Thru-hulls- what's the deal?

ufm82

Master Chief Petty Officer
Joined
Jul 29, 2003
Messages
827
I'm replacing the factory nylon thru-hulls in my boat with stainless and was wondering about fit-up. The holes in the hull are 1 1/2" on the nose. Yet I find that there are no thru-hulls out there measuring 1 1/2" O.D? I have 1 1/8" hose running to these fittings and all the fittings I find for that size hose measure either 1 5/16" or 1 3/8" O.D. That leaves a gap around the thru-hull and the hull itself. Am I to believe that that area is designed to be filled with sealant for vibration resistance? Durability? I can't see how that is correct. The fittings I removed had a 1/4" thick "spacer" ring of sealant around them if that makes sense. I want to re-install the fittings and make sure they are snug in the hull. So, my two questions are:
1) Why the size differential?
2) What sealant to use to seat the fittings? These fittings are submerged at times when I am in my boat.

UFM82
 

oops!

Supreme Mariner
Joined
Oct 18, 2007
Messages
12,932
Re: Thru-hulls- what's the deal?

my gas tank over flo is smaller than the hole.....

id use a gasket

or 5200....sikkaflex.....or pl 9000 if you dont wanna ever take it out :D
 

tashasdaddy

Honorary Moderator Emeritus
Joined
Nov 11, 2005
Messages
51,019
Re: Thru-hulls- what's the deal?

use 3m 4200 to seal around it. 3m 5200 is a forever .
 

drewpster

Commander
Joined
Oct 17, 2006
Messages
2,059
Re: Thru-hulls- what's the deal?

The reason the O.D. if the stainless thru-hulls are smaller than the plastic ones is because of the material they are made from. The plastic fittings need to have a thicker wall to be strong. The stainless is much stronger and therefore the wall of the fitting is thinner.
As long as you have a good amount of sealing surface under the head of the fitting against the hull you should be fine using sealer in the gap.
If the gap around the stainless fitting is is more than a 3/32" I would bond a backing plate made of epoxy coated wood on the inside of the hull covering the oversized hole. Then cut a new hole in the backing plate the proper size using a hole saw. Be sure to seal the the exposed wood in the new hole with epoxy. Then using a marine sealer (4200, 5200 ect.) reinstall the new fitting. WALLA!
 
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