[Rhodes22-list] Depthsounder

[Roger Pihlaja]

Jim,

I have Autohelm ST-50 instruments.  The depth transducer requires a 2" ID
hole & is about 3" long from the bottom of the flange to the end of the
threaded portion.  The cable is epoxied in placed & comes out the end of the
threaded portion.  In the location recomended by Stan amidships under the
cabin sole, there are at least a couple of inches of clearance between the
end of the transducer & the underside of the sole.

Doug,

I'm glad you are satisfied with your depthsounder installation.  Were you
aware of the potential echo return signal loss problem when you installed it
in that location?  Even though you got your unit to work, it's still
considered bad practice to mount it like that.  Another brand of depth
sounder may or may not work in that location.  By the way, are you sure it
works when you are heeled over on starboard tack?

As you are no doubt aware, depthsounders are not intended to look forward in
the manner of an active military-style SONAR unit searching for a submarine.
This capability would require that the transducer be mounted in a "blister"
which protruded downward from the hull or in a bludge on the bow.
Depthsounders are not intended to detect a sudden step change in water level
or obstructions ahead in real time.  Perhaps I worded my post a little too
strongly.  If I offended you, then I apologise.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Gardner, Douglas L. (LNG-DAY)" <douglas.gardner@lexisnexis.com>
To: "'The Rhodes 22 mail list'" <rhodes22-list@rhodes22.org>
Sent: Tuesday, February 18, 2003 4:42 PM
Subject: RE: [Rhodes22-list] Depthsounder


> 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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