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-   -   Mounting instruments into cabin: screws, and boatlife or epoxy? (http://www.sailnet.com/forums/gear-maintenance/29508-mounting-instruments-into-cabin-screws-boatlife-epoxy.html)

geary126 02-22-2007 04:16 PM

Mounting instruments into cabin: screws, and boatlife or epoxy?
 
So, I'm mounting a speaker into the cockpit. The hole goes right through the core, into the cabin. I'm planning on epoxying the interior of the hole, but what about the screw holes (outside the 5" hole, there are 4 mounting screws).

Is BoatLife enough? I've read at WestSystem.com about this process of drilling large holes, like 3/8", filling with epoxy, redrilling with pilot hole, but that seems over the top to me. Practically speaking, isn't boatlife going to do a pretty darn good job?

What kind of screws does one use for glass/ wood combos?


Finally, if the epoxy method is critical...do I use plain, or mixed with the silica? Do I let it harden, and then drill into it?

WestSystem suggests placing the screw in while the epoxy is wet, but that seems difficult, as it will have no holding power until dry. And can you screw it further in, or is it too late?

cpcohen 02-23-2007 12:41 AM

It's an unstressed joint -- good caulk _should_ be enough, IMHO. [Others may disagree.]

Use sheet-metal screws, short enough so they don't extend into the cabin.

Drill the holes in the outer fiberglass just a bit bigger than the screw shaft diameter. If the holes are too small, the screw threads will crack the glass when you drive the screws in.

Use masking tape around the edge of the big hole to control "caulk mess".
Put a bead of caulk on the speaker, a blob on each screw hole, a thin layer on each screw. Tighten the screws enough to get a bead of caulk extruding around the speaker -- don't try to drive them all the way in.

The "authorized" method is to remove some of the core around the screw holes. Then, fill with thickened epoxy. Let it set. Drill out the screw holes.

Then proceed with caulk as above.

I'm not willing to put screws into "wet" expoxy -- too frightened they'll never come out.

Charles

geary126 02-23-2007 01:57 AM

Thanks, Charles.

That's exactly what I needed.

PBzeer 02-23-2007 02:35 AM

I just finished installing my dinghy davits. A project I started before Christmas. The reason it took so long was the area of the rear deck core was saturated with moisture from the screw holes around the vents for the engine. The holes hadn't been backfilled. Yes, it takes an extra day to backfill them, but why wonder if it will work instead of knowing it will work?

Idiens 02-23-2007 04:56 AM

Back fill or rot
 
I always back-fill, troublesome though it is. Some "professional" did not do so with the windlass switches - the balsa core doesn't stand a chance over time and the cure is never painless or complete.

geary126 02-23-2007 09:39 AM

By backfill, you mean epoxy?

I guess the question becomes, why use BoatLife at all, in general, if it can't prevent moisture from moving in along...what, the thousandths of an inch of twisting, available space next to the threads?

Also, if I am to do it, could you detail the best procedure, please?

sailingdog 02-23-2007 09:40 AM

I would "pot" the holes with epoxy, after drilling them oversized... you really don't want to let water get a toehold into the core on your cabintop. Use silica filled epoxy, and then re-drill after it is dry. Why do a job half-assed... when it only takes a bit more time and effort to do it right...and prevent an expensive repair later.

sailingdog 02-23-2007 09:43 AM

I would dig out the core for about 1/4" around the big 5" hole and then fill the space the core was in with thickened epoxy. Then drill the holes for the screws about 1/4" bigger than they need to be...and fill the hole with thickened epoxy, and then after it has set... drill holes for the actual fasteners to pass through.

BTW, you should probably have some sort of waterproof cover bolted or screwed to the back of the speaker... a 5" diameter hole can let a lot of water through if the sucker get taken out by a wave... :D

PBzeer 02-23-2007 09:49 AM

Drill your holes oversized, then dig out the core around the hole (I have a allen wrench that the short side is cut back and sharpened, that I use with my drill), then fill with epoxy, let cure and then drill the proper hole through the hardened epoxy.

Caulk is for seams and secondary for holes. The epoxy is for preserving the integrity of your core. If you unscrewed something on the boat and found it wasn't backfilled with epoxy, what would you think of the person that put it in?

Valiente 02-23-2007 10:03 AM

I have a balsa core old C&C from the '70s. There is no headliner to speak of and where there is no paint (beneath the minimal headliner around the companionway hatch and in the forepeak), you can see the results of water intrusion into the core.

I have rebedded and recored virtually every piece of deck hardware. I have done actual core replacements (with encapsulated marine plywood) under the entire port genoa track, which ripped right out of the deck at one point during a blow. No, neither I nor my guests were amused with a car and block flailing at high speed as I tried to tack over to ease the sheet.

Consequently, I "drill and fill" exactly as is listed in the works of Saint Don Casey. It's a two-day job, but can take as little as 15 minutes per day per deck fixture. I grub out the old core (the messiest bit) with a small Allen key on a Dremel, or even just by hand). Then I run a hairdryer on the area to evaporate and further "honeycomb" the remaining dead core, which is only a matrix for fresh epoxy at this point. Then I tape over the hole from the bottom fill it with an epoxy slurry, and (here's the refinement) brace a flat plate of something wood or plastic from below and put some sort of metal weight above, compressing the glass layers and the setting epoxy. Cover and let harden. Drill out the next day (with heed to temperature, etc.)

A further refinement is that I put a spiral of caulk on the SS through bolt around the mid section. As the bolt goes in, the caulk seals the threads and makes a very neat and minimal "doughnut" as it seats in the cleat, winch base, whatever. This takes a little practice to gauge the correct amount of caulk, but it isn't much. I rarely have to wipe or Exacto-knife any surplus off.

Lastly, I cut 1/4" aluminum backing plates to spread the load beneath. I will put a fine bead of caulk on the topside of this plate, and I will torque down the bolts in two stages: finger-tight on Day 1 and then to final torque of maybe 35-40 ft/lbs. on Day 2. Having a helper with a screwdriver is handy here.

I end up with revitalized and strengthen core that resists water at the hole, at the caulked bolt threads and at the beaded backing plate, which spreads the load and mitigates the core damage. It's not as good as REPLACING the entire damaged core area, of course, but if I did that, I'd never go sailing!

My boat passed survey better in 2007 than in 1999. It is 34 years old.


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