Fixing a transducer thru-hull, advice needed!

[bigd14]

I am trying to figure out how to fix a transducer hole. The old speed log was a flush mount, which sat in a recessed cutout (see photos) The new transducer has a larger diameter body, but is not flush mount. Compounding matters, the interior of the boat around the speed log has silicone contamination, which i am concerned would prevent a good bond if I were to try to completely patch the hole and redrill it later.



So I have come up with the following approach.

1. Clean silicone contamination as best as I can.
2. Sand off bilge paint (trying not to contaminate underlying FG)
3. Epoxy G10 backing block in place over open hole
4. When backing block is cured, drill 1/4 inch guide hole in center
5. Place circular layers of 1708 and build up flush with hull.
6. Sand and fair flush with hull
7. Barrier coat
7. Redrill guide hole from inside of boat.
8. Using guide hole as center, drill new transducer hole through layers of 1708 and backing block.
9. Install transducer



I figure if the backing block does not bond too well it wont be the end of the world as the sealant and pressure of the thru hull should keep it together.

Does this sound OK? Will this give me enough strength around the existing hole? Or is there some other method I should consider?

Thanks!

Doug

[Triton106]

Doug,

I had a similar issue a while back with the head thru hull (not a transducer). I fixed it the way you described here. In hindsight I would not do it that way. I would just close off the existing hole and drill a new hole nearby. It's a better fix and easier too.

An issue I had with my Triton is that the inside of the recessed hole is not even with but stand proud of the surround area. This made adding the backing plate challenging. I had to grind it as even as possible with the surround area which took a long time. This does not appear to be an issue based on your drawing. Nevertheless, I still think closing the existing hole and drilling a new one is easier.

Ray

[Rachel]

What about patching the hole from the outside as you would to fill it in, and then re-drilling? That way you wouldn't be trying to get epoxy to stick in the contaminated area (except for the initial plug, which I don't think is really doing that much by the time you are done).

In other words, grind a 7" or so tapered circle on the outside and then fill it in from the outside with, say, three layers of biax 1708.

The reason I'm suggesting this is that I used this method to fill in a 1-1/2" through hull hole before re-drilling a 3/4" hole for a new, smaller through hull (not for a transducer but for a seacock).

I started by taping the hole from the outside and filling it from the inside with thickened epoxy. Once that had cured I ground the taper into the outside and then filled it with the concentric circles of cloth, then faired it. Then I drilled the new hole (which, since I had filled the old one, I moved over an inch or so to make better use of space inside the boat).

Rachel

[Hirilondë]

If due to silicone contamination you question how sound the bond of the G 10 will be then you really have to consider that it should not be considered a structural component of your repair. If this is so, then the glass/resin work should be structural. If the glass/resin work is structural, preferably as described by Rachel, then why include the G 10 at all? You can always use a piece of plywood or cardboard covered with wax paper and weighted in place as a mold during the glass/resin work to keep smooth finish on the inside.

[bigd14]

Good points all. I will go the traditional route and grind out a patch on the exterior, with a temporary block on the interior to preserve the surface. I was hoping to avoid any deep grinding, but I guess its better safe than sorry.

Thanks!

Doug

[barrybrown]

If I understand the problem correctly the new sleeve is larger than the old one, much easier to make the hole bigger than smaller. I have had to do this many times, just recently on my boat.
This should be easy, cut a slightly tapered plug from a 3/4" pine board and tap it tightly into the hole from the outside then center the pilot bit of your hole saw into the plug and drill the new hole from the outside. If part of the bevel is still showing it provides an excellent place for the bedding compound on the new sleeve.
If I have misunderstood the problem please disregard the above.

Barry

[bigd14]

Barry Yes, you have characterized it correctly. However I would be slightly worried about not having entirely solid material surrounding the edges of the plastic thru hull flange. I will have to check how much space there will be, but it seems like it will be about 1/4 inch all the way around. I will play around with it next time I am at the boat then make a final decision.

Thanks,
Doug

[bigd14]

OK, looks like I was wrong about the size of the hole. i took the transducer out about 6 months ago, and thought it fit tightly. This was an accident waiting to happen!



The new transducer fits loosely in the hole with about 1/8 inch all around it. Is this tight enough? In the photo the transducer is pushed all the way against one wall so the gap looks bigger than it is. I think I want it to fit very snugly. I may try to clean the inside of the hull off and do the patch I originally proposed, since the keel line will prevent me from getting a good bevel when patching this area from the outside.