Low range sonar

[ewkury]

Hi there
I need a small sonar stuff for my boat with as small as possible minimum
detectable depth. I want it to use as secondary sonar for measuring depths
as low as 4 inches.
Can anybody give me a hint?
Thank you

 

[Bayou Dave]

:welcome: Welcome to iboats!

I would be very surprised if you can find one that will give you accurate measurements at 4 inches. Hopefully, others with more knowledge of sonars than me, will chime in.

 

[aspeck]

There are others on here with much more knowledge of depth finders than I have, but I don't know any commercial sonar devices that will give you readings in inches. You are talking very precise measurements, and then you have o know how far the transducer is under the water, and that will vary with the amount of fuel in your boat, the amount of weight, the placement of the weight, etc.

 

[Bondo]

I need a small sonar stuff for my boat with as small as possible minimum
detectable depth. I want it to use as secondary sonar for measuring depths
as low as 4 inches.
Can anybody give me a hint?

Ayuh,.... Welcome Aboard,.... I suggest ya get yerself a Yardstick to carry with ya,....

 

[MRS]

Nope you need to learn the waters you are running in if 4" is your concern.

 

[Dave-R]

I have found the hard way that the skeg is a very accurate measurement of depth. Seriously though, even a flat boat with a pole for moving will draw close to 4". Maybe call hummingbird or lowrance for their advice. Dave-R

 

[alldodge]

Well dingbat might know, but I would like to know why you want that close a resolution?

 

[H20Rat]

Yeah I have to ask also, what possible use would there be for measuring that little of depth? At that point you could just make a mechanical depth finder. Put a flat metal plate on a pivot with a gauge on the topside, the bottom side would just bounce off the bottom.

 

[GA_Boater]

In even the murkiest waters I can look over the side to see I'm in too deep and about to go over my head.

Do you mean 4 feet, not 4 inches?

 

[ewkury]

AllDodge and smokingcrater: This is a sailing boat with 20 inches drag with the main sonar transducer located on the hull, 16 inches below the waterline*. Since my main sonar does not detect depths less than 2 feet, I was wandering if can install a small secondary sonar for use only in shallow water. The ideal minimum readable depth would be 4 inches but I think I can manage to install the secondary transducer at the height of 14 inches down from the waterline, so the required minimum readable depth would be 6 inches. I was thinking maybe there is a small low range sonar that has this minimum readable depth.

* Even though the main sonar may be set up to compensate the difference (on the display) between the transducer height and the height of the waterline, it still stops functioning at the depth of 2 feet plus the compensation of 16 inches, which will result in about 3 feet of water.

 

[H20Rat]

So you have a valid use case, but you don't need the accuracy. If your main transducer loses signal, you are already way too shallow. There isn't any point of detecting 4 inches because you are already in danger by that point.

The other option is a better main depth finder that will work at less than 2 feet. I know my Humminbird Helix will reliably display .8 feet before losing the bottom. This is with an exterior transducer, don't know how an in-hull would affect it.

 

[alldodge]

I think you in that area where as you have found its hard to find. Why place it 14 below the water line, what about off the bow or stern a few inches below the running water line? I'm thinking the only time your going to use it is when your barely moving, if you were going fast, there just isn't enough time to react

 

[roffey]

My depth finder will do 1.5 for sure. I know it will read in tenths but is it accurate, I doubt it. It sets off an alarm at 1.5 feet and I put boat in neutral and look over board to determine if I need to paddle or raise the motor. At three feet is dead slow ahead and all eyes on the water

This my be a stupid idea, you could install a transducer at the water line and measure how far down your keel goes. if its three or four feet then you set he alarm at three or four feet?

 

[Old Ironmaker]

With my engine running noise disrupts the first foot or so of the reading depending how fast I am going but you have a sail boat. Do you have a inboard or kicker that you use when that shallow to move the boat. If you do I don't think a traditional sonar will be accurate to 4", I actually would bet it won't. Like earlier said if you are in 4" of water you have a problem. From what I have learned transducers should be mounted at the water line not below it. I don't want to do arithmetic when looking at the screen. I don't get why your transducer is below the waterline as well either. I don't think the sonar you are looking for exists.

 

[dingbat]

I want it to use as secondary sonar for measuring depths as low as 4 inches.

Depth is a speed over time calculation.

The speed which a sound travels waters thru is heavily influenced by environmental factors (salinity, temperature, sediment load, etc.) so engineers typically use a "standardized" value for speed. However, neglecting the effects of the actual conditions can introduce significant errors. Add in the variables and limitations of the measurement of time (transducers and processors) and the accuracy can be quite poor at any given time.

The actual limits to performance are related to the quality of the transducer(s), electronics, and software. Shallow water sonar performance is poor at best. Too much (signal saturation) and to fast to process with any accuracy. Using Variable frequency (CHIRP) and Low "Q" transducers help but you still end up filtering the heck out of the returns and hoping for the best.

Having said that, your expectation of using 2" of clearance is unrealistic. A boat's pitch, roll and squat combined with inherent inaccuracies of a sonar based depth reading could easily exceed the allotted clearance.

My alarm is set at 10" below the skeg of my outboard and I've "kicked" bottom more than a couple of times.