Marine Sealant

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ashley1

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I’m fairly new to the world of boats and in the hopefully not too far distant future I will be removing and replacing a through the hull stainless fitting. I’ve had a look at marine grade sealants and there seems to be loads of different ones. The fitting is bolted in place so the sealant doesn’t need to to be an adhesive at all, just purely a sealant. Can anyone advise on any particular make. I’m probably over thinking this but rather than just choosing any old sealant based on no experience I thought it worth asking the question.
 
Puraflex40 is my GOTO sealant/adhesive for all of my waterproofing and gummy messy jobs.

Like you stated above it doesn't need to be an adhesive BUT it's sealant properties are brilliant and with it being a polyurethane based it is superb in marine environments.

Toolstation have it in black and white and its only £5.50 for a tube...way cheaper than what the dealers try to push onto you for the same result because it's labelled "Marine grade"

Wear old clothing and gloves as if it gets on you it is a pig to remove.
 
It is unusual to have stainless thru hulls - normally bronze or DZR, or increasingly non metallic such as Marelon or Tru Design. There are broadly 2 different types of sealant, non setting such as polysulphide or butyl or polyurethane (PU) which are also adhesive. The former have the advantage of allowing easier disassembly which is useful in some situations, but either would be suitable for thru hulls as if you use the correct material are virtually permanent fittings.
 
Below the waterline Sikaflex 291 for me. Am I worried about saving a couple of quid on a fitting which could sink the boat if it failed? Not really.
 
Tranona said:
It is unusual to have stainless thru hulls - normally bronze or DZR, or increasingly non metallic such as Marelon or Tru Design. There are broadly 2 different types of sealant, non setting such as polysulphide or butyl or polyurethane (PU) which are also adhesive. The former have the advantage of allowing easier disassembly which is useful in some situations, but either would be suitable for thru hulls as if you use the correct material are virtually permanent fittings.
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Through Hull Marine Skin Fittings
 
It’s the bottom gudgeon for the rudder and it certainly looks like stainless and it’s just slightly above the waterline.
 
ashley1 said:
It’s the bottom gudgeon for the rudder and it certainly looks like stainless and it’s just slightly above the waterline.
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OK. The weak point will be the threads of the stainless fasteners as I alluded to when talking about thu hulls. Is it through wood or GRP? If wood then suggest you soak the holes with epoxy to seal the wood - and would not do any harm to do the same with grp. The key thing is to keep damp away from the stainless as damp is the perfect conditions for crevice corrosion. Think I would use a setting PU sealant, liberally coating the threads and the face of the gudgeon. Don't tighten it fully initially as you may squeeze all the sealant out, but leave a turn or so then tighten up when the sealant is set. That will glue the gudgeon to the transom. Hopefully you won't ever have to remove it, but if you do you well break the seal by unscrewing the fastenings and putting a blade behind the gudgeon.
 
That was all very helpful. I’m glad I asked rather than just use what was available at the local chandlery.
Thanks.
 
ashley1 said:
I’m fairly new to the world of boats and in the hopefully not too far distant future I will be removing and replacing a through the hull stainless fitting. I’ve had a look at marine grade sealants and there seems to be loads of different ones. The fitting is bolted in place so the sealant doesn’t need to to be an adhesive at all, just purely a sealant. Can anyone advise on any particular make. I’m probably over thinking this but rather than just choosing any old sealant based on no experience I thought it worth asking the question.
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I'm following the boatyards now, many have moved on to 'BOSTIK SIMSON MSR' for below the waterline. Apparently because once opened it doesn't harden as quick as the others.
 
Tranona said:
Of course I know they are available - who does not know? I just remarked that they are unusual, except on steel boats. I could add for the OP that he should pay extra attention to sealing as one of the downsides of using stainless in this application is possible crevice corrosion in the threads.
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All of the above waterline skin fittings on mine are stainless. Looking outside, the boat beside me also has stainless, the two mobos i can see from here both have stainless. Lots and lots of mobos use stainless. They would appear to be not as unusual as you think.

Certainly important to seal them well, for the reasons you give.
 
PaulRainbow said:
All of the above waterline skin fittings on mine are stainless. Looking outside, the boat beside me also has stainless, the two mobos i can see from here both have stainless. Lots and lots of mobos use stainless. They would appear to be not as unusual as you think.

Certainly important to seal them well, for the reasons you give.
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Stainless may be common above the waterline. However I am not aware of any volume yacht builder that uses stainless thru hulls below the waterline, so literally 100s of thousands of mostly yellow metal thru hulls in use compared with a few MOBOs that might use stainless for underwater fittings. Also are you sure the above the waterline fittings are actually stainless and not the more common chrome plated brass often used for cosmetic reasons - and perfectly adequate for the job? Difficult to tell the difference visually when all you see is the outside flange.

Irrelevant in this context as the OP has now clarified that the component is a rudder fitting, although his original did say "through the hull" which I took to mean thru hull. My main comment still stands, there are risks associated with using stainless threaded fittings below the waterline that do not exist if you use the more suitable bronze, DZR or plastic fittings
 
Tranona said:
Stainless may be common above the waterline. However I am not aware of any volume yacht builder that uses stainless thru hulls below the waterline, <snip>
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I said "All of the above waterline skin fittings on mine are stainless. Looking outside, the boat beside me also has stainless, "

No mention of below the waterline.
 
I use butyl above the waterline for anything I think I might need to remove at some future date (eg window frames). CT1 or PU40 below the waterline and the same above if I will never need to remove it
 
Second CT1, or OB1 from Screwfix. Not clear or silver though, I didn't get as good results with the clear and noticed that those two have their own data sheets. The rest of the colours (black, white, grey) share the same data sheet, I've used them all and it's brilliant stuff, can be used under water...
 
V1701 said:
Second CT1, or OB1 from Screwfix. Not clear or silver though, I didn't get as good results with the clear and noticed that those two have their own data sheets. The rest of the colours (black, white, grey) share the same data sheet, I've used them all and it's brilliant stuff, can be used under water...
Click to expand...
Good call on the clear. I’ve heard before it isn’t as good.. didn’t know about silver though. Tx for the tip
 
Tek7 is like CT1 and available from DIY shops and builders merchants. So easy to get, and it sets in the wet. Even under water so they say. Sikaflex needs to be in the dry to work properly. Depends where your boat is being worked on. If it's in a heated shed then fine. If it's in a yard a dryed out between tides, or even in your garden, it'll be hard to dry things well enough.

I've used tek 7 successfully with no issues.
 
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