View Full Version : How would you mount a transducer?

Roger Stouff
11-05-2003, 07:48 AM
While awaiting the plans for the 16' skiff I'm going to build this winter, I am planning to equip it with some gadgetry, including a fish finder. How would I mount the transducer on a wooden hull and avoid the problem of rot from screws?

The only idea I could come up with right off hand would be to set a mounting bracket in epoxy or 5200, then mount the transducer to that, so the wood is not penetrated. But then, I don't want the transducer cable to be obnoxiously visible.


Best regards from the Rez,

11-05-2003, 07:54 AM
Buy the transducer which fires through the hull and then you certainly won't need top screw it anywhere and it will be invisible.....

Roger Stouff
11-05-2003, 07:55 AM
Really? I didn't know there was such a thing! Is it universal, or brand/model specific?

John Bell
11-05-2003, 08:05 AM
Are you planning to shoot through the hull or a transom mount? Shooting transducers through cored FG hulls doesn't work all that well, and sometimes not at all. Although I have not tried it on my own boats, I have my doubts about how well it would work on a wooden hull.

As for screwing to the transom, I think you might be worrying a bit more than is warranted about rot. I may be wrong, but I'm guessing your boat will live on a trailer when it's not being used. Dry storage is going to eliminate most of your rot issues, especially if you keep the boat covered to keep it from filling with rainwater. You can seal any screw holes with epoxy or 5200 or some other bedding compound to keep water out of the area. If you follow those steps, I doubt rot from your sonar transducer will be what kills the boat.

If screws still bug you, you could always glue a sacrifical block on the transom and screw the transuducer to that. Then if the block rots, you just chisel it off and glue on a new one.

[ 11-05-2003, 09:11 AM: Message edited by: John Bell ]

John Bell
11-05-2003, 08:08 AM
Also, for a fishing boat, I'd like two fish finders. One on the transom to use while running and one on the bottom of the trolling motor (assuming a bow mounted trolling motor).

11-05-2003, 08:25 AM
You could try using a rot resistant wood, like teak.

Roger Stouff
11-05-2003, 09:03 AM
Excellent input, all.

It will be trailered, of course, and the transom will likely be cypress, for rot resistance. A sacrificial block would be cypresss, too. I just don't like drilling below the waterline, especially where water can build up and hold.

It will have a bow mount trolling motor, so that's an option I hadn't thought of. It will also have a GPS.

I can't WAIT to get started! :D

11-05-2003, 11:10 AM
Find a suitable location for the transducer..you will probably use a low frequency unit on a boat that small..
cut a hole in the bottom of the boat the size of the outside of the transducer and to the next largest size of PVC pipe you find at the local plumbing shop..seal the inside of the wood and fill the hole with a fiberglass plug, and glassed over on the outside....lay up the glass in circles on waxed paper to just fit inside the hole and fit flush with the outside of the boat. Insert the PVC pipe or make a "new" pipe by wrapping fiberglass around the PVC to get an acceptable thickness. Insert the pipe in the hole..make a cap for the pipe so that with the transducer installed in the pipe it is about 1/4 inch from your fiberglass "window" in the bottom.
Use about a teaspoon of JOY liquid soap in a coup of water..coat the face of the transducer and the insides and window of your "well". then fill the tube with distilled water and ..with the transducer attached to the lid insert the thing in the well and seal it shut..
The JOY kills the surface tension on the water and couples the transducer Titanate surface to the fiberglass window and "pings" right through it......if you need drawings..send e mail and I'll sketch it up.....works fine.....

Todd Bradshaw
11-05-2003, 11:24 AM
On an interesting related note - one of the techs at Lowrance once told me that if you want to do a temporary test run to see if your transducer will shoot through a wooden or cored hull that you should smear a blob of "Dipity-Do" hair gel on the inside of the hull and set the transducer in that. If it works that way, then you can glue it in. And, you should have plenty left to do some sort of spikey punk hair-do.

Roger Stouff
11-05-2003, 11:30 AM
Originally posted by paladin:
[QB]...cut a hole in the bottom of the boat...QB]The hell you say! :eek:

Scared the living daylights out of me just reading it! :D

11-05-2003, 11:33 AM
I know my knowledge is a bit dated now, so there may be developments in transucers that I don't know about, but anyways. I'll make some comments that might clear up some concepts that have been touched on here.
A depth sounder (or fish finder) transducer can work quite well sending its signal right through a solid material before the water. On a SOLID fiberglas hull you can just epoxy the transducer to the inside of the hull (making sure there are no air bubbles trapped in the epoxy), or even mount it in a water filled box that is formed in the bottom of the boat. This method does not work on wood or cored fiberglass as the air pockets in the wood (or coring material) will scatter the sound waves so the transucer can't work. What Paladin is suggesting is to replace a small circle of wood from the bottom of the boat with a fiberglass disc (with a small water well above it) so the transducer can send its signal through the solid fiberglass. - Should work just fine & have no wires running down the transom, but you do have to bore about a 2" hole in the bottom & replace it with FRP..

11-05-2003, 11:34 AM
I see you guys were reading while I was typing. :D

Good comment Todd. I think you can use a variety of similar stuff.

[ 11-05-2003, 12:37 PM: Message edited by: nedL ]

11-05-2003, 12:03 PM
How accurate are these things for shallow water.

We have Pike in the river that like to sit in holes. The holes are about 1.5 to two feet deep when the rest of the river is at say 10" to 16" deep. Are they sensitive enough for that and to bring it back to Roger's thread would they need to be mounted differently to obtain that sensitivity.

Oh, the water where I fish tends to be muddy so, no I can't just look over the side.


11-05-2003, 12:33 PM
Thanks Donn,

About what I feared.


Roger Stouff
11-05-2003, 01:54 PM
The model I want to use has a side-view feature, but I don't want to use it so much for actually finding fish as finding structure, as well as keeping off the sand bars. :)In the Atchafalaya River and such places there is good depth, but most of the lakes I fish are 2-8 feet deep.


11-05-2003, 04:11 PM
Most interesting discussion. Thanks Roger, and all. As I began reading the thread I though I might suggest drilling a hole in the bottom of your boat and make the suggestions half facetious since I haven't been able to get myself to do on Prairie Islander. Thanks to the experts here I can not proceed. The idea of a transducer hanging off the transom did not appeal. No sense knowing the depth back there when the bow is stuck in a mud bank, eh. I think I can cut a hole from the inside and stop before I penetrate the two layers of glass on the outside and go from there. Thanks Paladin.

Go for it, Roger.

Roger Stouff
11-05-2003, 04:22 PM
Originally posted by Donn:
You had to mention the Atchafalaya River, didn't you? Now I have to dig out The Control of Nature again.Shoot, now I do too. :rolleyes:

Tom Lathrop
11-05-2003, 04:43 PM
I'm a little late getting to this but if you don't need the side looking feature that Roger mentions, this is how I've mounted a couple transducers. As has already been mentioned, I bore a 2" hole in the hull and taper both inside and outside of the edge. Fill with epoxy and glass until it is fair. Press a piece of something flat faced with plastic on the outside to avoid having to sand it fair.

Make the inside surface level and press a piece of wood shaped like the face of the transducer and covered with plastic into the resin on the inside. After it hardens, remove the wood and press the transducer in place with a bit of silicone for adhesive. Works fine and can be easily removed.

Since the fishfinders I bought only came with transom mounted transducers, I just used those and avoided the added cost of the through hull units. Dollar for dollar, fishfinders are far better than depth sounders.

[ 11-05-2003, 05:45 PM: Message edited by: Tom Lathrop ]

11-05-2003, 05:22 PM

I have the transducer for a depth sounder shooting through a plywood hull. To make matters worse, it is held in place with Silastic.

It does not seem to affect the performance of the unit. The greatest depth I have seen so far is 140 feet.

Do the following to try using the transducer through your hull.

Locate the boat in a known depth of water.

Try the unit outside the hull by placing the head of the tranducer in the water and turning the unit on. Record the depth indicated and position of any adjusting controls on the unit.

Pour some water into an area of your hull deep enough to cover the head of the tranducer.

Place the transducer vertically in the water. Just hold it there by hand.

Turn the unit on. Adjust the unit controls as per the unit's manual if required.

If it works, compare the previous depth and position of the controls.

If it does work and you are satisfied with the readings, locate the unit inside the hull with RTV where required. Make sure there are no air bubbles in the RTV. Push the head of the unit against the hull and rotate it slightly.

Give it ago. It may save you cutting a hole in the hull.

I have put mine through the hull as I beach my boat and would prefer not to have a transducer head outside to get damaged.

11-05-2003, 06:37 PM
Yo Norm - you'll know when you're aground :eek: - what you want is a knot meter :D

Back in the days when we were courting, Dave bought me a depth sounder for the little boat. Heck fire, the boat draws 18" with the board up, and I promise when I lack water I'll know it without looking at a gauge :eek: :eek:

Also, he hasn't forgotten you and the windless/winch instructions. Just really busy lately, and I think he wants to see how it works before passing out advice on the subject. ;)

End of hijack.

11-05-2003, 06:44 PM
Donn, et. al.

So which way should the transducer be pointed on a sailboat? If pointed straight down it will record the diagonal to the bottom when the boat is healed but it would be the same error on either tack. If not pointed straight down it might read an accurate depth on one tack and a greater error on the other.

John Bell
11-05-2003, 07:16 PM
I'm no expert, but I thought about it a little. I think the answer is to point it straight down on centerline. Yes there's some error as the boat heels, specifcally the depthfinder will read a bit deeper than the actual depth. However, most sailors find 15 degrees of heel to be excessive and generally sail their boats flatter than that. In the case of a 15 degrees of heel, the actual depth will be cosine(15) * indicated depth which equals 96.6 % of the indicated. So the difference is less than 3.5%.

If you run your 3.5 foot draft sailboat aground because your depthfinder read 1.5 inches too deep, I can't help you!

11-06-2003, 08:23 AM
Roger, has your question been answered to your satisfaction. If so....

How to mount on the center line? A hole through the keel? That wouldn't be as bad as it sounds the way Prairie Islander's keel flares out fore and aft of the ballast bars and centerplate. A knotch in the front of back of the centerplate trunk? Better I think. Ah, the possibilities are multiple.

11-06-2003, 10:22 AM
JimJ, I was thinking of mounting my tranducer inside my 1/2" plywood hull, but was just waiting for someone to say they have done it and it works. Thanks. I'll bet that ply might not
interfere with the signal as much as 3/4" plank would. Like most everyone else, if i can avoid cutting a hole in the hull, I'll feel that much more secure.

Roger Stouff
11-06-2003, 11:06 AM
Originally posted by NormMessinger:
Roger, has your question been answered to your satisfaction.Yes! Thank you all!

11-06-2003, 01:23 PM
If'n you izz gonna put the transduce on a puff boat......buy two transducers and figure out the most normal angle of heel while sailing..locate the appropriate spot for each transducer on the appropriate side....
run the cables to a coaxial switch. Install a mercury switch level under the cabin sole so that neither transducer is selected...then when the boat heels the switch will close on the downlooking side.....a third transducer can be incorporated that will only look straight down when you are motoring........simple......

11-06-2003, 02:07 PM

Yeah, that's easy for you to say. smile.gif

I guess one could mount three with the Hg switches arranged properly. But then I'd get used to depths in fresh water and run over something in salt. Or is it the other way 'round.

We will get to salt water next summer won't we Self?