new skin fitting

[paulburton44]

I am fitting a new skin fitting below the water line !!!!
I read the bit in pbo a couple of months ago.
It suggest a ply backing plate. If there was a slight leak would this piece of ply not absorb water and delaminate.
The skin fitting does have a large loose washer that will spread the load ??
The hull is 14mm thick.
Now i have drilled the hole would you suggest I epoxy the hole due to the now expossed glass, before installing the skin fitting ?
When i do fit i will be using underwater flexable sealant !! would you add epoxy inside ???
Thanks Paul...

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[sailorman]

the idea of the ply backing washer is to take-up any curvature in the hull on the inside.
it is not nessesary to epoxy the cut edges as they arnt exposed due to the mastic, likewise the ply isnt exposed to water ingress either.
dont epoxy inside u may want to remove it somrtime

<hr width=100% size=1>Nil iligitmo carborundum

 

[ParaHandy]

Is the hull laminated with a core material like balsa? i only ask because 14mm is quite thick. if it is, then sleeve the hole with a short length of pvc/pp water pipe of suitable diameter and seal the sleeve with resin ... otherwise, unless the hull has a serious curve to it, forget the ply and fit it directly with marine sealant. If you do need to back it, i would use a teak block which you can sand/abrade away to fit. ply does laminate easily unless you protect it with many coats of varnish ...

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[john_morris_uk]

Sincerely hope that the hull beneath the waterline is not balsa cored! Can't think of any production boat for many years that has been built with balsa core below the W/L. See previous threads on GRP construction to see why.

If you do have to put a fitting through a balsa cored area, a better technique is to drill the hole and then dig the balsa out from an area round the hole (Try a very stiff wire in an 'L' shape in an electric drill). Then tape over the bottom of the hole and inject an epoxy mixed with thickener (Not micro balloons - you want strength!). Redrill the hole through the middle when set.

Even if there is no balsa core, there is an argument for epoxying the raw GRP inside the hole you've cut.

Regarding backing pads:

The transducer will work fine without a pad, but it isn't bext practice.

The advantages of a plywood pad is that it doesn't split under load if you haven't shaped it perfectly for the hull. Good marine ply shouldn't delaminate! Why not coat it in epoxy (two or more coats of clear epoxy is pretty good and much more durable than multi coats of varnish)

That said, a decent piece of teak is extremely durable. Just make sure its shaped well to conform to the hull.

Bed the skin fitting on an appropriate bedding compound and it shouldn't leak.

(I suppose one could add: Finally tie soft wood bung near by in case of some unforseen catastrophic failure of the entire fitting.)

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[oldsaltoz]

G'day Paul,

The old Snapdragons were solid glass as I recall. I don't like ply or any other timber around skin fittings, it rots, delaminates, cracks of finds some other way to fail over time.

I have glassed in dozens of skin fittings and never had one fail, some are will over 30 years old and still as good as the day they cured.

Fist drill a small pilot hole from inside, that way you know you will have good access. give the inside a grinding to remove any coatings that may have been applied and to provide a good surface for bonding. Draw an oval 400 mm fore and aft and 200 mm above / below the hole. Glass the area with as many patches as it takes to give you a flat area to seat the new fitting on. start with a small patch 20 mm larger than the washer on the fitting, then another 20 mm wider and 40 mm longer, and so on till the last patch reaches the oval you drew to start with.

Now drill your hole using the pilot hole from outside; make this at least 6 mm larger that the fitting; now back inside and offer the fitting up to the hole, do any final shaping of the seat area as required, also check the outside flange, now that you have the extra glass you can do some 'minor' trimming so the flange sits flush on the curve, (you can't do this with timber backing). Now apply at least 3 coats of resin over the entire job, hole, new glass and seat area.

Add a dab of sealant around the flange only just prior to fitting and wipe off the access at soon as possible. do dot fully tighten for 48 hours, and even then make sure the flange does not turn when tightening the nut; if it moves, remove it, clean it and try again.

Note: Use only epoxy resin and cloth designed for use with epoxy, not chopped strand mat.

And avagoodweekend.........



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[dickh]

Good advice as usual - just make sure you use a proper BRONZE skin fitting, you don't want it dezinctifying in a couple of years. Do NOT use TONVAL ATD which is really a Brass but sometimes sold as 'dezinctification resistant' bronze.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[paulburton44]

it is solid glass (no balsa core) 1970 snapdragon 26

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