Backing plate and transducers

[Cdn_bacon]

First let me say this is my second season with a sailboat (Cal 33) and I’m learning as I go. I’m getting ready to launch and decided to replace my non-working instruments with new Raymarine (spd/depth transducers) and reuse the existing holes from the old.

There’s a bit of slop in the fit of the new instrument thru hull housings (few mm) but I’m ok filling it up with some extra 4200 when I bed them. I am looking for some advice though:

1.) is a backing plate required? The outside and inside surfaces are pretty flat although the old transducers had teak backing plates. If yes, I have some 1/8” and 1/4” G10 available. IF no, I’m convinced the locking nut would sit flush to the inside of the hull.

2.) there is no strength required for the housings, they just need to sit flat so I didn’t want to epoxy the plates to the hull (I’m concerned with botching the job and getting the plastic thru hull stuck with epoxy accidentally before I can get it out and bed it). My thought was just to use 4200 under the G10 and bed the plate and house if all in one go. Any issues with doing this? Worked for the last set of instruments with teak backer for 36 years (factory installed).

3.) if I were to epoxy the backing plate, what’s the process to get the plate on and aligned with the hole but not getting the plastic jousting stuck?

Any advice helps. Thanks.

[tkeithlu]

My concern would be the fit of the through hull nuts if the inside of the hull is rough or curved. The backing plates may have served to give those nuts a smooth and flat sealing surface. You might consider using 5200 to glue in a backing plate, and then a removable goo such as 4200 to seal it all up when you tighten the nuts.

[Cdn_bacon]

Originally Posted by tkeithlu

My concern would be the fit of the through hull nuts if the inside of the hull is rough or curved. The backing plates may have served to give those nuts a smooth and flat sealing surface. You might consider using 5200 to glue in a backing plate, and then a removable goo such as 4200 to seal it all up when you tighten the nuts.

Thanks. Why 5200 for the backing plate vs 4200?

[tkeithlu]

5200 is permanent. I'm proposing that you use it to make a bed for installing the through hull. 4200 is tough, but not quite permanent. I'm proposing that you use it anywhere you might want to disassemble the sensor in the future, such as replacing the speedometer.

Good luck with your project. It's an opportunity to do some pretty work.