Transom size - Can you make it bigger

[bcobbett]

The transom in my 16.5 deep V bass boat with a 40hp needs to be replaced. I have noticed several posts that would indicate it should be 1.5" or three pieces of 1/2" plywood. However the one that I just removed is 1 and a quarter inches which is 1 sheet of 1/2" plywood laminated to 1 sheet of 3/4" plywood.
My question is do I replace what was there or should it be 1.5" as I have read in other posts.

 

[LP]

You could go thicker, but you'll be making the boat heavier. Perhaps the weight gain for a 1/4" across the stern is a bullet you are willing to bite. I see there are transom gussets in your photos. Do you have room for the additional thickness in these areas? If not, I might avoid creating the additional work of trying to make the additional thickness fit. Most likely, a double thickness of 3/4" ply is desirable to you as both pieces can come from a single piece of ply rather than having to purchase two differing sizes. If the thicker section fits the current structure, you could get creative with the in layer and add lightening holes in non structural areas to remove excess weight where it isn't needed. You could end up with a stronger transom this way for the same amount of weight.

 

[bcobbett]

Thank you for your reply and advise. I am not sure if it will fit the additional width so I will remeasure to make sure. However I am not familiar with the term or process of "lightening holes" can you refer me to a site which would explain in detail how this process is achieved.

 

[sprit]

Thicker transoms are required to hold modern outboard motors.
If you have a 40 HP outboard, by all means, go thicker.
If you have an inboard engine, the transom does not need to be so strong.
Please consult a shipyard!
Bon voyage...

 

[Jamie Kennedy]

bcobbet,
I also live here in Saint John. Lightening holes are just sections cut out. You would drill large diameter holes with a hole saw and then connect them with a jig saw. You would only do this one sheet and only in places where you did not need full strength. This will allow you to put more weight and strength where you do need it, like the edges, and gussets, and where the motor mounts. You might also be able to reuse the pieces you cut out somewhere else.

I am not sure if you are also replacing the floorboards and gussets. They look like more plywood over to of the aluminum. If they are dried out and in not too bad shape you might be able to get some more life our of them by getting them to soak up some epoxy. There is epoxy specific for that purpose but regular epoxy will do also. You don't want to use too much as it is expensive, but it also bonds well to the aluminum, if that is what you want. Best time to apply epoxy to wood is after the wood has heated up, like in the sun, then move it into the shade and apply the epoxy and the cooling wood will suck in more epoxy that way as some of the water vapour in the wood condenses. A hair drier will work also. The wood should be as dry as possible though. I wouldn't mind taking a look at it over a beer if you want to PM me, just to make sure I am not steering you in the wrong direction. I live in the North End.

 

[bcobbett]

Thank you all for your input. I do however have another question. The 40hp outboard for the boat is a long shaft and as you can see from the pictures the rear aluminum has been cut down. When I put the new Transom in place (with the intent to raise the motor to the proper height and align it with the keel) will it be an issue with the lack of Aluminum backing. If so what is the low cost remedy for this.

 

[Jamie Kennedy]

I don't see a problem there but the longshaft will push and pull higher up. Maybe add a board across the top to spread the push and pull so it doesn't bow in and out where the motor is and put more load on the middle gussets. If you can tie that board in with the sides of the boat somehow that would be best. Doesn't have to be plywood, or even hardwood, just a nice chunk of lumber whatever looks right. I would bolt it to your transom in 3 or 5 places, and then bolt it to or near the gunnels with piece of aluminum bar bent 90 degrees. Leave room for a decent radius when you bend the aluminum so it doesn't crack. Don't go too thick so you can bend it. You can get it at Princess Auto. Maybe reinforce where you bolt it to the sides and gunnel. Whatever looks right should be strong enough for a 40 HP. Just reef on it when you are done, yanking back and forth on the transom where the motor will mount. Should feel solid everywhere, bottom and sides. You might not need the board if you have gone 1.5" but something replaceable to take the wear might be a good idea anyway. Whenbolting through the aluminum use stainless steel nuts and bolts, but insulate between the aluminum and the stainless steel with something like epoxy or wood or plastic or the aluminum will corrode around the bolt holes, especially in the saltier water of Grand Bay, and the salt water of the Bay of Fundy if you head out there. Use aluminum washers and backing plates, but stainless nuts and bolts.