Optimum Depth Sounder / Transducer hull location

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Optimum Depth Sounder / Transducer hull location

Postby epocat » Fri Oct 12, 2012 11:03 am

Ahoy Fleet 1,

New to the community and owner.

I recently purchased a 74 SJ21. It was never rigged for a depth finder.

I will be sailing the shallow waters of Biscayne Bay which averages about 7 feet.
I have been reading about fishfinders / depth finders / transducer for sailboats and the concesus seems to be, transom mounted fishfinders should be avoided. For sailboats, the choice seems to be a flush mounted Transducer.
Having said that, can anyone suggest where in the Hull would be best place to install it. Also any suggestions of the equipment would be appretiated.
I am thinking about getting a $99 transducer and installing it on a nylon in hull mount $99.

Thanks !
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Joined: Fri Oct 12, 2012 10:33 am

Re: Optimum Depth Sounder / Transducer hull location

Postby jeffd » Mon Oct 15, 2012 4:55 pm

Hello, and welcome to SJ21 Sailing! Here are a few things I've learned about Depth Sounders:

I use a Humminbird piranhamax 160 (about $90), with the stock transducer (the one that comes with the depth sounder), with the display mounted on a piece of wood. I am kind of a cheapskate, and used to (27 years ago!) have an old "flasher" depth sounder (the kind with the rotating light), but it finally quit working, so I "upgraded" to a solid state fish finder, with no moving parts. I basically bought the low end (cheap), mass produced fishing type, but it gives me the depth in feet, and a graph as a bonus. Side benefit of being portable: I can bring it inside, and when I'm at anchor, I can keep an eye on the depth - like when the tide goes out, or maybe the anchor drags! I can also put it inside the companionway, under cover, when I'm getting splashed by saltwater. And I normally remove it from the boat during the off-season (like now). Here is a photo of my cheap/portable mounting:

DS3.jpg (112.7 KiB) Viewed 1278 times

I agree with you that you don't want to mount the transducer on the stern of the boat. You want it AHEAD of the keel, so you can see what the depth is AHEAD of the keel, so you know BEFORE you run aground, not AFTER you run aground!

I used the transducer that came with the sounder, and mounted it in the well, below the V-Berth. I heard the trick is that the sounder must not have any "air" between it and the water. I learned of a 'trick' - fill the well below the V-Berth with a bit of water, enough so the transducer put into the well is under water (no air between the bottom of the transducer and the hull). Then, test the system, with the boat in the water, maybe at the dock, or someplace where you can drop a lead line to check the depth. The risk involved in this test is if it doesn't work, then, maybe you just spent money on a sounder that won't work without more $$ for a transducer, or maybe its the wrong sounder. I read one idea - if the "core" between the inside liner and the hull is making it so the sounder won't work, you can cut a hole through the inside liner, then remove a bit of the the core, and epoxy the transducer to the inside surface of the hull - although at that point, maybe its easier to just cut a hole in the bottom.

Years ago, I did this test (water in the well, to cover the transducer) with the flasher, and it worked, and it was accurate, so I just went with it. I secured it with Silicone Seal, maybe 1/4" thick or so, right in the middle, and made sure there were no air bubbles in it. I also secured it with the mounting brackets, although I think that is unnecessary. After many years of good use, and not good maintenance, the flasher finally quit, so I bought the Humminbird. I removed the old transducer, and installed the new one. That was about 5 years ago. I don't remember why I epoxied it it, after having to remove the old one. Here is a photo of the transducer in the well, below the V-Berth:

DS1.jpg (95.73 KiB) Viewed 1278 times

Here is a closeup of the transducer, mounted in fiberglass epoxy (with the old holes patched up). I apologize for the dirty appearance - I have a leak problem and haven't figured out where it's coming from, so the well gets flooded regularly.

DS2.jpg (81.99 KiB) Viewed 1278 times

I'm pretty happy with the setup. It gives me a good indication of the depth, and it is pretty accurate - the keel bumps the sand, and I glance at the sounder and it reads 4 feet. The display mounting could be better, but I haven't figured out a better way. I don't really want to permanently mount it outside, to the boat, and I don't really want to cut a hole with a window and mount it inside. A 2" round depth sounder or a smaller rectangular one that mounts on a bulkhead would probably be better, but they seem to cost a whole lot more. I guess I don't like to drill holes in the boat, unless there is a good reason to, although I guess most depth sounder installations involve cutting a hole in the hull.

You probably already know this - Having a depth sounder probably will not prevent you from running aground. I've run aground many times. I guess I kind of enjoy 'gunkholing' - exploring shallow lagoons and bays, and end up finding the bottom often (I probably shouldn't post this - no bareboat charter will ever consider renting to me!). But that's the beauty of a swing keel - its easy to get free.

I hope that helps.

Jeff D.
Last edited by jeffd on Wed Nov 04, 2015 11:48 pm, edited 2 times in total.
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Re: Optimum Depth Sounder / Transducer hull location

Postby Dark'n'stormy » Fri Jan 31, 2014 5:14 pm

I too am reluctant to cut a window in my boat for my chart plotter/gps/sonar. I have an hds5 which is kind of big...

Has anybody made a mold out of mdf and made an insert that the display can be mounted in so the unit itself is flush mounted with the bulkhead?

I have my unit inside, so when I need to see where I'm going (at night mostly) I can sit in the companion way and drive via tiller extension.... It kinda sucks, but it works in a pinch...
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