Conference unifix::sailing

Gasoline or diesel engine? Sounds like electrical noise from the engine 
is affecting your depthsounder (digital, flasher, or what? made by 
whom?). If the noise is from the alternator, a filter at the alternator 
output or the depth sounder 12v input might help. If the noise is from
the ignition system (gasoline engine), try calling the depthsounder
manufacturer for advice. Or get another depthsounder or first mate
(whichever is cheaper). 

    
    Could be one of two things.  First, where is your transducer located?
    Prop-wash bubbles will most often give you an incorrect reading.
    If your transducer is located aft and in-line with your prop, I
    would consider this to be the prime suspect.
    
    Secondly, I might suspect some kind of electrical interference,
    although my Standard "Horizon" transducer is located within 3 ft.
    of my alternator and the only problem I've had has been caused by
    marine growth on the unit.
    
    Back to the bubbles theory for a minute....I remember that the
    installation instructions supplied by Standard also noted the 
    "bubble-effect" generated by some keels.
    
    Bob

If the problem is electrical noise, it should occur with the engine 
running and the transmission in neutral. If it is due to bubbles from 
the prop wash, it should occur only in gear. Hmmmm, if the bubble theory 
is correct, then it would seem that your transducer is either just ahead 
of the propellor or aft of it, which is an unusual location.

    If it is not  the "bubble" effect, I would suspect a break in the
    ground sleeve section of the feed coax. You might look at the ground
    system if your unit is a part of a general instrument panel.
    Does it read OK if you switch other electrical devices on without
    the engine running. Things as simple as running lights, to as
    complex as your VHF and when transmitting. That would point to
    a ground situation.  
    
    Don

    Alan;
    
    My Pearson 30 Coaster has an Atomic 4 gas engine.
    
    The tranducer is located on the port side near the turn of the bilge
    where it joins the keel. It is mounted on a filler piece to keep
    it in a flat downward direction. This filler piece is faired.
    
    The type sounder excapes me as I have only used the boat three times
    before putting it up for the winter. I am not fully familar with
    all systems yet. However, it is a large (5 in) analog type dial
    mounted into the after cabin bulkhead (next to the companion way).
    
    The transducer inside the hull is located beneath the chart table
    about four feet ahead of the engine. The readout is about 4 1/2
    feet above the engine.
    
    There is no filter or noise supressor on the alternator.
    
    I hope that this helps.
    
    Thanks
    
    Paul

RE:.0 by ABE::HASKELL

> It's my depth finder. When I have the engine going, my finder indicates
> 80 -90 feet of water, when there is only 10 or so feet of water.
> When the engine is off, the finder indicates the correct depth.

As has already been stated,  this sounds like an electrical interference
problem.

1) Quick fix:  have you tried changing the gain control?  There may be a
 setting that will get the correct depth  (and then again there may not).

2) House cleaning:  Check connections and clean any suspect corrosion,  and
 reassemble with silicon grease (sold as heat sink compound).  Check wires
 and cables for visual defects.  Replace as needed.  Still have the problem?

3) Isolate the problem:  If it is an electrical interference problem,  the
 interference is happening on Power (12V) or on Ground.

  a)  Disconnect the power connection,  and wire two 6V lantern batteries 
  in series to provide 12V power to the depth sounder (making sure to match
  plus to plus!).  If the sounder works now with the motor running,  you 
  have some kind of power problem.  I would try a "spark noise suppresser" 
  sold for auto radios and/or rerouting the power wiring (see below).  Also,
  a .1 uF ceramic cap across the power connections at the unit is worth 
  trying.

  b)  If you have the problem with the two 6V's supplying power,  the problem 
  is a ground noise or a radiated noise problem.  You might try disconnecting
  all ground connections,  see if you still have the problem,  and then 
  reconnecting one at a time.

  To correct either a ground problem or a radiated noise problem will require 
  better electrical "housekeeping" of some sort.  Possible fixes include
  changing where wires and cables are run,  adding shielding and changing how
  the circuit is wired.  The power wire could be twisted around the ground, 
  and the power and ground could be changed so as to "share" less wiring with
  the alternator (or ignition) circuits.  A "noise suppresser" is also worth
  trying. 


Phil

From experience, I can tell you that getting a new first mate is relatively
    cheap.  So is getting a new boat as compared to getting rid of the
    old First Mate !
    
    Good luck.
    

    Keel or prop turbulence, or bubble effect will cause early return 
    of the ultrasonic single resulting in LOWER than actual readings
    which is contrary to your situation. Happens to me (full keel),
    occasionally, and more often under motor. Depthsounder periodically
    leaps from a Boston Harbor depth of 42 feet to 3.8!!! Scary at first.
    
    
    HIGHER (deeper) than actual readings imply delayed or missed return
    of signal. Intermittent missing of a series of return signals could
    cause 10X readings. I'd look at connections in transducer, etc ...
    
    regards,
    
    Tom
                                 

    Tom,
    
    Sailing in Boston Harbor you probably WERE getting a solid echo
    at 3.8 feet as you sliced through the raw sewage.  Then again, it
    might just have been a Panamanian submarine making a delivery!
    
    Encore

RE:.8 by BUFFER::FLEISCHMANN

> Depthsounder periodically leaps from a Boston Harbor depth of 42 feet to 
> 3.8!!! Scary at first.

Fish can cause this as well,  but I hear that there aren't many in Boston
Harbor.  Except there are a lot of "finless browns" over by Deer Island.


:-)
Phil

    I noticed that in a busy harbor I get this shallow reading effect
    every time I cross the wake of a power boat.  This occurs even quite 
    some time (several minutes) after the power boat is long gone. 
    I guess it's because of either turbulence left in the water or small
    bubbles, or a combination of both.  I haven't tried to play agound
    with the gain and sensitivity controls or anything.  I would guess
    that is what you are experiencing as well.