In-Hull Transducer Issues

[Tom.B]

I've been having some issues with our in-hull transducers and thus, depth sounder problems.

I have just bought a new Humminbird HDR 650 and a dedicated shoot-thru-the-hull XP920 transducer to replace the old one. A simple unit I know, but it's really all I need. We have two. One on the flybridge and one at the lower helm. Each has its own transducer and neither operate at the same time via a 3-way power switch. We also have a solid, non-cored hull.

In the past, the old unit would, apparently, not be able to make an accurate reading and would only flash the last recorded depth until it "recovered". After a few basic trial and error tests, I never had an "ah-ha" moment and figured that since it was cheap enough, I'd just replace the flybridge unit, get a dedicated transducer, and task the unused one to the lower helm.

After the install of the dedicated shoot-thru-the-hull transducer, the same problem occurred. Frustrating beyond belief. I even tried a couple of different spots with the same results. After a few streams of loud cussing ended, I decided to try the transducer that came with the unit. I mounted it with toilet wax in the same spot and it seemed to work. Not perfectly, mind you because it does still have some flashy moments in shallow water, but it's better.

Now, here is the thing. I think it's happening becaue of the blister job we had done over the winter. I am wondering if there was a blister under the old transducer and when I mounted the new one, either there is maybe a bubble in the epoxy barrier coat or the epoxy itself restricts the beam. The bottom was covered with small blisters and we sand blasted and coated the bottom (not the BEST solution, but it's what works for us).

Am I missing something? Is transom mounting an exercise in futility on a 35' trawler? Seems to me that the more forward you mount it, the better. But then again, if you go into shallow water at 7 knots, by the time your sounder goes off, you'll be unable to stop your boat in time to avoid hitting bottom. I have also heard that the prop spins out so much wash, there are too many bubbles in the water to get a reading.

(as you can see... I have a lot on my mind about this )

Tom-
m/v Skinny Dippin'

[noelex 77]

If you put the transducer on top of a sealed plastic bag filled with water and move it around you can find the best spot, but the return and therefore the performance is rarely as good as an external transducer.

I think the depth sounder is one of those near vital bits of equipment that should be performing its best, so prefer an external transducer. In fact I have two so there is always a backup.

[Tom.B]

I am thinking that maybe it's time to just bite it and put a proper thru-hull one in, but that's lots of effort and cost I would rather not bare.

[Lake-Effect]

I used noelex's trick with the water-filled baggie to find a good spot, then I made a sort of tank out of PVC pipe, and the transducer sits in some water in that lil tank. Works fine, and yes i've remembered to remove it before winter I've not had good luck with epoxying transducers; and I am a bit suspect of the toilet-ring wax too.

Diagnostic steps:
- since your transducer's loose, dip it over the side to confirm that it still works fine
- try the baggie trick to check the location

Drilling and mounting an external transducer isn't that hard. I'm certain MaineSail or another sage has detailed it somewhere. If you're still concerned, a good tech should be able to drill and mount a transducer properly in about an hour, if access is good and the boat is on the hard and dry.