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 Speed/Depth transducer location

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Volksaholic Posted - 02/20/2023 : 22:03:11
My '81 C25 has an old-school SR Mariner knotmeter and gimballed compass with no other instrumentation. The transducer for the knotmeter is installed under the starboard quarter berth with what appears to be a 2-1/8" hole for the housing. I bought an Airmar DST810 which requires a 2" hole and recommends installing forward of the fin keel. I'm wondering if installing where they recommend is worth the extra work of dealing with the old transducer location for whatever gain in accuracy I might get.

Theoretically I like the idea of having the depth sounder ahead of the keel so I can see what I'm going to run into rather than aft so I only see what I already hit, but practically it probably doesn't make a difference. Alternately I could probably use the existing 2-1/8" hole as is or build it up enough to make it 2" per the Airmar instructions.

If I move the transducer I either need to put the housing plug in permanently, think of something else to use that hole for, or glass it in. I've still got keel filling/fairing and bottom paint to do so maybe if I want to patch the hole it's probably the right time.
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Volksaholic Posted - 02/24/2023 : 23:13:18
Thanks for the input. I'm on the fence between drilling the new hole and patching the old one or taking the simple path of reusing the existing hole. After pondering it, leaving the old through hull fitting with a plug leaves me a little uneasy. I don't know how well that plastic holds up, the boat's been on the trailer for about 6 years so the fitting's been exposed to UV, and I'd hate to bump and break it on the water if it's gotten brittle with age. As I mentioned, I've got some keel rust repair/filling/fairing and paint to do so I'm going to have epoxies, mixing containers, spreaders, squeegees, and sander on hand and I've got more than enough glass cloth. If I think I'm making good progress toward getting on the water I might drill and fill. If I'm in a crunch as I usually am, I'll reuse the old hole. The lake is seasonal anyway, unless I want to put skates under her, so I'll have other opportunities when I pull her in the fall or before launching next spring to drill & fill if I use the existing hole and decide I don't like it.
Leon Sisson Posted - 02/23/2023 : 20:39:46
Yeah, what they said.

Best quick&dirty solution for now is reuse existing transducer hole and let bedding compound make up the difference in diameter.

As Steve said, it's a lot easier to do more fiberglass work if you're already doing some anyway.

I installed sonar transducers under the V-berth in two Catalina sailboats.  The only minor location-related problems I've had there are occasionally the depth reading gets confused by aerated water from engine in reverse or whitecaps.
bigelowp Posted - 02/23/2023 : 13:39:16
I filled the hole and mounted the transducer inside the hull for depth, and I use a portable GPS for speed. I believe the fewer thru-hull "holes" the better.
JB Posted - 02/22/2023 : 13:16:51
Personally I would reuse the old hole.
islander Posted - 02/21/2023 : 09:56:27
The difference between the two holes is 1/8". That would mean you would have a 1/16" gap around the sensor. It's really not that big and can easily be filled with the bedding compound if you decide to reuse the existing hole. My depth sounder sensor is in that same spot under the quarter berth and it works very well and rarely if ever loses bottom lock. This is just my opinion but I think having it farther aft gets the sensor out of the air bubbles coming under the bow and into a cleaner stream of water. Again just my opinion.
Steve Milby Posted - 02/21/2023 : 07:00:02
There's no perfect solution. One of the choices that you're considering is to do fiberglass work to reduce the size of the existing hole and then install the new transducer in that location, which is not the recommended location. Alternatively, you propose to cut a new hole in the recommended location to install the new transducer, and then repairing the old hole. The third alternative is to cut a new hole in the recommended location to install the new transducer, and then put the plug in the old hole. The third choice makes sense to me. It puts the new transducer in the correct location. It requires no fiberglass repair work. The plug in the old hole can last indefinitely. You can go sailing. If, at some time in the future, you have to do a significant fiberglass repair elsewhere on the boat, you can fill the old hole at that time, while you have all the materials and equipment out anyway. The boat will sail just as well if you repair the old hole or just plug it. The third option would be my choice.

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