Depthmeter facing forward

Has anyone tested or heard about setting a second depthmeter or fishfinder facing forward? The idea is to avoid underwater colisions against rocks or containers, which cannot be seen on radar or eyes.

 

Forward looking Depthfinder

It would be great if it works. I fear you will be swamped by surface revereration, making it difficult to see anything. This is where the sonar waves reflect off the surface irregularities (ripples, waves, entrained air etc.)

 

There are many Obstacle Avoidance Sonars (OAS) out there for ROV's and the like and similar systems have become small enough to fit on the boards for fish finding with mid-water trawls. These systems can be expensive though (100K-500K+ USD) and are not intended for high speeds (~5 knots max) due to their limited range.

Theoretically, you could just point an cheap depth sounder forward, but I'll say right up front that the "information" you get displayed back would be very suspect for various technical reasons of aperture angle, cut off, side lobes, cross talk, mutli-pathing, etc.

 

Sounds as if you understand sonar. Probably the biggest problem with pointing a fishfinder forward is they normally use a single beam facing down. It is stationary rel. the boat."Scanning" comes from the forward motion. OAS that I am aware of have mechanical scanning from side to side to generate the "picture"

 

While we are on the subject, maybe someone can answer a question I have. I have never owned a fish finder so I was wondering if the transducer has to be oriented a certain direction? The screen shows a 2 dimentional surface. Does that surface represent a thin section of the bottom in the for, aft plane or in the side to side plane. If it is in the side to side plane, could you rotate the transducer 90 degrees to get a for, aft view. This would make it easier to see a trend if you are heading into shallower water or deeper water.

 

John David;
No, I’m not a tron, but I’ve done enough advance concept design to learn some about what can and can’t be done with most ship systems and why. I find it sad that a fair number of NA’s never desire to learn about the ”hows and whys” of ship systems. On the flip side, I also see a lot of monstrosities out there as ME’s and EE’s try to design marine systems.

des;
Most non-commercial fish finders and modern depth sounders are what is referred to as "waterfall" displays (from the old days of paper displays where the output paper looked like a waterfall coming unit). That means that every vertical row of data is a snapshot of what is directly under the transducer (just to keep it simple and not talk about what could be represented). Now most cheap units just update at regular intervals which means that the bottom "contour" is not real but is a representation of the depth at each ping (i.e. if you are not moving the bottom appears flat regardless of the true contour and if the boat is rolling the bottom may appear wavy). Better units use ping timing tied to ships velocity and give a better representation of bottom contour. The best units are software modified multi-beam acoustic doppler units that map the bottom with each ensemble (bursts of pings) and have all the other bells and whistles such as motion cancellation, bottom track, current profile, bottom type estimation, etc.

 

Well, gentlemen, seems to me that if this thing worked, would solve our problems related to crashing against something underwater, like a container or rocks, isn´t it? Is there a chance to adjust the transducer for a paralel beam to the water surface? That would avoid the reverberation from the surface and keep a strict beam keeping an eye in some portion between the keel and a little below the water line.

 

SamAlec;

Hydroacoustics and transducers don't work that way. It's not like optics in air. You always get spreading in the medium and transducers measure all the pressure fluctuations reguardless of direction. Modern OAS systems go to great lengths processing out scatter and multi-pathing. They do this with arrays of transducers and some pretty complex signal processing and head drivers (mechanical scaning, digital scanning, Constant Transmission Frequency Modulated (CTFM), doppler, digital beam steering, etc.) Yes, you can buy a system to do just what you asked, but be prepared to pay more than the boat for it and it will be a real drain on the electrical system to run it all the time.