My 15’ Gregor skiff needs a new transom. I replaced it 3 or 4 years ago but the wood is warped and cracked. It isn’t soft yet but I’m going to get ahead of the game before I go on a 600 mile walleye fishing adventure.

My plan is to replace it with aluminum tubing and be done with it, forever. Needs to be 1.5” thick and at least 6” tall. I’m thinking a length of tubing bolted in place like the wood one is but with the gussets welded to it and the floor instead of welded to the floor and bolted to the transom.

I plan to use 1/8” wall tubing with a hole drilled and round tubes plug welded in where the motor bolts go through to keep from crushing it. May add a few more plugs down the length to help the tube keep it’s shape. I’ll either TIG it or more likely borrow a buddy’s spool gun.

The real issue is that I read that 6063 aluminum is soft and has a tensile strength of 15k whereas 6061 tensile strength is 32-35k. Problem being 6061 I can only find in 1.5x3 tube but 6063 can be had in 1.5x6 or maybe even 1.5x8. My thinking is to go with the 6061 and stack two pieces together and strip weld them clamped down to the table before installation.

Anyone see anything I’m missing or any better alternative? One piece of 1.5x6 would be lighter but is 6063 strong enough? A good reputable place I can order tubing from would be appreciated too.

ETA. Motor is a 20hp Suzuki 4stroke that weighs 97lb. That’s likely to be the only thing that ever hangs back there.

I replace mine when needed UHMD sheets 1/2 inch thick glued together, cut to fit. Same stuff cutting boards are made of. Strong as steel and never rot.

Osky

Be aware that wherever you weld this to your hull better be super rigid or the hull flex will cause a crack at the weld joint. If you want uber rigid, run multiple smaller tubes stacked and plate it....you'll have to have bad luck and a big wrench to crush it.

I personally would avoid welding to the floor unless you can plate a section to ease the flex on the weld area. If the transom is rigid and well attached....its not going anywhere that a 20hp is going to hurt it.

I'd go 4043 on this one myself.

I hardly ever use 1/8" for anything structural. The alloy isn't a strength problem for your application. There's no place for any wood in a boat.

I recently put a new transom and cabin on my son's boat. Coosa board is my go to for a transom, it's pretty much indestructible when you put fiberglass on both sides.

This brings up a good point. I currently have a small crack right in the stop of a weld on the transom brace that’s welded to the floor. This is what prompted this project and made me want to get on it before it gets worse.

Due to the shape of the hull I can’t go full width with the tubes. I can only get within about 1/2” of the sides of the hull. Should I stack two 1.5x3s and stitch them together then run a gusset at a 45deg angle to each gunwale? I thought I’d weld the current crack and dress it down then relocate the braces to a fresh area and put them on a plate like you suggest. I hate welding on aluminum any more than necessary due to the structural issues it can cause but I want to get it scotched once and for all.

isnt "tensil strength" a tension measure?

or is it deflection?

you have compression from the clamps, gravity from the hanging weight and torsion from the lever of tge operating motor shaft.


another idea might be to get some fiberglass box beam to frame it and skin it with glass coated wood...

I replaced the transom on my boat several years ago. Aluminum boat with 1 1/4" board in transom for strength. Welded new 1/8" metal for transom with 2 extra gussets at the top and 2 at the bottom. 1 1/2" aluminum angle across top of transom. Glued 1/2" and 3/4" marine plywood together for transom strength. 75 hp motor and no problems.

I’m not sure how tensile strength relates to deflection. I won’t pretend to be a physicist or anything but a decent fabricator. I just assumed that for the same weight stronger is better.

I need to put up a pic or two so you guys will better know what I’m up against. I don’t feel like this is a difficult problem to tackle but I’d rather not have to do it again later.

looked it up..
tensil is described as "pull apart" strength. like a snapping steel cable.

for a 15' skiff your transom will be what? 7-8 square feet?

three sections of 1" square tube bonded/welded together should provide sufficint clamping surface. you might go with a thicker wall for that part and fill it with spay foam ...
or you could just use a aluminum plate like they do on inflatables and RIBs..
I wonder what material inflable transoms are made from?

you could pick up a trashed inflatable and cut the transom out...

Did you try onlinemetals.com. I purchased a bunch of stuff from them recently. The S&H was bit ouchy.

I looked at several places online and they all wanted a firstborn child for freight.

I called over to a local metal shop after work today and they have 21 foot sticks of 1.5x1.5x.125 for $63 so I think I’ll go get a stick and make a stack of 4 tubes to span the transom. That should eliminate the need for adding tubes to keep it from collapsing. I will probably plate the center section where the motor mounts to tie it all in and give a smooth area for the motor mount.

outboard clamp plates usually have hemispherical dimples that help keep the clamps from moving around...
just an FYI..

if you want to minimize the amont of square tube you need...
you could do an aframe in the middle (clamping area) inside a larger perimeter box and cover it with a plate/skin..

think an inverted trapazoidal anarchy symbol with a skin over it..