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i28 hull thickness
Out depth sounder had stopped working and its installation had begun leaking. I found hull thickness of 1/2"-5/8" in the flat area just aft of the keel stub, a bit more where the depth sounder was installed under the galley sink. The side of the keel stub where the speed is installed has more material, perhaps 3/4"-1".
I did this project two years ago, you may find this helpful.
You should try to have the outer and inner hull surfaces parallel to provide even clamping of your new thru-hull fitting. You may want to include a scrap of marine plywood and belt sander in your tools for this project.
I found a loose scrap of plywood and a bunch of dried out (and weeping) stuff in the old depth finder original installation. Their installation was NOT a good effort and the transducer nut was not clamping evenly. After removing the hardware, clean the area well. This is a must before you sand anything. Sanding will spread around any oil, grease, soap or waxes so clean first.
After removing the old fitting and cleaning the area, sand about a 6"x6" centered on the existing hole. Then to make a form fitting block, I nailed a piece of scrap 1/2" marine ply (thin nails) to a larger chunk of wood. Then I was able to belt sand (the nails will wear off as you go) the necessary curve to better fit the hull. I tapered the outer edges of the block to help avoid making a hard edge. This would help lay glass down if you decided to add a top skin.
I had a helper run a construction screw through a hole in a block placed outside the hull, into my leveling pad to clamp & hold it in place till the epoxy cured.
I used West System epoxy and filler 403 or 404 to attach the block and coated the cabin side too. I don't remember if I covered it with a layer of glass but it does seem a bit like overkill. My new sender was a bit smaller then the original. The flange on the transducer head was still larger than the hole but I wanted more boat and less sealant....
With the block installed as a backing I was able to mix some very thick epoxy and build up the hole. Messy is OK. If you have to do all this you'll run a hole saw through both the block and the filler.
I used a burr file in a drill but a round or half round file would also work well to chamfer the inside lip of the hole. The chamfer helps hold a bit more sealant where you want it most and will reduce the shearing pressure on the head as you tighten the nut.
I gave the finished hole including the installed block a coat of epoxy. Let this coating cure before you install sealant. Raw epoxy and sealant don't seem to cure well together. Let the epoxy cure before installing with sealant.
I used Boat Life's Life Caulk. I like the way it adheres, cleans up and stays a bit flexible. It also will come apart without resorting the dynamite...