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Through hull indecisiveness

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Hello People, 

Currently, I am in the process of replacing my old through-hulls, and it has got me a bit stumped. 
The current ones are Blakes bronze ones (yes bronze, they are that old) and are possibly original to 

my 1972 Contessa. The problem is that these through hulls were installed using countersunk bolts 

from the outside of the hull. I would like to change these to more modern ones but I am unsure about 

what to do about the bolt holes. If I'd upgrade to ,for example, tru-design plastic through hulls, I would 

have to enlarge the holes to such an extent that I'd have 12mm (1 1/2 inch) and 17mm (3/4 inch)mm

between the centre of the bolt hole and the edge of the through hull hole.  Could this be a structural

concern if I fill up the bolt holes with thickened epoxy?  or should I glass the boltholes somehow?

Thank you and kind regards.


Screenshot 2021-03-02 at 17.56.33.png

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No problem filling them with thickened epoxy. You're already cutting a big hole for the new thru hull :)

Do you know the trick with a hole saw to enlarge an existing hole?

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21 minutes ago, Max Rockatansky said:

Regardless, please share. We may learn something new

Use old hole's size holesaw as a guide, spun onto the stud inside the new larger hole. Cutting wax is helpful on the bottom bit for heat.




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This is a great opportunity to glass a thick, flat mounting base to the inside of your hull for the new seacock to rest upon (and screw/bolt to, without the need for external fasteners.) Some sing the praises of G10; I like a phenolic countertop material called Paperstone, mostly because I have many leftover chunks of it lying about the shop. :) Shaping these to fit the hull curvature is unpleasant but not difficult work (50 grit sanding belts are the ticket.) Bed to lightly-ground hull FG with thickened epoxy, with blue tape over the exterior to capture the drips and simultaneously fill your old holes. Drill new holes, install thru hull, screw seacock to thick base plate, you have a monster strong and rot-pruf installation. 

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Thank you for the replies. That all makes sense to me. I was aware of the 'trick' of mounting a wooden plate to the interior, to get material 

to drill the pilot hole in, but I must say I'm charmed by this method as well. 

I will definitely be installing a backing plate, and making it cover the existing holes with a lot of overlap does make a lot of sense. I will probably 

have to slightly redesign the heads area but it is worth it to fix this properly once and for all.


I'd love to use G10 but it is a very hard material to come by here, and usually only in very thin plates. I'll look into it. 
Do you have any recommendations for thickness? I was thinking something in the neighborhood of 10mm or higher

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