on?

A guy a few blocks from me has a 16 ft Wellcraft Air Slot(no motor) and nice trailer for sale for less than $400.

3E73K63F15N55G25K5d5e9155f7973eb11dd4(1).jpg

joebob14 · May 25, 2013

Re: new transom

jestor68 said:
My gut feeling is that an old tri hull with a 15 inch transom height is not rated for a 90 hp motor.

Since it's rotted out anyway, why don't you get a proper boat(with a 20 inch transom)to put your 90 on?

A guy a few blocks from me has a 16 ft Wellcraft Air Slot(no motor) and nice trailer for sale for less than $400. View attachment 195401

That sounds easy enough but its not easy to find a boat with a 20 inch transom, would it be possible to replace the lower unit wit the shortshaft?

joebob14 · May 25, 2013

Re: new transom

Its an 18ft rated for 120hp

NYBo · May 25, 2013

Re: new transom

How about a picture of the current transom? If it has a cutout like most, I don't see why it would be a problem with reducing or eliminating the cutout when you rebuild. You should consider additional bracing, however, due to the increased leverage from the motor on the higher transom.

joebob14 · May 25, 2013

Re: new transom

NYBo said:
How about a picture of the current transom? If it has a cutout like most, I don't see why it would be a problem with reducing or eliminating the cutout when you rebuild. You should consider additional bracing, however, due to the increased leverage from the motor on the higher transom.

That was pretty much my plan, it does have the cutout. Of course some extra bracing would be needed as I didn't even find any rot when I drilled the transom but it does flex about a 1/2 inch under hard acceleration. I'm thinking this is from the extra leverage of the long shaft. I'm sure the splash well does give some support but I'm thinking if I'm doing all this work anyway it wouldn't be that hard to remove it and replace it with something a little beefier, its just a flimsy piece of fiberglass. I'll get pictures up in a minute.

Frank Acampora · May 25, 2013

Re: new transom

This was originally a 15 inch transom boat which I raised to a 20 inch transom. Since I was simply raising the transom 5 inches, I plated it with .080 aluminum and added corner gussets for strength. You can also see that I added aluminum plating to the sides of the splashwell to further reinforce the transom, demonstrating the structural need for you to keep the splashwell in your boat.

In my opinion, it is the better way to go to raise the transom, especially since you are repairing/replacing it

OH! BTW: Your splashwell may draw water through the drain hole while sitting but it should not be a lot or cause a problem

jigngrub · May 25, 2013

Re: new transom

Raising your motor height on your boat either with a jack plate or taller transom will change the center of gravity on your boat possibly decreasing it's performance.

NYBo · May 25, 2013

Re: new transom

jigngrub said:
Raising your motor height on your boat either with a jack plate or taller transom will change the center of gravity on your boat possibly decreasing it's performance.

It will raise the CG vertically, but it won't change fore and aft. We're also not talking about much of a difference in any case.

joebob14 · May 25, 2013

Re: new transom

Sorry I'm having trouble getting the pictures to download but I do have the motor on the boat at the correct height and it performs better than just ok in my book. I just don't like the way the top of the motor bracket sticks up 5 inches above the transom. I don't ever have any water in the splash well unless I back up to fast but I do see how it gives extra strength so I will be keeping it and beefing it up a bit. The only problem I have the way it is would be the flex in the transom.

NYBo · May 25, 2013

Re: new transom

joebob14 said:
I just don't like the way the top of the motor bracket sticks up 5 inches above the transom. I don't ever have any water in the splash well unless I back up to fast but I do see how it gives extra strength so I will be keeping it and beefing it up a bit. The only problem I have the way it is would be the flex in the transom.

I think the flexing, if not caused by rot, is happening because of the way the motor is mounted. But if the transom isn't rotten, just add the necessary structure to properly support the motor rather than completely rebuilding it.

jigngrub · May 25, 2013

Re: new transom

joebob14 said:
The only problem I have the way it is would be the flex in the transom.

Any flex in the transom is unacceptable.

How old is your boat and when was the last time the transom was replaced? Never?

NYBo said:
It will raise the CG vertically, but it won't change fore and aft. We're also not talking about much of a difference in any case.

Until the boat fills with water, then the difference could be between capsizing and level upright floatation... but boats never fill with water eh?!

NYBo · May 25, 2013

Re: new transom

Raising the motor 5" is unlikely to make much of a difference. It's already there, according to the OP; it just isn't properly supported.

metriccrescentwrench · May 26, 2013

Re: new transom

jigngrub said:
Until the boat fills with water, then the difference could be between capsizing and level upright floatation... but boats never fill with water eh?!

I think that's kind of overly dramatic. I'm just making some educated guesses here, but that engine probably weighs around 375 or 400 pounds. With a 5 inch longer shaft the cg of the engine might be about 4 inches higher than the cg of the short shaft engine (gearcase stays at the same height). An 18 ft trihull must weigh at least 800 pounds or so, maybe more. So you essentially raise 375 or 400 pounds by maybe 4 inches, at least 800 pounds or more doesn't move. The overall cg is only going to go up by an inch or so.

new transom

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