Make and Make Do

By Rob Rohde-Szudy - Madison, Wisconsin - USA


The Hamernik Hull Drain
How to mount a drain in a thin-bottomed boat

In my article about the Chicagoland Messabout, I referred to Tom Hamernik as “the smart one”. Tom is very self-deprecating about this, but he’s lived up to the title once again.

The problem at hand is how to mount a hull drain in a thin-bottomed boat. Expanding rubber drain plugs need to have good support over their entire depth. If you try to insert one in a hole through half-inch plywood, you’ll be lucky to get it to lock in place at all. I doubt it would work at all in quarter-inch ply. Even if it could, in either case the plug would be sticking below the bottom to collect mud and weeds.

Yet these bottom thicknesses can be found in boats plenty big enough to herniate a disk or two if you try to flip them, so dumping the water out isn’t always an option. My light schooner is a perfect example at over a quarter ton. Sponging gets old pretty quickly in a boat that size, especially when you have to sponge several gallons of freezing cold water during a winter thaw. (Sure wish I could find where that tarp is leaking…) And you might want one of these even in a small boat if it lives on a trailer. It’s a huge pain to drag it off just to dump some water out. So how to install this drain with that thin plywood bottom?

The easy approach would be to bore a hole down through the false keel, providing plenty of wood to support the plug. Unfortunately the centerline is exactly where you don’t want the drain. It would never drain all the water unless the boat were perfectly level! Practically, the drain needs to be in the corner that is lowest when your boat is parked for storage. For me this is the forward port corner of each cockpit, since I park the trailer with the starboard wheel slightly elevated. (If I were ever unsure I’d park it with that wheel on a scrap of lumber.)

But I don’t have any convenient chunk of timber beneath the bottom plywood at the cockpit corners. Some powerboats have skids that work nicely, but I don’t. I could have added a faired section of skid, but it would increase my draft unnecessarily and seems inelegant to glue on a tiny bilge keel just to drain the cockpit.

Trust Tom to come up with the elegant solution.

The Hamernik Hull Drain

The plug has to be supported throughout its entire length, but the water doesn’t have to drain into that channel at the top. Tom’s method involves adding a block inside the cockpit and boring a plug hole through it and the bottom. This supports the expanding plug. A channel or channels cut in the block allow water to drain into the vertical plug channel. The picture is worth 1000 words.

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The Hamernik Hull Drain

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Some might point out that with a ½” bottom it might be easier to epoxy a stainless nut into a hole, then seal it with a regular bolt and Teflon tape. Or a nylon bolt. Maybe, but I wouldn’t feel very comfortable installing such a system in mere ¼” plywood. Furthermore, it is harder to get that system crammed deep into a corner without cutting away material we’d rather keep. You have to get a wrench on it, after all. Tom’s drain channels can be guided to wherever the hole needs to be.

Devil in the details

But construction is not as simple as it seems. I tried test molding some tube assemblies in fiberglass. It is surprisingly difficult to get the bore of the drain tube correct. Either the plug doesn’t want to slip in or it cannot be tensioned. I have a feeling there is a reason the commercial suppliers use metal. Back to the drawing board.

It saves a lot of effort to use a $3 commercially available brass tube, and these have a flange around the top edge, which helps with strength. We will need to cut a slot for the water to drain, but it will be easier to locate that slot after we build the parts that will surround it.

The block

Now we need to fit the wood piece that will support the drain tube. First we need to know the right thickness. This depends on the drain plug’s dimensions. The only part where the plug will actually seal water out is in the thickness of the bottom. We need at least two of the plug’s rubber rings compressed in that space. This might mean a little of the plug’s brass center rod will protrude below the bottom of the boat. This should present little trouble in the water or beaching on sand, but if you beach on concrete a lot, you might want a plug that is flat on the bottom.

My plug fills a hole about 1” deep. ½” of that is the bottom, so I needed to add a ½” thick block inside the hull. I used ½” plywood because it was easiest. (A ¼” bottom would probably require about a ¾”block.)

First we fit the piece into the corner where our drain will go. With a flatiron skiff this is pretty easy, but it’s not terribly hard with a stitch and tape boat either. In the age of epoxy, gaps aren’t bad. Just make sure it’s snug against the bottom so the tube length is right. I made these by cutting a 3.5” square of plywood, then cutting it diagonally. A belt sander did the final fitting.

The drain block dry fitted

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But don’t glue it yet!

Drain channels

First we need to locate the drain hole. I already had holes bored in the bottom because of my earlier experiments. So I traced their location on the bottom of the blocks. You can make it easier, though, by just boring from the top. You don’t want this hole so tight you need to hammer the tube in. Leave a little space for epoxy. I used a 1-1/8” drill.

Next we mark a 3/8” slot where the water will drain into the tube and cut it through to the hypotenuse side of the triangle.

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Drain blocks cut

Now we’re ready to nail down our blocks. I drilled pilot holes in the block for two ¾” ring nails. Folks with thinner bottoms will have to do this the other way around and put in screws from underneath. I wouldn’t try to nail it that way, as it would surely knock your block out of position.

Drain Tube

Now we need to cut the tube to fit. First we mark the slot to match the slot in the block.

Marking the drain tube

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Then we use another scrap of plywood (the same thickness as the boat’s bottom) to mark the cutoff line.

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Marking the cut-off line

Drain tube marked

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Then we drill the tube along the lines. Drill a little inside of them and file to the lines. This is quickest if yo udril a few holes and connect them by snipping out the intervening metal with a side cutter. A Dremel or similar tool is helpful in finishing to the lines, but a file works fine. Finally we hacksaw off the bottom of the tube and file to the line. Make sure you smooth all those edges now, since you won’t be able to later.

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Drain tube finished


This step takes a little preparation. Lightly sand the outside of the tube, and coat the inside of the tube with a couple coats of automotive paste wax so it will shed any epoxy you manage to get on it. Finally, carefully wipe the outside of the tube with acetone shortly before mixing the epoxy. Don’t let it take off any of the wax. Finally, slap a piece of duct tape on the bottom of the boat to cover the hole.

When you mix the epoxy, paint the entire wood block and hole with unthickened resin. This is also a good time to glass any limbers you had to make. In many of these skiffs, the bottom panels are held together with butt blocks. This get in the way of water draining, so we need to cut a limber along the chine on the “downhill” side. It’s usually a good idea to glass it after the ugly chiseling that’s required.

Glassed limber

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Add thickener to the rest of the resin to get something between the consistency of mayonnaise and peanut butter. Butter up the block and nail it in. Then butter up the inside of the hole and the outside of the tube and press it into place. But you’ll need a chopstick or something similar to work the epoxy out of the channel leading to it. You’ll definitely want plastic gloves, but I’m assuming you are already taking appropriate epoxy precautions. Don’t try to scrape epoxy out of the bottom of the drain tube. You’ll only scrape away the wax so it can stick. You might also want to prod the epoxy with toothpicks to work out any air bubbles. Just don’t pierce the duct tape.

Important note: Do NOT use an epoxy filler with FIBERS! They will drag when you’re trying to shape it in the channel. You will not be happy. Stick with wood flour or something similarly non-fibrous. Without fibers I could have done this in one shot, but with fibers I had to go back and fill a gap.

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Drain epoxied

I hate spending half the day scraping at epoxy, so I added heat here to speed up the process. But check it frequently so it doesn’t get away from you. An incandescent light bulb up close makes a lot of heat!

Heating the epoxy

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Once the epoxy reaches “green cure” we can finish up. This is where it doesn’t sag anymore, but it is still rubbery. Carefully pull off the tape and push the epoxy “plug” out of the tube. Push downward, or you’ll dislodge the tube. I used a chisel, but I’m sure other things would work. If it doesn’t want to move, scrape the epoxy off the bottom of the metal tube. Then leave the light on it overnight for a fast cure.

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Drain cut out

This is also a good time to add any other fiberglass you plan to add. I didn’t bother, opting to paint the whole arrangement with epoxy and call it done.

When cured hard, a light sanding is needed to remove the gloss and any “blush” so paint will stick. You might consider re-waxing the inside of the tube if you’re messy with paint. Here it is sanded and painted. The finishing touch is a screweye and lanyard so we don’t lose the plug.

Lanyard tied on

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And from below…

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From below

The test

With a ½” thick bottom, I have experienced no leakage. It doesn’t seem to leak even when I retract the plug by ¼”, which is promising. The true test will be when someone builds this into a boat with a ¼” bottom. I think the plug will have to project a bit, though, to get a good seal within ¼”. (Maybe Tom will test this on his Michalak Mixer.)

Tom also asked me how fast this drain clears the cockpit of rainwater. I always tarp the boat, but I unintentionally tested the draining ability. I’m told that you only have to back the trailer in with the plugs out once. I hope that’s true. Here’s the water running out.

Water running out

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It drains about like a garden hose with no water pressure. It slows down a lot when the level drops to where only the small slot is draining water, but by then you’re in sponging range anyhow. And obviously you have to be alert to any blockages. Any little chunk of refuse can plug the small slot, and leaf can about shut it off completely. Though I suppose that is no different from any other hull drain. Perhaps better, since we have a slot and a top hole, and with any luck at least one of them will be draining. I would want more drains if I stored the boat uncovered, but I think that’s a bad idea anyway because the boat’s bottom turns into forest floor pretty quickly. And a polytarp cover is cheap and easy to make.

A caution to the really cheap people

It is tempting to think this can be done even cheaper, but I bet it can’t. 1” copper plumbing pipe has an inside diameter larger than 1” and won’t work with commercial drain plugs. You probably could make it work by buying rubber stoppers and tensioning them with a carriage bolt, wingnut and a couple washers. But I am almost certain this would cost at least as much as buying ready-made parts unless you’re a really good scrounger.

Rob Rohde-Szudy
Madison, WI

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