hi, I was reading the thread on cutting a larger thru hull hole for a fitting which brought up a guestion I have. Do I need the backing plate for support of my depthfinder thru hull fitting? I drilled the hole from the outside as my hole saw attachment for my dril wasn't strong enough and wobbly tocut through the acking plate first. Cutting through the hull the plate popped off. The hole was the correct size but ragged. Trying to reattach it meant cutting the subfloor to fit both the plate and large plastic wingnut through the floor. I sadler the fitting with 3M 4200 sealant and tightened the wingnut to the hull. It is a wide(3 inch edge) fitting. I plan to launch my C&C 24 and test it as I have to test my meters as new power and transducer cable were installed for both depthfinder and knotmeter. Will it HD or should I reinstall with backing plate? Thanks,ahab211

Yes, you really should have a backing plate on the through-hull. Ideally, it should be a fiberglass backing plate that is epoxied to the hull.

Thanks, I will reattach it. Luckily, it's a removable transducer cable fitting so I should be able to drop the plate over the fitting and tighten it to the floor eith the wingnut using epoxy to seal it.

While the backing plate should be epoxied to the hull, the through hull itself probably should not be.

I tried to work the wingnut off but it wouldn't budge. I must have got some 3M 4200 sealant on it from the fitting. It must have oozed through the hull when I tightened it. I tried using a knife to cut away any sealant on it then used a pry bar hitting a hammer against it to loosen the wingnut. Nothing moved. The only way i'll get this off is to destroy the fitting in the process. I might as well leave it as is. I don't think I'll have a problem with leakage as it is solid as a rock. It is a removable transducer so don't have to worry about that as I can put in the dummy plug and send it in if it needs replacing or repair. I'll call the manufacturer Signet Marine tomorrow as they are sending me some new O rings and might advise me further. Any other suggestions? Thanks Ahab211.

Just watch it closely when you splash the boat.

Thanks

I haven't looked at depthfinder installations in that many boats, but I don't recall ever seeing a backing plate used in that application. This advice comes 4 days too late to matter for me, just giving me one more thing to worry about! I wonder, though, how many others have installed these without any additional backing?

The backing plate I had on was at least two to three inches thick. A trusted marine supplier today told me he has a resolve 2000 that will remove the 3M 4200 sealant from my transducer but will check with a local sailing consultant on the thickness of my hull as he thinks I don't need a backing. he'll let me know tonight as I need VC17 to treat my hull and I will be dropping in anyway. I get good advise on this site too bad sometimes it's after I made a blunder. I think most things I've worked on this boat have been trial and error, Good thing I've only about $4000 invested as this is my fourth sailboat and I've always got my money back. I was offered $5000 for it a coule years back but like the way this C&C 24 handles.

A lot of times, especially if the knotmeter or depthsounder is installed by the builder, the backing plate will be incorporated into the hull as a thicker section laid up for the transducer through-hulls, so it may not be apparent. Also, on some boats, especially older ones, the hulls are thick enough that a backing plate is superfluous.