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Rod
McInnis has been using an Interphase Probe around San Francisco Bay aboard
his 36-foot Carver since 1995 and was the least enthusiastic of my correspondents.
While he reports great service from Interphase, he says, “The reality
of a scanning sonar didn’t exactly match my expectations.”
His main gripe is the limited range. While a similarly priced fishfinder
might find bottom at 2,000 feet, no FLS is able to pack the same punch
into a multidirectional transducer. Interphase claims a forward range
of 1,200 feet for its top model, EchoPilot about half that. These are
small numbers, especially compared to the human eye or radar on the surface,
and they really only apply to ideal conditions. An important, and unfortunate,
limit to FLS range is depth; due to surface and bottom echo interference,
FLS can generally see ahead only about six times the depth of the water
column. Thus, in the 25-foot soundings that McInnis often encounters in
the California Delta, his forward view only extends 150 feet beyond his
bow (minus the distance his transducer is mounted aft to avoid aeration
when he’s planing).
McInnis
also points out the limitations of the Probe’s display in shallow
waters. Since its forward scale is one and a half times its depth scale
(see illustration on page 82), he must set it at 90 feet just to get a
135-foot forward look; hence, in 25 feet of water, he’s only using
about a third of the 3.25"x4" screen. It’s a small image
easily obscured by clutter. He concludes, “If you are expecting
something and are going slow, you could pick up an obstacle in time to
react to it. But if you are just cruising along, you will never get enough
warning. If I had to do it again, I probably wouldn’t buy another
forward looking sonar.”
However,
one man’s disappointment can be another’s delight. David Hoar,
writing in the April issue of Pacific Yachting, fairly raves about how
his trawler’s new Interphase Outlook helps him to gunkhole the outer
shores of British Columbia’s Queen Charlotte Islands. Hoar is an
old hand at this and had even equipped his dinghy with a sounder for reconnaissance
work in these famously rocky and poorly charted waters. Now he enters
a new anchorage at dead slow ahead, then makes a mental 3D survey by “kicking
the stern in a circle, using the forward-scanning sonar to determine the
distance to both visible and underwater obstructions while noting their
depths.” He says that he can even pick out the anchor chains of
other vessels, though his Outlook is a lower-powered (and lower-priced)
version of the Probe, and declares FLS is “without a doubt the answer”
for his adventurous cruising style.
Next page >
Sonar, Part 3 > Page 1,
2, 3, 4
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