Transom Design

The goal is to convert my 1973 SeaCamper project boat from 225 hp inboard-outdrive to 225 hp outboard. An Armstrong motor bracket is part of the plan, also a ~10 hp kicker mounted directly to the starboard side of the transom.

I've removed the rotted transom, stringer and motor-mount wood. What remains of the transom is the 3/8" outer skin and layups (tabbing may be the correct word for these fiberglass strips?) that join the skin to the rest of the hull. The original fiberglass layups failed where the transom joins the integrated steps (see photo below) and there are other layups that need reinforcing after grinding out the voids in these layups.

The entire inner surface of the transom fiberglass skin would be flat if not for the edge layups. That rise varies between 0" and 1/4" depending on the thickness of the layups. The width of the edge layups vary between 0" and 4".

If I attach the new transom wood (~2 inches of laminated marine plywood) so that the edges of plywood are inside the existing layups, I can bond a nominally flat service to a nominally flat surface. If I extend this wood all the way to bottom, starboard and port inside surfaces of the hull, I'd have to shape (route out) the wood surfaces near the edges so that it overlaps the old layups and I'd probably have some voids.


Inside of port side of 3/8" transom skin where it joins integrated hull steps. Note the failed layup joint between the transom and steps.



Inside of starboard side of transom fiberglass 3/8" skin.



Outside transom 3/8" fiberglass skin and steps integrated into the hull

Knee braces (triangular plywood), that attach the new transom wood to the new (not yet installed) stringers, are an option.

The current plan is to use thickened polyester (peanut butter) for bonding the wood to the inside transom skin. Mat between the layers of plywood and between the plywood and skin may be useful. All plywood surfaces would be primed with resin before they're bonded to anything.

I'm looking for advice on how to design the new transom.

1. Should I run the transom wood edges all the way to starboard, port and bottom hull surfaces and then join this wood to these surfaces with layups (tabbing)? My concern here is voids between the plywood and the transom skin since the transom isn't completely flat.

2. Should I first reinforce the fiberglass hull joints with multiple layers of fiberglass layups and then place the edges of the new transom wood inside these layups? If so, then do I somehow fill in the space (as much as 4 inches) between the transom wood edges and hull surfaces and attempt to join the new transom wood to the other hull surfaces?

3. Are the knee braces worth the trouble of fabricating them?

4. Should this novice (me) use epoxy resin rather than polyester resin?

5. Other advice?

 

1. Rebate the core edges, use thickened resin to fill any voids.
2. No.
3. Knees are mandatory for this type of conversion.
4. If you use plywood as a core epoxy is the better option. Polyester is for foam.
5. Sand and degrease all bonding surfaces really well. There are several youtube series where you can see such modifications in detail.

 

Thank you for the response Rumars.

I'm not familiar with the term "Rebate the core edges," please explain.

 

The core is the plywood. The rebate is the step that you router out around the edges to accommodate the additional thickness of the original fiberglass layups. Since you're not doing an inlaying job of perfectly matching the wood to the fiberglass, you butter up the recess with thickened epoxy and press the whole thing against the fiberglass.

 

Thank you for the explanation.

I don't know for certain what the original resin is on this boat, but I assume it's polyester, because polyester costs less.
1. Are there any issues trying to bond new epoxy resin with old (but clean) polyester resin?
2. Which epoxy resin do you recommend?

When I get to the outside of the boat, I'll have some fiberglass exposed, but some of the layup edges will be over sanded gel coat, which I think is a polyester resin. To get an even surface, I'll need to use a filler, a fairing compound, on the outside repairs. My instinct tells me I should be using the same type of resin on the outside for all the compounds, which suggests polyester.
3. Are my instincts correct here?
4. Should I switch to polyester when I repair the outside of the boat?

You're recommending epoxy for structural work because:
5. It cures slower and gives this novice more time to get my bonding act together?
6. It's stronger than polyester?

I've watched several videos that use bracing and fabricated plywood clamps to squeeze the outside glass skin to the new inside transom plywood; most brace only from the inside. The transom skin on the SeaCamper is slightly warped but flexible. I'd have to brace from both sides. Inside braces seem straightforward, but not the outside. The glass skin currently has 54 holes to fill. I'm thinking of using screws to squeeze the skin to the new plywood and make use of some of these holes before I fill them.
7. Is use of temporary screws a bad idea here? I haven't seen this technique used in any of the videos.

 

2nd Rumars.

The transom converts the forces from the engine into the hull. To do so requires the transom to be well tabbed to the hull and knees and stringers.

The only reason to use esters is for cosmetics and to use gelcoats on the exterior. It is silly to use them on the transom repair.

Make sure to precoat the plywood with epoxy to avoid the material sucking resin from the fillers and making dry bonds.

Thickened epoxy cannot be made more than about 1/2” thick or it will crack or catch fire or melt the outside finishes or such when it gets hot, so keep the margins less than 1/2” or so. Use cabosil as a filler and mix about 2:1 by volume or until the mix stands no sag for 10 seconds on a pile.

Use only slow hardeners as an amateur.

When you apply the plywood; you’ll need to overbutter the routed out areas and butter all of it with 1/8” or 3/16” or 3-5mm vee trowel on thickened material that won’t sag as above. Typically, some fashion of bolting is used. Only bond a single piece of 3/4” material at a time so you can edge fill and be certain the bond is good. For the follow on pieces, I prefer 1/16” vee trowel on both surfaces. I’ve done some tests and a single epoxied surface is not as good as both. You could also do this on the first surface, but it does take a lot of time so you really need to be ready to butter and mix both sides in under 60 minutes. You can’t allow it to gel before clamping really. Make sure things fit dry first..I run 12 ounce batches of epoxy when I do this type of work. More gets to be a lot to stir when thicker. Put all mixed and filled resin on a hawk or board with a 6” wide trowel nice and flat to keep gel times down and never leave it in the pail.

 

I suggested epoxy because of the plywood. Polyester isn't a wood glue, epoxy is. You can do a perfectly adequate repair with polyester but you have to use foam (for a transom typically coosa or other high density stuff) as the core. Yes, epoxy is stronger especially for secondary bonds (sticking to cured laminates), but that's not the point here, the wood is. Once you use epoxy you can't use polyester fillers and gelcoat over it, only epoxy fillers and paint.
Your strategies look like this:
1. Use coosa, and do everything in polyester, or use plywood and do everything in epoxy. You can use screws with 2x4's as washers to apply pressure to the core from the outside (place the screws wherever you like and make sure to not bond the 2x4's or whatever else you use to the transom). Then you patch the holes (old and new) with glass and continue with poly filler/gelcoat or epoxy filler/paint as appropriate.
2. Repair all the holes in the transom first using poly, then you can refinish the outside using gelcoat. This means bracing from the outside with something when you glue in the plywood. This something (usually cheap sheet goods like OSB or MDF) is braced against a wall or the floor, and the core is pressed in with jacks, wedges, etc. acting against the interior structure of the boat. The elegant version is to use a vacuum bag for this.

Now I won't say you can't glue in plywood using polyester products, but it's not advisable. Epoxy is cheap enough that the time invested doesn't warrant the use of polyester with wood. It doesn't matter what epoxy brand you use, West, East, Totalboat, Raka, etc. they will all do the job.

 

A very helpful post, thank you.

There is a phrase I don't understand: Typically, some fashion of bolting is used. Bolting could mean running bolts through the laminates to squeeze them together, or something else. Please clarify.

 

A very helpful post, thank you.

2. Repair all the holes in the transom first using poly, then you can refinish the outside using gelcoat. This means bracing from the outside with something when you glue in the plywood.

If I've bonded the interior wood to the interior of the glass skin using screws and then laminated additional layers of plywood inside, maybe also using screws to squeeze each layer together, what's the purpose of outside bracing? Unless I somehow anchored something to push against to the back of the trailer, I don't have anything solid inside the barn to push against and I don't want to attempt this outside. I was hoping to do all laminating with inside bracing and screws. I'm missing something here.

 

Look man, epoxy over poly is good, poly over epoxy is bad. Since there are several ways to skin this particular cat, it's down to you to choose. If you want to use gelcoat on the outside of the transom instead of paint, you need to repair all those holes using poly, before you start buttering up the inside of the transom with epoxy. This means you also can't run screws from the outside of the transom into the plywood core to apply pressure when glueing the plywood because epoxy will squeeze out. Since the transom is flexible you can't just push the plywood from the inside against it and expect it to stay flat. So what you do is put a piece of thick ply/mdf/etc. on the outside and brace it to something like a wall or some posts burried into the ground, then press from the inside.

If paint isn't a problem for you, or if you use coosa with all poly, there isn't any need for any bracing, inside or out. You just take a bunch of screws (half thread wood screws) and something to act as big washers and run the screws trough the transom into the plywood or coosa, pressing both together. When the epoxy is cured you remove the screws and patch all the holes from the outside, the old existing ones and the new you made with your screws using fiberglass and epoxy. Then you fair the whole outside of the transom and paint it. When you install the new stuff on the transom you drill new clean oversized holes, fill them with thickened epoxy, redrill to bolt size and install.

The question is how do you want to finish the outside of the transom, gelcoat or paint.

 

Thanks again, Rumars.

"Look man, epoxy over poly is good, poly over epoxy is bad." That's key info, thanks.

I'm not shooting for a perfect finish on the boat.

The outside surfaces of this 51 year old boat have many gouges and gel coat cracks in addition to the holes in the transom skin; I intend to patch all of them. I'll need resin, fiberglass, filler, fairing, topside primer paint, topside paint bottom barrier paint and bottom ablative paint to do this. If I can find epoxy compatible versions of all these components, I'll be a happy camper, boater, with this boat, both.

"When you install the new stuff on the transom you drill new clean oversized holes, fill them with thickened epoxy, redrill to bolt size and install." That solves the wood coating problem for new holes, I'm liking the idea. Thanks.

I'm considering Coosa but I'm a bit paranoid about screwing up use of material this expensive. I can buy marine plywood locally in Escanaba at about 1/4th the cost of Coosa. Pretty much all the other materials I need for this project have to be ordered online.

 

Which sterndrive unit did it have originally?

 

I think Rumars answered well enough. The target bond thickness for thickened resins is 1millimeter. Wholly impractical for the skin with all the router work, but you use mechanical fasteners to apply some pressure so you don’t have a 3/8” (10mm) bondline with 1/8” (3mm) of thickened resins. Okay?

 

It was a Mercruiser 225 II-TR.

 

Okay, thanks for the post.