Replacing Transom on Aluminum Boat

Hi,
A couple years ago I replaced the transom in my 22' Aluminum Starcraft Holiday. Not knowing better at the time, I used pressure treated plywood and now have the pitting in the aluminum to deal with as well as replacing the transom again. I plan to use two layers of 3/4" marine plywood. I have a few questions: First one is what should I use for glue to laminate the two layers? With the pressure treated wood, I used a water proof glue and stainless steel screws. Do I need the use the screws with the marine plywood? Will it hurt anything if I do just as an extra precausion? Also, do I need to coat the transom with anything to help waterproof it or can it just be laminated and then installed? As for the pitting in the aluminum, I was planning on patching it from the inside with something like JB weld or aluminum epoxy or something like that. Any suggestions for that?

 

Epoxy with a metal load is fine for small pitting repairs. Use epoxy to seal and glue the ply too. The screws are fine, belt and suspenders. Good luck

 

Sorry, I'm a novice here so please forgive me for not understanding a couple things you said, gonzo. Not sure what you meant by "Epoxy with metal load'. So you are also saying to use epoxy instead of a glue to laminate the layers. Any suggestions for Epoxy? Also I got that the screws were fine but was lost on "belt and suspenders". Thanks

 

You could save a little if you cannot get 2 pieces from a single 4 x 8 sheet of ply by using ABX or ACX (exterior) ply and you would have 2 A faces on the outside. Unless it is covered by aluminum of course. No mater what you use after gluing them together coat all sides and especially edges with 3 coats of epoxy so buy enough. This is for water infusion prevention and you will probably never have to fix it again. ditto to what Gonzo said about screws or boat nails.

 

JBWeld is epoxy with a metal powder load. You can add it to the epoxy you will use to glue and seal also. For example, aluminum powder. If there are only a few pits, JB may be easier although more expensive.

 

I wouldn't recommend JBWeld for anything, it's viscosity and formulation is all wrong. You can use aluminum oxide as a filler in laminating resin, but I wouldn't recommend this either.

I also wouldn't recommend ACX or other construction grades of plywood for your transom, if you have much more the 50 HP on the old gal, which I suspect you do. Also If you are hanging more then 100 HP on the transom, you should consider three layers of plywood, not two. Over 100 HP you should have at least 2" of thickness, not 1.5" and if you do have 1.5", it should be three layers of 1/2", not two of 3/4", though it's preferred to have three layers of 3/4" when big, heavy, high HP outboards are hanging there.

The epoxy should be a straight laminating resin with the hardener to suit you conditions (temperature). I'd recommend using Marinepoxy from www.bateau.com, as it's the cheapest around that will do the job right. Coat the aluminum with straight epoxy, immediately after hitting the aluminum with a 80 grit flap wheel and cleaning it with acetone. Let this coating get tacky then apply a slightly to moderately thickened coating of thickened epoxy. The thickening mixture should be 50/50 silica (west 404) and milled fibers or other long, fibrous material like cotton flock (west 403). Trowel this on to fill the craters and bond the plywood in. Next install the plywood in to the wet goo. Both the transom and the plywood need to be well coated. Lightly clamp everything in place, insuring ooze out around all the edges.

Log onto www.westsystem.com and www.systemthree.com and download their users guides and instruction manuals. It'll explain how to work with these materials and goos.

 

It is probably far better if you can take some pictures of the troubled area too. Since what you call pitting may be called something else, who knows? since by your own admision:

Thus a few pictures would clear up any ambiguity and address the issues more concisely.

 

PAR,
I do have a new Nissan Marine 115 HP on the boat. It originally had a 150 HP on it. The boat is made so that it will not take a transom thinker than 1.5" so It sounds like I should get two sheets of 1/2" instead of one sheet of 3/4. So are you also recommending that after I laminate it with the epoxy, that I coat the whole thing a few times with the same epoxy? As for coating the aluminum to fill the voids from the pitting, do you think it would be a problem if that hardened before I put the transom in? My concern is that if I put it in wet, I would never be able to get it back out if I had to for some reason. The transom is bolted in with 32 bolts (1/4") plus the 4 engine bolts and the two towing U bolts. The original transom wasn't bonded to the aluminum in that way. It was only connected with the bolts. Also, somewhere on this site someone mentioned drilling out the engine bolt holes larger and filling them with the thickened epoxy and then redrilling with the original size. I can see how this would prevent water from getting into the wood around the bolt wholes but my concern is, will that stuff crack under pressure and eventually make the wholes loose?

 

Mechanically fastening the transom core only, will invite problems in the furtive, but you can do it this way if you like (as you've probably already seen). It would be wise to use a bedding compound of some sort. Polysulfide would be my recommendation or a low tack polyurethane. Avoid 3M-5200 as removal of the transom core at a later date will be nearly as difficult as if you epoxied it into place. I suggest 3M-101.

3 layers of 1/2" makes 1.5" and each sheet should be cocked to a 11-12 degree angle to the previous sheet for maximum strength. This increases the cross grain stiffness of the assembled core. These plywood layers should be bonded together and the whole thing sealed with 3 coats of unthickened epoxy, to prevent moisture from getting at it.

It's always a good idea to "bond" fasteners holes, (the drill out over size thing you read about) in wood. The user's guide I mentioned previously will explain this technique.

 

Ok, I'm just getting back to this project. I have the old transom core out of the boat. The pitting isn't as bad as I thought but it will need to be filled. So here is my plan based on what has been told to me here and research I've done.

1. I will by 2 sheets of 1/2" marine plywood so that I can use three layers of 1/2" instead of 2 layers of 3/4". Question on this: I will offset each layer by 12 degrees as mentioned. Do I just cut two layers straight and offset the center layer at 12 degrees or do they each have to be an additional 12 degrees?

2. I will purchase the Marinepoxy online as suggested to laminate the plywood. Question: Do I have to wet out each surface first, or just apply the epoxy to both sides and stick them together. Question: Should this epoxy be thickened? After the lamination is done I will coat the whole thing with three coats of epoxy. Another question: Is sanding needed between coats?

3. Next I will slide the new core into place and drill all my holes. (32) 1/4" bolt holes, 4 engine mount holes, 4 holes for the tow eyes (Ubolts) and (2) 1" holes for the drain tubes. I'll take the core back out and drill each hole out 1/4" larger. I'll cover all the holes on one side with masking tape or duct tape, lay the core on a flat surface with the tape down and fill all the holes with the epoxy thinked with west 404? After this hardens I will slide the transom core back into place and redrill all of the holes to the original size. I have about a half dozen screws that will need to be screwed through the aluminum and into the core to hold transducer wires and the tranducer mounting board. I thought I would just drill a pilot hole from the outside and then either pump epoxy into the holes or just put it on the screw threads and then screw them in. Does this sound ok?

4. Filling the pitted aluminum: I guess my plan here is to sand it (I've already wire brushed it), then clean with acetone, then apply the unthickened epoxy, and then the thicked epoxy with the West 404 High Density Filler and the West 403 Microfibers and let this cure. Here is where it appears I have 2 options:

A: I can use the Polysulfide or low tack polyurethane as a bedding compound as suggested which will appearently make it possible to slide the core back out if I ever needed to. This core slides in a pretty tight spot between the aluminum transom skin and the two braces that connect to the stringers. I wonder how easy it will be to remove it if I do this.

B: The other option (which I believe you think is better, PAR) is to Bond the core to the aluminum right after applying the thickened epoxy mixture. I would also coat the back of the core with this before sliding it into place. I would think a lot of this mixture might get scraped off when sliding the core in but I imagine enough of it would remain for the bonding. Then once it was in place I would install and tighten the (32) 1/4' bolts and the Ubolts. I suppose I should use marine sealant in the bolt holes and on the bolt heads as I did in the past? I suppose if I do a good job using this option, there really shouldn't be any need to ever have to remove the core anyway.

I do have on last question: How much resin and hardener should I need for this project. (Laminate the three layers, coat it three times, bonding the fastener holes, fill the pitting aluminum and bond the core to the aluminum if I use that meathod)? I assume if I get one container each of the West 404 and 403, that would be enough?

So this is my plan with some questions. Sorry for being so long winded here but I want to make sure I have this right. If you could read it over and make comments, it would be greatly appreciated.

Thanks,
Steve

 

The gallon kit should be enough

 

1- Place the middle layer with the grain level to the water. The layer facing the transom skin (the aluminum) cant 12 degrees to starboard, so the grain is higher on the starboard side then the port. Do the opposite with the inboard layer. To recap, the inner most layer (against the actual transom) is cocked 12 degrees up on the right, the middle layer straight across and the last layer is tipped 12 degrees the other way from being level. This places the grain of the plywood in an optimum location to transfer loads from the outboard bracket to the hull shell.

2- For epoxy to be effective as a moisture vapor seal, you need three good coats. You can get by with 2 if you have some mad skills and a very controlled environment, but the average person needs 3 to be sure it's sealed. Special attention to the end grain is important, as this is the usual place moisture gets in.

To save some steps (coats), you can use a bonding coat (when you glue the sheets together) as the last coat of the 3 coat requirement. In fact, you could probably get by with just 2 coats on the faces that are glued to each other, assuming you'll really coat the crap out of the laminated assembly afterward. on these facing layers, the first coat is straight epoxy, then come back with a thickened second coat, using both 404 and milled fibers. Mix this to a catchup consistency. use drywall screws to hold the pieces together as the goo cures, then remove the screws when the epoxy cures. Start screwing in the middle of each piece and using a circular pattern, spin out from the center with the screws. This will force out the air between the layers. You should have epoxy oozing out all around the edge of the transom core when you're done with the screws. Make sure you remove them and fill the holes when the goo cures.

If I was doing this I would test fit each piece dry (several times) and then glue and screw the whole thing as an assembly, all in one shot. 30-40 years from now, when someone else has to fiddle around with this transom, they cuss a lot trying to get the remains of this job out of the boat, but I think this is the whole point.

3- During the testing fittings, you'll drill all your holes, right through all the layers. These will then be drilled over size, in anticipation of the install which will fill them with thickened goo. Drill an overly deep pilot hole for the screws, then squirt some epoxy in the hole, before you screw then sounder or what ever to the transom. The extra deep pilot hole serves as a well, where the epoxy can drain, when the screw tries to push it out of the hole, while being driven home.

4- The choice to bond the core or not is up to you, though you seem to have answered it already. The way I look at things now is, if it will outlast me, then what the hell do I care. I'm in my 50's so I got time yet, but I'd be surprised to make 80, so if I fix something that's got this much durability in it, then it's good enough for me.

If you elect to not bond the core, just use 404 and skip the micro fibers, as these are only necessary if you bond the core.

An unbonded core can trap moisture between it and the transom skin. This is a big concern for me. Anytime you place a metal against wood, you better have a serious bedding or the wood should be bonded. Moisture is going to get in there. The idea is to minimize the ways it can and the space it has to live.

I'm not sure how much filler you'll need, as this is dependent on many variables. There's a lot of surface area in a transom core, especially 3 layers, so a gallon would be a good choice. You'll find other uses for this stuff too, it's the wood butcher's friend.

 

Thank you both for your reply. It's great to have all this knowledge available.

I have decided that I will go ahead and bond the core to the aluminum.

So the 3 individual sheets need to be coated once or twice before lamination? Should I allow this to harden before spreading the thickened laminating epoxy or just let it get tacky and then apply the laminating epoxy? Any need to sand between applications?

So I don't need to fill the oversized holes and let them harden before I install the core? If I got this right, they will fill on thier own when put the core in place into the goo. What prevents the stuff from just running out of the holes on the inside or will it be thick enough so that it won't? Also, once it's in place do I put the bolts right through and goo and tighten them or wait for everything to harden and then redrill them?

Looking at the website that you suggested, it looks like the smallest kit is the 1.5 gallon kit which I plan on ordering. I'm not sure about the cure time though. I'm doing this in my garage. With the heater I have out there, I can get it up to mid 40's or maybe 50 degrees if it's not to cold outside. Is this warm enough and if so, should I get the slow, medium or fast cure kit?

Again, thanks for you input. I think I'm just about ready

 

Cut your plywood to fit, all three pieces, then insert all into position, and drill out all the holes. Remove the core pieces and realign them on a work bench so you can drill the bolt holes over size, and screw them together.

Since you boat isn't very big, nor more then a few hundred pounds, what I usually do is stand it up, so the transom is level. The easy way is to hoist the bow until the transom is square with the ground, then place some blocking under it. This lets the work proceed in the "down hand" position and also lets the goo settle evenly, without sags. Naturally, you'll want to tape all the holes up, so the goo stays where you put it.

In a perfect world, you'll want to apply subsequent coats of epoxy just as the previous coat just loses it's tackiness. This can't always be done, so sanding between cured coats is necessary, but if the previous coat is still "green" (it dents if you push a fingernail into it), then no sanding is necessary.

Yes, you'll need to drill and fill the over size holes, but they will partly fill as the pieces get screwed together. Yes, when filling these holes, use a thicker mixture, about the constancy of peanut butter. This will stay put and not run out of the hole. It might sag, but you can lightly tack a scrap piece of plywood over each hole. Use small brads and cover the scrap plywood with plastic packaging tape so the epoxy doesn't stick to it. Knock them off when the goo has kicked off.

Yes, Marinepoxy has kits from 1.5 gallons and up, but since it's a 2:1 goo, you just need twice as much resin as you do hardener. So, buy 2 quarts of resin and one quart of hardener and you're good. In fact, they use to sell a 3/4" gallon kit, so talk to Joel over there and see if he can hook you up.

Lastly, the hardener you'll want is medium. Actually, most in your area will use slow, but you'll need the working time of the medium cure. If you have space heaters, tent the work area around the transom, when you're ready to walk away (all the goo tossing is over) and place the heater inside it. Leave it like this for a day and you'll probably be good to go. It's difficult in cold areas to recommend hardeners, but if you can raise the temperature for several hours, then you can usually get it to cure.

 

Ok, I think I've got it. My boat is 22 feet long with and 8 feet wide, so I won't be able to lift it up as you suggested. Also, the whole core has to slide down in between the aluminum transom sheet and the brackets that are riveted to the stringers. That gap is 1.5".
So I thought I would cut the three pieces, make sure they fit, laminate them together the way you suggested, then slide the core into place and drill out the holes. Then I would remove the core, oversize the holes and then fill them with the thickened epoxy and let it harden. Then I will put it back in place once more, redrill the holes through the hardened epoxy, then pull it back out, put my three coats of epoxy on and let it dry. Then I'd sand and clean the aluminum, sand the back of the core, apply the goo to both surfaces, slide the core in place and bolt it in. Will this work or do the holes have to be filled during the process of bonding the core to the aluminum.
You think that 3 quarts total will do this job fine and I don't need the 1.5 gallon kit? Would that still allow a little room for error? Also, whats a ball park figure on how much time I have to work before the epoxy hardens?