Going Deep (CHIRP Technology)
The captain who piloted us around is well known for his swordfishing and general bottomfishing prowess. He told us what any captain would tell you: Knowledge of location is paramount. (Ironically, we are keeping his name and his boat’s name secret so you don’t follow him.) To show us the value of CHIRP technology, he ran and set up on six fishing spots in the span of three hours.
Then it dawned on me: We ran and set up on six wrecks in three hours. And, en route to these spots, our captain passed over new “undiscovered” structure and marked it with his MOB button. One of the key facets of CHIRP sonar is that it holds ground deep, and at speed.
“At 25 knots I can mark a fish at 1,000 feet,” said our amazed captain. (A Garmin unit recently held bottom at below 17,600 feet.)
CHIRP is one of the biggest game changers to filter down from the military to date. Here’s why.
Traditional sonar operates at a single frequency, though you’ll likely get a choice of two, like a short, fast, high pulse (or ping) of 200 kHz for shallow returns or the long, slower pulse of 50 kHz for deep readouts. Some fish finders even let you run two frequencies at once, but the limitations are documented: lost returns on intermediary marks, lost bottom at high speeds and so forth. One of the main problems with deepwater returns at 50 kHz is that the long waves required to probe the depths do not pick up detail. So structure readouts come back as flat bottom, slight contour changes or lumps.
The Simrad BSM-2 box, paired with the proper Airmar transducer (see boatingmag.com/transducers), solves those problems. Instead of a single-frequency “ping,” CHIRP sends a modulated pulse — a chirp — that sweeps through a range of frequencies: 130 to 210 kHz for the high band and 25 to 45 kHz for the low. The BSM-2 module translates the best frequency based on the best return and constantly adjusts it in motion to show detailed reads from the depths.
What does this mean? Our captain could run to a designated spot and quickly scan the structure to see if fish were holding, and exactly where they were holding. So he’d know where to set his anchor or his drift and the depth to set his lines to get a bite. Oh, yeah, he marked a large, free-swimming pelagic fish at 700 feet while we were at cruise.
What does this mean for nonanglers? The price of Simrad’s BSM-2 is steep ($2,495, plus a four-figure sticker for the transducer), so unless you’re a wreck fisherman or a swordfisherman, you may not yet have a justification for it. Or maybe you do.
As our captain noted, “On our last trip to Bimini we marked a big ol’ mountain range at 2,500 feet.” He went on, “I’ve made that crossing hundreds of times and never knew it was there.”