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Old 11-19-2020, 10:44 PM   #1
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Thru Hull's What Would You Do?

I am currently working through rebuilding my 1990 Island Gypsy 36 project boat. I have come across 5-6 old thru hull fittings below the water line that have no purpose and are seized closed. They are 1" to 1.5" factory installed and I have no idea what they could have been used for aside from scuttling valves. My original plan was to just cap them with appropriate sized threaded caps and move one. However a good friend who is also a boater and a mechanical engineer pointed out they are just extra failure points and should be removed. I looked at removing them but they appear to be threaded from the outside (male) and glassed into the hull on the inside (female). I am not familiar with this set up where the actual valve body inside the hull is not threaded. I'm not a big fan on having 11 or 12 thru hulls below the waterline when half of them have no purpose but it looks like a lot of work to cut them out. How prone to failure is this style of thru hull? My main worry is crevice corrosion where they cant be inspected before the valve body. What would you do if this was on your boat?
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Old 11-19-2020, 10:51 PM   #2
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I am doing it this winter, actually just finished. I get rid of them and glass them closed. I just glassed 6 holes closed on the starboard side, 2 transducers and a bronze sinistered plate with 4 bolt holes. Last winter I did the port side and glassed 2 old transducer and a thru hull closed. It makes for less hull penetrations and lesst possibility of leaks. And the engine room looks much better. It isnít that hard to do. I had a thru hull that I couldnít get loose. I took the grinder and ground the mushroom head off. Didnít take long. Glassing them closed was easy also. Then I painted the glass work with epoxy barrier with 6 coats expoy and 2 coats bottom paint. Good luck.
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Old 11-19-2020, 11:03 PM   #3
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In 2016 we had the yard glass in 3 thru hull valve holes and half a dozen bolt holes thru the transom in. Cost about the same as changing the thru hulls, now we only have two thru hulls left, one for raw water for the engine/generator and one for raw water wash down. Sure sleep easier...........
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Old 11-19-2020, 11:21 PM   #4
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We now have 5 through hulls, 2 for main engines, 1 for genset, 1 for A/C and 1 for forward head and sea water wash down.
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Old 11-20-2020, 12:27 AM   #5
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The answer depends upon how comfortable you are doing fiberglass repairs. The repair is simple but must be done correctly. Not a place for mistakes.
Interior photos would help. Original through hulls were typically a mushroom cap on exterior hull, a threaded extension going through the hull, then going through a teak spacer or mounting block, and then the valve or seacock. (or just a nut in the case of a transducer.)
The teak spacer is usually glassed in using mat and resin.

If it is something that was added by PO, then anything is possible.
It is preferable to minimize number of hull openings for obvious reasons.

Destructive removal is easy. Remove any seacock if present. Use an angle grinder to cut through the threaded extension or stem. You could also use an oscillating saw with a blade cutting metal. A Sawzall would also work. Cut through the spacer. punch out the remaining extension and mushroom. Grind the interior fiberglass flat.

If there is no glassed in spacer, then you could simply cut off the mushroom and punch the extension in.

https://www.boatworkstoday.com/videos/ has the best videos on fiberglass repairs.
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Old 11-20-2020, 01:22 AM   #6
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I use epoxy for hull repairs below the waterline since it is stronger and will stick to whatever resin was used before. I like 1708 biaxial for strength. It isnít too technical but can be a pain working upside down. A Tyvek suit helps keep you clean. Use a roller to get it solidly in control with the hull. I taper 12:1 and taper both sides.
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Old 11-20-2020, 06:59 AM   #7
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https://www.youtube.com/user/Sailvayu

Everybody has slightly different methods but his are as good as any. Epoxy is better then what he uses.
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Old 11-20-2020, 07:02 AM   #8
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There's sure and argument to do it when hauling for something else, as the hauling costs will most likely exceed the cost of removing the thru hull fitting.


Now, one could remove one while in the water. It's just a bit harder. The really good underwater tools are brutally expensive, but a cheap Harbor Freight pneumatic grinder or drill will work.



Obviously, one would grind off one side first and seal well and then do the other end.



I've sealed off a few and installed a bunch over the years, but always out of the water. Not horribly hard.
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Old 11-20-2020, 07:10 AM   #9
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I could be wrong, but have always assumed that through hulls and associated hoses were sized by the builder for the job they need to do. I wouldn’t want to share a through hull for one of the main engines and any other function.

Our boat had 14 underwater through hulls when we bought it. All intakes are on the seachest (2 main engine, one genset, AC and raw water wash down; there are also two plugged spares and a vent at the top of the seachest that is technically above the waterline to prevent a vacuum from forming). There are also five drains connected to through hulls for galley, heads and overboard holding tank discharge (which we’ve never used).

One of our first projects was to remove and glass over one I could never see using and replace all the others. Ours were the old Groco rubber plug design that parts have been unavailable for over the last 20 years. Two of the biggest concerns were the soft, delaminating or soaked plywood backing plates and the fact that the only thing holding the through hulls and seacocks together were some mismatched threads and caulk (they were not through-bolted).

We installed all new Groco gear, including their composite backing plates, which are a vast improvement over the original. We had the yard cut out the old stuff and my wife and I installed the new. As others have noted, it’s a straightforward job. Having 13 underwater through hulls (12 if you don’t count the vent) would bother some folks, but I’m confident in the equipment and just don’t worry about it.
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Old 11-20-2020, 08:21 AM   #10
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I agree with many who have confidence in quality thru hulls such as Groco. But I am a proponent of using a sea strainer wherever possible - so many inlets do not have a strainer in them, or the strainer ends up undersized and often located in awkward locations. A sea chest such as fitting on most Defever is a great solution if there's room in the ER.

A properly sized and designed manifold off an oversized strainer and thru hull can minimize sea strainers and place them in an easily accessible location for service and inspection.
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Old 11-20-2020, 08:46 AM   #11
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I agree with many who have confidence in quality thru hulls such as Groco. But I am a proponent of using a sea strainer wherever possible - so many inlets do not have a strainer in them, or the strainer ends up undersized and often located in awkward locations. A sea chest such as fitting on most Defever is a great solution if there's room in the ER.

A properly sized and designed manifold off an oversized strainer and thru hull can minimize sea strainers and place them in an easily accessible location for service and inspection.

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Old 11-20-2020, 09:12 AM   #12
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Separate the SW to the gen and the main engine.

Generally, one piece of equipment, one through hull.
There are some exceptions PERHAPS but not, the ME(s) and GEN(s). These also require a separate sea strainer.

My thought is a clam-shell on each SW inlet plus a SW strainer as necessary.
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Old 11-20-2020, 09:59 AM   #13
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Clamshells on some genset installs are either not encouraged or suggested the scoops be installed backwards to prevent flooding while underway.
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Old 11-20-2020, 10:46 AM   #14
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The longer I own a boat the more I can see the advantage of a sea chest per Defever.
Of course it the sea chest begins to leak at the hull joint, the more concern I have.
I have never heard of that happening.
Plus, with the sea chest, you can look down and count the fish and see the bottom. wink
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Old 11-20-2020, 02:37 PM   #15
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Clamshells on some genset installs are either not encouraged or suggested the scoops be installed backwards to prevent flooding while underway.
I installed a scoop backwards on my genny intake. I also upsized it so I could also T the strainer for a washdown pump and run them both at the same time. Worked fine and I never had to clean that strainer again in the 12 years after I made that change.
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Old 11-20-2020, 02:48 PM   #16
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Going oversize also has the benefit of the water entering the thru hull slower and is not as likely to pick up trash.
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Old 11-21-2020, 11:05 AM   #17
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Thanks for all the information and advise! It really helps me feel more confident about the decision to cut them out! Will use thickened epoxy from both sides then glass and fair out. The main ongoing project at the moment is fairing the entire hull below the waterline ahead of barrier coat and bottom paint anyway. Looking to have everything below the waterline done in next 3-4 weeks. Will post a before and after picture then.
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Old 11-21-2020, 11:30 AM   #18
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Thanks for all the information and advise! It really helps me feel more confident about the decision to cut them out! Will use thickened epoxy from both sides then glass and fair out. The main ongoing project at the moment is fairing the entire hull below the waterline ahead of barrier coat and bottom paint anyway. Looking to have everything below the waterline done in next 3-4 weeks. Will post a before and after picture then.

I tore out the uneeded 5 or so...glass and epoxy filled with 12:1 bevels (or best as I could eyeball0.


9 years later...no issues.
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Old 11-21-2020, 12:22 PM   #19
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I had 5. Removed and epoxy sealed 4. Added 1 for holding tank discharge, that I now regret wasting the effort on. Repurposed the original 2" seacock with Groco strainer to a bronze manifold and valves for all raw water needs (engine, generator, air conditioning, and anchor wash down pump). Lot to be said for being able to close all raw water inlets with one valve, in an emergency.

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Old 11-21-2020, 12:36 PM   #20
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Thanks for all the information and advise! It really helps me feel more confident about the decision to cut them out! Will use thickened epoxy from both sides then glass and fair out. The main ongoing project at the moment is fairing the entire hull below the waterline ahead of barrier coat and bottom paint anyway. Looking to have everything below the waterline done in next 3-4 weeks. Will post a before and after picture then.
I donít use thickened epoxy first. I grind out the hole to about a 12:1 bevel. Then I coat the ground area with unthickened epoxy. Then I add 1708 biaxial glass and wet out each layer as I go and use the roller to roll them down into good contact and squeeze out excess epoxy. When I get the ground out area filled with layers of 1708 I use thickened epoxy to fair out the area so it is pretty smooth. Then let it set up and after it has gone off I wash it with water and a scrub brush to get the amine blush off before sanding. Then sand and fair again if needed otherwise I paint it with epoxy barrier coat then bottom paint.
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