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DrB 04-26-2011 03:25 PM

Thru-Hull Thread count
A standard mushroom 3/4" thru-hull has 2 inches of threads. Based on the thickness of my hull and base mounting plate for the seacock, I estimate count that the I can get 6 full turns before the thru-hull is flush with the hull and therefore can not be turned anymore. I would need to purchase the Long version if I wanted more. The seacock flange will allow 12 full turns before it bottoms out.

Here is the $64,000 question, do I need more threaded turns or is 6 adequate? If 6 is okay, what is to few? I was told in my youth that if you can at least 4 full turns, you have reached essentially the strength of the thread and more turns doesn't buy significantly more strength. T or F? There is adhesive on the threads too.

Any machinists, Mech E or generally smart people care to weigh in?



WDfrmTN 04-26-2011 05:22 PM

If it helps, for aircraft and general mechanics, three threads beyond a bulkhead nut is sufficient.

Here is an article that might help: Installing a Seacock by Don Casey

klem 04-26-2011 05:33 PM

There are rules of thumb for the required thread engagement but most of them are for coarse threads in hardened steel nuts so they do not apply well here.

The correct way to do this would be to figure out what the thread stress area is (it is a function of the diameter and pitch as well as number of threads and can be easily looked up in Machinery's Handbook). Then, you can look up the mechanical properties for the seacock and nut material (note: both need to be strong enough). Multiplying the stress area by the yield strength of the material will give you an axial force which can be transmitted. When you do this calculation for a fastener, it is straight forward since you just need to make the threads stronger than the rated yield strength of the fastener. For a thru hull, the same is true except that knowing the strength of the thru hull is a bit tougher (although a force/area approximation is a good start). Additionally, it is always wise to check the induced tension from tightening the nut and make sure it is below the thread yield strength and overall fitting strength. There are many online calculators that will let you figure this out, you just need the diameter and tightening torque.

Since this is a marine environment, you are going to want to leave yourself a significant factor of safety. There really is no excuse not to, it won't add a lot of weight. The marine environment has the unique ability to corrode or otherwise degrade just about anything which will greatly impact the strength. In addition, the normal fatigue assumptions about a bolted joint do not apply very well here because the fastener (thru hull) is stiffer than what it is clamping together.

Sorry that I didn't give you a number answer, there are too many variable. It will give you a place to start (or maybe just convince you to buy the longer ones).

bwindrope 04-26-2011 07:01 PM

I'm no ME and don't portray one on TV, and can only add that the Groco IBVF flanges I just installed on my boat are designed to mate to Groco thru-hulls and IIRC they accept about 6 turns before topping out. Same for the Groco ball valves that are designed to fit the IBVF flanges on top. About 6 turns.

I don't know squat about the technical specifications of the situation, but trust that Groco does and that this thread fit is sufficient for full strength in their bronze. Just a data point for consideration.

eherlihy 04-26-2011 08:09 PM

I'm in awe of Klem's 2nd paragraph...

However, using a NPS thread - like on the through hull - in an NPT fitting - like on an unflanged seacock - only allows 3 turns. I know this because that is what the yard mechanics did with the sink through hull on my boat before I purchased the boat and fixed it.

Maine Sail has posted a good illustration of this problem here.

eherlihy 04-26-2011 08:17 PM

...And the reason that they make through hulls with NPS fittings is so that you can cut the through hull fitting to the correct length, and still screw the fitting into the adapter, or flanged seacock.

DrB 04-27-2011 10:09 AM

It's in
Thanks all for the responses. It' turns out, no pun intended, that I had >6 but <8 full turns before I reached tightness. I am confident that I have enough NPS thread into the flange to insure a good seal and one that won't easily unthread. Before I threaded it in, I cleaned the hole well with lots of OMS and Acetone, allowed to dry and then coated the inside of the hole with a layer of 291. Then I gooped up the threads of the thru-hull, threaded it in and really snugged it down.

Next week, when I get splashed, will be the test.


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