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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 03-30-2010
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Making Through Hulls larger?

I've decided to replace an old flush mount depth transducer with a new mushroom style extractable unit. I'm looking for any hints for drilling out the hull to accomodate the larger transducer. The hull is solid fiberglass I think (1989 Catalina 36) and I am hoping to leave the transducer in the same space due to the liner and access to it. The current hole is 1 3/4" - new transducer requires 2". Guiding a hole saw into an existing hole has always proven to be difficult for me. Your thoughts? Drilling, sealing, bedding?
Thanks
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Last edited by bakerha51; 03-30-2010 at 09:37 PM.
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Old 03-30-2010
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we just talked about this a few weeks ago. my vote is a piece of ply wood in side or out and use it to guide the hole guide bit then the hole saw.

to hold the ply in place there are lots of options, i think the best is hot glue. easy to do, should hold well and be easy to remove. another really good idea is to make a new backer plate for inside the boat and epoxy it in place then drill both the plate and the hull in one shot
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Scotty's is a good idea, or you could plug the hole with a wooden cone/dowel (like the ones they use for emergency hull-breach repair). If you drive it in hard enough and cut it flush to the hull, you may then be able to get the pilot bit of a hole-saw to penetrate well enough into it to start the teeth of the hole-saw to cut the desired diameter without the plug spinning loose.

Good luck.
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Old 03-31-2010
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hole saw guide

If you have a hole saw that fit snug inside the existing hole (in your case 1 3/4"), you can install that one inside the one you intend to use (2"). The smaller one that is install last will be a little proud of the other on and can be use as a guide. Once deep enough the guide can be remove to prevent it to become over tight.

Good luck
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Old 03-31-2010
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The conventional logic (and probably simplest method) is to plug the hole then redrill it. Get a 1-3/4" hole saw, cut a plug from scrap wood, glue it in place. Scribe the center carefully, come back after the glue has set up hard and drill the 2" hole as if it was new wood.

If you get them from a lubberly hardware store, the drill-saws shouldn't be expensive. The cheap ones are designed for cutting wood and will do quite well enough, the expensive ones are for installing lock sets in steel-jacketed doors or for commercial use, and hopefully you won't be making THAT many holes in the boat.

Of course if you prefer elbow grease, you can use a sanding wheel with coarse paper, or a wood rasp, and just chew out the existing hole 1/8" deeper in all dimensions. But that won't quite be round, unless you're a very very good worker.

Expect surprises--like wood core that may need sealing. And if you drill through from the OUTside instead of the inside, you'll get a cleaner cut in the gelcoat on the bottom.
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All - great input. I really like the hot glue idea and if I can cut a piece to fit the hole with a hole saw - it will already have the center hole set to use as a guide.

Interested to see if I can make the two saw idea work too. Amazing idea stefbro!

I'm planning to Epoxy coat the hole and 5200 the housing in place.

Thanks for the advice guys - I'll touch back when things are completed to let you know how it went.
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Old 04-24-2010
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Well here's the skinny. Turns out that the existing hole was over-sized for the transducer that was installed. More like 1 7/8 than 1 3/4 and beveled to accommodate the flush mounted transducer - not leaving much mating surface for the new housing. So - I am having the hole glassed over and straight drilled for the new 2" transducer. Would try it myself but the 450 mile trips to the boat get old (and expensive) so having the yard do the work is more timely and could be less $. Any way - hope to launch in 2 weeks and wanted to get it done. Thanks for all the ideas - I'm sure this info will come in handy for future jobs.
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Old 04-24-2010
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There is a product called the Starrett 'oops arbor' that is made for this, it is a cheap solution and enables a hole to be opened up by using a hole saw sized to fit the original hole as a pilot. Toolmonger » Blog Archive » Enlarge Holes With The Starrett Oops Arbor

Last edited by r.furborough; 04-24-2010 at 08:12 PM. Reason: correct typo
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