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Plugging bottom holes

Aug 29, 2017

Our very well-preserved 1972 Tartan 34.5 had two paddle wheel-type knot meters and two 2-inch thru-hull depth sounder transducers. All four were more than 30 years old and hopelessly out of date. The four thru-hull fittings were made of very durable plastic but required 2-inch tapered holes in the half-inch hull bottom. The thru-hulls were tapered inward, making the bottom seal areas slightly more than half an inch. Nevertheless, the age of the thru-hulls and the seals made us nervous. Occasionally one of the knot meters would dribble seawater from the end of the paddle-wheel fitting located inside the hull. I would carefully adjust it, hoping the O-ring would seal better and stop the leak. On one occasion I couldn’t stop the port knot meter leak. At anchor, Kathy went over the side, swam around under the boat and held our small toilet plunger over the port knot meter so I could remove the paddle wheel from inside the boat and replace the O-ring. The toilet plunger sealed the hole very effectively, and I was able to replace the O-ring. Kathy removed the toilet plunger; the leak had stopped. Kathy frequently reminds me of that trick.

Enough already! New chartplotters with GPS replace the need for thru-hull knot meters. Sure, we don’t know the current speed with GPS, but we’re not racing and most times we can estimate the current close enough. New depth sounders are much better than the old ones. We replaced both old depth sounder transducers with a forward-looking, dual-frequency sounder linked to our chartplotter. The amazing thing even hints at the size and species of fish under the boat! We removed all the old electronics in the cockpit, leaving us with two 5-inch instrument holes in the cockpit plus the four 2-inch holes in the bottom. We removed most — not all — of the old wires, and we installed a VHF radio and a stereo speaker in the two 5-inch cockpit bulkhead holes. Plugging the four 2-inch bottom holes was a much different story.

We bought West System epoxy with both fast and slow hardeners and a 4-foot length of 3-inch-wide fiberglass tape for the bottom hole plugging job. To make the holes accessible inside the boat, we moved the stores and cleared a space around them.

Removing the depth sounder transducers and knot meters required a little force — they’d been in the hull for more than 30 years! Once they were out of the boat, the old tapered holes were cleaned of any old sealer. This is best done from outside the boat; a Dremel tool with a small grinding disk removed the sealer. The area around the four holes inside the boat was cleaned and then sealed with two crisscrossed layers of the 3-inch-wide fiberglass tape over the holes and West System epoxy with its red-colored fast hardener (we love West System epoxy with the pump handle mixers). We let the inside hole covers set overnight, put the stores back in place the next day and then started on the outside.

The outside required cutting 2-inch diameter circles of tape, soaking them in West System epoxy with slow hardener and stuffing them in the holes, making certain there were no air bubbles. Each hole required four 2-inch diameter fiberglass cloth plugs to fill the holes to the outside edge of the bottom. We allowed each layer to harden overnight. The last application covered the holes completely with no air bubbles. Again, after letting it set overnight, we sanded the area smooth. When we painted the bottom it was nearly impossible to tell where the holes had been. We sleep better with four fewer big holes in the boat. Needless to say, the four plugged holes haven’t leaked.

Richard de Grasse and his wife Kathy live on their Tartan sloop Endeavour in southern waters in the winter and summer at Islesboro, Maine.

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Aug 29, 2017 03:36 pm
 Posted by  Craft Advisory

Those knot meter through hulls come with a plug, so that you can remove the transducer, quickly insert the plug, clean the paddle wheel then swap the plug for a clean wheel. When I tried this, I had a temporary geiser and looked like one of the three stooges putting the plug in. Plus the insertion / reinsertion weakened the old sealant around the through hull and developed a slow leak. I fixed the seal with 3M marine premium filler and BoatLife. I should just leave the plug in but it is nice when it is working and will give you an idea of the current.

Aug 29, 2017 05:57 pm
 Posted by  danpfeiffer

Two tips for this sort of repair.

First, all surfaces that receive epoxy should be sanded to 80 grit for best secondary (mechanical) bonding. You should grind a taper around the hole and use circular patches of decreasing size to get overlap of the edges. West System recommends a 12:1 taper. For a 1/2" hull the taper should be 12/2 wide or 6". This provides the needed bonding surface for a solid repair. That's a 14" diameter area for a 2" hole in a 1/2" thick hull (11" for 3/8" hull). For a hole like this I taper inside and out so I am at 7" overall but maintain the 12:1 taper. The patch is hourglass shaped in cross section. All this is for a solid (not cored) hull.

Second, by letting the inside part of the patch set before doing the outside you loose the great advantage of primary bonding (cross-linking) of the patch layers. Primary bonding is many times stronger than secondary bonding. Also, if you don't clean the surface of the inside patch before applying the outside you will have amine blush on the surface that would further degrade the bond.

To get a smooth surface outside you can cover the patch with some heavy but bendable plastic conforming to the hull shape and hold it in place with a spring loaded curtain rod while the epoxy sets. This greatly reduces the grinding work to get it fair. I would do this patch with biax fabric because it's very strong and wets out easier but I might do the last inside layer with a piece of cloth for a smoother finish. On the inside the aesthetics may be less of a concern in a locker space but a smooth finish will be easier to keep clean.

And as a safety note, the dust generated by grinding and sanding fiberglass is quite nasty and can lead to permanent lung problems if inhaled. You need to have dust collection and proper protection in the form of an asbestos rated respirator. And make every effort to keep the dust from spreading throughout the boat.

Dan Pfeiffer
Mentor, OH

Aug 29, 2017 08:36 pm
 Posted by  Jeff Harding

Dan has it right, sad to see this poor advice/article in this mag
Jeff Harding

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