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Hi Pete,
It doesn't look like your Commander has any more curve/bend to the coamings than on my Triton. I didn't really have any trouble getting the boards to conform to the shape. I used a jack the first time, but they have been off the boat every year since, and I've had no trouble pulling them tight just with screws. I start in the middle of the board and bend it as far as I can (usually leaving just a small gap) with my knee while I drive the first screw. Tightens right up with no problem.
Aside from suspending boards at ends with weight in middle.
Have a flat wood surface, or metal table? Could bend the uncut boards, full width, using shelf supports screwed to the table in a fair curve smaller than the curve needed. Start clamping, using the deepest throat clamps around and full width vertical pieces under the clamp against the mahogany. Screw the clamp in a little more as resistence lessens. Leave it for awhile in clamps, days, weeks.
If you faircurve a bend in longer wood than what you want for the coamings, like a couple feet, I'ld guess you'ld be more likely not to get the straight runoff on the ends that Nathan mentions. You'ld waste the end pieces (good bread boards) but you'll have curve running past the ends so to speak when you cut the pattern. Probably get better results in a warm room.
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Haven't done this:
Same clamp mold as above and in a warm shop. Mist with water, heat with heat gun - creating steam. Do this to both sides. Jackson Pollock ballet. You might get the plank to relax this way. If you can remove the clamps and lock the wood in with more shelf standards that would be good too, I think. Use wood cleats between work and metal. Especially after you do your first steaming and see if anything has happened by taking it out of clamps. If the chord looks permanent, you could put it back, spray and heat some more, lock it in with all shelf supports. Steam some more.
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Build a steam box out of cheap exterior plywood and galv grabbers just big enough to house both planks, stickered, separated. Using campstove, or electric plate if indoors, and water in a gas can, attach nozzle with hose and some kind or plumbing fitting to one end of box. Make it removable, as in Lid. This will really get hot. The box won't. much. Never tried this: but wrapping that 6" wide shrinkwrap around the box might seal off everything until the job is done (water comes out everywhere.) - two, three hours? You need an exit hole too out the far end which you have tilted a bit to get runoff, you can stuff with rags. See Roy Underhill, Yankee Woodshop.
You really can't put both pieces in the same mold, so maybe you decide to do them separately. Steam one board at a time. Your temporary set up has to last that much longer. Now, the classic fire drill. When you think it's cooked (the plank good and hot clear thru) use oven mits and throw it into the clamping mold as quickly as you can. Quicker. You have all your pieces ready to go: clamps, cleats - you hopefully will be real surprised how easily it bends. Hold that bend at the deepest with a clamp as you force the plank flat on its edge to the table. Now clamp from the middle out to the ends, fast. Make adjustments after it's in the form.
When we did it an age ago, we included a try piece in the steamer we could pull out and test. We got hot 2 X 2 white oak to wiggle when we shook it!
I can't attest to water stains on mahogany. I haven't bent M. nor a wide plank. I think steam time was one inch an hour but a wide plank might take a little longer. Generally clamping pressure is modest. Don't want to distort the wood under the clamp. Or in the mold. Eye the piece to see if the curve is fair.
If the mahogany keeps the overcurve once you release it, that's a plus for installing. You may want to keep it in the form, loose, til you work on it.
Most wood favors a side it would want to bend on. You can tell by sighting its length on edge which way it'll be more comfortable. The curve shows up.
Once cooked long teak rails in a steamer (sort of successfully). Had to take the pieces out to reverse them, as the steam didn't keep all its hot down to the far end. It's really a gas!
In terms of steambending, the curve of the coaming is very very modest. Once you get them bent, store them bent. Good luck.
PS. Just occured to me that somebody might try a flat mold. Both coamings could be done same time, by screwing cleats of graduated heights to the table that would make that nice fair curve (a little tighter and a little longer) you'ld bend the planks over the rack, securing the ends. Then pour boiling water over all. Lots. Any tendancy for the work to raise, screw a cleat down over it into the cleat under. I think you'll get better results bending full planks before shaping. I would get the thickness close, tho. Think it might work?
New work or 'wooded down' -
Pretreating the wood befor assembly with thinned epoxy is IMCO the best rot preventative. There isn't much penetration of any liquid into the pores, but 2-part epoxy does help protect the wood better than plain varnish or alternatives that sit on the surface. Use the epoxy to prepare the varnishing surface by sanding between thin coats. If you get the pores filled at this stage the finish will come out like glass. With less coats.
If the varnish has UV inhibitors in it that will help the epoxy last as the weather degrades the surface between maintenances. Maybe avoid wooding down all together.
Epoxy is very good at protecting end grain where there is some penetration.
A word of caution about using epoxy on wood. It will keep the wood from breathing. Any condensation on the inside or existing moisture will not be able to evaporate. If that happens the wood will either rot or the portion will pop off. This is a particular problem in these cold climates where we experience that white stuff and our boats are on skids six months of the year.
Talking about, in this application, epoxy used as a primer.
Exposed epoxy by itself as a finish could be a problem where it has been built up as a coating. Putting a boat up wet might create a raft of other problems. The boat should not be closed up but have air moving through at all times. Constant fresh water damp will get through ANY finish.
If you are worried about you own mix of thinned epoxy, buy CPES which is formulated to be flexible (ie, work with the wood.) You can't build up with this stuff - if you try it will get gummy. Also, you need warmth for CPES to go off properly. It has been known to blush. CPES is made with some bad solvents, that's why I don't use it any more, I used to bathe in it. "Penetration" into sound wood is a myth. Well: microns more. that's what 'protects' the wood surface. But it will stick better than the finish.
Therefor make your own up from 100% solids low viscosity laminating resin with slow hardener adding a small amount of xylene. Each man's epoxy is probably different. Each man's climate situation, different. Each man's method of application, prep, materials. Each man's lay up facility. Too many varibles.
If you insist on varnishing your exterior teak or mahogany, your finish is better off on a (thin) primer base of 2-part epoxy. And your maintenance of the film must be impecable. You probably shouldn't cover a boat, whose woodwork suffers from the last season's wear, without a new coat of varnish.
If the thinned epoxy is sticking to the wood, and you fine sand the cured epoxied surface in prep for the varnish, then your varnish will stick to the surface better than it'll stick to plain wood. You are looking at wood here with something thin soaked in on it - not a coating of something on top of it like the varnish will be. The varnish will be less likely to pull away from the wood. I'm sure the wood will breathe when it has to.
However, 20 coats of varnish may make wood unbreathable.
[The way I understand it, for what it's worth, the finish job will last as long as you keep the sun and weather from getting to the surface of the mahogany. UV inhibitor(s) in the film is the only way to go. With regular light sanding and touch up coats. Keeping the tops of the coaming boards and hand rails shiney. I sure didn't invent this epoxy primer business. So far as I know it is accepted practice. Don't know what the first steps are for systems like Epiphanies or Bristol. The above works for traditional 'soft' varnishes: spar varnish. Helps the finish last longer. I find 'wooding down' failed or tired coatings very disagreeable.]
Coaming Repair -
My coaming boards are both cracked about half way from the stern end. They are still holding together, but I'm worried that if I don't try fixing them they will only get worse and completely break sooner or later. What to do? Simply put glue (Gorilla glue?) into to crack and somehow press them together?
A possible fix:
A previous owner ("PO") attached brass plates at each end of the coamings on Maika'i. (Approx 1" by 6" fwd and 1" by 3" aft, if I remember correctly.) They support (in my case) the two boards that make up the coaming assembly. I'll take photos next time I'm at the boat.