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Old 30-06-2016, 08:26   #1
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Flush-fit thru hulls

Hi
My boat has four thru-hulls, all of which are of unknown age and flush-fit on the external part of the hull - i.e., the thruhull mushroom head is buried in epoxy so there is no 'bump' and drag is minimised. Sorry, I have no pics of this.

They appear to have been epoxied or glassed in, and even the hollow tube is very narrow so there may be a layer of epoxy inside the thruhull as well. The flush fit is, I have been told, a racing thing to reduce drag - she is after all an old race boat (Jim Young Rocket 31). I am assuming there is a rebate the thruhulls are fitted into, and then this is all epoxied in to create the flush-fit on the outside of the hull. The hull is strip-planked red cedar with dynel. All thru-hulls appear in reasonably good condition, but because they are of unknown age my confidence in them is not as high as I want.

Because they are unknown age (possibly never changed in the thirtyone years since she was built, at the pending next lift for antifouling etc I want to replace them.

Thing is, I have never replaced thru-hulls like this. I have only ever done ones that are a straight forward fit and the mushroom head standing proud (you know - just fit them into a hole with backing plate and sealant etc).

So, here is the first question - how the heck will I remove them? Do I grind out the epoxy to get to the thruhulls - sounds a bit rough, but probably doable. Smash them out - that sounds wayyyyy too rough so I doubt that would be right. Use a hole saw to cut away the epoxy - which might make sense. Or something else. I guess another option might be to drill out the thru-hull to the full size of the thru-hull shaft/tube, leave the mushroom head buried in the epoxy and just do a standard fit?

Second question - should I redo the flushfit using the rebate, or accept the mushroom head standing proud of the hull and accept the (probably minimal) increase in drag?

Hope my explanation makes sense - sorry for nil pics. I've googled etc but finding nothing. Help!
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Old 30-06-2016, 08:40   #2
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Re: Flush-fit thru hulls

Cutting through the wall of the through hull using a hacksaw blade will minimise damage to the hull. Halves will usually work. Sometimes quarters.

You'll have some leverage too. Assuming you cant just undo the retaining nut / plate and work the old through hull out.

You'll need to rebuild the mating surfaces anyway so removing material wont be a problem.

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Old 30-06-2016, 08:43   #3
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Re: Flush-fit thru hulls

My friend with a Cheoy Lee 30 (definitely not a racer) has the flush fit through-hulls but he has not (yet) considered removing them. Are you sure they need to be removed? Any signs of trouble with them (as in turning pink?) In any case, I'd put the mushrooms in when the time comes. I'd venture to guess they wouldn't slow the boat down one iota, jot, whit or tittle.
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Old 30-06-2016, 08:49   #4
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Re: Flush-fit thru hulls

Don. No I am not sure they need replacing. They do not show major signs of corrosion - just the standard light green one sees. But the unknown age bothers me. I am starting to think I should get a surveyor to have a look - probably when I lift her in Aug/Sep. .

I tend to agree about just replacing with mushrooms. Obviously way easier to do, and as you say will probably be an unnoticeable drag increase.
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Old 30-06-2016, 11:01   #5
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Re: Flush-fit thru hulls

Tony,

I think you are making a major issue out of something that isn't even a problem. High quality bronze will last pretty much forever, it's just the low quality brass that is currently being installed that has issues.

For instance your thruhulls are likely so low in zinc they probably can't suffer from de-zincification which is pretty much the only failure mode to worry about.

If you do decide to remove them then first things first is to just unbolt everything and try see how badly they are stuck on. To do this you need a thruhulls wrench. It slots inside and grabs the inner ridges. Put a cheater bar on it and see if you can get it to turn. If not the next step is to heat the thru hull with a blow torch, to try and break the sealant that's holding it in place.

The sealant, not epoxy is probably what's holding it in place btw. No one has ever made it a habit of epoxing thru hulls in place. Judicious use of heat combined with the wrench should allow you to spin it out.

Again I wouldn't bother going this route unless you had a specific reason to.
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Old 30-06-2016, 11:12   #6
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Re: Flush-fit thru hulls

You should be able to remove them with a thru hull wrench. FLush fit thru hulls are usually not so much a mushroom as a countersink taper type. They used to be quite common, not sure now as I haven't raced for many years. Are you sure they are epoxied in or maybe just faired in with sealant. Properly installed they don't need much done.
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Old 30-06-2016, 11:42   #7
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Re: Flush-fit thru hulls

Mine are 54 years old and seem fine.
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Old 30-06-2016, 12:11   #8
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Re: Flush-fit thru hulls

Hard to say without having a look, but I can tell you how I did mine. I installed Marelon thruhulls, and I just took epoxy thickened with fairing compound and filleted it between the thruhull 'mushroom' flange and the hull. No rebate etc, just a fair fillet on the outside edge of the 'mushroom'

If yours are done like this you should be able to chip away the thickened epoxy which will leave the (in your case) metal thruhull and then you can just unscrew it as usual.

But hell if they ain't looking bad nor leaking, I'd leave well enough alone...

btw I didn't just fair these in to smooth waterflow, I like the idea of a belt-and-suspenders solution for leak possibilities
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Old 30-06-2016, 13:02   #9
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Re: Flush-fit thru hulls

If they are truely flush, and you truely need to replace them, then rather than tearing them out and damaging your boat in the process, why not just fill them in with epoxy and mount new mushrooms beside them?
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Old 30-06-2016, 14:02   #10
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Re: Flush-fit thru hulls

Quote:
Originally Posted by Don C L View Post
Mine are 54 years old and seem fine.
When I gutted and re-built my 1965 Rhodes Reliant 41 in '94 - '98, I removed all the 5/16" solid flange ( not tapered Cheech), thruhulls and put back new mushrooms into the sockets, and built them out flush with filler. I then acid cleaned the old ones, only to find out I had just wasted BIG $, They were all perfect after 30yrs.
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