Back Up Sounder

Hi guys!
The old original Dart sounder (1988) finally died. (Bet no one gets 22 years off today's electronics!) My main sounder is a Garmin mounted on top of the console. Works fine.
The Dart is like a thick panel mounted flat (horizontally) near the throttles. A neat installation. I think that I would like to install a small Uniden sounder in this location and go with an "in-hull" transducer - the kind that is not "thru hull" but shoots trough the hull.

http://www.uniden.com/products/produ...6&filter=Depth

The boat does not need to come out of the water at this time and I would hate to haul it unnecessarily to install a thru hull transducer.
What am I doing wrong (again)?

I've put xducers in in the water. its not a real problem. You drill the hole from the inside when the drill goes thru you drop a string with a weight thru the hole and jam a rag in it. Reach under the boat with a long boat hook or gaff and snag the string. Tie the xducer wire to the string coat the flange with 5200 and pull it thru the hull. thread the nut on and hold the stem of the xducer with something to tighten it up. Not that much water comes in the boat.

I put in an in-hull (shoot-through) transducer/Raytheon ST40 as a back up to the garmin chartplotter through-hull on our 53MY. Works great and I heartily recommend it.

I Have Had Bad Luck With Garmin Shoot Through The Hull Transducer
Had It Professionally Installed Last Spring And It Worked Ok Unless You Really Needed It And ThEn Crapped Out. I Am Having A New Thru Hull Type INSTALLED This Next Haul Out.

I tried the same thing using a Hawkeye sounder as a back-up. It would not work, shooting through the hull.

Airmar is the company that makes most (all?) of the transducers used by sounder/chartplotter manufacturers. The Airmar P79 is a purpose-made shoot through.
http://www.airmartechnology.com/airm...=All&PageNo=99

I guess at some point hull thickness would be an issue but the 1" thick hull of our 53 MY is not a problem at all. The shoot-through and the through-hull agree within a couple tenths of a foot at all depths we have encountered in the Ches Bay. It's critical to set it up properly and have the tube full of liquid - non-toxic antifreeze is recommended by Airmar.

I mounted the transducer about a foot away from the through-hull transducer to ensure they both saw the depth from the same place. ALSO, the frequency of the transducers is important. I don't recall the specifics now though I read it thoroughly back when I installed them, but there can be an issue with the frequency of the two transducers (in hull and through hull) interfering so that you will lose the reading. I can't remember if they need to be the same or different but you can find that info on airmar or other sites.

As I said, the Airmar P79/ST40 works perfectly on our 53 and I plan to install another one so I have a back-up sounder at both helm positions.

I had a "shoot through the hull" transducer put in to use as a digital back up for my primary bottom machine. It has been about a year and it works great. Never had a problem with it not reading properly. We mounted it in a small resevoir which you need to keep filled with liquid. ( I just use water) I do have to occassionally top off the fluid level due to evaporation.