Water gushes in after removing the thru hull

Thru Hull Failure

Like many boats manufactured since the early 1990s, Priorities has Marelon thru hulls and ball valves made by Forespar. Marelon is a pretty strong plastic, and since it doesn’t corrode it can be a good choice for thru hull material. However, they do occasionally fail… and I experienced this failure a few days ago.

After testing the seawater pump for my refrigeration system, I attempted to close it’s related seacock. It wasn’t jammed, and I didn’t force the handle in a strange direction. Unfortunately, as I moved the handle to the closed position, the handle popped off, and seawater began pouring in fairly quickly!

It just so happened I was about to repair (how many things on the boat am I “about to repair?”) the automatic float switch for the bilge pump, so the bilge pump wouldn’t automatically activate, either.

Attempting to reinstall the handle or close the ball valve proved futile, as it seemed either the screw hole or the valve’s teeth were stripped. Fortunately, the leak would slow to a trickle with the handle pressed into position.

After 20 minutes of messing with the broken handle, and occasionally running over to the bilge pump switch to manually activate the bilge pump, I decided to regroup by holding the handle in place with a cable tie. Though precarious, it worked as a temporary solution, slowing the leak to a mere drip while I developed a better plan.

Damaged thru hull ball valve handle held in place with a cable tie.

Being a member of a yacht club or local boater’s club is wonderful for many reasons, one of which is there are people around who can help with ideas on how to solve the never ending problems that occur on boats. I called a few friends, as well as emailed the technical reps at Forespar and we made a few conclusions. First, the handle was broken and probably needed replacement. Second, the ball valve was a ½” Forespar “93 Series,” which is modular and can be dissassembled in an emergency like this. And third, the repair could be done with the boat in the water. We considered cannibalizing another similar size thru hull that was above the waterline, but I wasn’t THAT desperate and didn’t want to open another can of worms.

I ordered a replacement online, with overnight shipping, which arrived late the next day.

Forespar Marelon thru hull and ball valve assemblies come with a white plug attached to the handle. This plug is to be used to plug the thru hull hole from the outside in the event the valve needs to be serviced. Brilliant! I would need to hire a diver to install it, but it would make things much less messy during servicing. Unfortunately, on my boat, this particular thru hull is covered with a screen, preventing us from inserting the plug… how unfortunate! Access to the valve inside the boat is excellent, though, so I decided to do the work anyway without a haulout and live with the mess.

This white plug, stored on the ball valve handle, is a plug for the thru hull that is inserted on the exterior side of the thru hull… if you don’t have a screen there like I do!
New thru hull and ball valve partially disassembled.

Prior to touching the existing thru hull assembly again, I disassembled the new thru hull assembly to compare it with the old. It revealed a minor difference the Forespar rep had cautioned me about. The old assemblies, like mine, use O rings between the base, the valve, and the elbow. The new assemblies use rubber gaskets, but are otherwise interchangeable with the old bases.

After reviewing the plan with my crew, we swapped the ball valves.

Getting ready to fix the thru hull with my dad
My Dad and I reviewing the thru hull repair plan.

While my Dad held the old valve in place, I unscrewed all four screws. With a TruPlug ready to take it’s place, I removed the old valve assembly and witnessed a 15 inch high fountain of seawater flooding the boat at about 7 GPM. The TruPlug actually worked great, and almost completely stopped the flood.

Fountain of seawater after removing the old ball valve.


TruPlug sealing the hole quite well.

Comparing the two valves side by side revealed the screw lengths were different, as well. I decided to reuse the old screws to prevent any chance of stripping the threads of the base, which was NOT replaceable in the water.

I wiped the old base to make sure it was clean and ready to seal well with the new gasket.

The new valve assembly consisted of the elbow, the middle gasket, the valve, old screws, and the bottom gasket. The bottom gasket was held in position with the four screws and a flat piece of plastic I could remove later. I didn’t want that gasket to get swept away by the water pressure!

Placing the assembly onto the hole, I quickly began screwing it into place. Upon the advice of Forespar, I did not use an electric drill for fear of stripping the base. Seawater still came in quickly, though not as forcefully as I was afraid of. I left the valve open to lower the pressure on the valve and gaskets during tightening. When all four screws had been partially threaded, I gently lifted the assembly slightly and removed the plastic holder. Looking back I should have spent a little more time pushing the gasket into position before tightening the screws… I ended up having the reseat it a few minutes later.

Screwing in the new ball valve. Note the white plastic piece to hold the bottom rubber gasket in place.

Once the screws were tight, I closed the valve. I checked for leaks and watched for any deformation of the gasket. After reattaching the pump hose and drying out the bilge, it was done!

Final tightening of the screws. I kept a careful eye on the gaskets and made sure they were seated properly.

Subsequent inspection of the old assembly revealed the teeth inside the valve had broken, explaining why I hadn’t been able to reinstall it.

Broken teeth inside the ball valve assembly led to all my problems.

So would I do this myself over again? Yes, though having help would definitely be a requirement. I plan on taking an inventory of all the ball valve sizes on the boat this summer for my records. This winter I plan on making sure the white plugs still fit in each thru hull, and no bottom paint has clogged their fit.

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