Aluminum transom beef up

[Ry Scott]

I need some help on a 20' all aluminum Texas Maid boat. Any advice would be appreciated since no one around me seems to know.
My boat was a 20' cabin boat that I have removed the top and am rebuilding as a large run-about (not cabin boat). Its 8' wide and built very heavy with aluminum stringers.
Question 1.
The transom is aluminum on the outside and wood on the inside with an aluminum cap. The boat was originally rated in the early 60's at a max 100hp outboard or 150hp IO. Now that my boat will hold 6-8 people I was wanting to increase the hp to a 150hp outboard. Can this be done by using coosa board or is there a better way to strengthen the transom to safely upgrade my motor. I know the old 60's motors probably weighed more than newer outboards. Or do I need to stick with the factory specs even tho I have changed a lot about the room in the boat?
Question 2. Is there a formula to determine the capacity of this boat. The old capacity was 4 because there was only 4 seats. The rest of the area was a sleeping area.

 

[bajansailor]

Welcome to the Forum Ry.

What is the thickness of your aluminium plate transom?
And how thick is the timber (plywood?) on the transom?

Rather than using timber, I think I would be inclined to beef it up by welding on some additional stiffening, if you have access to welding equipment (and maybe a friend who is a welder)?

The cat in my avatar has 2 x 150 hp O/B motors; her transoms are 6 mm thick ally plate, with additional 6 mm thick doubler plates welded on in way of the securing bolts. The top of the transom is stiffened by a 1.5" square hollow section welded on.

Re the photo of your transom, are we looking at the wood on the transom, or has this been removed?

If welding is not feasible, you could glue on another layer of plywood with epoxy, assuming that the existing timber is in good condition?
If it is in poor condition, then you could remove it all and replace with new, thicker plywood (?).

 

[Mr Efficiency]

Is the old ply still stuck to the alloy transom ? As bajansailor says, you could get an all-alloy transom fabricated, but replacing what is there, with new ply, would be much easier. Your boat is quite flat bottomed and beamy, and I really don't think it would need much more than 100 hp to reach its potential.

 

[fallguy]

Cool project. You gonna foam that all in?

otherwise; that boat is a sinking disaster waiting to happen

 

[ondarvr]

Newer motors are heavier than those old ones.

Coosa isn't designed to add strength in an application like that.

Beefing it up with aluminum is the best option. And you may need to consider tying the transom into other structural components to transfer the added stress from a larger motor.

I did about the same thing to my boat, it was far more usable after the remodel.

 

[kapnD]

Interesting that the ribs are discontinuous from the deck to the bottom, looks like a weak spot to me.
The hull may not take well to high speeds, I’d be wary of overpowering it, especially since you have removed a significant amount of weight, it should require less hp.

 

[Mr Efficiency]

I would imagine any structural weaknesses would have expressed themselves by now, being such an old boat, but I agree stripped out, it should not require 150 hp, unless heavy loads are envisaged, even then a somewhat smaller outboard correctly propped would likely be more than sufficient.

 

[fallguy]

Yeah, gp Ondarvr.

needs some knees I'd say that form a motorwell perhaps and buttressed with a full height transom for safety(good home for foam)..

Foam might also quiet things.

 

[Dejay]

Nothing useful to add, just that I really like picture 3! That teardrop trailer looks awesome. And did you clear coat that beetle?

 

[Ry Scott]

Yes the wood in the pic has already been removed. No it was not glued in any way between the aluminum outer or inner structure. The top cap that goes over the wood has a stringer on each side (if that's even what they are called) that extends about a foot and attaches to the side walls. I like the idea of welding more reinforcement on the transom. I can weld but not tig. So I would need to find someone who can design and weld it further. It had a 100hp motor for 60 years so I'm sure the transome is "good enough" but if i have it tore down this far...why not do it right.

Yes i was planning on doing the closed cell expansion foam in the floor. Why is it a sinking problem if i don't?

Good to know about coosa. What would i use kn between the 1 1/2" aluminum walls instead?

 

[Mr Efficiency]

Certainly if you are going to go the ply-backed transom, be sure to properly prepare and paint the alloy first, otherwise it is an invitation to corrosion.

 

[Ry Scott]

Nothing useful to add, just that I really like picture 3! That teardrop trailer looks awesome. And did you clear coat that beetle?

Lol yes 1 gal of clear. Hand built teardrop. Both car and camper have been in 2 magazines and recieved 9 trophies.

 

[Mr Efficiency]

I would not use pouring foam, it would make welding repairs a nightmare, or any repair for that matter, that required access to the inside. Starting fires, you do not want.

 

[Ry Scott]

Certainly if you are going to go the ply-backed transom, be sure to properly prepare and paint the alloy first, otherwise it is an invitation to corrosion.

Yah I'm learning as I go. I can fiberglass and I'm pretty handy. I'm just not sure how to strengthen

 

[Ry Scott]

I would not use pouring foam, it would make welding repairs a nightmare, or any repair for that matter, that required access to the inside. Starting fires, you do not want.

I liked the pour foam because it make the boat virtually unsinkable. What would you suggest instead?