[Public-List] depth transducer

[Stephen Gwyn]

Hi,

I've gone through two depth sounders, both with
interior transducers. The unit that came with
the boat was mounted under the port settee,
sitting a plastic box full of mineral oil.
It worked fine for a few years, but then
started giving different depths if you switched
from fathoms to feet. I recently saw an ad
for the unit in a sailing magazine from the 1970s.
Apparently it cost 2000$ then.

I replaced it with 100$ Humminbird unit 9 years
ago. I put the transducer in a puddle of epoxy
under the port settee against the forward
bulkhead. I built a dam with duct tape, poured
in the epoxy and then smushed the transducer
in. Use slow cure epoxy, to avoid bubbles.
It's worked fine ever since. If the water
is getting deeper, it will follow the bottom
down to 600 feet (the maximum for the unit).
Coming in from deep water it usually won't
find the bottom until gets as shallow as 400
feet, so maybe the performance is being
slightly affected, but I can live with it.
The only real problem is that I sail near the
Fraser River, so I get fresh water over salt layers.
There's a change in the acoustic properties of the
water, and you can get a false return off the
boundary, usually at the alarmingly shallow depth
of 12 feet, when the bottom should be 300 feet.

To test whether your favorite location will work,
fill a zip-lock bag *completely* with water. Let
some spill out as you close it.  There should be
no bubbles in the bag whatsoever. Put the bag
in your chosen location then put the transducer
on top of the bag. If you get a good return,
then the epoxy puddle method will work.

Alberg 30 hulls are solid fiberglass, which
is a good sound transmitter. Cored hulls on
the other hand don't allow an interior mounted
transducer to acoustically couple to the water.

SG

-- 
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Stephen Gwyn                       | Tel: 1-250-363-3136
Canadian Astronomical Data Centre  | Fax: 1-250-363-0045
Herzberg Institute of Astrophysics | Cell: 1-250-885-6969
5071 West Saanich Road             | E-mail: Stephen.Gwyn at nrc-cnrc.gc.ca
Victoria, BC, V9E 2E7, Canada      | http://astrowww.phys.uvic.ca/~gwyn/
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