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Old Mar 11, 2012, 11:09 PM
It's a really big number.
Brisbane Intl, Australia
Joined Jan 2001
1,315 Posts
Question
WTC Gasket material

Hi all,

I've ALMOST finished by Robbe U-47 - but doing the pressure tests on the WTC I discovered that the supplied gaskets are not so great. Apart from a large (2mm) gap between the main hull and the stern sections when used that looks plain ugly, they leak!

Of the 3 I have on hand, and despite a the WTC and end cap being machined really smooth, all of them leaked slowly, and a close examination revealed all 3 have perished and gone hard and when stretched over the end cap, develop tiny surface cracks that allow the air out no matter how tight the WTC is done up. With 2 of the o-rings being replacement parts and only a few months old and only just removed from their packaging yesterday, I'm not so happy with the life of them

Anyway, ditching the o-rings, I found that 3 wraps of plumbers teflon tape (the stuff you use to seal threads on taps etc) around the slight taper of the end cap seals the WTC far better (and no gap in the hull sections either!)... While effective (the WTC still had +ve pressure 45 mins after pumping it up), it doesn't strike me as a real long term solution if I need to do this each time I go for a cruise in the sub...

So my question is, what materials do others recommend to make WTC gaskets from?
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Old Mar 11, 2012, 11:28 PM
Registered User
Joined Feb 2012
38 Posts
On my Robbe Sea Wolf, the WTC sealed much better after I lubricated the O-ring with petroleum jelly. It also acts as a water block.
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Old Mar 12, 2012, 04:56 PM
Formerly Nuke Power
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United States, NY, Gloversville
Joined May 2007
426 Posts
Some O-rings depending on material dont react well to petroleum jelly and it will cause then to deteriorate. I like to use silicone grease or white lithium paste for mine.
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Old Mar 13, 2012, 10:24 AM
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United States, CO, Loveland
Joined Apr 2009
126 Posts
Most O-rings are shore A70. This indicates hardness. Shore A50 is softer, and meant to conform more to irregularities. They are also much easier to slide an outer wall of WTC over, since they compress more.

There is almost no way to tell hardness. The 50 does feel more squishy than the 70. You want Buna-N type rubber.

http://www.mcmaster.com/#o-rings/=gn994k

It sounds like your O-rings got cooked by UV sunlight maybe? That may do it fast- left on the dashboard a few times.

Another trick I was taught is to build up the bottom of the groove that the O-ring rests in with thin strips cut from Monokote (?)...the stuff that RC planes are coated with, adhesive backed. Put a couple layers in at a time, then check to see if when you put the O-ring back in the groove, the outer WTC tube wall now compresses the O-ring equally all around.

When it does, remove the O-ring, and carefully dribble a little thinnest CA glue into the groove to bond down the Monokote strips and seal any micocracks in the bulkhead/endcap near the groove.
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Last edited by H2Ohaze; Mar 13, 2012 at 10:30 AM.
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Old Mar 13, 2012, 02:53 PM
Man from Atlantis
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London
Joined Nov 2003
882 Posts
The seal seems to be the achillies heel of this boat. It would have been better if Robbe had designed the pressure hull so that seal compresses radially on the inside of the tube instead of axially, as the tube is very accurately moulded.

Quite a few people upgrade the boat with an aluminium bayonet seal, which is available from some German retailers and is a big improvement on the Robbe design, but owing to the design of the boats tech rack, it has to be fitted before construction begins.

Most people use nitrile o-rings for sealing boats (also known as buna-n). You can use pretty much any grease on those including vaseline. If the mating surfaces have been machined, and are as smooth as you say, a nitrile o-ring should work fine, and should last a long time unless you have some bad/old stock.

O-rings can be found in other materials too- neoprene is a little more resitant to ozone than nitrile, but less resistant to mineral based lubricants, so use silicone grease on those seals. Silicone and EPDM are two other materials.

Silicone o-rings tend to be softer than nitrile or neoprene, so that makes them more compliant and better for sealing slightly irregular surfaces. You mustn't use any petroleum or silicone greases on them, use lard or vegetable oil if they need lubricating.

EPDM is the most expensive material. Almost totally inert to sunshine and ozone- this is the rubber used on modern car windscreen rubbers and is also extensively used for flat roofing. Hardness rating is similar to nitrile and neoprene, you can lubricate it with silicone grease.
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Old Mar 13, 2012, 09:03 PM
It's a really big number.
Brisbane Intl, Australia
Joined Jan 2001
1,315 Posts
Quote:
Originally Posted by H2Ohaze View Post
It sounds like your O-rings got cooked by UV sunlight maybe? That may do it fast- left on the dashboard a few times.
Just the heat I think - it's the tail end of summer down here and the workshop often gets to +35C for weeks on end, and combined with the average 90% humidity this time of year, if it can rot, rust or perish, it often will .

On the up side, epoxy and CA cure real quick

I'll have to dig around for some of the materials specified to make replacement gaskets. There are a lot of silicone and composite suppliers not too far away, so hopefully one of them can help me out with better gaskets.
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Old Mar 14, 2012, 02:07 AM
Man from Atlantis
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London
Joined Nov 2003
882 Posts
I prefer one piece moulded o-rings rather than joining them together from cord- the glued section is a potential point of failure.
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Old Apr 27, 2012, 06:28 PM
Wasserkuppe wannabe
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Central coast NSW Australia
Joined Aug 2006
108 Posts
Try using Anhydrous Lanoline. It does not go hard with water and retains its properties really well. Petroleum products can not only cause problems with Rubber but I believe have also be seen as a problem with Acrylic tube "crazing "

dave
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