Transom replacement. Not 1 1/2"

[61mysteryboat]

I began replacing the transom in my 1961 starcraft safari. After 11 hours I finally got all of the rotten wood removed. I then noticed the space between the remaining fiberglass was only 1 1/4" . Will that be a problem. I know the standard is 1 1/2". I will only be mounting a vintage 75hp Johnson (200 lbs more or less).
I have both 3/4" and 1/2" premium veneer core ply. Also, since plywood is slightly undersized I will have a gap of more or less 1/16" even if I use both sizes of ply. Should I fill the space with additional layers of veneer and then fill with epoxy?

 

[gm280]

61mysteryboat, take a few pictures from different angles of your empty transom setup now and post them. Sounds like you didn't cut the inside fiberglass section out and that may be okay. We just need to see what you are working with.

 

[61mysteryboat]

Unfortunately it is dark out right now. Basically I cut out the rear of the transom and left a 4" perimeter on all sides. The space between the outer and inner wall is 1 1/4" and the top is open. ( I removed the topside of the boat to ease rebuild of the transom, floor, stringers, and inner walls of hull). Hence, the replacement transom will slide down from the top. Thanks for the help.

 

[gm280]

Unfortunately it is dark out right now. Basically I cut out the rear of the transom and left a 4" perimeter on all sides. The space between the outer and inner wall is 1 1/4" and the top is open. ( I removed the topside of the boat to ease rebuild of the transom, floor, stringers, and inner walls of hull). Hence, the replacement transom will slide down from the top. Thanks for the help.

Okay I understand better. However, you really need to get ALL the old wood out before replacing new material. But when you can, post some pictures because there are some very good folks on these forum that can offer the best advice for you particular setup. The usual way folks replace their transoms, is to cut the inside fiberglass out and grind the remaining outer fiberglass layer until it is clean and free of all old material so the new transom has something to attack too. So post the pictures. JMHO!

 

[61mysteryboat]

All wood has been removed. Although it looks like wood in photo it is only a thin veneer stuck to fiberglass. I plan on grinding out all remaining surfaces to get to clean fiberglass. Transom wood will be treated with cpes and installed with epoxy. I decided to install from the back because the inside was still fairly solid and had a number of corners while the back was heavily crazed and needed to be ground down and refaired. Also, my back has been yelling at me and I found it easier to work sitting on a bucket rather than laying in the hull. The topside in the photo has been separated from the hull and will allow the transom wood to slide in from the top. My main concern is the 1/16 space that will be left if I install one piece of 3/4" and one piece of 1/2" ply. I am a cabinet maker and could layer up additional veneers to bring it to the final thickness if needed. Thanks for any input.

 

[gm280]

Okay... This is going to be a lot more work then the typical transom effort. You absolutely need to get every little bit of wood out from even the sides of the area. And that is because you will need to inject Peanut Butter into those area when installing the transom replacement. Peanut Butter is a term we use on these forms and it is actually a mixture of Polyester Resin, Cabisol (fumed silica) and 1/4" chop fiberglass strand. You mix those ingredients up to a consistency of peanut butter and use it as a filler in the corners and other areas.

The problem I see here is the ability to get a good adhesion to the inside fiberglass remaining panel when you have to slide in the transom. As you slide in the transom, you will be scrapping down any PB you applied initially. The actual transom thickness isn't the problem. We usually use exterior grade quality plywoods (not pressure treated) for our transom built ups. We usually take two pieces to built up our thicknesses glued together with waterproof glues. TiteBond 3 is a very good option for that effort.

You will have to be a lot more concerned with your thickness going this way now .We then apply polyester resin to all the sides and then CSM cloth as well to basically waterproof the wood before installing any transom. And then we coat the remaining fiberglass transom of the boat with a coating of PB and install the waterproofed transom and tons of clamps to hold everything secure for it to cure.

So your install with be a little different. Maybe others will chime in and add to this issue with better more informative steps. JMHO!

 

[ondarvr]

As for thichness, just put a layer of glass on the plywood before you intstall it and you then have the correct thickness, or the vaneer, either way works.

Skip the CPES, it's nothing more than thinned epoxy at a very high price, and is of no value in this application.

Most people do the transom from the inside because the stringers and other things are rotting along with the transom, so it all needs to be opened up anyhow, but doing it from the outside is fine if you don't have other work to do under the floor.

 

[61mysteryboat]

Would it be feasible to saturate the ply with penetrating epoxy prior to install. Install the transom from above then drill small holes through glass and inject epoxy into the areas where the voids are until they are filled.

 

[gm280]

61mysteryboat, are you planning on using Epoxy or Polyester risen? It does make a difference in how things will be done. Most folks use Polyester 435 laminating resin. What are you going to use?

 

[61mysteryboat]

I would prefer epoxy.

 

[ondarvr]

You're making this harder than it is, don't overthink it. Slather the glass surface with epoxy putty, put a little extra down in the slot, slide the plywood in place and clamp it.

Penetrating epoxy is almost worthless, it will be of no benefit in this application. While you could use it, the cost is higher and it's an inferior product to normal unthinned epoxy. Coat the plywood and let it harden a little prior to placing it in the slot, no need for a different epoxy.

 

[61mysteryboat]

Sounds great. I appreciate everyone's input. What a helpful group we have here at iboats.

 

[ondarvr]

You don't need to make the plywood the full thickness prior to to putting in place.

If you fill most of the slot with putty it will ooze out as you push the plywood down and fill any gap in that area. Once hard and clamped in place you now know the exact amount of buildup, if any, the plywood needs to make it even with the rest of the transom. If it just needs a very small amount, add an extra layer of glass, if it needs more, you could add a veneer. it may need nothing though.