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Old 01-06-2010, 09:18   #1
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Through-Hull Material on Steel Boat

Hello,

I have to replace my depth transducer and through hull, and I can get plastic, bronze or stainless steel. I am tending towards the metal option, as the rest of the hull is made of steel and the previous (raymarine) depth transducer came off when I gave it a little pull (on the hard luckily)..

I am not sure whether to get bronze or SS since bronze is more robust but further away on the galvanic scale from the mild steel plate around it. Also, how would you insulate this?

Thanks for your time
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Old 02-06-2010, 01:35   #2
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On a steel boat, your best bet is plastic. Try to mount it in a recess so that floating debris do not damage it. Regards, Richard.
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Old 02-06-2010, 03:02   #3
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Rolling dice?

When I brought Boracay she came with bronze through hulls. Looked like they were bedded in well with a simple mastic. Only a tiny leak on one of them.

So I replaced the one with a tiny leak with stainless. Next time I hauled I found evidence of electrolytic corrosion.

Now I think I've found the source of the corrosion (bilge pump switch always in water, slightly hot, took anodes...) but I'm never going to feel the same again about stainless.

So I'm not going to pull out an otherwise perfectly good through hull. However I'd go with stainless again in any position where mechanical failure could happen. Knock against a rock, wharf, marina, kicked accidentally, overtightened or whatever. Electrolysis/corrosion may only give a small leak that can be attended to in time.

I'd favour welding the through hull to the hull with an internal backing plate if possible, but there seems to be a body of thought that this is not the way to go.
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Old 02-06-2010, 03:40   #4
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Thanks Bora, I think both materials would have to be insulated from the steel below the waterline, so I can't see welding doing much good there?
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Old 02-06-2010, 05:34   #5
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Plastic is the way to go even though it sounds counter intuitive. No corrosion issues.
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Old 02-06-2010, 06:38   #6
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Shouldn't be any corrosion issues with bronze, either. I believe you could weld SS pipe directly to the hull, too. Perhaps Brent Swain (or another steel boat builder) will chime in with his experience.
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Old 02-06-2010, 07:02   #7
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Stainless steel should never be used below the waterline.
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Old 02-06-2010, 07:13   #8
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I'd go with the plastic as well. I'm not a big fan of plastic below the waterline but the transducer assemblies have a very good reliability record and the transducer should be well protected by the keel and hull.
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Old 02-06-2010, 13:21   #9
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Thanks for the advice all around.. Plastic seems to be a bit of a winner.. Grovel..

I'd heard stainless is a bit of a nono underwater, although my seacocks are all stainless and the popular suppliers do seem to be producing them?
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Old 02-06-2010, 14:10   #10
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Quote:
Originally Posted by DeepFrz View Post
Stainless steel should never be used below the waterline.


Ummmm this sounds like a booming voice from the temple

Would the great god Boojum care to explain to us lowly mortals why stainless should not be used below the waterline?

pretty please?


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Old 02-06-2010, 14:24   #11
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From my experience with three different steel boats, common practice for the cheap set is to weld a pipe nipple onto the inside of the skin and then put the seacock on it.

However, this isn't truly a great idea as the threads are mismatched. I've never seen or heard of a fail here but I'd be leary.

Second posibility is to weld stainless nipples on to a MS hull. This can be done but the weld area is subject to corrosion due to the mixing of material in the weld puddle. Then there is Boojums proclamation (TM) which expressly states: Never to use stainless below the waterline. So far I have not heard of failures there either.

Third possibility is either Stainless or Bronze fittings mounted to the hull on insulated pads to prevent galvanic corrosion.

Fourth option is plastic. As mentioned, depthsounders seem to have a pretty good record of staying intact so they present the least complicated method. Plus they have proper threads so they will mate up with seacocks. There has been concern expressed as to shear strength of the fitting if something heavy were to hit them from the side, ie a dive tank slipping down the hull and impacting the fitting. To my mind, this can be prevented by welding a fence around the fitting to stop any impact.


I will be doing my refit this summer and I will be doing Sabre Dances' seacocks with marelon thruhulls and seacocks. I will be fitting fences around them to prevent any damage.


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Old 02-06-2010, 14:34   #12
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SabreKai, I don't know anything about the "great god boojum", but I do know about crevice corrosion. If you don't, look it up.

idpnd, I would second SabreKai's recommendation on using Marelon. I'm not sure that fences are necessary, I guess it depends on how vulnerable the through hull is.
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Old 02-06-2010, 16:35   #13
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I have been welding in stainless type 316 pipe nipples for thru hulls, with stainless ball valves on, for 34 years, with no problems whatever. I recently removed a plastic thru hull from a steel boat by lightly kicking it. Scary. Any piece of gear shifting a bit could easily do that.
Never had any noticeable corrosion .With a tiny amount of stainless welded to a huge amount of steel, corrosion is no problem . With a huge amount of stainless attached to a tiny amount of steel, there is a big problem ( galv chain link on a stainless anchor)
Plastic would be a big mistake.
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Old 02-06-2010, 16:48   #14
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I bow to your experience Brent. Was the plastic through hull that you removed Marelon?
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Old 02-06-2010, 18:49   #15
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What do commercial and military vessels use for thru hulls? I'm sure it ain't plastic.
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