The boat yard says they don't drill thru the hull. Almost everything I read shows that except one article shows screwing to backing plate. But not sure if less holes in boat is worth going against the majority.
Installing new seacocks. How to bolt?
- Thread starter Tripaddle
- Start date
There is an acceptable alternative method where the flanged seacock flush-bolts to a backing plate and the backing plate is clamped to the hull by threading the threaded thruhull into the flanged seacock. Completely adequate, fewer holes in the boat. I have attached install guidance from the folks at Groco.
If you have a cored hull, you must dig out the core around the thruhull hole and fill it with a material like epoxy putty so it won't compress when to you tighten the bolts (NOT SCREWS!!!!).
In well over 50 years as a professional mariner, I have never had a problem with any hole, in any boat, that was properly done. I am always shocked by those who are terrified by holes in their hull, especially to the point of completely removing and sealing up any unused thruhulls or removing all of them entirely. It seems I always need an extra thruhull for installations of things like a watermaker or a deck wash system. As long as they are properly done, they are no danger to the integrity of a vessel.
In well over 50 years as a professional mariner, I have never had a problem with any hole, in any boat, that was properly done. I am always shocked by those who are terrified by holes in their hull, especially to the point of completely removing and sealing up any unused thruhulls or removing all of them entirely. It seems I always need an extra thruhull for installations of things like a watermaker or a deck wash system. As long as they are properly done, they are no danger to the integrity of a vessel.
I'm working on through-hulls on my rebuild project now. I used two thicknesses of 1/2 inch Meranti plywood epoxied together and will epoxy to the hull and seal all with epoxy. The bolt holes I overbored and filled with epoxy mixed with the drilling chips and fumed silica. These I will drill out to size for SS lag bolts which I will insulate from the bronze to prevent galvanic action. 
Since no question is a dumb question, what is the problem with proper sized lag bolts? The only reason I see for even having fasteners there is to keep the flange from turning, thus backing off the thru hull.
Its prob. better to use milled fiber in your resin-silica mix in place of anything that could soak up water.
A boat's hull works and flexes. Lag bolts would probably work loose after some time. If you are going to put a hole in your boat it must be properly done or you could have a catastrophic failure and loose not only the boat, but possibly your life. I can not remember ever seeing lag bolts being used on any sailing vessel, for any purpose. They are not secure and there is no way to insure they do not work loose or back out. See Gunni's post for the acceptable method of installing thruhulls in a boat. No need to reinvent the wheel. It's already been worked out for anybody who needs the info.
Nope; I also did not want bolts going all the way through the hull. I also debated the merits of encapsulating the bolt ( or the nut) in the backing plate but decided since I am epoxying the backing plate to the hull I wouldn't want to destroy the whole thing if ever a galled/cross thread occurred. There is "one big bolt" holding it all together ; it's the thru-hull threading into the seacock. With some seacock arrangements this is all you have with a little backing nut to hold in the thru-hull. However, with a flanged seacock you have the bolts to take the loads will trying to turn a stiff or frozen handle. Wouldn't want to snap off that bronze thru-hull.
Good point MM; I had short strand fiberglass I could have mixed in and thought of doing so. However; since the wood shavings are small enough to thoroughly saturate I concluded it would be a non issue.
[QUOTE.... I can not remember ever seeing lag bolts being used on any sailing vessel, for any purpose. ....[/QUOTE]
Certainly you've sat on the head and been happy it has not fallen over while in use???
Certainly you've sat on the head and been happy it has not fallen over while in use???
I read the Groco article, and they say that there are 2 acceptable methods. Bolts all the way through, or mechanically attaching the flange to the backing block ( would not using lag screws to do this qualify?). If your block is wood, I don't see any other way to attach the flange to it. I doubt, once tightened, a lag bolt would loosen up in this situation.
Good point Ron, the head is usually lag bolted down, as well as many engine mounts.
Good point Ron, the head is usually lag bolted down, as well as many engine mounts.
Hi All,
This is my first post..
Oh, I could tell you a little story about when a young lady and the head rolled out into the passageway...
This is my first post..
Oh, I could tell you a little story about when a young lady and the head rolled out into the passageway...
You don't need to epoxy the backing plate to the hull, and the thru-hull fitting is completely up to the job of holding the assembly in place once you stabilize the flanged seacock with a bolted backing plate. Caulk the thru-hull and threads. The danger is not coming unscrewed, the danger is stabilizing against any side loads.
Yeah Gunni you're right..... If you're gonna use bolts all the way through.
I'm not phobic of holes, just don't want them where not needed. Epoxy backing plates is the answer for me.
Side note; backboring plates and encapsulating bolts might have been a better solution as I'm having to make thread cutters on a bolt to use as a tap so I can have tight hole clearances without shattering epoxy plugs.
We're not holding wings on airplanes here. Just supporting seacock against shear and bending loads created by levering a stiff handle. Honestly, it will take a hammer and chisel to remove these plates.
Anyway for the OP and all interested, there is a lot of good info given by all posters; consider all and choose.
I'm not phobic of holes, just don't want them where not needed. Epoxy backing plates is the answer for me.
Side note; backboring plates and encapsulating bolts might have been a better solution as I'm having to make thread cutters on a bolt to use as a tap so I can have tight hole clearances without shattering epoxy plugs.
We're not holding wings on airplanes here. Just supporting seacock against shear and bending loads created by levering a stiff handle. Honestly, it will take a hammer and chisel to remove these plates.
Anyway for the OP and all interested, there is a lot of good info given by all posters; consider all and choose.
Fellas we have wood screws all over our boats. A lag bolt is a wood screw with a hex head instead of a slot, Frearson, or Phillips. Just don't use them on a boat like you would building a pole barn; you must make a proper clearance hole or they will destroy your work just by installing.
No, even if your bolts only attach to the backing plate you do not need to epoxy the backing plate to the hull. The backing plate is there to stiffen the flange base area. If it makes you feel better, glue it on with 3m 4200 so that it can be removed.
The reason to epoxy in place, is so that the backing block and hull have a 100 % contact area, with no point loading. Unless your hull is as flat as the block, or you massage the block to match the contour of the hull, putting a layer of thickened epoxy between the two will give you good contact area and no point loads. If the two surfaces don't mate completely flat together, you could split the backing block when you tighten the thru hull to base. you can make a nice fillet around the block by spreading the epoxy that oozes out around the block with your finger (rubber glove on of course).