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Old 06-07-2023, 11:22 AM   #1
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In-hull vs. Thru-hull Transducer choice

My thru-hull transducer is dead. It is an older Airmar bronze with a fairing and is connected to a Garmin GPSmap. That connection requires a Garmin specific interface. Speed through water and temperature had never worked and the depth was only reliable under 35', spotty up to 85' unless going slow, with an occasional deeper reading. My backup, an ancient DataMarine depth sounder, gives consistent readings up to 900', but doesn't interface with the Garmin MFD.

Airmar apparently doesn't make a new model thru-hull that fits our existing fairing. Two boatyards said that, if installing a similar model, it is very likely that the old fairing will have to be ground off and a complete new unit fit. That would require fabricating a new fairing block. $1,200 for the new similar transducer and $1,200 for haul, labor, possible 3 days in the yard.

A well reviewed Garmin/Airmar in-hull transducer is $170. It doesn't have a speed paddlewheel (which we don't need) or a temperature sensor (which we don't need) or any fancy fish-finding capability (which we don't need). I read that a transducer with a fairing may provide better readings on larger vessels (>35') at higher speeds, again something that I don't think we need.

Installation of the in-hull looks to be a DIY afternoon. The in-hull transducer can be tested pre-installation by dropping it into a bag of water against the proposed hull position. Seems fool proof.

Is there something that I'm missing by not spending the time, hassle, and extra $2,000?
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Old 06-07-2023, 11:45 AM   #2
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As long as your hull isnít cored it should work. You may loose a bit of depth due to shooting through the hull but it probably wonít be too bad. It should still be accurate just may not reach as deep.
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Old 06-07-2023, 11:50 AM   #3
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One of sounders I have is an In-Hull Simrad unit. It has worked well for the last 3 years and it correlates perfectly in terms of depth (up to 800') with the more advanced through-hull unit.

The sensor sits in a solution that I check and maintain as needed on a yearly basis. A pretty simple task.

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Old 06-07-2023, 12:00 PM   #4
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I installed an Airmar in hull transducer in our Camano Troll in 2005 and it worked just fine. It usually worked to a depth of over 250 feet and also picked up fish on the fish finder.
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Old 06-07-2023, 12:19 PM   #5
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I pulled my fairing blocks and unused transducers and fiberglassed the holes. I was surprised at the bad condition of the blocks.

I've got a cheap Hawkeye that shoots through the hull. Worked good last year. No reading this year though. I need to pull it and check see if it is still working, if it is I will remount it. It is depth only.

I also have a transom mount for my Garmin Echomap. Does depth and temperature.
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Old 06-07-2023, 03:57 PM   #6
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I did an inside transducer in 2 boats and accuracy never suffered.
On one I epoxied it to the hull on the flat of the hollow keel.
On my Albin it was in a cup type fixture that came with the xducer that I had epoxied to the hull and I filled it with pink antifreeze.
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Old 06-07-2023, 04:24 PM   #7
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Originally Posted by jleonard View Post
I did an inside transducer in 2 boats and accuracy never suffered.
On one I epoxied it to the hull on the flat of the hollow keel.
On my Albin it was in a cup type fixture that came with the xducer that I had epoxied to the hull and I filled it with pink antifreeze.
Me too...lots of transducers installed thru the years and the new Airmars with dial in hull deadrise are the way to go for cruisers.
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Old 06-07-2023, 07:12 PM   #8
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I'm not really clear on what type of transducer you have thats failed but an airmar thru-hull transducer in the $1200 range is a 1kW unit that would see the bottom in depths of 900'. The in-hull you quoted sounds like a single-element transducer that would be hard-pressed to read over 3-400' in the real world. Do you need to see 900'? If not , an Airmar B-45 thru-hull bronze single-element transducer can be bought for less than $300 with a fairing block included. Or you could go with a through -hull flush mount B-60 that doesn't require a fairing block. Either of those will read the bottom up to 5-600' and have a temperature sensor built in. A high-performance 1kW is less than $1500 and comes with a fairing block if you need to see deeper.
Maybe you should research your options with a qualified electronics dealer instead of a boatyard to be sure you'll have what suits your style of boating.
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Old 09-02-2023, 11:32 AM   #9
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I went with the thru-hull $180 transducer. Installation was simple. It cuts out occasionally, but my vintage DataMarine usually picks up when the Garmin isn't hitting. Both of them seem to lose the bottom for a few minutes for no apparent reason. The Garmin can be good to about 300' but, once the bottom is lost, it sometimes doesn't pick up again until back into <40'.

It would be nice if both of them were always getting signal at <300'. As it is, I have slowed almost to a stop in shallow tricky passages to let them both find the bottom before proceeding. It could be that if I'd spent $2K more I would have better readings. Or maybe not.
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Old 09-04-2023, 06:47 AM   #10
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Originally Posted by Marco Flamingo View Post
I went with the thru-hull $180 transducer. Installation was simple. It cuts out occasionally, but my vintage DataMarine usually picks up when the Garmin isn't hitting. Both of them seem to lose the bottom for a few minutes for no apparent reason. The Garmin can be good to about 300' but, once the bottom is lost, it sometimes doesn't pick up again until back into <40'.

It would be nice if both of them were always getting signal at <300'. As it is, I have slowed almost to a stop in shallow tricky passages to let them both find the bottom before proceeding. It could be that if I'd spent $2K more I would have better readings. Or maybe not.
I've found that if I take my sounders out of the auto ranging mode ,they track the bottom with much better reliability. In the automatic range setting, if it looses the bottom, it starts searching for the bottom to display a depth. In the manual mode, it might loose the bottom for a moment but picks it right back up as soon as the turbulence is cleared from the transducer.
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Old 09-04-2023, 07:47 AM   #11
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In the past I have found that at certain depths two can interfere their signals.
I had two that would get screwy around 30-35 feet if they were both on.
Turn one off and the remaining one would read good again.
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Old 09-04-2023, 10:15 AM   #12
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Quote:
Originally Posted by Marco Flamingo View Post
I went with the thru-hull $180 transducer. Installation was simple. It cuts out occasionally, but my vintage DataMarine usually picks up when the Garmin isn't hitting. Both of them seem to lose the bottom for a few minutes for no apparent reason. The Garmin can be good to about 300' but, once the bottom is lost, it sometimes doesn't pick up again until back into <40'.

It would be nice if both of them were always getting signal at <300'. As it is, I have slowed almost to a stop in shallow tricky passages to let them both find the bottom before proceeding. It could be that if I'd spent $2K more I would have better readings. Or maybe not.
I went with the same transducer as you and in the same boat. Where did you mount yours? Mine in on the Starboard side near the front of the fuel tank just outboard of the main stringer. Its been working good. It does occasionally loose bottom in my muddy slip. But out of the slip it works great.
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Old 09-04-2023, 12:06 PM   #13
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I have also found that if I switch from chart to sounder on the Garmin that often picks up the bottom. If I go to a split screen chart/sounder it doesn't seem to help. Have to go full sounder, then back to chart. I'll have to look further into what my options are.

I put my new transducer behind the starboard fuel tank just outside the second stringer. In front of the sb fuel tank the PO had built nice shelving for oil, coolant, filters, etc. Now having owned the boat long enough to have crawled through everywhere and lifted every floorboard, I think I know all past transducers. One portside (removed and patched) in the lazarette. One port under the floorboard forward of the engine (missing its paddlewheel but still working otherwise). Three behind the starboard fuel tank, one of which is the new functioning one.
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