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The Test Bench

The Test Bench

For a long while I have been meaning to test the strength of a ball valve threaded directly onto a thru-hull as many production builders do for seacock installations. These same builders very often display the ABYC logo proudly suggesting their vessels are built to meet the safety standards. In all fairness to the builders I suspect they believe these installations do or would meet the minimum safety standards but as I found out they certainly may not. We can't blame the builders for installing UL Marine parts and then not having them meet the minimum standards. You would assume the manufacturers would have tested these fittings for the application into which they are sold..

As I just found out the 3/4" Marine UL rated bronze valve threaded to a 3/4" Marine UL rated thru-hull fitting I purchased, does not meet ABYC H-27 Standards. It failed at just 404 pounds and I was an 1 3/8" below the "innermost" hard piping so had slightly less leverage than I would have had I been able to apply the load at the absolute innermost "hard piping". This 3/4" installation should have easily handled 500 pounds for 30 seconds if it were to meet the ABYC H-27 standard.

Does this test mean it will fail in normal use? Probably not, but we had one fail on us when a spare alternator slid across a shelf and hit the in-board end of the assembly so it can/could potentially happen.

My point here is that when buying a boat with this type of installation please be aware of and know that it is, and can be, a potential week point that may not meet the minimum ABYC safety standards for seacocks and thru-hull fittings, at least in the 3/4" size range..

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Graeme 14-Aug-2014 19:04
I've always wondered about the standard threaded thru-hull fittings sold. Just looking at them, they seem to have marginal wall thickness, considering the reduction in area from threading, and the stress-risers created by the thread roots. This even without considering any in-service degradation from corrosion.

Why not increase the OD a little and use a larger nut? With only a slight increase, you could easily double the stress area (based on root diameter of the thread). The inboard end could be threaded with a smaller diameter, proper taper thread for the seacock.

ABYC or whoever should write a standard for the thru-hull fitting *itself*, specifying the minimum bending moment to be withstood with the thru-hull mounted in a thin, rigid plate. This would add responsibility on the fitting manufacturer to produce a fitting than might have some chance of meeting the standard in an installed system.