Seems like every project I do involves drilling a hole. But this is the first one I’ve done that could let water in.
Bella Star is currently receiving an electronics refit. We’re installing a new chartplotter, HD radar, anemometer, transducer, secondary instrument display and SSB radio.
The depth sounder that came with the boat usually works fine. But even with a clean bottom it frequently reads irregular and alarmingly shallow depths when we have 500 feet of water under us. This one will read speed through the water and temperature as well as depth. Also it will be networked into the chartplotter, so we don’t have to have a separate display for it. Should just be a matter of taking out the old transducer and screwing in the new one, right? Easy cheesy. As it turns out, I had never really looked too hard at the old transducer and just assumed that it was a through-hull fitting.
So with the boat hauled out I got ready to make the switch, only to find that in fact there was no through-hull for the depth sounder. It’s just bedded in a big blob of epoxy to the inside of hull.
We decided to go ahead and put the new transducer in anyway. So out came the hole saw.
The old transducer is located almost all the way aft, pretty close to the prop wash. I wanted the new one up forward, where it would be easily accessible and get an undisturbed flow of water over it. The prime spot is already taken by our forward looking sonar transducer on the starboard side (which we’re happy with and want to keep) so the new hole was going in approximately the same location on the port side.
So here’s how I did it.
1. Drill a small pilot hole from inside the boat. Find the hole outside the boat and make sure the location looks good there as well.
2. Drill 2” vertical hole. I drilled it half way from the inside and half way from the outside to get a clean cut on both sides)
(I do recommend doing this before the beer break. And if you drill the hole in the wrong location, don’t sweat it. Just keep drilling them till you get it right.)
3. Build fairing blocks so that the transducer points straight down, at least when the boat isn’t heeled.
I used two sheets of 1/2” StarBoard marine polymer with the recommended epoxy to glue them together. I built several of these blocks anticipating screw-ups.
4. Cut the 2” hole in the blocks and slice them in half at the same angle as the deadrise in the hull. (I used a piece of cardboard held against the hull and a bubble level to find the angle)
Here’s the one for inside the hull. This ensures the locking nut on the through-hull has a flat surface to mate to.
For outside the hull I made four with slightly different angles, knowing full well if I only made one it would be off by 1.4 degrees. Unacceptable!
5. Trim down the fairing block.
I anticipate that the efficiency of this fairing block is so hydrodynamic that it will actually add .25 knot to our boat speed!
6. Sand to bare gelcoat around the hole.
7. Using appropriate epoxy I set it up on the hull with two wooden blocks bolted from either side to hold it in place.
(If Seaview Boatyard asks what happened to one of the legs of their office chair I don’t know anything about it.)
8. Mount the through-hull into the fairing and hull with adhesive sealant.
9. Paint with antifouling.
Since we don’t have any of the electronics installed yet we just inserted the blanking plug before we splashed. The idea is you can pull the transducer out if you’re not going to use the boat for awhile and put a blank plug in the hole. This will help to prevent growth on the transducer and also allow us to clean it. You do this while the boat is in the water, so I would think it would be a pretty wet process changing them out. According to the instructions, “With practice this can be done with only about 10 oz of water entering the boat.” I’ll be interested to see what the “without practice” volume is.