Looking Ahead Page 2

Looking Ahead - Forward-Looking Sonar - Part 2
Looking Ahead

Part 2: However, one man's disappointment can be another's delight.

By Ben Ellison — July 2002

 More of this Feature

• Part 1: Sonar
• Part 2: Sonar
• Part 3: Sonar

 Related Resources
• Electronics Index

 Elsewhere on the Web
• Interphase
• Pilot Marine

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

This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.

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