I pulled my one and only below water thru hull last weekend. What I found was a little surprising. It's a typical mushroom through hull held tight to the hull with a nut. Then there's a backing plate that was glued to the hull with an adhesive that failed ages ago and finally the ball valve was snugged up against the backing plate. The flanged valve was screwed to the backing plate with large wood screws. But, here's the odd thing, there's about a 3" circle that had been machined to make the hull thinner in this area. This does give the nut enough room to sit between the hull and the backing plate. This seems like a strange way to install a vital component. I'm not opposed to adding more glass to bring the hull to proper thickness. At the bare minimum, I'd like to make a larger backing plate and use 5200 or epoxy to secure it to the hull. I'm not too keen on installing and countersinking screws below the waterline. Unfortunately I did not grab a pic of it.
Thru Hull Backing Plates
- Thread starter Project_Mayhem
- Start date
Yes, bring the hull back to its proper thickness.
I would bed the backing plate with thickened epoxy as this will smooth out any rough areas under the backing plate. 5200 won't do that as well. Covering the backing plate with a layer or two of glass would also be a good idea. If you do this, be sure to a fillet around the edge of the plate to avoid a sharp corner.
Done properly there is no need for a nut on the mushroom fitting. The flanged valve, ball or tapered cone, screws directly onto the mushroom fitting. The lag screws on the flange are there primarily to keep the seacock from turning.
A couple of good references:
I would bed the backing plate with thickened epoxy as this will smooth out any rough areas under the backing plate. 5200 won't do that as well. Covering the backing plate with a layer or two of glass would also be a good idea. If you do this, be sure to a fillet around the edge of the plate to avoid a sharp corner.
Done properly there is no need for a nut on the mushroom fitting. The flanged valve, ball or tapered cone, screws directly onto the mushroom fitting. The lag screws on the flange are there primarily to keep the seacock from turning.
A couple of good references:
Replacing Thru-Hulls and Seacocks - Marine How To
Tip: If you would like a higher resolution or larger photo simply click on the picture and it will get bigger. A Poor Idea The picture on the left is what not to do if starting from scratch and installing new seacocks.
marinehowto.com
Seacock & Thru-Hull Primer - Marine How To
These are called Thru-Hulls This item is called a thru-hull or sometimes a mushroom-head or even a skin fitting. They are straight threaded (NPS thread) so they can be cut to the correct length, for your hulls thickness, and then be
marinehowto.com
Instead of wood, use G 10 glass material epoxied to the hull .
Do you mean the edge of the flange should be slightly ground down to make it a bit smoother?
Good call. I'll double check to make sure there's no liner in that area
The wood is in very good shape even after being exposed to small amounts of water over the past couple of years. If it's coated in epoxy and/or cloth it should last longer than I plan on owning the boat
@dlochner beat me to it - and he even referenced our good friend Rod's excellent instructions. Fully agree with using the G10 to avoid the rot. If you are doing the work to layer up the formerly ground-down hull to the same thickness, you'll have the materials on hand after it sets up to mix up a new batch, add some chopped up glass mat to thicken a bit, and cement the backing plate down to the hull. If you pass the through-hull mushroom up through the hole as you work with the epoxy, it will avoid having to drill out the hole after it all sets up, but do not forget to put molding wax on the threads or you will have to buy a new one! If you didn't have spare epoxy and glass, West Systems Six-10 is pre-mixed thickened epoxy in a caulking tube that is really handy for such small jobs.
The fillet comment @dlochner made was not about damaging the metal / Marlon through-hull, but ramping around the sharp edges of the backing plate so that it is a smooth transition from backing plate to hull on top. Honestly, it more cosmetic and avoiding snagging fingers, but that may be worthwhile if you have the materials and time.
The fillet comment @dlochner made was not about damaging the metal / Marlon through-hull, but ramping around the sharp edges of the backing plate so that it is a smooth transition from backing plate to hull on top. Honestly, it more cosmetic and avoiding snagging fingers, but that may be worthwhile if you have the materials and time.
Precisely the philosophy that makes second hand older boats require in-depth inspections.... Jus' sayin'
dj
This seems to be a thing with O'Day. I had a 23' O'Day that had the keel bed counterbored, leaving just the skin of the hull to support the keel. The bolts finally did pull through.
Mayhem, you are saying the hull was “drilled out” partial thickness on the inside, under the backing plate? I could see if was bored out on the outside to countersink the mushroom, but doesn’t mare much sense to me on the inside, under the backing plate. Unless it is designed to hold sealant “in the groove”.
My O’Day 322 isn’t bored out, as far as I know…but I havent pulled the backing plates off under the thru-hulls. I did rebed one thru-hull over the winter…but backing plate was solid and attached. The backing plate seemed to have a slight recess where the nut tightened down…which also doesn’t make much sense to me…made it harder to keep a wrench on the nut….but who knows…
Greg
Are you saying the through hull nut was under the backing platte, i.e. between the hull and the backing plate?
If so, what was that person thinking?
I found a pic of that area and it looks like @DeniseO30 is right about the liner. I'll confirm and take pics this weekend
Yep. I think it'll make a bit more sense once I get better pics. The real head scratcher is why was this thing bedded with butyl?
There aren't any keel bolts in the shoal version of O'Days. The "skin" of the keel is about 1/2" thick if memory serves me correctly. I'm not sure about the deep/fin keel versions however
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My O'Day was a centerboarder. The keel was iron, and extended about 2 feet below the hull, with about 6" tucked up into the keel bed. Definitely had keel bolts. No skin at all on the keel, just epoxy.
The keel bolts are those shiny metal things shaped like a hexagon. The thing hanging under the boat is the keel.
Here are a couple of pics. It definitely looks like it's the liner that has the 3" hole. Items were found in this order:
Ball valve and flange assembly
Wood Backing plate
Flange nut and liner
Hull
mushroom
The nut was hidden under the backing plate
Ball valve and flange assembly
Wood Backing plate
Flange nut and liner
Hull
mushroom
The nut was hidden under the backing plate
That's very different than the one's I've seen up close. Is that an early 70's model?
The thru hull is the one located under the aft sink, next to the keel
So what's the best approach to this? Carefully screw a new backing plate into the liner? Secure it with epoxy? I'd like to try to avoid making more below water holes if reasonably possible
Yep. The valve was tightened against the backing plate which at one point was glued to the liner. If I had to guess, someone tried to use wood glue between the backing plate and the liner
I guessed that yours was an earlier model because of the position of the pendant tube. The later 25 models scrapped the winch in the the cabin and replaced it with a line in the cockpit. As far as I know, there are two models - the swinging centerboard and the deep fin keel. How heavy is your centerboard?
Here is what I would do. Cut a backing plate that fits into the liner cutout. I'd use G10, YBYC.
Then I'd use a hole saw from the outside using the existing hole for alignment to drill through the backing plate. Use what ever thickness needed to bring the backing plate above the liner a little bit.
Then I'd install a Forespar Series 93 OEM valve with King Nut getting the kit that includes the thru hull. Series 93 OEM
Then I'd use a hole saw from the outside using the existing hole for alignment to drill through the backing plate. Use what ever thickness needed to bring the backing plate above the liner a little bit.
Then I'd install a Forespar Series 93 OEM valve with King Nut getting the kit that includes the thru hull. Series 93 OEM