The Offshore Voyaging Reference Site

Elliptical Hole Hack

I have to cut an elliptical hole in the stern of our boat to accommodate the heater exhaust fitting above. And just to add to the fun and games, I need to also make two high-temperature gaskets out of silicon sheet.

So how the heck do I get a nice clean hole just the right shape in both the transom and the gasket material?

My first inclination was to use a hole saw to cut a hole in the transom the same as the minor axis of the ellipse and then start shaping with a Dremel and burr bit. I guess that might have worked, but I’m not either the neatest or the most patient guy around, so it also could have gone very wrong.

And what about the gaskets?

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Richard Ritchie

Maybe try a pilot hole first at the required angle to the stern? Start small and work up. If you have a good pilot to provide the centre guide of the hole cutter, this should be able to feed in cleanly and will create the elliptical hole you want…. The guide rod is the key. (But I have only done it in wood….)

Richard Ritchie

Sorry not to be clear.
A tube cut at an angle gives the ellipse.
Drilling a hole at an angle is much easier than “hole-cutting” a hole and can be fine adjusted if you make a small hole first and check angle. This hole then guides the guide “axle” sticking out of the central front of the hole saw. Torsten below has an equally good method if you don’t have a central guide pin to your hole saw, but there is more grab and friction his way.

Torsten Hansen

That’s one way to do it but if you view the elliptical hole as a tilted round one, there is a better way. Take a block of wood to your drill press and drill a hole the diameter of your exhaust pipe plus any clearance required. Then find the angle between the faceplate and the pipe and cut the wooden block to match. Use hot melt glue to temporarily attach the block to your transom, use the hole as your guide and drill through, remove the block.

Francis Livingston

The one refinement I would make on the solution that you have come up with, John, is to make a plywood template first that is approximately the same thickness as the fibreglass that you are cutting through.

That will allow you to refine both the fit and angles beforehand and if the first try is a botch the only cost is a piece of plywood.

Once you are satisfied with the template it can be used as a template for your gaskets and also for the hole. Jigsaw or holesaw out the hole undersize, hot glue the template in place and then hand file the opening to final size and shape using the plywood template as a guide.

I also think that Mr Hansen’s solution is a really good one and much faster. Just glue the block down really well. You’ll also probably need to buy an extender shaft for your holesaw shank. They are inexpensive and readily available.

The more I think about it the more inclined I am towards Mr Hansen’s solution. It saves a lot of handwork. It even provides you the shape of the inside hole for your gaskets. I can’t imagine that finding a holesaw of the correct diameter will be much of an issue.

In my experience using a holesaw to cut an angled hole using only the 1/4” pilot drill as a guide only works, sort of, for shallow angles. Getting and maintaining the correct angle is difficult and the pilot drill is too willing to open up the guide hole. Even if one replaces the pilot drill with a plain 1/4” steel shaft deflection of the shaft remains a problem.

A guide block solves all those issues.

James George

they make a hole saw you push the hole saw up the bit drill at angle you want then push the hole saw back down and drill your elliptical hole i have one works great but i think it was like 15 for bit and hole saw sie i needed I’m sure in one search you will find it its a well kn own manufacturer

James George

Made by spyder they have you tube demo take you about a couple minutes to drill a hole at any angle. The bit is fairly long and sturdy drill your angle push hole saw back down shaft and in a minute you hole is done

James George

Spyder has website or video even how to make the hole bigger with their hole saw and bit combo . I wish it was near me i would take a phot and then you would go oh yeah now i see

James George

I should be calling it an arbor but it comes with long pilot bit

James George

spyder tct hole saw…video of makeing an angled hole and more

John Gulliver

I like the last idea of cutting a pilot block(?) to the needed angle and hot gluing it to the transom. Very slick. I have often used a long pilot bit in a hole saw and eyeballed, (with the visual aid of a sliding bevel gauge), the angle, and drill until just the pilot bit penetrates the material. Then, using the long bit as a guide, mount a block or any handy board that has a hole the same size as your pilot bit on the inside of transom. You should have two pilot holes lined up at this point at the same angle. The aim is to have the pilot bit engaged in both holes before the actual hole saw starts its cut. When cutting through the intended surface with the hole saw you still have the inside guide to keep the angle correct as hole saw undermines the first pilot hole. Sorry for the long winded explanation. In most cases I’d probably go for the hot glue method, but it’s always good to have another solution handy if needed.

David Eberhard

Mark the center of where you want the hole saw pilot to go. Only the pilot drill, no hole saw yet. Start drilling at 90 degrees to the surface. At this point, all you want to do drill just a little bit. You are making a little divot to keep the drill from sliding out of position. Now with the drill in the divot turn it on. With the drill spinning, tip it the angle you want the finished hole to be at. Complete the pilot hole all the way through. Hole saws come in deep versions, use a deep version. More than likely the pilot hole drill bit will be too short. Replace it with a steel rod of the same diameter. It needs to belong enough to fully engage the pilot hole. Add the hole saw. The saw will now be held at the proper angle. The rod pilot will keep it in place. Start drilling, but keep the speed relatively slow, and the pressure low, so that is controllable. It will not take very long to get the hole drilled in deep enough that it is locked in place. You can then increase the speed as you get comfortable that the saw will stay in place. You may find that before you finish the hole, the saw has bottomed out. Stop and remove the saw. If you are drilling through wood, stick in a screw driver and break the piece off. Continue drilling. In fiberglass that may not be possible. Use a saber saw to cut out what is stopping the saw from cutting. The farther away you are from 90 degrees, and the thicker the material is, the more times you may need to do this. You willend up with the perfect shaped ellipse with the perfect bevels all around. Practice first in a block of scrape wood until you get the hang of it. Take your time and enjoy the new experience.

Now you can use the completed hole to make a paper template for the inside hole for your gasket. Do this by laying a piece of regular paper over the hole. Tape it in place. Using the side of a pencil, rub the pencil around the edges of the hole. The edge of the hole will make a nice dark line. Do this all the way around. Cut out the center. Slide it over the hardware. Use the flange as your guide for the outside edge for your gasket. Cut it out and you are ready to go. Have fun with it.