Re: [PSUBS-MAILIST] SONAR

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Ed Greany" <crest25@attglobal.net>
To: <personal_submersibles@psubs.org>
Sent: Friday, August 10, 2001 8:25 PM
Subject: Re: [PSUBS-MAILIST] SONAR


> The typical fish finder works just like a fathometer or as Capt. Nemo
> called it, a "sonar altimeter" which is a pretty good term except an
> altimeter measures the height above a horizontal plane such as a sea
> level. In water, it is a little different in that it measures the depth
> below the keel.

Hi Ed,

One last response before I dive.....

Yes that's exactly what it does: measures the distance between my keel and
the horizontal plane of the bottom.  That distance is usually thought of as
depth to the bottom; and a fisherman in a surface boat will always see it
that way.  ( I know you're a submariner, I'm just using the fisherman as an
example.)  But I'm on my way down, so I find it helpful to think of it as my
keels "altitude above the bottom".

Being a pilot, I find similarities in flying an airplane and piloting a sub;
and  operating my sub in the water above the seabed is a lot like operating
an airplane in the air above land.  When I descend in a plane, the altimeter
winds down to zero, and then I land.  In the water, I use the fishfinder the
same way.  My depth gauge tells me my depth (distance below the surface) and
outside water pressure; the sonar gives me a readout of how far below the
"ground" is, and when that number drops to about 1.2 feet, I know I'm about
to make contact.  (The transducer's up in the bow, a little higher than the
keel itself.)  Depth or altitude: really only a question of semantics saying
the same thing for all practical purposes when operating a little sub like
mine.  We're on the same page here.

> I thing two meters would be very valuable in a P-Sub. First, and most
> important, is a depth gauge. You HAVE to know your depth. Second, a
> fathometer (or fish finder) would be very helpful for locating
> underwater "invisible" objects such as big rocks, pier pilings, sunken
> hulls, or other items you don't want to connect with - just view.

Yep, it does do that, and pretty well too.   Funny thing: the only time I
bottomed it  I had the darn thing turned off.  Fortunately my keel is solid
steel 2 1/8" thick by  9" tall and covered with axe-sharp "sawteeth" , so I
didn't receive any damage.  A fishfinder is great to have, but not worth
much if it aint switched on.  ;-)

See y'all later!

Pat