Sealing Transom Through Holes

[marcusg]

As mentioned in another thread, I'm adding a swim ladder to the transom this year. In my research, I've come across the phenomenon of sealing transom through holes, which up until this point I wasn't aware of (I thought the washers/bolt heads sealed them, I guess.) So I'd like to properly seal stuff, including both my upcoming ladder installation, as well as the existing througholes, or at least the lower set of 4 bolts that hold the attachment point for the rudder to mount onto, just above the drainholes.

So far from what I've read, it's good to countersink your hole, at least for the screw/bolt head, so that you can get a thicker bead of butyl tape/3m 4200 sealant in there. But for all of the holes in question for me, the outer part doesn't come into contact with the bolt head, since there's a piece of SS hardware in between the bolt head in the transom. For example, my ladder bracket will be sandwiched between bolt head and transom. And for my rudder attachment point, that part (whatever you want to call it, it has a little loophole that is the female piece for the rudder's male "hook" -- that is sandwiched between 4 bolts through the transom. So do I squeeze butyl tape behind this SS hardware? Do I still countersink the holes and try to stuff them before attaching the hardware? I don't get it.

[GreenLake]

@Marcus,
There are some general considerations when sealing some fitting against the deck. For simplicity, let's ignore the details of the actual fitting and assume you just have a flat plate that is throughbolted. Your aim is to keep water from getting into the bolt holes.

First, you would use a bead of butyl tape around the perimeter of your flat plate. That keeps water from creeping under the plate into the bolt holes.

Next, the bolts are usually not integral to the plate, but the plate has holes for them. You are correct, you also need to seal those. Normally, if the hole is countersunk, the head of the bolt should wedge itself against the sloped flank of the hole and that should produce a pretty good seal. If you are worried that it's not perfect, you can lay a bead of butyl around each bolt where it exits the underside of your plate. When you tighten the bolt, that should squeeze against the bolt, completing the seal.

If your transom is a sandwich, you should seal the inside of each hole with epoxy. If the core is a soft material that can give under pressure, you need to excavate a bit of the core and fill the void with epoxy, thickened epoxy, or short bits of glass mixed with epoxy into a paste. You then drill a new hole through that plug. The plug helps the sandwich from being crushed as the bolts are tightened and also seals it against any water getting into the sandwich (which is always worse than a few drops of water getting into your cockpit).

If you have some at hand, 3M High Strength Marine Filler would also work to seal and strengthen a hole through soft core material. For the excavation the tool usually recommended is a "bent nail in a drill".

The names for the rudder parts are pintles and gudgeons. The pintles are the ones that look like pins. Terms worth knowing.

[marcusg]

Gudgeons and pintles, got it. Thanks.

... if the hole is countersunk, the head of the bolt should wedge itself against the sloped flank of the hole and that should produce a pretty good seal. If you are worried that it's not perfect, you can lay a bead of butyl around each bolt where it exits the underside of your plate. When you tighten the bolt, that should squeeze against the bolt, completing the seal.

I'm guessing you meant put a bead of butyl tape around the underside of the plate, so that it squeezes up against the bolt when everything is tightened. Or do you actually mean to put the bolt (or bolts) through the plate, and then wrap a piece of butyl tape around the shaft, that then, when it's all snugged up, travels sorta into the bolt hole of the plate?

I think I have some butyl tape left over from when I did my last roof on a shed. Can I just use that stuff, or is there a special boat version?

And man...I love 3m High strength marine filler, but I wish I'd stocked up on it more a couple years ago. I just had a few blobs left (but a ton of extra creamer,) however the price is now 2-3x what it used to be.

[GreenLake]

If you can put the bolts through first, it would be easier to place a bead around it, just so. But in general, I was aiming for the pressure to form a collar around the shaft of the bolt, making a seal and stopping any water from getting in through the countersunk hole from continuing along the bolt.

I don't have any advice on shelf life. And I hear you on the price of materials. I can currently only find it offered in qt size which is more than I used to purchase.