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Catalina/Capri 25/250 Sailor's Forums

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Proper way to bed knotmeter transducer

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Dan Greer
1st Mate

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USA
36 Posts

Initially Posted - 01/26/2009 : 18:54:18

Odyssey's previous owner installed a knotmeter transducer under the quarterberth, about 18 inches behind the trailing edge of the keel, and about 8 inches off centerline.

It's not the recommended location, but it seemed to have worked well for the last 20 years. When I had to replace the knotmeter, I figured I'd use the same location.

When I got the old thru-hull out, I was surprised to learn that my hull has a balsa core down there.

The balsa was nice and clean and white and dry. The old mushroom thru-hull bedding had done its job perfectly.

The new thru-hull is not a mushroom, but a tapered flush fitting. I bedded it with Marine silicon from West Marine. (It says its good to use below the waterline.)

The thing is - by its structure the old fitting had maybe 3/8" of bedding compound between the balsa and the water, but the new tapered fitting has maybe 1/8" of bedding compound (silicone) between it and the water.

If the hull were solid glass, I wouldn't worry. If it doesn't leak into the boat, it's good to go.

But with the balsa core I worry that water might get into the balsa without being apparent to me from inside.

Should I have sealed the balsa with epoxy or something before I bedded the thru-hull?

Dan Greer
1989 C-25 SR WK
Phoenix, AZ

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Deric
Captain

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USA
408 Posts

Response Posted - 01/26/2009 : 19:09:53

Dan,

I am throwing out an idea. I wonder if you could grout out the balsa, enough to fill the area with epoxy in order to create a solid structure for your thru-hull.

Deric

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 01/27/2009 : 12:23:54

I can not believe the Catalina 25 hull is balsa cored. I've never heard this before and find it very unusual. This is only done in high end race boats, and any kind of coring below the waterline is very risky. If any water at all gets in the core the boat can be ruined.

My boat is full of plywood core (deck, transom) but I'm sure the hull is solid glass, maybe some plywood around the keel mounts.

I'd like to see a photo.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 01/27/2009 : 19:13:28

I've never heard of Catalina using balsa below the waterline--it's never been there style. I suspect that what you're seeing is something like Coremat, a "fluffy" polyester material with microspheres embedded in it. It absorbs resin, but remains a lightweight whitish layer in the layup--always surrounded by layers of woven roving and/or other flat mats. I don't know whether Catalina used Coremat in 1989 (or uses it now), but I really doubt that they used balsa down there. I know virtually for sure they didn't in mine, but it was a 1985 fin.

So, if I'm right, what about that layer of "core"...? I would put a bent nail in an electric drill, use it to carve out a quarter to half inch of that "core" between the outer laminates, and make sure your silicone fully fills that area as you install the transducer. Why not epoxy before installing the unit? Well, if you prefer epoxy, go for it. But if the core actually was balsa, I'd be concerned that the slightest crack between the epoxy and the hull laminates might allow just what you're trying to prevent.

By the way, regardless of what that layer is, you don't want water migrating in between the fiberglass strands in the roving, which is a prime culprit in delamination. The first and most important line of defense is the bedding of the transducer on the outside of the hull. The second defense is sealing the inside of the hole with something--epoxy or a suitable below-the-waterline caulk.

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Dan Greer
1st Mate

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USA
36 Posts

Response Posted - 01/28/2009 : 01:55:01

Thanks very much for the good info, guys.

I'm gonna pull that thru-hull back out and follow Dave's advice to ream some of the light stuff out and then work some silicone into the space where it was.

I'll also try to get some good pictures at that time. It will be a couple of months before I get back to it, but Odyssey is sitting safely dry on the trailer.

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britinusa
Web Editor

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USA
5404 Posts

Response Posted - 01/28/2009 : 06:05:32

As you are pulling the plug so to speak, I would read Don Casey's book on [url="http://books.google.com/books?id=isEFQaXQ9vwC&dq=don+casey+hull+and+deck+repair&printsec=frontcover&source=bn&hl=en&sa=X&oi=book_result&resnum=5&ct=result#PPP1,M1"]hull and deck repair[/url]

Paul

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DaveR
Master Marine Consultant

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USA
2015 Posts

Response Posted - 01/28/2009 : 07:23:46

Isn't a non-mushroom thru hull a bad idea?

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 01/28/2009 : 08:36:22

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by DaveR
 Isn't a non-mushroom thru hull a bad idea?<hr height="1" noshade id="quote"></blockquote id="quote">If the "tapered flush fitting" has a flat "head" (like a nail) that is tapered on its edges such that the outside has a slightly larger diameter (like the valve in an engine), it is meant to be countersunk slightly into the hull so that it is close to flush. That still means that there is an external surface securing it, just as a mushroom does, as long as the hull is thicker than the countersink. My 1985's thru-hull drains had countersunk flush heads like that--perhaps for trailering purposes.

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.