[Rhodes22-list] Depthsounder

[Gardner, Douglas L. (LNG-DAY)]

Jim, just for clarification:

Underneath the potti the hull does not have a pronounced V, and it is not
yet forward of the leading edge of the keel, but it is SLIGHTLY forward of
the deepest portion of the keel and the centerboard.  If you go further
forward, yes it does get more pronounced.  Where my sensors are, they are
nearly horzontal (although not as flat as further aft in the same location)
and they've never been out of the water on a heel or been in dirty flow.  To
my knowledge, I've never lost a signal, although I admit that I don't
usually spend much time looking at my instruments when I'm at 25 deg of
heel.  
;^)

Instrument placement is as much an exercise in trade offs as anything else,
and your cruising grounds may bear on your decision. It is important to know
the trade-offs you make, and to do it with your eyes open. Personally, I
still prefer the common location of under the potti.  Other locations are
quite suitable, and are superior in certain respects (as I noted in my
original post).  

Oh, Roger...  regarding your statement: 'Anyone that thinks such
a short distance is going to keep them out of trouble in rapidly shoaling
water doesn't know how to properly use a depthsounder.'

I find that statement offensive.  If you targeted that to Jim's desire to
have his transducer forward of the keel, I find it patronizing and
unfriendly at best. If it was in response to my reply on the sailnet list,
then I find it insulting. If you read my post you should have noted that I
limited that advantage severely.  However, I stand by it.  If your sensor is
in front of your keel, you have SOME chance of detecting an object before
you hit it.  IF you detect an object, you may not have much time to react,
but some time is better than no time.  You may even have enough time to
brace yourself and keep your face off the deck. If your sensor is behind
your keel, you have no chance.  Is this proper use of a depth sounder?  No.
Depthsounders are to get macro (general) depth information and not to detect
specific obstructions.  Could it be useful information?  You betcha.  Do I
rely on it?  No way.  Either way, lighten up.  

Jim, if you bothered to read this far... consider all of the alternatives
and weigh them accordingly.  Think about what you need and weigh the
alternatives. Choose the one you think will work best for you and you
probably won't go wrong. In fact, the biggest limitation has not yet been
discussed: Please make sure you have enough clearance ABOVE the transducer
(for the sole or other obstructions).  It'd really bite to drill the hole
only to find that the transducer sticks up a half inch higher than the cabin
sole!

Good luck, 

--Doug


-----Original Message-----
From: Roger Pihlaja [mailto:cen09402@centurytel.net]
Sent: Tuesday, February 18, 2003 1:08 PM
To: The Rhodes 22 mail list
Subject: Re: [Rhodes22-list] Depthsounder


Jim,

When I installed the depthsounder on Dynamic Equilibrium, I had many of the
same questions as you.  So, I asked Stan at GBI for his recommendation re
tranducer location.

As per Stan's instructions, I mounted the transducer on the starboard side
just inboard of the ice box as close as possible to the keel.  The inboard
edge of my depthsounder transducer is tangent to the point where the hull
shape transitions from the flat "garboard" area to the tight radius fillet
to form the vertical keel.  I wanted to have quick access to the backside of
the transducer.  So, I mounted a 6" diameter Beckson screw-in inspection
deck plate in the cabin sole right above the transducer.  My transducer is a
thru-hull installation.  So far, the installation has been absolutely
bulletproof & has never leaked a drop.  I never lose the bottom return echo
signal on either sailing tack + it always stays submerged in bubble-free
water.  The transducer is located well inboard of the trailer bunks.  I've
never had a depth sounder transducer/trailer interference problem while
trailering the boat in dozens of launches/recoveries & thousands of
trailering miles.

There is a couple of fundamental problems with mounting locations up forward
of the keel, like in the head area.  The R-22 hull has a pronounced deep-V
shape anywhere forward of the keel.  Echo sounder transducers do not
transmit & receive outside of a fairly narrow cone.  If you mount one of
these transducers on an angle, such as would be required up near the bow, it
may work fine on one tack or while motoring in level trim.  But, when you
get over onto the opposite sailing tack & the hull heels over, the
transducer will probably lose the bottom return echo signal.  The hull shape
in Stan's suggested location is much flatter & thus is not subject to this
affect.  In addition, the water flow over the transducer is much more likely
to have entrained bubbles when mounted close to the bow, especially on the
sailing tack wherein the transducer is lifted closer to the surface of the
water.

There is only about 6' of hull separating possible mounting locations
forward of the keel & Stan's recommended location.  Anyone that thinks such
a short distance is going to keep them out of trouble in rapidly shoaling
water doesn't know how to properly use a depthsounder.  At 4 knots, the 6
foot forward mounting location translates to about 0.9 sec extra warning
time! Of course, the transducer may not be getting a bottom echo return
signal, so you may not get any warning at all.  I'd much rather have an
absolutely reliable transducer mounting installation than worry about such a
trivial amount of potential extra warning time.

Roger Pihlaja
S/V Dynamic Equilibrium


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