[Rhodes22-list] transducer thru-hull

[Roger Pihlaja]

Bruce,

I have the Autohelm ST-50 tridata system, which is the immediate predecessor
to your ST-60 system.  Assuming your ST-60 transducer is like mine; then,
the thru hull has a key which prevents inserting the transducer out of
alignment.  When you say your transducer is out of alignment, I presume you
mean the entire thru hull is installed such that the alignment of the keyed
transducer is not parallel with the centerline of the boat.  Assuming we are
on the same page, there is no reason why one couldn't unscrew the big
flanged nut on the thru hull, break the caulk loose, remove the thru hull,
clean up the old caulk, & reinstall the thru hull in the correct alignment.
In the stern area of the hull under the cockpit, there is a nice handy CL
molded into the hull running from the back of the shoal draft keel
practically back to the cockpit drain.  A chalkline from the transducer thru
hull & running parallel to this molded-in CL would ensure transducer
alignment to less than 1 degree of [Error].  The existing transducer
misalignment will not affect the water depth or water temperature
measurements.  However, the knotmeter and knotlog will be off by the
following amount:

[Error] = 1 - cos(Theta)

Where: (Theta) is the misalignment angle in degrees between the boat's C.L.
& the transducer axis

For example, suppose (Theta) = 10 degrees:

[Error] = 1 - cos(10 deg) = 1 - 0.9848 = 0.01519 or about 1.52% low

Note that this transducer misalignment will always cause the speed &
distance measurements to be in error on the low side.  I leave it to you to
decide if 1.5% error is enough to worry about.

The Seatalk system is Autohelm's proprietary communications bus which
enables all the instruments connected to the bus to share and display data.

Hopefully, this helps, Bruce.  Good luck.

Roger Pihlaja
S/V Dynamic EQuilibrium

----- Original Message -----
From: <bgreenwald at optonline.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Sunday, March 28, 2004 1:19 PM
Subject: [Rhodes22-list] transducer thru-hull


> Hi all,
>
> I have an ST-60 tridata with a thru-hulll speed transducer,
> installed in the lazarette by GB 2 years ago on my then
> new 2002.  I have had some troubles since the beginning
> achieving accurate readings  - I tried various fixes until
> last summer when the whole thing failed - sent it back to
> Raymarine for a cable replacement.  In that process, I
> discovered that the thru-hull is not aligned perfectly with
> the center line of the boat - its probably off 5-10 degrees
> fro parallel.  I am curious if this is a material problem in
> accurately recording speed and, if so, if there is any way
> to remove the thru-hull and reseat it on the proper line.
> Any suggestions would be appreciated.  Thanks.
>
> Bruce Greenwald
> S/V Ruach II
>
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