Should I glue or screw these cabin sides?

Hi folks,

I've a question for you experienced builders out there. I've prefabricated the parts for my 73 Silverton's cabin and am getting ready to do the final assembly. This cabin is for a dual station small cruiser of 27'. I'm going to retain the original side window assembly. These cabin sides utilize an upper and lower beam (they are substantial), the aluminum window frame runs the length of the cabin and is installed with 1/2" clearance on top and at the windshield pillar, this prevents binding of the horizontal sliding glass.

Back in 73 they used some kind of glue and many, many silicon bronze screws to secure 1" mahagony trim to the outside of the cabin beams. These mahagony boards were doubled back by the rear cabin wall and extended along the side of the boat into the cockpit area to provide support for the rear cabin wall and tie everything together back there. So far this appears to be pretty standard construction.

The issue is the screws. It seems to me that in '73 that may have been the prefered way to attach those mahagony boards. Those screws seemed to be the source of rot over time. Now, with epoxy, 3M 5200, 4200 etc., I'm wondering if there is a prefered technique to laminate those 1" mahagony exterior boards to the outside of the cabin beams.

Is it acceptable to use 5200 or 4200 or epoxy and a couple of screws to draw it all together and clamp everything up? Is there another product or technique that I'm not aware of?

Thanks in advance for any thoughts you might have.

Regards,

MIA

 

Most likely, barring poor building practices, what caused rot was lack of maintainence. The screws would have been bunged, and the mahogany varnished, and all parts bedded in something.
When solid wood has any appreciable width, say over 3", AND that wood is not constantly well protected by a waterproof paint, varnish, or oil, adhesive (esp. epoxy) on one side only creates a potential for working the wood because the adhesive side doesn't allow moisture cycling, and hence the fasteners work too, especially seasonally between summer and winter.
Many points of moisture ingress are then created; bungs pop out or loosen, board edges cup outwards as the outside dries faster than the inside, cracks appear at the board ends, and so forth.
If I were installing boards as you are about to do, I would varnish the backside of the boards three coats, bed in a good compound, screw on with stainless (or better, bronze) screws (1" mahogany should use at least 2" #10 screws), and then apply 8 coats of spar varnish or equal finish to the outside.
Then, each season, apply two more coats to replace both sanding and UV loss.
Keep that finish up. and you'll have no problems. Yes, you could epoxy the wood, and adhere it to the wall as well, but as said, only narrow pieces should be done this way. There are different ways to mill wood too, where quarter-sawn wood moves less than plain-sawn by as much as half, but nowadays, count on having access to plain-sawn wood alone due to scarcity and markets. You don't want movement, but movement is inevitable to a small degree; therefore you want very gradual and very consistant movement involving not enough exchange of ambient moisture/dryness occurs in a short amount of time to cause enough of a difference within the wood (front to back) to cause warpage. That's all any warpage is--- one side exchanges water content faster than it can transfer internally to the other side, and the most moisture-laden side gets bigger before the dryer side can catch up.

 

Thanks Alan.

Looks like I'm on the right track. I've got plenty of 2" silicon bronze screws.

Based on what you wrote it would seem that applying epoxy to bond 12" wide by 1" mahogany boards would potentially cause problems due to expansion & contraction based on how the wood itself was sawn, right? The epoxy would be too brittle.

I'm probably going overkill here, but here is my thinking based on your reply.

1. I'll sheath each prefabbed panel with 4oz cloth / epoxy. All 6 sides. I've gotten pretty good with the router, beveling edges so putting a thin layer of cloth on is straightforward.
2. The next day I'll install the parts. I'll bed the boards with 3M 4200 and secure with #10 screws countersunk, bunged.
3. After the cabin sides are in I'll either fair with another coat of resin and microballoons or perhaps sheath the exposed wood with a second layer of 4 oz. cloth, depending on how things look after installation. If I do a good job in step 1 then I doubt I'll need another layer of cloth in this step.
4. After it's all sanded down and prepped I'll use interlux epoxy prime coat (2 coats) and then a couple of coats of interlux perfection as a finish on the exterior. I'll finish any exposed surface facing the inside of the boat with a few coats of spar varnish over the epoxy/cloth that was applied in step 1 (I hate to paint all that mahagony, it so nice looking).

One last question. I understand that 3M 4200 is not as strong as, but more flexible than 5200. Since I don't really need a brute strength bond here but I'd want a little flexibility I'd think that 4200 would be the best bedding in this situation. Would you agree?

Thanks again Alan,

MIA

 

Your plan of action seems well thought out MIA. You've come such a long way "grasshopper".

 

The mention of grain orientation was a bit misleading. You want to give your job the best chances and since it is no longer as easy to get good quartersawn wood, you are usually dealing with the maximum shrinkage and expansion of plain-sawn----therefore, you gotta do everything right!
Epoxy coatings ideally should be either all sides or not at all, and so epoxy as an adhesive for the boards means having to apply epoxy under the varnish on the exterior surface too.
That's in itself overkill, and expensive, so don't epoxy them on. The idea is to equalize moisture take-up and drying cycles on all edges and sides of the wood, hence a few varnish coats on the board's back sides.
It isn't so much brittleness (though that matters too), as the idea that you've just sealed the back in adhering it to the wall, and now you are doomed to epoxy-coating the face and edges as well.
4200 would be best, I think, with solid boards that wide.
The second layer of 4oz cloth on the walls may be overkill, but I would maybe do that to fir plywood. I'm guessing you have a different plywood though, one with a tight grain, which needs no more than a single layer of cloth (or none at all if desired). Try fairing with West #410 microlight. It feathers better than balloons. Once faired, a couple of coats of neat epoxy and sand and paint.

 

Thanks Alan as always I appreciate your input.

I'll post a few photos when this is done.

MIA

 

When I built the cabin for my '68 Luhrs 25' Flybridge, I glued and screwed every joint with thickened epoxy and either some of the few hundred bronze screws I removed from the original build or used either stainless or even (yikes) deck screws. But I also filled every screw hole with thickened epoxy to ensure it (a) stays put and (b) corrosion proof the fastener. Like in your boat, Luhrs didn't use any glue to hold the boards together 40 years ago, but I did as it makes me feel safer in rougher seas, than just having screws only. Here's some links to my project

http://www.bloodydecks.com/forums/b...-worth-extra-money-over-vinylester-resin.html

http://www.bloodydecks.com/forums/check-out-my-boat/113113-savannah-ii-update.html

http://www.bloodydecks.com/forums/check-out-my-boat/117970-savannah-ii-progress-update-cabin.html

http://www.bloodydecks.com/forums/check-out-my-boat/119162-more-pics-savannah-ii.html

 

Well, I'm on my way to the yard in a few minutes to finish cutting the paor cabin side and maybe a couple of other things if I have time.

Thanks first to PAR for the compliment. Ahh..the old kung fu episodes . I learned a lot and still have a lot more to understand. I have a lot of work in front of me but she's starting to look like a boat so I can see the light at the end of the tunnel. The other day I was talking with my wife and I said "you know, I'll be done with this boat by spring. I think I'll take a little time to enjoy her but I was thinkin' about maybe building a small wooden sailboat."

Boy you should have seen the look I got. I think it's the sawdust.....or all the days I'm missinginaction at the yard.

Thanks to you too Mongo. That Luhrs looks remarkably similar in profile to what I'm doing. Looking at the original build photos the angles are just about identical. You're doing a great job.

As always I appreciate the feedback.

MIA

 

silicon bronze screw source

Go to www.j2depot.com. You will find top quality 651 silicon bronze screws with non-compitable price.

 

Thanks mike. This post takes me back to last summer. Those are very good prices on the screws, I'll keep that retailer in mind.

Thanks,

MIA