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Discussion Starter · #1 ·
Greetings All

I'm in the process of replacing several of my (frozen) 30 year old bronze Apollo valves and mushroom thru hulls with flush mount Marelon 93s (not looking to ignite the Marelon debate again thanks ... already spent hours on that and the spec sheets).

Solid glass hull ranging from 1/2 to 3/4 thick. The old thru hulls are already removed.

A few tool/technique questions re: just the thru hulls ...

1. A few of the holes in the hull will need enlargement. Therefore hole saw. Oops or Bosch HE1 preferred?

2. Appropriate type of hole saw for thick fiberglass? Bi-metal, carbide, etc. ?

3. My biggest question. Any guidance re: recessing the flush thru hulls? I'm leaning towards concentric hole saw cuts to required depth, then chisel as required. I don't intend to cut the outside bevel, just a straight cut, then seat the thru hull in thickened epoxy (well waxed thru hull) to get the perfect fit.

Sounds good?
 

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Courtney the Dancer
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If you haven't already read Mainsails thread on through hull replacement I highly recommend it. I think it's a sticky in the maintenance section.
 

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I am not sure with the marlon flush fittings, but the groco bronze have a 45 degree face. Which allows them to work perfectly with a 45 degree router bit set to the right depth which ranges from .3 to .57" deep.
 

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I just remembered this article in Good Old Boat about using an adjustable hole cutter to make a tapered shoulder, bevel cut, for a seacock. I've never used this method so I'd advise trying it out on some scrap fiberglass to make sure you don't get excessive chipping:

Adding Holes In: New through-hulls require diligence
http://www.goodoldboat.org/pdfs/Addingholesinyourboat.pdf
 

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Discussion Starter · #10 ·
@kentobin. Yes, that is the primary source I'm following. Just using the dremel instead for the outer bevel.

@stumble. Have you actually done this with a router? Seems a template would need be fixed to the hull, and would not result in a flat surface for the thru hull.
 

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If you use a 45 chamfer bit with a 1/2" bit with a bearing on it it's pretty easy actually. Once the hole for the shaft is drilled the bearing effectively becomes the template. It does take a bit of skill with working a router by hand, but it isn't difficult.

I have done this a number of times, it was the standard way to instal them at the boat yard I worked at. But you might want to practice some first. Just get a 2x6 and drill some holes in it, then practice doing the hand routing before working on the glass.


It is also critical to know the depth of cut, and the angle you need. The Groco ones are 45 degrees, I don't know what the marlon fittings are.
 
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