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Old 02-13-2019, 02:09 PM   #1
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Transducer Depth

My Mainship 400 was recently on the hard and I decided to take an accurate measurement of the depth of the transducer from the bottom of the rudder, which is the lowest point on my boat.

The previous owner told me that the transducer was 18" from the bottom of the rudder. The transducer is mounted on the port side bottom just under the Racor in the single engine version of the Mainship 400.

I had set the depth finder gauge at 18" relying upon the former owners information. So after measuring the transducer, it's not 18" from the bottom on the rudder. It's 29" from the rudder bottom. I've had the boat for one year and had an actual depth of the boat that was 11" deeper than the depth gauge was set for. Never went aground but I guess I was lucky.

If your boat is out of the water, good time to check the measurements to be sure your depth gauge is set properly.
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Old 02-13-2019, 03:45 PM   #2
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Good point. Im planning on checking and adjusting mine. I've always assumed 2' but as you found, might not be correct. best to actually know.

Up here in the NE where the bottom is so variable, I always want several feet or more, but in some places, one can be within a few feet of the bottom for miles and that's normal.

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Old 02-13-2019, 05:28 PM   #3
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Measure twice / Len


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Old 02-13-2019, 06:09 PM   #4
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...and make sure you get the +/- sign right.
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Old 02-14-2019, 03:12 PM   #5
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If an 11" variance is the difference between floating and grounding, the water is too skinny for me to traverse.
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Old 02-14-2019, 03:46 PM   #6
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During refit last year, to align both depth instru, recorded measure from xdcr faces to deepest keel depth (running gear in pocket between keel & hull-protected). After launch adjusted instru so displays show water depth under keel,alarms set at 2ft. Installed forward sonar (2 dimensional picture) that sees 6 X current depth ahead; we only use when in skinny water.
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Old 02-18-2019, 09:22 PM   #7
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Just measured and adjusted the dedicated depth gauge while in water using the old fashioned way, with a weight and line. I have a 4’ draft to bottom of keel, I offset reading by 5’. This gives me 1’ buffer before depth gauge shows 0’. We’ve seen it go to 2’, a little too shallow for comfort.
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Old 02-19-2019, 12:29 AM   #8
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To help with this, when the boat is on the hard, can the depth sounder be used to check this depth or will it damage it?
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Old 02-20-2019, 04:46 AM   #9
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I set my transducer to the actual depth of my boat, then set a 2' alarm for low water.

Here is SWFL, there is a lot of low water and you've really got to be aware of the tides, current, wind, etc. Keeps you on your toes!
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Old 02-20-2019, 03:10 PM   #10
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Hey Jeffery,


Interesting topic. I can't recall, do you have a single or twins?


Personally, I've never messed with an offset, though I know most people prefer to do it that way. I know my boat draws 3'8", a little more when fully loaded, so I just pay attention to the actual depth. If it's less than about 4'6" I don't go there.


A lot of places we sneak into in the Bahamas are super shallow, 11" under the keel is not comfortable, but doable. Eyeball navigation there is the most effective way to get around there anyway. A few years back our transducer went bad and we went on a month long cruise without it. I used a lead line to check anchoring depths, we had no issues.



Heck, my sounder just blinks in anything shallower than about 4' anyway.
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Old 02-20-2019, 04:46 PM   #11
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Quote:
Originally Posted by Brisyboy View Post
To help with this, when the boat is on the hard, can the depth sounder be used to check this depth or will it damage it?
If you are on land, yes the DF can be turned on, but it will not read anything or nor damage it short term. The transducer ultrasonic pulses will not travel through air. If you put your ear up near the TD, you will hear a faint clicking sound when on. Sometimes you can feel the pulses with your hand.
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Old 02-21-2019, 05:51 AM   #12
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Dougcole,
I have a SE MS 400. The problem with just going where you have 4'6" is how do you know without having a properly set depth reading?

The charts are unreliable. The tides affect depth, shoaling, etc.

Setting it to depth below the keel makes sense to me.
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Old 02-21-2019, 07:44 AM   #13
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I understand the thought process of having the depth set at "0" for the lowest point on the boat. I still prefer to set the gauge at the actual depth. I know I need 3'6" of water.
If I'm in water less than 4'6" I turn around. I just don't like using my math skills to figure the depth. Personal choice I guess?
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Old 02-21-2019, 10:11 AM   #14
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Dougcole,
I have a SE MS 400. The problem with just going where you have 4'6" is how do you know without having a properly set depth reading?

The charts are unreliable. The tides affect depth, shoaling, etc.

Setting it to depth below the keel makes sense to me.

Hey Jeffery,


Yes, this is for sure one of those personal preference things. Both ways work the same, it's just which one is easier for you.


I asked if your boat was a single as we have the same boat, so I was interested in your findings. Though my boat is a twin, the draft is a little different. Also, some of your mods that you were nice enough to share on here in the past have really helped me, specifically how you moved your holding tank vent. I appreciate your in depth understanding of your boat.


As to how do I know the depth before I get there? Well, that's the rub isn't it? To me, sounder really isn't much help in keeping me from going aground, unless it is as forward scanner. By the time I get a reading I am likely already in trouble.


What it does tell me though is what the water in a particular area of a particular depth "looks like." Personally, I trust my eyes more than my sounder, though this doesn't always work in dirty/dark conditions, so in those times I'm extra careful. I assimilate what the chart, the sounder and my eyes are telling me. I'm sure you do the same thing, even if you don't consciously think about it.


I'm a flats fisherman, I was a tarpon guide for 13 years. A big part of that sport involves looking for minor differences in depths of shallow water. It's a skill that I worked hard to acquire, but it's not rocket science. I look for water color, catspaws, current flows etc. I also pilot exclusively from the Fly Bridge in shallow water. That water reading skill has helped me a lot.



Thanks for all you have added to the forum.
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Old 02-22-2019, 06:12 AM   #15
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Dougcole,

My boat is a single engine. It turns out that it does draw the 3'8" that Mainship states. My transducer, which is mounted on the port side, by the forward bulkhead in the engine compartment below the fuel filter, is 29" above the bottom on the rudder.
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Old 03-01-2019, 02:06 PM   #16
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Old 03-02-2019, 11:17 PM   #17
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I understand the thought process of having the depth set at "0" for the lowest point on the boat. I still prefer to set the gauge at the actual depth. I know I need 3'6" of water.
If I'm in water less than 4'6" I turn around. I just don't like using my math skills to figure the depth. Personal choice I guess?
So don't you have to perfom some mental math with the screen of your sounder reading zero at the face of the 'ducer? Or do I misunderstand the quoted portion of the post?
Personally, I have always set the sounders on my boats to read zero at the lowest part of the boat. If I see 2 on the sounder, I don't have any math to do to know that I have 2 feet to go before I am aground. Thus, an alarm setting of 6, common with me, means that when it goes off, I have six more feet to go before going aground.
However, I like exploring (slooooowly) in shallows when the tide is incoming and a reading of 1 or below is not uncommon with me.
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Old 03-03-2019, 03:22 AM   #18
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To help with this, when the boat is on the hard, can the depth sounder be used to check this depth or will it damage it?
Ensure your transducer is /never/ powered up when out of the water. At best, it'll do no good -- air doesnt carry the signal well enough for it to work at all. At worst, it'll damage it beyond use or repair.

Transducers are water cooled. Water has a specific heat capacity about 1000x more than air and a thermal conductivity about 25x greater than air. Even warm water cools much better than cooler air.

Some people may report that theirs was powered up on land and it hurt nothing. That might be true. And, it might offer you some reassurance should you goof. But, it shouldn't encourage you to do intentionally.

Whether and how much damage is done depends on many factors, e.g. how long it was powered up, model, power setting, age, etc. And, unless the damage is immediate total failure, it might be hard to assess and correlate back to the out-of-water event later on, e.g. premature aging, reduced sensitivity, frequency drift, power loss, timing variability, noise, etc.
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Old 03-03-2019, 07:51 AM   #19
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Ensure your transducer is /never/ powered up when out of the water. At best, it'll do no good -- air doesnt carry the signal well enough for it to work at all. At worst, it'll damage it beyond use or repair.

Transducers are water cooled. Water has a specific heat capacity about 1000x more than air and a thermal conductivity about 25x greater than air. Even warm water cools much better than cooler air.

Some people may report that theirs was powered up on land and it hurt nothing. That might be true. And, it might offer you some reassurance should you goof. But, it shouldn't encourage you to do intentionally.

Whether and how much damage is done depends on many factors, e.g. how long it was powered up, model, power setting, age, etc. And, unless the damage is immediate total failure, it might be hard to assess and correlate back to the out-of-water event later on, e.g. premature aging, reduced sensitivity, frequency drift, power loss, timing variability, noise, etc.
What cools transducers that are mounted internally, to the hull?
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Old 03-03-2019, 09:42 AM   #20
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What cools transducers that are mounted internally, to the hull?
The hull itself, which in turn is cooled by the water it is in.
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