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What Makes a Thru Hull Suitable for Below Waterline?


Tao of Wake

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I am going to install two new Tsunami 1200 pumps to fill two Fly High 750# sacs, and I am looking for the correct thru hull to purchase. I want to go with a 1" thru hull to avoid restricting flow. I have already purchased the 1" ball valves and 1" elbow (MPT to barb) to connect to the thru hulls to go to the pumps. When searching for thru hulls, some say they are not suitable for below the waterline, very few say they are suitable and others don't say whether they are or not. How do I know?

Also, I will be installing these on Friday before Memorial Day weekend, and I would really like to use the boat that weekend. The boat is 3 1/2 hours away, and I can't make it up there before Memorial Day. I found these Attwood thru hulls that have a solid silicone gasket. According to Attwood, this eliminates the need to use a sealant. This sounds attractive to me so that I don't have to wait for the sealant to set. The problems are that I cannot find this ANYWHERE and is this fitting suitable for below the waterline? Does anyone know where to purchase a similar gasket that could be used with another 1" below the waterline thru hull?

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Realistically, it's the construction, in which case any metal fitting should be sufficient for below the waterline operation.

Be careful though, many water pickups have straight thread (a convention going back forever) and require either an adapter or a straight thread to tapered thread ball valve.

I'm not aware of a source for those Attwood, but if you're just looking to use the boat for the weekend you should be able to get by with a quick set non-adhesive silicone for the weekend. Then when you're done at the end of the trip use a marine adhesive approved for below water line applications and let it setup when you leave.

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Do you know of any thru hulls with NPT threads? In searching around, it looks as though the Buck Algonquin ones do, but I haven't been able to confirm. Just at every site that I find the thru hulls, they all say NPT rather than NPS or NPSM.

Groco has an adapter to go from NPS to NPT, but the 1" is $35!

I already purchased NPT ball valves and elbows, so I really don't want to have to buy all new hardware. I would really like to find some NPT thru hulls.

Also, do you know if anyone else makes a solid silicone gasket like the one included with the Attwood thru hulls? I don't like the prospect of using a temporary silicone solution for the weekend and then having to disassemble everything to apply the marine sealant. The boat will be put on a hoist for the summer, so I could do it in the water, but I would rather not. What I was thinking was to use the gasket but also put a bead of marine sealant around the gap between the hull and the thru hull. The gasket will keep this dry and allow the sealant to set up.

EDIT: Have you ever used the 3M 5200 Fast Cure Adhesive/Sealant? It claims to be tack free within 2 hours and fully cured within 24 hours. If this is true, this looks like my best option.

Edited by kernaltao
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Check out marinehardware.com's online stuff. Under their thru hulls they say they move from a straight to a tapered pipe thread - Page 7.

http://www.marinehardware.com/downloads/import.pdf

"Our parts are cast with a very sturdy wall thickness and machined with straight pipe thread , which blends into tapered pipe thread at the end of the neck."

-Chris

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Check out marinehardware.com's online stuff. Under their thru hulls they say they move from a straight to a tapered pipe thread - Page 7.

http://www.marinehardware.com/downloads/import.pdf

"Our parts are cast with a very sturdy wall thickness and machined with straight pipe thread , which blends into tapered pipe thread at the end of the neck."

-Chris

Excellent! I just ordered 2 of these. Thanks, Chris.

-Carl

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Yep, Marine Hardware is the only manufacturer I know of that has transition threads from straight to NPT.

I'm not aware of a silicone gasket, but I'm sure you could make one yourself if you're dead set on going that way.

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Yep, Marine Hardware is the only manufacturer I know of that has transition threads from straight to NPT.

I'm not aware of a silicone gasket, but I'm sure you could make one yourself if you're dead set on going that way.

I've got some silicone tape that I may use to put around the outside perimeter of the thru hull and then put the fast cure adhesive just inside of that. That way, the silicone will keep the water out - at least for the weekend - giving the adhesive plenty of time to cure.

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