TOPIC: wood transom

still looking for suggestions for what mastic to use under wood transom on 57 Cambridge

Don't know what you are referring to. Could you please explain what you are looking for? I'm sure there is someone here who could help.

MFG transoms of this year consist of thin fiberglass sandwiched between 3/4" wood. Mine was rotted and needed replacing. Underneath was evidence of some type of mastic applied with a toothed trowel over the whole surface.
Thank you for any suggestions

Ah-ha. I've replaced a few transoms over the years. Do not cut off the outer skin of the transom. That should not be compromised.
If you remove the inside shell to get the old rotted 3/4" plywood you should be able to clean the whole she-bang pretty good.
I use some tag-board/cardboard and trace out a pattern of the transom that you can lay out on your new wood and cut to size. I cut out two pieces of 3/4" and stick them together with fiberglass resin. You could use 3M 5200 or some good waterproof construction adhesive if you want. When you get the complete wood setup dried you more than likely will have to do some trimming around the edges and round off the inside to fit the curves. When you are satisfied with the fit cover the whole thing with glass mat or cloth so you have one solid 1 1/2" transom. Then it's just a matter of gluing it back in place. I really do recommend the 3M5200 product, it takes a week to set but once done it will NEVER let loose and you will have a rock solid transom that is completely sealed in glass. When that is set in you can then use a can of Bondo long strand to fill in the voids around the edges. Some guys call it "peanut butter" but it's just chopped strand glass mixed with the resin to make it thick. The long strand stuff is easier to work with and you can buy it in quarts at a local box store or auto store.

Thank you very much for the reply. I'm not sure that I explained it very well. I have a 3/4" mahogany outer transom piece varnished and cut to size. I also have a similar inner piece varnished and ready. Now I'm ready to bolt them together with the 1/4" fiberglass transom of the hull. This is to simulate the old wooden Lyman wood transom. I can see that something was put on the fiberglass with a notched trowel before the wood pieces were put on. I don't know what to use to replace that stuff. Is it to waterproof or stiffen, does it need to stay flexable, what brand to use, ect?

Thanks Peter

Can you take a picture of this? I think it would help the discussion. It sounds like you are leaving the fiberglass and adding an inner and out wood? That is not normal to a fiberglass boat.

Those 15' MFGs were made differently from the usual fiberglass boat. Rather than plywood in the center, with inner and outer fiberglass skins, these had a fiberglass inner core (part of the pressure-molded hull) and then inner and outer layers of mahogany, thus the reverse of the usual construction. The mahogany was glued on, but I don't know what MFG used.

I do know this - if you used 3M 5200, it would never loosen up, and would be hard, but not impossible to get off again. I'd use 5200.

I looked into the 3m 5200 ind it looks ideal. Thanks for the suggestion.

To emphasize what has been said, I'm only replacing what was there originally not trying anything differently. It's the way the older MFG's were made.

Thank You to all who responded,
Peter

I wonder if anyone can comment if there is any value in Grandpas old method from the 1940’s of protecting the wood from water. He would take Linseed oil that was thinned with turpentine and coat his raw wood. First coats drank in fast. He would give a little time between coats and coat it until the wood wouldn’t drink any more. Not sure how long he would let it stand, but eventually applied several coats of varnish. His boats were all wood and built from Popular Mechanics design plans. Not sure if magazine recommended this or how he came about that method. The theory was that if the boat got surface gauged from car topping or shore or stumps, the oil would repel the water and not let it soak in to prevent rotting. He re-varnished ever spring to help. He wanted protection when he had the boat out every day for 2 or 3 weeks of fishing. Not sure if the Linseed oil thing is worthy of repeating and not sure how today’s epoxies or resins would stick. Since that outer board will see water, just thought I would throw this out there to see what our wooden boat experts think of the old method and if they have better advice with newer methods.

I am no expert, but ,Yes I have heard that before. It's the oldest and first version of CPES (Clear Penetrating Epoxy Sealer) as a wood preservative.
And Yes you can varnish over linseed oil/turpentine, Because Varnish has oil in it.
BUT some of the newer varnishes are water based, so you have to be careful to get a oil based varnish.
I have no idea what would happen using a water based varnish over oil, but I bet it wouldn't be pretty ?

That makes sense. Wonder if an epoxy would de-laminate? Epoxy on dry wood would likely soak in a bit as well as bite to a surface tooth. Wonder if the oil would prevent that bond? Maybe I will do some test pieces when things warm up in April or early May. The mfg should look great with fresh replacement wood. Would be cool to see pictures of finished project. I have also read a few times that some people drill oversized holes for motor bolting... then they epoxy closed and later re-drill smaller holes to ensure water doesn’t sneak into the wood if not sealed perfectly. Is that best practice? For any wooden transoms and motor bolts below the waterline? I am thinking I want to do that if that’s best because I want my kids and grandkids to have my boat when I am gone.

Mark and Chuck.
You hear guys talk about the varnish smell on those old boats but what the "smell" is really the linseed oil. It does seem to keep the varnish pliable also. The admiral is an oil painting artist/teacher and linseed oil in in her book of tricks as a medium and when she uses it there is that definite varnish smell.
If anyone has used CPES you know the smell will drive you out of a room and stay with the work for a week or so until it dispels. Linseed oil and turp are more user friendly but being an oil it doesn't dry on itself so adding the turp thins it enough to soak into the wood. Did ya ever run a fingernail over an old table that was finished with linseed oil? after many years it attracts dirt that builds up and gets rubbed in causing the "patina" and also gets under your fingernails
For the transom you could use the CPES and after a week put several coats of gloss polyurethane, I used Helmsman(?), think it was a Minwax product, on my Century woodie a few years back and never had a delamination problem and it was rock hard and could take some dock hits.
www.minwax.com/wood-products/clear-protective-finishes/interior-exterior/minwax-helmsman-spar-urethane

More great info, Cal. Sounds like there are 2 professionals in the house! Great to have extra resources.

I've used boiled (not raw) linseed oil and turpentine 50/50 for a hand-rubbed finish on furniture I have made. (Rubbed on using 0000 steel wool.) Frankly, I wouldn't expect a table top finished this way to fair well out in the weather. I've said it before but I'll repeat myself. When doing marine work, use marine products. The difference in cost is minimal when compared to the cost of redoing something because the wrong product was used in the first place. I'll stick with my method of 8 coats of Captain's varnish.

I use (mahogany colored) 5200 for any/all exposed wood edges. That includes the ends of planks, all plywood edges, the insides of holes drilled through the transom...tank vent, exhaust pipes, trim tabs, etc. I rub it into the end grain with my finger (or a thin bolt, whatever) to make sure the exposed end grain is 100% covered. I don't have much faith in CPES doing the job on end grain. Naturally, this application is only for edges that will not be seen.

Appreciate the added info from Nautilus. Grandpa used to heat his Linseed oil and turpentine on a stove. He did tons of coats of varnish as well. That was 1940’s and 50s. I am on board with using good products and modern techniques because I want to do this work only once. End grain protection is a must. Seems like an absorbent paper towel drinking up water without protection. I just picked up some 5200 to use like Nautilus suggested.

Here's what I've used on wood for over 40 years.

I'd use thickened epoxy to laminate the wood to the fiberglass. Scuff the joining sides with 40 grit and then clamp in place.