Transom rebuild 14' aluminum boat

Hi guys, I have a 14' aluminum boat that's rated for a 15hp outboard but I want to put a 20hp on it. It works fine but the transom flexes sometimes just a little so I'd like go rebuild it. Currently, the wood portion is about 5 inches high and maybe an inch thick (see pictures). It's being supported by a thin aluminum bracket that's held to the boat by about 12 rivets. My plan is to drill out the rivets, close the holes they leave, move the bracket down to allow room for a larger transom, drill holes in the aluminum, rerivet the bracket on with the new, larger transom.
I was wondering if I actually needed to move that bracket down at all or if it's unnecessary to drill more holes in the transom. I could just remove the bracket, rivet the holes shut and bolt the new transom wood onto the skin. Is the bracket actually necessary?
My other question was what is a recommended sized for the new transom wood for it to support a 20 hp no problem. What is the minimum height and thickness that I'd need.
Thanks for the help!

Pictures will help advice a bunch.

Thanks for the reply, I thought I had uploaded a picture but guess I didn't do it properly. Here they are.

Simple really ... cut out that 'shelf', trim to width and lower a good 8" or more (I'd keep the rivet holes above the static waterline). Add in new wood, perhaps even adding in a wooden (encapsulated in epoxy) or tin 'knee' to support the transom and move the thrust stress to the entire hull.

Here are some links to recent transom replacements I've done, as maybe the pictures will help you think things through, plan and execute:

https://forum.tinboats.net/viewtopic.php?f=21&t=36465 Shows the step-by-step process for an epoxy-encapsulated wood transom

https://forum.tinboats.net/viewtopic.php?f=3&t=47405 Similar, but lots of pictures

https://forum.tinboats.net/viewtopic.php?f=21&t=43039&p=434035 See the 'knee' support in the center of the photo

Appreciate the reply! I only have a couple concerns or questions. When you say cut it out in guessing you mean drill out the rivets? Or do you suggest leaving them and cutting the bracket leaving the part of it that is riveted to the boat?
As it is now, the rivets are about an inch above the water line so moving them down any amount will be below the water line. This is why I'm not sure if it's worth reattaching that bracket lower or simply get rid of it and bolt the transome right to the skin without the bracket. Taking a look at the pictures you supplied, it seems your transome doesnt have that bracket and instead the wood goes all the way to the floor. I'm wondering if that would be better as it would save me having to drill 12 new holes below the water line and hoping the new rivets don't leak. I don't think I have access to an air hammer so I'd most likely be using closed end blind rivets.
Regarding the knee brace, I was also hoping to add one but that would require drilling into the bottom of the boat which again I was hoping to avoid. I saw a video on YouTube where someone made a brace that goes from the transom wood straight to the back bench, so I was thinking of doing that instead (skip to 8:40 in the following YouTube link if you'd like to see it) (https://m.youtube.com/watch?v=iCnEndD3Mig)
What do you suggest for both questions? Thanks again for the help

Properly secured (see my links about using the SS fasteners) and you won't need the shelf at all. Just keep above the bottom in case the hull holds water.

On a similar hull, I once put large 'L' brackets from the wood of the new transom to the sides of the hull, riveted there. But otherwise your support idea to the seat is sound; intent it to just tie the structure together. On another one, I used 'U' channel vertically on the transom to hold a triangular piece of plywood (as a 'knee') that just rested on the bottom of the hull, but epoxy glued in place. Worked good for as long as I had it ...

Avoiding drilling holes in the bottom of your boat is a good strategy.

Another idea may be to tie the outboard ends (next to sides) of the new transom plywood to the seat frame too. The ties could be upside down knee braces.

The middle of the plywood will be held well by the motor bolts.

If middle & ends are secure, the motor force should go into the hull, & not just flex the transom.

That makes sense, I think I'll put L brackets as I can drill the holes above the water line. I also read your post on the SS fasteners and will look into that. So when I drill out the 12 rivets that currently hold that bracket in place, would you recommend riveting them shut with closed end blind rivets or drilling 12 holes in the new transome wood and bolting through the holes? The only issue is the holes are very small so I'd have to drill them bigger to accomodate for bolts and I feel like that might be too many large holes. I'm also gonna have to drill a couple holes higher up and lower down to secure the wood properly so it's not just being held in the middle so I'm not sure if using the 12 holes for bolts is a good idea. If I were to rivet them shut the rivets would come out the other end a bit (inside the boat) and I'm worried that won't allow me to secure the wood snug against the aluminum so I might also have to drill a mm into the wood where all the rivet ends would poke through. Thoughts?

Sorry cedarriverscooter, I'm not sure what you mean by the outboard ends. Do you mean just the very left and right of the transome plywood where it touches the side of the boat? So there would be a brace on the center to the bench and one on each side? Thanks

I'd rivet the old river holes shut by simple closed ones (preferred) or goop-filled blind ones, if open.

That's likely what I'm gonna do. Appreciate all the help!

By the way, if I make the transom wood about a foot high and an inch thick, with a brace to the bench, I'm guessing that would be good for a 20hp? I don't want to have to worry about any flexing again. I'm also thinking of making it 1.5", which would you go with for my application?

In one of my posts I bought a 4x4 sheet of 3/4", which isn't a true 3/4" thick anyway. Cutting the wood on the angle (wasn't a wide boat) gave me the 2 layers to glue together, one taller than the other, but pay around with the layout before buying or cutting.

That makes a new strong transom that 1-1/2" aluminum 'U' channel then makes a slick transom cap for. In fact, I have a length left over you could use - if long enough, just pay for the shipping.

Jon13245 said:

Sorry cedarriverscooter, I'm not sure what you mean by the outboard ends. Do you mean just the very left and right of the transome plywood where it touches the side of the boat? So there would be a brace on the center to the bench and one on each side? Thanks

Click to expand...

Exactly.

BTW, my old fiber glass runabout had 1-1/2 inch thick plywood in transom (the glass skin has little strength) & it had an 80 hp on it.

I suspect that cap comes off of the transom to allow replacement of the wood.

I did something similar to this:

Add a couple of pieces of all-thread from the transom to the rear bench seat. I actually installed a piece of stainless rectangular tubing behind the seat but the seat will work just fine. These rods will add an enormous amount of strength to your transom and prevent flexing. Would definitely be strong enough to support a heavier motor - assuming the boat itself won't be too heavy in the back.

Hey sorry I've been a little busy. Dale I really appreciate the offer but I think my buddy's got some material I could use.
As for the brace, I do like the idea of the two braces. Do they need to be bolted to the aluminum skin or would screwing them into the wood be good and securing the wood to the skin?
Also, I was gonna epoxy my wood bi also read that the epoxy doesn't hold up to uv.. What coating do you guys recommend?
Thanks

Jon13245 said:

Hey sorry I've been a little busy. Dale I really appreciate the offer but I think my buddy's got some material I could use.
As for the brace, I do like the idea of the two braces. Do they need to be bolted to the aluminum skin or would screwing them into the wood be good and securing the wood to the skin?
Also, I was gonna epoxy my wood bi also read that the epoxy doesn't hold up to uv.. What coating do you guys recommend?
Thanks

Click to expand...

FYI, Raka epoxy is 1/3rd to 1/2 the price of West Systems and works just as good, www.raka.com. Most boat shops around here have switched to it, including that of my good friend (at my recommendation), at a 5th generation wooden boat shop. As a bonus, you don't need mixing pumps. I use small cat food tins and add whatever I need, 1/4" in each at a time or whatever is needed, or even bottle caps for small amounts. They use a simple 2 parts resins to 1 part hardener for most of their mixes (less the highly specialized ones).

I paint anything I epoxy on tin boats with good ol' Rust-Oleum enamel paints.

I'll definitely look into both of those, spent a bit more on restoring my motor than I was hoping so I wouldn't mind saving on the epoxy if it's still good.
I saw somewhere online someone was saying to use non thickened epoxy first on each of the sheets of plywood, I'm guessing because the wood absorbs it better and you can saturate the wood more before then applying thickened epoxy (they said to thicken it with wood flour) to glue then together and give them one more thicker coating. I'd probably drill out all my holes first before that last thick coating so I can fill the holes with thick epoxy and let it harden and redrill it so none of the wood is exposed to the elements. Then I'd sandpaper and rustoleum paint it and I think that should be solid and well protected from uv

Also I'm leaning towards using epoxy but I see a lot of people use waterproof wood glue or some other type of glue.. What are the pros and cons of each and what would you recommend?
Also one person not only glued the plywood together but used about 20 3/4" screws evenly distributed to fasten them together and then covered the screw heads with epoxy. Is that a good idea or is it unnecessary? Seems a bit overkill if the epoxy or glue provides a good bond.

IMHO if you're going to use epoxy to seal the wood, just use it to glue the 2 together - simple! I also see no need to add extra 3/4" screws, but all tin transoms I've re-done also has SS fasteners in a few places and they'll hold it together too. All but 1 of the topics in my signature have pictures of the epoxy-encapsulated wood transom and SS thru-bolt process.