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Through Hull Maintenance

“Krabby, will you show me where that through hull fitting is that you replaced on Sally back in September?” asked the East Coaster on a chilly fall afternoon, “Now is a good time while we are cleaning up for the next race and checking systems.


“It’s right here in the sail bin, just forward of the bulkhead,” I said, shining the flashlight on the shiny new bronze seacock.


“Why did that one fail? I thought that you replaced all the through hull fittings four years ago, they should last longer than that, I should hope,” she said wearing into the sail locker.

“Well, I broke my own rule about exercise,” I replied.


“Wait, what are you talking about? Like a New Year’s resolution? ‘I will exercise regularly in 2017’. The boatyard and the 10 meter give you plenty of exercise!” retorted the puzzled engineer.


“No, no, not me, it’s the through hull fittings that need to be exercised regularly,” I explained.


“What kind of exercises do you do with valves in the bilge of a boat, Pilates? What am I missing here?” queried my frustrated plumbing student.


“All right, it’s simple, you open and close the through hull fitting to ensure it is operating. This frees up corrosion or other build-up that can cause it to freeze in place. Usually, every six months is often enough; but the fitting on the head discharge might need more regular exercise because of the calcium deposits. As a rule of thumb, if they are hard to free up at six months, then you need to exercise them more often. When you do this, it's a good idea to inspect the plumbing fittings (hoses, clamps, etc.) to see if there is a failure looming.”


“Ok, that makes more sense - but my engineering background is not comfortable with a hole in the bottom of a boat. What is done to reinforce the structure of the hull, and what is the safety redundancy should the fitting fail?” she asked.


“Great questions. Yes, I agree. A hole in a boat is the first place water could come in, usually on a dark and stormy night at 0300, but they are a necessary evil if you want to have engine cooling water, a head, and other niceties. Hey, speaking of dark and stormy, I am kind of thirsty - could you fix us a couple while I explain further?” I asked with a parched voice


“Sure, you keep talking while I handle the bartending” she answered.


“Thanks. Ok, back to holes in the boat: a well-engineered through hull fitting has adequate reinforcement around the penetration, such as a doubling block made of wood or fiberglass. This not only maintains the hull structure, but it forms a foundation to carry the load of the fitting in the event it is over stressed when attempting to close it.” I explained.


“Oh, like when you broke the stem off the one on Sally when it wouldn't close?” she smartly poked.


“Yes, sometimes I don't know my own strength. For added safety, a tapered soft wood plug is attached to a lanyard to the fitting; if you succeed in breaking it off, you can drive the plug into the damaged fitting to temporary stop the egress of water.” I continued.


“Let’s hope we never need those plugs. Now I have another question, why do these fittings on Sally look very different from the ones on my Dad’s Pearson 35?” asked the observant East Coaster.


“Another good question. There are two basic types of through hull fittings: seacocks, like the ones you see here, and the type that are valves threaded to the fitting held in place with a nut, like on your Dad's Pearson. Seacocks are bolted to the hull by way of a flange cast into the base. The fitting that penetrates the hull is threaded into the straight cut threads in the base of the fitting. The other side of the fitting has tapered threads to accept standard pipe fittings. With the other type, the straight cut (non-tapered) threads of the fitting that penetrates the hull is held in place by a nut on the threaded section of the fitting then a valve with standard pipe threads is fit to the reaming threads beyond the nut. This may be a little hard to visualize, but basically the fitting on Sally is more robust for a larger offshore boat and the other type is more standard equipment on smaller inshore type boats. Both types are quarter turn type ball valves. Some older fitting have a taper core valve that is a little more complicated to operate but can be rebuilt and severed in the boat.” I explained as I took a long deserved drink.


“So, what you are telling me is that seacocks are the way to go if you want a super strong fitting. Are you telling me that a tapered female pipe fitting is fitted to a non-tapered fitting on the one on my Dad’s boat? That is unsound for an engineering point of view. Only the first couple of threads are holding the valve on? Seems a little sketchy to me.” said the concerned engineer.


“You are right. It certainly does not meet best engineering practices, but it is the accepted standard on smaller boats with fittings that are generally smaller in size. The more robust seacocks are larger and won’t fit in many of the tight places on smaller boats.” I said.


“Ok, I guess it works. You’re right about the tight places the valves are installed in. How does anyone get in there to service them? That’s probably why they don't get exercised. Maybe the skipper will have to get his exercise first, in order to fit in there, to exercise the through hull fitting,” concluded the East Coaster.


“Good point, exercise yourself, and then your through hull fittings will be the New Year’s resolution.”

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