Reducing size of thru-hull sonar transducer

[bikesherpa]

Hi,

I must replace a very old sonar thru-hull bronze transducer on my GRP sailboat. The old transducer has a 26.5mm diameter hole (1+1/16"), the new transducer (also bronze) will require an approx 22mm hole.
What is the best way to do it? I was thinking of 3 options:

1) Ask a machining shop (a friend of mine) to create a bronze threaded adapter (22mm inside, 26.5mm outside). The adapter would have a very small thickness, so I am not sure about the reliability (cracking, etc.)
2) Fill with epoxy (with glass micro-spheres) the current hole and re-drill the hole of the correct size
3) Just fill the extra void with sealant (3M 5200)

Any advice would be welcome.

 

[lw395]

The question is how big is the flange compared to the old hole.
And how oversize was the old hole.
2.25mm of sealant does not sound excessive.
You might put a piece of tube, or o ring etc in there, just to make sure the transducer sits centrally in the hole?

 

[prv]

The proper way is to fill the hole using the classic tapering-layers-of-GRP approach, and then drill a new one. But that's a lot of effort. I like the bronze sleeve idea.

Pete

 

[pvb]

The question is how big is the flange compared to the old hole.

This is the crux of it. If the new transducer's flange overlaps the old hole by, say, 3 or 4mm it should be OK. Adding some sort of tube or bush to centralise the transducer in the hole is a good idea. There's effectively no water pressure on the transducer, and if you also use an adhesive sealant it's not going to fall out in a hurry.