replace through hull from plastic to SS

[The Boss]

I need help on the replacement proceedures of a thru hull fitting on my aluminum boat. I must use a stainless steel one to pass the commercial requirements. With all the concern of the galvanic action I need some solid advice on how to do this installation correctly. Is there such materials out there that will provide me the isolation of the stainless from the aluminum? (marelon washers? sika flex etc.? rubber backing?) I also have seen the nuts on the ss thru hulls which have a bonding lug. Is this essential to use and where is the appropriate place to bond it?

Thanks in advance for any advice,
BOSS

[LarryMcI]

Is Marelon not suitable? If not (God knows why?) then your washer/collar approach is appropriate. Do not rely on bedding compound to radially isolate the two materials. Any lateral creeping, during clamp-down, could result in metal-to-metal contact. Machine some radial-spacers from non-conductive material (nylon, Delrin etc) then use similar washers on both sides to absorb clamping-loads.

[alan white]

I'd trust good quality plastic before mixing metals. Boy, governments love to spend your money. And transducers and knot meters? How many of them have metal thru hulls?

[LarryMcI]

How true. I have NO metallic thru-hulls on my 37' cat, except one ... the damn thru-hull transducer from Furuno. Grrrrr!

[alan white]

Exactly proving my point almost.

[TollyWally]

The nanny state certainly is a nosy bitch. I'm still waiting for her to pay me for the pain she caused adding ethanol to my fuel tanks.

[CDK]

A rubber or neoprene grommet would be the best solution but may be hard to find. An alternative may be a sleeve cut from pvc hose, shaped with a heat gun, or two washers, one flat, the other with a collar to center the thru hull fitting.

The bonding lug is mandatory if the feed thru is connected to a fuel tank; in that case it must be connected to the tank.

[The Boss]

Thanks for all the good info. I am thinking that maybe I could have a washer machined out of an insulating material for both sides of the hull and then use sika flex as well??

The Boss

[alan white]

I think painting epoxy neatly onto both thru hull and boat is all you need to do. Then, yes, bed it in a sealant. I like polysulphide, but any good PU for underwater use should work. I can't imagine a problem if you epoxy both surfaces. Machining washers will cost way too much unless you do it yourself. If you do, use an old or unneeded piece of countertop and a hole saw. Drill to size while laminate is still cemented to the particle board. Use a hole saw for the outside diameter and a spade bit for the inside. Once you like the result, tease the piece of countertop up to the tablesaw blade so that the laminate is sawn off. Instant custom washer.

[LarryMcI]

There are molded lip-washers, available at any plumbing supply, which are used for drain connections under sinks. Like the ones on P-trap flanges, only bigger for kitchen sink drain connectors etc. They even have a spilt to facilitate installation. Nylon flat washers are common. Just peruse the plumbing supply aisles.

[2TALLTARY]

delrin is probaly your best bet. found a good site with more info and material...http://www.iplasticsupply.com/materi...alux-sheet-rod

[michael pierzga]

This is a Stainless steel valve installed on an Aluminum hull. The SS fastenings are isolated from the flanges with plastic bushings. The valve is set onto the flange face with gaskets. I have no corrosion problems.

[micspoko]

maybe triclad
http://www.triclad.com/

[CatBuilder]

I see this is a old thread, but Alan White's epoxy idea sounds great. Mix up a batch of epoxy and dunk the through hull in it completely, using a string. Do this a couple times and you have, essentially, a stainless component to pass inspection, but a plastic component for all intents and purposes.

[michael pierzga]

The sea water exposed surface area of a SS valve is so small that it presents little problem. Keep the alloy stand pipe, flange, tall enough to get the valve out of bilge slop water.