Flush Thru-Hulls replace with Mushroom Type

[trogers122]

Question for the knowledgeable folks here:

I just pulled my boat to replace my thru-hulls (amongst other things) and I hadn't realized it but they are flush mount thru hulls rather than the typical mushroom style. I pulled them all and it looks like they are from the factory this way (nice clean gelcoat underneath and a uniformly built up hull on the inside).
Now, I had purchased some new groco flanged seacocks and mushroom type thru hulls for this project in advance and I'm wondering if there's a way to pivot with this new information. I think there's a few options that I can take, but I'm not sure what the best on might be:

1. Fill in countersink with a thickened epoxy. Redrill out the hole from the inside. Install mushroom and seacock as usual.
2. Treat the exterior as if I'm filling the hole. Grind out a nice taper, fiberglass outside, install as usual.
3. Buy flush thru hulls instead, plan to either modify countersink/chamfer based on new thru hull profile or take to machine shop to modify new thru hull to existing profile.
4. Dont do anything to the exterior. Instead install the mushroom with a boatload of 4200 or lifecalk. Not totally on-board with this one, soo not a real option I think.

Which of these seems like the best idea? Or maybe there's another option?

Google hasn't been terribly helpful since most people are concerned with the opposite problem rather than replacing the flush thru-hulls with mushrooms.

Thanks in advance!

 

[dlochner]

Welcome to SBO.

I'd take Option 3.

 

[Ward H]

Catalina flush mounts the thru hulls on their boats, at least earlier boats. Mine are flush mounted on my 93.
Forum member @Hayden Watson had the same issue on his C30 and used mushroom head thru hulls that sat fully into the recess. He then filled with G Flex epoxy. I followed his lead and did the same.
At least on Catalinas, where the recess is on the outside of the hull, the inside is raised so the hull is just as thick as the surrounding hull.
I don't see how mounting a mushroom head thru hull in the recess would be any weaker than anywhere on the surrounding hull as long as the mushroom head sat flat against the inside of the recess.
By filling in the area with epoxy or a sealant makes for a cleaner install.

That said, my marina told me Buck Algonquin makes a bronze flush mount thru hull that fits the Catalina flush mount recess. They did this on another buddy's C30 and it looked fine to me. I don't know for sure if the taper perfectly matched. I didn't use them because I was installing Marelon valves and used Marelon thru hulls.

A search may find the thread where Hayden posted a drawing of how he did his.

 

[rgranger]

...and used mushroom head thru hulls that sat fully into the recess. He then filled with G Flex epoxy.

This sounds like the easiest and lest expensive way to go ... BUT!

If you flush and fair with epoxy, it is a one-and-done fix and you will not be able to ever replace those through hulls again without a LOT of extra work. If you go this route, I'd do some research on alternative fairing compounds....

Seems like you should be able to find something that will last but could be chiseled out later if needed. (4200?)

 

[trogers122]

Catalina flush mounts the thru hulls on their boats, at least earlier boats. Mine are flush mounted on my 93.
Forum member @Hayden Watson had the same issue on his C30 and used mushroom head thru hulls that sat fully into the recess. He then filled with G Flex epoxy. I followed his lead and did the same.
At least on Catalinas, where the recess is on the outside of the hull, the inside is raised so the hull is just as thick as the surrounding hull.
I don't see how mounting a mushroom head thru hull in the recess would be any weaker than anywhere on the surrounding hull as long as the mushroom head sat flat against the inside of the recess.
By filling in the area with epoxy or a sealant makes for a cleaner install.

That said, my marina told me Buck Algonquin makes a bronze flush mount thru hull that fits the Catalina flush mount recess. They did this on another buddy's C30 and it looked fine to me. I don't know for sure if the taper perfectly matched. I didn't use them because I was installing Marelon valves and used Marelon thru hulls.

A search may find the thread where Hayden posted a drawing of how he did his.

Huh, so I searched around for his solution. I'm not so sure about using a g-flex. Like @rgranger said, that's a one and done fix. Something like 4200 afterwards might be better, and I'm already planning on that for the sealant anyways.
Of course, that's dependent on whether the mushroom fits flat on the recess. I'm heading down to the boat today and Im going to check it out and see if that's even a solution.

 

[Ward H]

I know I said I followed his solution but I did use Sikiflex 291 sealant to fair around the mushroom head and recess. Hayden had his reasons for using G Flex and I didn't want something so permanent so I went with the Sikiflex which is what I used for sealing the thru hulls.
I agree with you and rgranger.

Sikiflex 291 is one of the products Maine Sail recommends in his article on doing thru hulls.

 

[BigEasy]

I would avoid any epoxy alternative; its more expensive, time consuming, & an absolute nightmare if you fair around the fitting with epoxy and have to replace the fitting in the future. Find a flush mount or mushroom fitting that fits the diameter of the recess, seat it with 4200, and be done with it. I prefer mushroom fittings because I simply grind off the exterior mushroom portion to replace them. Unfortunately, with the mushroom seated in a recess, wouldn't be able to grind it off in future.

 

[FastOlson]

Just about Any thru hull installation - done 'right' - will last 30+ years.
While the OP seems to be going with a good bronze product, there are alternatives just as long-lived.
We changed out all the '88 OEM Marelon valves in out boat, in the 90's, to the then-new and still-in-production Forespar Marelon type 93 versions.
Method: FRP backing plates, each thru hull base sealed in thickened epoxy when tightened up against the flanged outside piece. After the epoxy kicked the outside protruding flange/rim was ground off flush with the hull.
Note that this valve series can be rebuilt from the inside, if ever needed.

If... the boat in question already has the recess molded in for a flush-set thru hull fitting, even if he angle of the new bronze one is slightly different, you could set it into thickened epoxy to re-orient the flange angle. Just finish up the install with new frp backing plates inside, set in epoxy mush.
(One small caveat: if using bronze thru hulls be sure to avoid 'hot' marinas due to potential risk for galvanic corrosion.)

As for the "someday" hassle of removal, pretty much regardless of sealant or epoxy, just use a heat gun. That future owner or yard worker person may still have to remove the old thru hull with a hole saw, anyway... !

 

[BigEasy]

Just about Any thru hull installation - done 'right' - will last 30+ years.

True; however, you get a neighboring boat that is putting out stray current, that damages your underwater gear, and you will have second thought about using epoxy to seat the thru hulls.

 

[rgranger]

I prefer mushroom fittings ...

... but but but.... that extra 0.001 kt/hr might be the difference between glory and footing the bill for a round at Foxy's

 

[trogers122]

Lots of good replies here! Thanks guys.

Down at the boat today, I did a quick check on the thru hulls to see if the mushrooms will fit flat. Unfortunately, it's probably going to be a no-go for 3 of the 4. Only the 3/4" fitting seemed like it'd work.
I found another surprise though, the new fittings are going to need bigger holes. Ugh, I'm starting to regret purchasing these things in advance.

I'm starting to think the simplest solution might be to fill the exterior recess with a thickened epoxy, faring, then drill out the new holes to the size needed, and call it good. The hull should be thick enough at these locations since it was built up on the inside to accommodate the flush fittings.
And it should make fitting the mushroom fitting now and in the future simpler.

Any concerns with that approach?

 

[trogers122]

I would avoid any epoxy alternative; its more expensive, time consuming, & an absolute nightmare if you fair around the fitting with epoxy and have to replace the fitting in the future. Find a flush mount or mushroom fitting that fits the diameter of the recess, seat it with 4200, and be done with it. I prefer mushroom fittings because I simply grind off the exterior mushroom portion to replace them. Unfortunately, with the mushroom seated in a recess, wouldn't be able to grind it off in future.

That was how I was planning to remove these thru hulls. I was a bit confused when I saw the hole, but no mushroom. I ended up having to chop them off from the inside and grind the nut off two the get the darn things out!

 

[Tally Ho]

Lots of good replies here! Thanks guys.

Down at the boat today, I did a quick check on the thru hulls to see if the mushrooms will fit flat. Unfortunately, it's probably going to be a no-go for 3 of the 4. Only the 3/4" fitting seemed like it'd work.
I found another surprise though, the new fittings are going to need bigger holes. Ugh, I'm starting to regret purchasing these things in advance.

I'm starting to think the simplest solution might be to fill the exterior recess with a thickened epoxy, faring, then drill out the new holes to the size needed, and call it good. The hull should be thick enough at these locations since it was built up on the inside to accommodate the flush fittings.
And it should make fitting the mushroom fitting now and in the future simpler.

Any concerns with that approach?

I think this approach would work.

Greg