My thru hull repair

[Scott T-Bird]

This thru hull is at the bow for the head discharge and it appears to be a 1-1/4" thru hull. The fitting is a RC Marine (marelon) fitting. What I first noticed to be a problem was the seal to one of the thru bolts had popped out when I scraped a little paint off. The material is spongy so I assume it was a 3M 4200 adhesive sealant. The photo shows the plug which is about 3/4" in diameter and 1/2" thick; however, I pried more material out of the hole as shown and the hole appears to be about 1 inch deep until the bolt head.

The inside photos show the RC Marine fitting with the washer and nuts holding the flange in place. I happened to twist on the bolts with my fingers and one of the screws just snapped right off ... I guess crevise corrosion took its toll.

I wasn't planning on a doing a complete swap-out of the thru hull fittings this spring but now I'm a little worried. Next off-season I had planned to strip off all the bottom paint and inspect all the thru hulls and replace them if I deemed necessary. I've had no leakage issues to date whatsoever.

Is it feasible to simply remove the 3 bolts and replace them without disturbing the seal to the thru hull and the flange? I would then fair the bolt holes with 3M Fast Cure 4000 UV as I have a fresh tube of it on hand.

 

[kendall]

You could probably get by with simply replacing the bolts, but by the time you did that, you'll have enough work into it that you may as well carry on and do the rest of the job.
That way you don't have to worry about it.

Ken.

 

[Roger Long]

I guess crevise corrosion took its toll.

Reason enough to replace all the stainless bolts with bronze. I don't see any reason why replacing just the bolts this year wouldn't be a reasonable thing to do. They are clearly deteriorating by a different process. Once the bolts are out, give the thru hulls a wiggle and twist and look carefully at the bedding at the edges. If they look and feel solid, they are probably OK.

Another issue to look at carefully though is the backing blocks if they are wood and the thru hulls are bronze. If enough water has gotten into cause crevise corrosion of the bolts, there may have been some galvanic corrosion, possibly speeded up by some current differential. Sometimes, this will cause softening of wood. Stick an ice pick up at an angle in the holes and probe around.

If you have time, re-doing everything would be best.

 

[sailingdog]

Roger—

That's a Marelon Seacock and through-hull. Galvanic corrosion shouldn't be an issue...

Scott—

If you're sure you haven't accidentally broken the seal on the through-hull, then replacing the bolts and nuts should be fine. The two concerns to watch for are: 1) if the backing block was plywood, even epoxy coated, it may be rotten, 2) not breaking the seal on the throughhull when putting the new nuts and screws in place.

I would recommend going with bronze screws and nuts because they don't suffer from crevice corrosion issues like stainless steel does.

 

[seadaddler]

Change

I would change the whole thing while it is out of the water,I don't like those plastic thru hulls at all.
Change it and check it all out now and one down and one less to worry about.
Nick

 

[Tom S]

Those are not "plastic" Thru-hulls/Seacocks those are Marelon - To me the only weakness I have heard & seen are the handles - some models have beefier connection points for the handle to the ball. Even silicone bronze Thru-hulls/Seacocks are not perfect, I know more than a few boats that have had their Thru-Hulls fail through electrolysis or galvanic corrosion - I guess you pick your poison .

Like he said the Marelon Thru Hull seems to be fine and there are no signs of leaks, so if he can just "drill" out the screw/bolt holes he'll probably be ok

 

[Hermit Scott]

If you are going to just remove the old bolts and replace them I would do them one at a time to minimize the risk of breaking the seal. Starting with the one that's already broke.

 

[sailingdog]

One thing about Marelon Seacocks—the maintenance procedures really MUST BE FOLLOWED. They need to be exercised regularly—opened and closed—at least once a month, preferably every time you're on the boat. They need to be greased once a year. It ain't rocket science. If you do this, they'll last a good long time, with little danger of electrolysis or failure.

 

[Scott T-Bird]

Thanks for the comments ...

For this year, I am going to replace the bolts, one at a time like Hermit says, because the seal seems to be intact. Hopefully, I can get a phillips head or a straight screwdriver to grip the head without stripping the metal. I'll have to look for bronze bolts. I would like to strip all the bottom paint at make a close inspection of all the thru hulls but the marina will not allow any paint removal and I can't hire them to do it either, so I have been waiting until we move the boat, which should be the end of this season. I do use white lithium grease twice a year and open and close all sea cocks on a regular basis, so I am satisfied that I don't have a maintenance issue with valves.

This picture shows the water intake for the head ... as you can see, the boat manufacturer built up the hull substantially where the thru hulls were installed (oops - it looks like the clamps are a little discolored by corrosion). I don't know if there is a plywood core covered by the fiberglass. It would be nice to assume that it is solid glass, but I don't know ... When I poked with the pick from underneath, it feels like solid glass and the metal head of a screw ... there is nothing that feels suspicious.

As you can see, this 3/4" intake was fastened with lag screws mounted to the hull from inside. The underside photo indicates that they are not thru bolted.

 

[Don12364]

Marelon

Marelon is nylon filled with glass powder.
DuPont make it and list it as a thermoplastic.
So yes it is a plastic - but a very good one for underwater use.

However it needs to be in thicker sections than an equivalent bronze fitting and it is still significantly softer than the metal so could suffer damage more readily from unfairly applied loads both inside and outside the hull.
It will also embrittle with old age and sunlight - though not much of that under a boat!.
In seacock use one trades the cost, need to cast in a foundry and possible electrolytic problems of bronze with the cheaper material, ease of injection molding but thicker sections, significantly lower hardness and long term possible embrittlement leading to sudden fractures.

Neither material is perfect but both are fine - just as long as one knows what to look for. Sadly older fittings made from each material can fail without warning as a result of its own shortcomings and from quite a small knock.
That's why we all worry!

BTW:- I second Roger's suggestion of replacing the stainless bolts with bronze ones. The Blakes seacocks come with bronze fastenings. http://www.blakes-lavac-taylors.co.uk/blakes_seacocks.htm

 

[Ron20324]

Scott, I looked at your original post, and you say the hole is one inch deep to the bolthead. Add another 5/16-3/8 for the bolthead, and just how thick is the hull there? If that's so, there is not much glass left in there???

 

[Scott T-Bird]

Ron, I am surprised by the depth to the bolt head myself. The manufacturer did build up the hull for the thru hulls, and I'm guessing he added about an extra inch at least. This was done for the head discharge and the head intake. The thru hulls for my depth transducer and paddle wheel are a different type (not flush with the bottom) and may have been added after the boat was built. The only other thru hulls below the water line are back by the engine where there is the cooling water intake and the galley sink drain ... I haven't paid attention whether or not the thickness was increased there. I'll have to look next time.

Like I said earlier, the head discharge is thru bolted while the water intake seems to be lag screwed from the inside.

Looks like I do have a photo ... it appears that the thru hulls back by the engine also have a section of built-up hull. The flanges also appear to be lag screwed from the inside, not thru bolted. (I had the engine pulled out of its compartment this winter to install a PSS shaft seal and work on the tranny).