Thru Hull Transducers

colbysmith

colbysmith

Active member
It's been a while since this was discussed and I'm opening a new thread for current technology. I've always wanted to get a transducer that would measure water speed, along with temperature and sonar. In recent communications I have learned from Garmin that their GST43 Thru-Hull transponder, with the GST10 package, would allow either my GPSMAP 541 or GPSMAP 840 to read water speed via a NMEA 2000 connection. Is anyone here familiar with the GST43, or have experience with a thru hull transducer? (Do not confuse this with a transducer that shoots thru the hull. A thru hull would involve cutting a hole all the way through the hull, and then placing the transducer into the hull. I would overdrill the hole and epoxy or fiberglass in so that the transducer would not leave the hull core exposed to moisture..)

I don't believe there are any transom mount transducers with water speed that would continue to transmit downview data for the 840, or work with the 541.

I'm also curious about how easy/often water speed transducers become fouled? I imagine a thru-hull would be harder to access for cleaning while on the water.

Colby
 
I'm not familiar with this exact model, but we did have a Airmar 1kw thru-hull transducer on our Parker. Bottom paint did not stick to it, and for that matter I don't believe it was suppose to be painted... so it was always the first thing to grow barnacles and foul on the boat, followed by the trim tabs. This happens relatively fast where we live.

Performance wise I didn't feel like it gave much of an advantage but this newer unit might be better. I actually prefer the picture quality of the traditional Garmin transom mount transducer we have now but it is newer technology.

From a peace of mind standpoint, I never liked the thru-hull. It came on the boat when we purchased it and was marketed as an expensive added feature, but I would have preferred if it hadn't been done. Those reasons include how hard it was to clean when fouled and potential water intrusion, or failure of the seal. The least below water through hole features the better the way I look at it.

I think it would be a neat project to follow along if you go forward with it. Id like to see the step by step if you'd be willing to document it in your album.
 
Colby-

Hmmmm..........???

Interesting problem.

Do you moor the boat much? Experience marine growth?

You could always go with a transom mounted transducer with a paddle wheel on a flip-up mount of your own design that would store it up out the water when moored or overnight when on cruises, employing the KISS Principle.

Just a thought. :lol:

Joe. :teeth :thup
 
My project boat has a thru-hull transducer, which protrudes beneath the hull about an inch. I'm going to remove it and won't replace it because I want the ability to beach the boat as well as park on tidal flats without worrying about damaging the transducer or the hull.
Scraping a shallow bottom is one thing, but what about skimming over a rock and popping off the transducer, creating an instant fountain of watery panic on the cockpit floor. I have no experience with these things so perhaps my fears are un-grounded. Are they available flush-mounted?
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I'd follow up any project with photos in Midnight Flyer's photo album, and a step by step record posting in text. Personally, I would prefer a transom mount, but do not believe I can find one with water speed input for the 541, or that would keep the down view data for the 840. I did reply to Garmin asking if they had anything transom mount that would work and am waiting for their response.

I trailer the boat, and it mostly sees fresh water. However, I have had the transom mount transducers get fouled with weeds and such, so I have to imagine a paddle wheel would be worse. The only other option I can see for determining water speed, is to float in the current for a short distance... Colby
 
pcg":3s6qsa9m said:
Scraping a shallow bottom is one thing, but what about skimming over a rock and popping off the transducer, creating an instant fountain of watery panic on the cockpit floor. I have no experience with these things so perhaps my fears are un-grounded.
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By chance, just this morning, I was watching a YouTube made by a couple who were crusing a sailboat in the topics. The title hooked me "We are sinking!". They got 3 feet of water in the boat before they discovered that the transducer for the speedo (which never worked on their boat anyway) had popped out of its thru-hull in the bow.

colbysmith":3s6qsa9m said:
I have had the transom mount transducers get fouled with weeds and such, so I have to imagine a paddle wheel would be worse.
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I have a transom mounted paddle wheel speedo. Lots of ell grass up here in the San Juans, but I almost never have problems with stuff on the transducter (OTOH, I do get problems of grass hanging up on the engine.
 
Not sure why you want speed thru the water? I used thru hull paddle wheel transducers for many years in racing sailboats. They are a real pain in the keister. They come with blank plugs which will stem the flow of water when you pull the paddle wheel to get kelp and other items off it. Also with time, there may be variation, so they had a method of calibration--which meant running a measured mile both with directions.

The drift method is so much easier--and for the most part one wants to know how far then went, and precise location, rather than water speed.

. If you really want all of this--here are the Airmar units which work on NMEA 2000. Should work on Garmin.

Gemeco may gave some of the older transom mount transducers, with all 3. You may have to do some re-wiring to fit the input pins for your chart plotter.

Ultrasonic speed transducers are better for water speed--but they also have some dis-advantages.
 
Thanks again Bob and others for more good info.

Not sure why you want speed thru the water?
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I figure it's good to know the amount of current, just for helping with fuel planning, and also when salmon trolling. Probably really no other reason. I wonder if one could take a thru hull, and make it a transom mount? I might be more tempted to do that, if I hang on to my desire to have water speed. If I did that, I would just pull the 541's transducer, and fabricate the thru hull into that bracket perhaps. I know my current 541 transducer has dual frequency, and water temperature, but not sure if the Garmin recommended speed transducer has dual frequency. Then again, I rarely use the dual freq on the 541 anyway... Colby
 
colbysmith":850xw5iq said:
Thanks again Bob and others for more good info.

I figure it's good to know the amount of current, just for helping with fuel planning, and also when salmon trolling. Probably really no other reason. I wonder if one could take a thru hull, and make it a transom mount? I might be more tempted to do that, if I hang on to my desire to have water speed. If I did that, I would just pull the 541's transducer, and fabricate the thru hull into that bracket perhaps. I know my current 541 transducer has dual frequency, and water temperature, but not sure if the Garmin recommended speed transducer has dual frequency. Then again, I rarely use the dual freq on the 541 anyway... Colby
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Colby, I find that a fuel flow meter is best for determining fuel use and planing than current speed. Yes, current speed can be useful in salmon trolling. I have done a fair amount from sailboats, where we looked at the action of the bait, and angle of the down rigger to determine proper speed. You might adapt one of the thru hulls to a transom mount--but I don't think it would be worth it. I have used those transom triducers before GPS came along. Another way we determined speed was with a "chip log". the really old fashion way...

As long as you have clear quick access to the bilge, it is not all that bad to pull the transducer for cleaning--keep a sponge or cloths handy to cover that hole as you pull the one plug out, and put the other in. About an inch and a half diameter hole in the bottom of the boat--water can come in quickly. To me the issue with even a flush thru hull transducer, is that with a paddle wheel, you may damage it when loading on the trailer or grounding the boat.

Have you checked out this ultrasonic transducer?:

The Garmin Airmar Smart Triducer,Plastic Thru, NMEA 2000 010-11051-00:

This plastic smart thru-hull mount transducer provides depth, speed and temperature data to your NMEA 2000 network.

This 100 W transducer has a depth of 330 ft; an operating frequency of 235 kHz; a beamwidth of 10x44 degrees; and a 0 to 22 degree deadrise angle.
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I have the GT23M-TH transducer which is a thru-hull w/o a speed wheel, providing signal to both a 841xs and a 1040xs. Personally, I have always installed thru-hulls for improved cruise speed signal reception. Transom mounts are much tougher to install to get a fast cruise speed quality signal. My signal is a quality reception up to my max speed of 40 mph. A transom mount is much more susceptible to being knocked out of the mounting bracket by debris in the water. I spent an entire week at Barkley Sound without the depth finder because it had been hit, so the transducer was laying in the motor well because the bracket had been broken.
Regarding speed wheel, why ? You get the same info from your chartplotter.
There are flush mount styles, I have installed both.
 
Thanks Bob. I'll look into that one. I guess after listening to just a few of you, I would be concerned about having a hole in the bottom of the boat with a thru hull transducer. But I'm intrigued by my thought of fabricating one to a transom mount. I understand a transom mount can be kicked up by logs and such. And in fact have had that happen, but fortunately to only one or the other at a time. (I have two chartplotters, each with their own transducer.) Then very easy to push back down. (My mounts allow them to just kick out of the way and fold up.)

Regarding speed wheel, why ? You get the same info from your chartplotter.
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The chartplotter only gives you GPS or over the ground speed. Not water speed, unless you have a speed sensing transponder.

Colby
 
Hi Colby,

When I bought new electronics in 2012 I spent extra for a wheel on my transducer, but the Garmin 740 doesn't handle that, despite my several attempts to make it work.

What has appeared to me over time is that I am only concerned about water current at slow speeds. For example, at 1900 to 2100 RPM is my hull speed (depending on how heavy the boat is loaded) = 5.2 kts. So when I see 4.7 on the chart plotter I know that I am fighting a 0.5 kt current. And when I get 6.0 kts I've got 0.8 current behind me. I used that a lot last year on both the St John and Apalachicola Rivers. Since I mostly go hull speed the math is simple.

If you want it for higher speeds get in a lake and build a chart for RPM and GPS speed in still water.

Chuck
 
Colby, since you have a Mercury, one option for water speed should be via a built-in pickup tube in your lower unit, and if you have the NMEA 2000 Gateway installed, you can add the sensor and transmit data to your Garmin, SmartCraft gauge or other NMEA 2000 device. I don't have the speed sensor installed on my outboard, but I do have the Gateway and SmartCraft installed and connected through my Garmin, and love it. Perhaps pitot tube is not that accurate at low speed, but I see they do list paddle wheel speed as one of the many functions available, so maybe there are other options worth exploring...
My Merc 90 is a 2016 but I thought I read once that all mercs 40 hp and up, from about 2004 on are SmartCraft compatible. Don't quote me on that. The SmartCraft manual talks about engines 2007 as "Gen1", so either way I think your 115 should be compatible with some of these features...

NMEA 2000 Gateway

SmartCraft Gauges Manual


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Pardon my ignorance, but why do you need to know the water speed? With a GPS (most chartplotters have them) you get your speed over the ground which to me seems to be more useful. My fuel flow system uses GPS to get the boat speed and computes the range based on the current fuel flow and speed.

If you want to know the water speed, just put the motor in neutral for a minute and the GPS will tell you what it is.

Maybe I'm missing something here...
 
So that there is so much concern about another hole in the boat below the waterline. Most important item, " The job needs to be done properly ".
Fixing up boats has been a bit of a hobby to me.
I have rebuilt many transoms. From experience I can tell you many shops are rather sloppy when installing an outboard to the transom. The result being a leaking lower outboard mounting bolt.
Tell me. How many of the shops are taking the time to seal the transom core ?
I will bet you, NOT 5 %.
Jackin Alaska just spent a ton of money, because installers failed to properly install / seal the attaching bolts of the boats Armstrong Pod.
Do it yourself, most likely you will take the extra time and steps to do the job properly. Then, you will have no worries.
 
I epoxied my Garmin transducer in the small bilge under the sink in my 22'. I use it for depth, temperature, and fish finding. Works great and no new hole in hull. Speed comes from GPS. Second boat with installation like this. Garmin was ok with installation. In place now 9 years with no problems and good results with small bilge pump next to transducer.
Pete
 
Thanks Mike for that info. Yes I have a merc, and I did recently install a smart gauge and wiring to that from the engine. (I don't think I can link that into my NMEA 2000 system, altho that would be nice since my Garmin Chartplotter has an engine gauge page. But maybe I should look more into that also.) I'll have to research more if my engine has the speed pickup somewhere, I know speed is not registering on the smart gauge.

Regarding why I want water speed? GPS over the ground speed gives us just that, over the ground. But for trolling or just want to know, it'd be good to know what the current is or speed of the boat over water. Yes, I can just stop and drift to get that. But sometimes that's not always possible or ideal. And having a water speed readout would give one instant notification of the current. Is it really necessary, no, not really. Just nice to have....

Chuck, thanks for that advice. I do have a decent tach in the boat now, so maybe after recording some of my numbers, I can do exactly as you say. Colby
 
Colby, I have a transom mount transducer with a paddle wheel. (And it has twice in 1400 hours actually kicked up.) The ducer remains intact and functioning after those 2 times.

As to why you want water speed. I get what you are looking at/for. I make the comparison of GPS and water speed frequently, checking on that current flow rate happening right under me at the moment.

Harvey
SleepyC :moon

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