Depth sounder for shallow water

Discussion in 'OnBoard Electronics & Controls' started by tom28571, Feb 21, 2007.

  1. tom28571
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    tom28571 Senior Member

    That title sounds superfluous but noneof the spec lists ofor depth sounders address this. They tout power and deep water capabilities but don't cover what is most important. I much prefer the fishfinder for its display and cheaper cost over the usual digital numeric display only. Much easier to navigate in narrow channels with and less ambiguity of the reading.

    The questions is, does anyone make such a reasonable cost sounder that will read very low water depth? This would mean a high transmit frequency and a very narrow pulse width. Most sounders quit at a bit over 3 feet and that just where things start to get dicey with an outboard.
     
  2. Pericles
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    Pericles Senior Member

  3. safewalrus
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    safewalrus Ancient Marriner

    Stick yer leg over the side, when it gets caught and you fall out of the boat cut your engine! Seriously tho' for that sort of depth a pole and slow speed would be the best idea - if you paint it alternate dark and light colours every foot it helps judge - works for me! 'cause if your a 'seal' or the like dodging bullets and going hell for leather that a different problem that's got nothing to do with this forum, but for shallow water slow is safe!!
     
  4. Willallison
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    Willallison Senior Member

    Tom, I can help on one of two counts...
    We've had Koden, Furuno and Raymarine fishfinders on a variety of boats over the years. All have a dual range facility - 50 /200kHz and all have been capable of reliably providing 'pictures' until the boat alomots touches bottom. Granted you have to 'fiddle' with the controls a bit - and the picture becomes a little less obvious than it is a greater depth, but it is possible.
    Sadly - none have been exactly cheap.....
    My experience suggests that it's at least as important to select the correct transducer. These are often common across a number of brands (Airmar on our units)
    I had a Furuno plotter/sounder/gps aboard my Searay. I had a transom-mount transducer on that, but for several years actually 'shot' it thru the hull bottom after setting it in a substantial dollop of grease. When I finally got 'round to mounting it on the transom, I found that there was a barely perceptable improvement in image return at low speed - but this was soon negated once I was on the plane: no doubt a result of the turbulent water in which it was operating. I doubtless could have improved this somewhat, but we sold the boat not long after, so I never got the chance to try.
    Our bigger boats have all had thru-hull fitting transducers and these have all been quite succesful.

    These days, of course, where equipment is 'dumbed down' so that ignorant 1st-time users can be spared from doing anything so outrageous as reading the owners manual, it's almost impossible to tune your own sounder. Our old (papar chart) Furuno allowed you to alter virtually everything - including the pulse rate. It gave fabulous bottom images - so long as you knew how to drive it....
     
    Last edited: Feb 21, 2007
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  5. ted655
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    ted655 Senior Member

    Interphase makes several mocels. You didn't give a price range. Their cheapest one is a PC version @ around $600.00, you supply the computer & monitor. Be aware! A laptop will NOT work. It requires a "paraell port. Laptops quit installing these ports several years ago. NO (i asked) an adapter from paraell to usb won't work either.
    My choise is the twinscope model. Shop hard! prices vary a lot.
    I wonder what the far sounder cost? If you have to ask, I guess you can't afford it, as the saying goes.
    Do you need to "see" the bottom or just be made aware of shallow water? If the latter, then why not a numeric guage with an alarm?
    http://www.interphase-tech.com/
     
  6. charmc
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    charmc Senior Member

    Tom,

    I sympathize with you, I operated for a long time in Barnegat Bay in NJ, notorious for going from 6' to 1' depth in a few yards. After chuckling over the obvious suggestions to drag a foot in the water or use a stick (although amusing, both have actually proved quite effective over the last 10,000 years or so), here are a few thoughts:

    By all means get a unit with manual adjustments for gain and power. The automatic modes are truly for dummies, and just read averages over such wide areas as to be virtually useless for navigating in shallows. If cost is a major factor, I think there are still a few mfrs making an analog display with a lighted "pointer" moving across a fixed numeric scale. Properly tuned, I found these able to show 6" increments (the pointer would be clearly between 2 numbers) in shallow waters. I was able to get reliable readings down to 18" depth. I doubt any transducer can or should read any closer. Single frequency, dual cone width transducers are usually best for shallow water.

    Garmin's GSD 21 line should meet much of these criteria. Garmin claims it will work to 1' depth. As others have recommended, Lowrance and Humminbird make good units. Avoid the bells and whistles and look for 200khz transducers and manual adjustments, and you ahould find a unit to fit your needs.
     
  7. tom28571
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    tom28571 Senior Member

    While I have sounders installed on my boats, none have been able to work at depths less than 3' and often get squirelly above that. The thing that brought this on is that I am helping build a 22' sailboat for the Everglades Challenge race starting March 3rd. The later parts of this race take place over the Florida Flats. You can see info on the Challenge at: http://www.watertribe.com/Default.aspx?Expand=7&ucPtr=Everglades/EvergladesTeaser.ascx

    It's a very challenging race with many different kinds of factors that limit what type of boat can be successful. Just fast is not good enough. Last year my building partner and crew won the 300 mile event in a 17' cat ketch of his design over two catamarans. You have to be able to move the boat into the water from high tide line with the sailing crew of one or two, take masts down to pass under bridges, pass through narrow openings, and row or paddle to reach the three check points. Stopping or sleeping counts against your time.

    There are shoals as the boats turn into Florida Bay from the Gulf. Between Flamingo and Key Largo, the "real" flats appear. Very shallow, channels are not officially marked, channels are often narrow and winding and even they are not deep. The boat will take along makeshift "snowshoes" for getting off the bottom iif it becomes necessary (it has in the past) because the mud is so soft you sink with each step. Of course tide is something of a factor also.

    A plotting type sounder that could help with this tricky navigation would be of great aid but bells and whistles are superfluous. All that is needed is a small unit with a transmitted pulse width narrow enough to allow reading in 1' or less of water. I think a pulse with of 0.25msec would do the trick but I have not been successful in gettting answers from manufacturers so far. A pulse of this width should allow the pulse to hit the bottom and return without running into itself at the transducer.

    Oh, and the faster boats will reach the last and most difficult shallow water at night. Power requirements on a small boat is also a large issue so fancy high-power units need not apply. Other than that there is no problem.
     
  8. tom28571
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    tom28571 Senior Member

  9. Pericles
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    Pericles Senior Member

    Hello Tom,

    An interesting conundrum How about a strong underwater light, scanning forward? Or composite cat's whiskers (lengths of fishing rod) mounted on strain gauges and positioned down at the bow to warn when bow is in 14 inches of water?

    Correctly mounted and adjusted so boat speed does not affect the strain gauges until they hit mud ,they will give you about as much warning as a forward scanning asdic. Not a lot, but :D

    BTW, sailing boats on the Norfolk Broads mount the mast in a swivel with lead counter balances so the gaff rigged boats can lower the masts on the move to pass under bridges. Just unclip the forestay and the mast is lowered toute suite.

    http://images.google.co.uk/imgres?i...orfolk+broads&svnum=10&hl=en&newwindow=1&sa=N

    Regards,

    Pericles
     
  10. tom28571
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    tom28571 Senior Member

    Thanks for the link Pericles. One beauty of the fishfinder display is the ability to see the changing contour of the bottom. I have navigated through difficult channels in heavy fog with only the sounder display as a guide from mark to mark. Mechanical means can work if you are not in a hurry but in a race, I'm very doubtful except in emergency.

    My wife and I plan to be in England in August for a cruise on the narrowboats. I thought about the Norfolk Broads but that is too much like a lot of our coastal area so decided to pass in favor of something different. My son and daughter in law are taking us for our 50th so I'm happy to go where ever.
     
  11. Pericles
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  12. jehardiman
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    jehardiman Senior Member

    Tom, one of the problems with simple down looking fish finders is that you only see the bottom after you have passed. In order to see ahead, you need a forward looking unit, and that is going to be pricey. As far getting a 0.25msec pulse, most depth sounders won't go that short because the depth sounding feature will be defeated due to low signal to noise return of the bottom and high attenuation of low power signals (I know you could build one, but you would most likely hear the ping in the air). Once you are into 200+kHz frequencies you are starting to get into 200+dB doppler log / current meter terriory where you use long pulse continiously modulated freqencies to build statistical responses over 1024+ returns.

    If they really, really, really want to win then there is no substitute for local knowledge. Go the UDT method and spend a month or two down there ahead of time and work out all the tracks with GPS waypoints and sound them corrected to MLLW. Often up in SF bay we would take a little known back channel through the flats during a race and cut miles off the sailed course.
     
  13. tom28571
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    tom28571 Senior Member

    Graham has sailed the course twice and won last year so he is becoming familiar with the territory and you are absolutely right in that local knowlege is paramount. I know that a very shallow water sounder is a bit different but such low power is needed that I don't see how the signalnoise ratio is the limiting factor. It's just a pulse and is 50 cycles long at 200Khz so that does not seem to be insurmountable. We are not going to get into designing such a unit though. I gave that up 20 years ago when retiring to do more interesting things like boats.

    My only reason for the original question was to see if there might be a simple unit out there that was being overlooked. Such equipment is not likely to determine the winner but should be helpful. Anyway, I mounted the usual fishfinder that offers 3' today.

    Thanks for the interest.:)
     
  14. jehardiman
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    jehardiman Senior Member

    I leave the design to the PhDs, but as I understand it, the higher the frequency (needed to shorten up the wave lengths so you can have more cycles in transit), the more the "clutter" in the water and xducer ringing reduces the bottom return distinctness, hence the higher power needed to get a "hard" return so as to define the "bottom" (which may or may not be the "real" bottom given the matrix i.e. sand, mud, silt,grass, shell, etc).
     

  15. tom28571
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    tom28571 Senior Member

    My ruminations are just that. I have no special expertise in this particular area other than being a used to be EE.
     
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