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Old 16-12-2006, 10:01   #1
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transom mounted tranducer

Hello all:

I am looking at buying a new GPS Sounder. I am considering a Garmin 178C with an external antenna. I would like to get a thru hull transducer but the best prices are on transom mounted transducers. I am not so sure about the stern mounted transducer it seems like it would be a weak point does anyone have any experience with transom mounted tranducers.
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Old 16-12-2006, 11:24   #2
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What hull do you have? Flat planning hulls are suitable for transom mounts. Displament hulls require through hull.
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Old 16-12-2006, 11:40   #3
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Concur with Alan.......

The Transom mounts have to be in the water all the time, as do others. They are made for power boats who's transoms are always in the water. Sailboats transoms pitch up and down loosing the signal.

If your hull is solid glass the transducers can be mounted inside the hull if your not wanting to put in the hole. It's submerged in a solution of mineral oil inclosed next to the hull. Explanations later if desired.........................._/)
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Old 16-12-2006, 11:41   #4
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Thanks Wheels:

I have a displacement hull so I need a thru hull. Appreciate the info you think that Garmin would mention that on there web site.
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Old 16-12-2006, 12:07   #5
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DelMarrey:

If you could give me details I would appreciate it. I presume that would preclude using the temp gauge so that will save me some $. On another site it was recomended that the transducer be expoxied to the hull with West Epoxy.
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Old 16-12-2006, 12:49   #6
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First you need to determine if your hull has no core. Some manufactures do make a puck transducer that just glues to the inside of the hull. But the thruhull types can be used also.

The best place is just forward of the keel 2-3 feet. This keeps the air from interrupting the signal.

To start the hull has to be ground flat where the transducer (TD) will sit, for a positive contact area. You'll need to build a box to surround the transducer and epoxy it to the hull. It has to be high enough to cover the base of the TD. And a lid needs to be constructed to seal in the oil so it doesn't slosh out.

Once the box is set up the transducer can be placed in and secured in place with the lid. The TD can be epoxyed to the hull but there can not be any air bubbles. The lid needs to seal in the oil. Remember there's no pressure just gravity. A small hole can be drilled in the lid for a small pipe plug. This is where you will add the oil. Compass oil is best. It's clean and pure.

The main thing is to keep out as much air as possible and to keep the TD as snug to the hull as possible.

I may add more later but that’s the basics ………………….._/)
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Old 16-12-2006, 13:50   #7
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Thanks Del Marrey:

Is it ok to build up rahter than ground down the hull?
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Old 16-12-2006, 14:41   #8
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Do be ware of two issues with mounting inside the hull. Firstly, some sensitivity is lost, so your sounder will not read as deeply as it would outside the hull. Second, the tmp feature will not be accurate or fast to respond to temp changes.
Lastly, you need to mount the transducer, no matter if it is inside or outside, in just the right place. Usually the forward third of the hull and away from the keel by about a third. This is to stop getting flections of the keel. If you have a paddle wheel speed transducer fitted to the through hull, then this also helps with getting clear straight flowing water. Once the water curves around the keel and hull shape, the flow alters speed and gives you an error as well.
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Old 16-12-2006, 18:05   #9
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Build up?

Yeah! I would imagine but is has to be solid, no visable air pockets. But like Alan stated the thicker the hull the less sensitivity and it should be epoxy/polyester & glass.

To be honest, on a 40+ foot boat, I personally would put it thru the hull. If it's solid glass the hole and transducer could be put in while hanging on the slings, providing a yard would allow that. In about an hour.

If it has a core, the core should be sealed from the hole in case there ever was a seeping seal. That would take a whole day.

BTW Sailboat thruhull transducers are usually flush mount or have a deflector around it to protect it and redirect water flow.
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Old 16-12-2006, 22:46   #10
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Wheels and Dell

Thanks for the help. I'm going to be hauling the boat to put in a thru hull for the genset so I guess I could do it then. There already is a depth sounder (B&G) I don't know if I would take that one out or not. It's 20 years old but I tested it for accuracy and it works.
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Old 17-12-2006, 14:39   #11
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If you already have a thruhull fitting in place, that makes it simple. But 20 years old? Personally, I would replace it anyway. Plastic ones tend to shrink and leak and bronze may have some hidden corrosion/eletrolysis. The wires may even have blackened causing resistance. They are not that expensive compaired to the trouble.

But before, if you attempt to pull the old one, check the hole/shank size. They come in different sizes. On mine I pulled out a 2" transducer and reinstalled a 3/4". I had to make up a fiberglass bung, fair it into the hull and then redrill.

Also, SOMETIMES they're a bitch to get out. Unless there is something to put a wrench on to unscrew'm they end up getting a hole drilled thru them and go from there, whatever it takes (EZ-out, bigger drill or hole grinder)

Mine is actually a Fishfinder, dualbeam. Not for fishing per se but so I can get a definition of the bottom where I'm anchoring. It also has a depth alarm so when I'm sneaking up on an anchorage I know where to stop. But I did have to build a deflector for this one. If I happen to hit some large driftwood (lots of it here) it may compromise the 3/4" hollow shank. I'll post a pictures later today, if interested.
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Old 17-12-2006, 17:54   #12
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I have actually carved out the core in an area in the centreline of one of my hull's and reglassed the inner and outer glass skin's to each other making a solid glass spot big enough to put the transom mount transducer onto, shooting through about 1/4 inch of glass.

Then I use the trusty roof and guttering silicone to bond to hull, ensuring there is no bubbles in silicone.

I have done this on my last 2 boats and done it on several clients boats with no probs. This was how Humminbird suggested it was done to me.


I hate cutting holes in my boats

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Old 17-12-2006, 20:53   #13
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Dave and Del:

Thanks for the advice. I already have a hole in the boat so I have to decide what to do with it. I'll have to see what size the transducer is and then decide what to do from there. I'll be sailing up in PNW this summer so the idea of protection from flotsam etc sounds like a good idea.
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Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together through long periods of separation, it had the effect of making us tolerant of each other's yarns -- and even convictions. Heart of Darkness
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