Flush mounted thru hull

[FloatingMoneyPit]

I may have to replace some flush mounted thru hulls with larger thru hulls that have a larger flange. How do boatbuilders create the flanged-shaped cavity in the hull? Is there a specialized grinding bit for this?

[Figment]

1st of all.... don't fear the mushroom.

But if you have your heart set on flushmounts...

I've only done it once, and I kinda made it up as I went along. There probably is a better method. I over-ground the hollow after drilling the hole for the fitting, then coated the fitting with wax, slathered a huge gob of thickened epoxy into the hollow, and smooshed the fitting into place. After the epoxy had cured, I popped the fitting out and faired the squeezed-out epoxy, then reinstalled the fitting in a bed of sealant.

Because you're enlarging the fitting, make sure you have a big enough backing plate. I feel that because flushmount fittings eat away so much more of the skin of the boat, a much larger backing plate is prudent.

[FloatingMoneyPit]

Now why didn't I think of that? Thank you for that, sir.
This is for the head intake thru hull, and all the other thru-hulls are flush, so I don't want it to feel left out if nothing else.
Definitely going with beefy backing plates for all thru hulls. 3/4" marine ply bedded in epoxy/silica, with seacock fastening bolts counterbored into the underside of the plate (no screw holes in the hull).

[JonnyBoats]

with seacock fastening bolts counterbored into the underside of the plate (no screw holes in the hull).

I have seen people do this both ways, with screws through both the hull and backing plate and alternatively through only the backing plate.

What are the pros/cons of each method?

[Tim]

It's ultimately stronger if the bolts extend through backing plate and hull alike, but of course that's three or four extra holes in the hull to (in theory) worry about. The point of the fasteners securing the seacock is to ensure that the seacock cannot move, particularly if the valve freezes and excessive force is required to move the handle (or to try to move it).

Screwing into the backing plate only requires that the backing plate be bonded to the hull with epoxy or 5200, of course; otherwise, there would be no benefit at all. I think this method is OK when all is said and done, but is somewhat less strong in the end.

That said, I bolt through the hull, myself, and bond/seal it all with 5200 (including the backing plate to the hull).