Resealing the Stern Tube? - Page 2 - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 08-23-2012
DrB DrB is offline
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Butyl Tape for Below Waterline

I am not sure I would have gone that approach.

I have no issue using Butyl Rubber above water line for deck hardware, but for I would never use it below waterline for thruhulls. I would have used Sikaflex 291 or some other Brand Name of that type of material. Butyl rubber has NO tensile strength and weak adhesion, so all of the load on your stern tube is now on the bolts. Also, since the rubber has weak adhesion, if any there is any really poor bonded areas, water can push past it. Unlike deck hardware which has no water pressure, the stern tube has the pressure of the water holding up the boat. With an adhesive, like 3M 5200/4200 or Sikaflex 291, the the adhesive you have much more adhesive properties and the adhesive distributes the load across the bonding surface.

If I was you, and the boat isn't launched, I'd rebed the stern tube with Sikaflex 291 or 4200. If it is in the water, keep and eye an one it and replace it at the end of the season.

DrB
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  #12  
Old 08-24-2012
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Re: Resealing the Stern Tube?

Ok you have sufficiently worried me to redo the project. I'm still out the water and only rushed to get the stern tube down so that my new engine can go in. I will pull it apart tomorrow and start over. Thanks.
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  #13  
Old 08-24-2012
DrB DrB is offline
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Should not take you too long.

Quote:
Originally Posted by Trinidoc View Post
Ok you have sufficiently worried me to redo the project. I'm still out the water and only rushed to get the stern tube down so that my new engine can go in. I will pull it apart tomorrow and start over. Thanks.
The good news is that the tube will be easy to remove once you get the bolts off as the butyl rubber is pretty easy to remove.

Once you get the stern tube removed. Rough both the bottom it's flange and the hull interior with 100 grit sand paper, soak a rag with rubbing alcohol to remove the sanding dust, and then follow it with a thorough cleaning with an acetone soaked rag. Allow the acetone to fully evaporate (pretty quick) before bonding. Butter both the stern tube flange and hull with the adhesive (Sikaflex 291 or 3M 4200) about 1/8" thick, position the flange onto the hull, bolt tight an clean excess adhesive. Run your finger along the edge of the flange and hull to get a nice smooth transition of the adhesive. Go under boat and remove excess adhesive that may have squeezed out into the cylinder where the prop shaft will go. Should take no more than 30 min start to finish.

I know that I lot of folks on this forum think the butyl rubber is the best bedding agent because it flows well, is sticky, and is easy to apply and remove. But because it really has no adhesive properties nor structural strength, it isn't a good material for thru hulls and other below waterline fittings nor as the primary means of fastening something to another surface. It is also very easily degraded by organics like diesel, oil, and gas. "

Folks have to remember that adhesive doesn't necessarily mean something that is tacky/sticky. Honey and syrup are sticky/tacky, but have no significant structural adhesive properties. The best way to think of Butyl Rubber is a soft and deformable "o-ring and use it in in areas where a item like that would be okay.

DrB
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