Transducer Depth

[Jgutten]

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.

 

[kchace]

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.

Ken

 

[deblen]

Measure twice / Len

 

[High Wire]

...and make sure you get the +/- sign right.

 

[Shrew]

If an 11" variance is the difference between floating and grounding, the water is too skinny for me to traverse.

 

[bgillroy]

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.

 

[Djoub]

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.

 

[Brisyboy]

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?

 

[Jgutten]

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!

 

[Dougcole]

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.

 

[High Wire]

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.

 

[Jgutten]

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.

 

[Molly]

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?

 

[Dougcole]

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.

 

[Jgutten]

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.

 

[ben2go]

Measure twice / Len


View attachment 85301

View attachment 85302

If'n ya can't dodge it, ram it.

 

[rgano]

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.

 

[STB]

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.

 

[ben2go]

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?

 

[caltexflanc]

What cools transducers that are mounted internally, to the hull?

The hull itself, which in turn is cooled by the water it is in.

 

[STB]

What cools transducers that are mounted internally, to the hull?

Good question!

These are normally larger to have a greater surface air to better dissipate heat. They are in an antifreeze bath, which has better dissipation properties than air and further increases the surface area to dissipate heat. And, they are up against the hull, which has much, much better dissipation properties than air. (See below)

They are also usually designed to be thru-hull mounted, so I assume they are designed to handle heat better. They are usually not over 600W. They usually don't have chirp transducer or multiple transducer elements. Etc.

In other words, in addition to getting cooled better when installed "in hull" properly than by air alone, they are also designed to generate less heat, dissipate it better, and tolerate it better. They may even be designed to last less long (I have no idea).

And, again, I'm not suggesting that running a transducer dry will necessarily kill it. I'm just saying it will get hotter than it will otherwise (unless it has an internal thermal cut-off) and that it getting hotter than it should has the real possibility of damaging it very quickly or over time.

The antifreeze bath has a thermal transfer about 1/2 of water -- 12x better than air. And, it has a specific heat capacity of about 2/3 of water, about 2000x better than air (by volume).

The fiberglass hull has a specific heat capacity of about 1/3 of water -- about 1000x better than air (by volume).

-- Solids, Liquids and Gases - Thermal Conductivities
-- Specific Heat of common Substances


Corrections/clarifications:

1. I just looked back at my earlier post and realize that I mistyped. By volume, water has a specific heat capacity of 3000x air. (I wrote 1000x).

2. To be fair, I should note that salt water has a specific heat capacity of only about 90% of fresh water, but my numbers were too rough to make adjusting for that meaningful.

3. I assumed dry air. Moisture would improve transfer and specific heat. But, at human scale, even 100% relative humidity is a small absolute percentage of water, so it doesn't move the needle much. Maybe up to a small few percent. Again, my numbers were just too rough for this to be meaningful.

4. I've compared specific heat capacity by mass, except for when comparing to air, where I have done it by volume. I did that because, as a gas, air has so little density to make the transfer across that large a volume not particularly meaningful.

5. I assumed a 50-50 epoxy glass layup, by mass.