Look All Around

HOW EX­PERTS CHOOSE AND USE SCAN­NING SONAR TO FIND FISH Capt. Jeff Jones knows scan­ning sonar in­side and out. Not only has he used these sys­tems for more than 30 years to find fish, but he has also in­stalled them — most re­cently aboard the 36-foot Freshon

Saltwater Sportsman - - Float Plan / Electronics - By Jim Hen­dricks

Soon after com­plet­ing the in­tri­cate in­stal­la­tion of the Fu­runo CH-270 scan­ning sonar on said boat, Jones put the sys­tem to good ad­van­tage, find­ing a school of white seabass un­der a kelp bed at San Cle­mente Is­land. Hav­ing spied them at a dis­tance of more than 50 yards, Jones set an­chor out­side the kelp and scored 10 of the prize croak­ers.

This is just one ex­am­ple of how scan­ning sonar ben­e­fits boat­ing an­glers when fish­ing rel­a­tively shal­low in­shore wa­ters. Born in com­mer­cial ap­pli­ca­tions and cost­ing far more than most fish find­ers, these sonar sys­tems also work ex­ceed­ingly well when look­ing for blue­wa­ter fish such as mar­lin, sail­fish and tuna.

True Sonar

The word sonar is bandied about loosely to­day in de­scrib­ing var­i­ous fish find­ers that trans­mit ver­ti­cally and side­ways. Yet only true sonar can search hor­i­zon­tally in a full cir­cle, cre­at­ing a dis­play im­age that’s much like that of radar. For this rea­son, it is of­ten called scan­ning or all-around sonar.

The trans­duc­ers also dif­fer sig­nif­i­cantly in size and con­fig­u­ra­tion from those used for fish find­ers. Re­sid­ing in tubes as large as 8 inches in diameter, the sonar trans­ducer as­sem­bly de­ploys from within a sea chest and pro­trudes well be­low the hull when in use, then re­tracts when the boat is run­ning at higher speeds. In­stal­la­tions are com­plex and ex­pen­sive.

In some sys­tems, the trans­ducer me­chan­i­cally ro­tates as it trans­mits. With more-ad­vanced and even pricier scan­ning sys­tems (what Fu­runo calls omni sonar) such as the Fu­runo CSH8L Mark 2, the trans­ducer is sta­tion­ary, but it’s equipped with an ar­ray of 420 el­e­ments that trans­mit elec­tron­i­cally, re­sult­ing in a sonar im­age that’s up­dated in real time.

Mo­tion sen­sors on all of Fu­runo scan­ning-sonar sys­tems com­pen­sate for the rock­ing of the boat in rough seas and keep the sonar beam an­gle aimed as level as pos­si­ble, help­ing to en­sure that the sonar sig­nal does not os­cil­late.

Sonar Ad­van­tage

Such sonar should not be con­fused with side-scan­ning sys­tems such as the Fu­runo DFF-3D, Garmin Sidevü, Hum­min­bird Mega Imag­ing, Ray­ma­rine Side­vi­sion or Sim­rad Struc­tures­can HD. With these, a beam sweeps ver­ti­cally side to side.

Scan­ning sonar’s hor­i­zon­tal sweep of­fers the abil­ity to search well be­yond what is to the side or un­der the boat. You can look 360 de­grees around the boat and as far out as 6,000 feet with some sys­tems, but you can also limit the search to a des­ig­nated sec­tor such as 120 de­grees in front of the boat.

Jones says that scan­ning sonar gives him a huge ad­van­tage in find­ing fish in a wide range of sit­u­a­tions.

“The Fu­runo CH-270 sonar on Fresh One lets me search more than 2,000 feet in a full cir­cle around the boat,” Jones points out. “That al­lows me to find schools of tuna that other boats (with­out scan­ning sonar) might drive right by.”

While search­ing ahead and to the sides proves use­ful in tuna fish­ing, Jones likes to look be­hind the boat when trolling for striped mar­lin. “I can see the fish swim­ming into the trolling spread and alert the crew to get ready with drop-back baits,” he says.

When fish­ing wrecks in shal­low wa­ter, Jones tilts the beam down­ward to look for schools deep or to get a bet­ter look at bot­tom struc­ture. Like all scan­ning sonars, the CH-270 lets you ad­just the sonar beam an­gle up and down.

Mat­ter of In­ter­pre­ta­tion

Skip­pers who possess years of ex­pe­ri­ence in us­ing scan­ning sonar have few is­sues with in­ter­pret­ing the dis­play, but new­com­ers might find it dif­fi­cult, says Eric Kunz, se­nior prod­uct man­ager for Fu­runo USA. This is particularly true when it comes to search­light sonar, which Kunz likens to ro­tat­ing a flash­light on a stick.

Search­light mod­els like the CH-270 ro­tate rel­a­tively slowly, tak­ing as long as 64 sec­onds to make a full-cir­cle sweep, or “train,” as it is called, at the max­i­mum range of 2,500 feet. This is be­cause the CH-270 sam­ples in rel­a­tively thin sec­tors — some as nar­row as 6 de­grees — at a time. It waits for each re­turn be­fore mov­ing on to the next sec­tor.

“It’s easy to lose tar­gets due to the slow ro­ta­tion, particularly at long range,” Kunz says. “By the time the sonar cir­cles again, a fast-mov­ing tar­get such as a bluefin tuna might have dis­ap­peared from the field of view.”

In shorter range set­tings, how­ever, train time is re­duced. At the 400-foot range, for ex­am­ple, the CH-270 re­quires just 11 sec­onds to make a full-cir­cle sweep, and that can help with in­ter­pre­ta­tion, but fish don’t al­ways co­op­er­ate by get­ting that close to the boat.

Au­dio De­tec­tion

With a rel­a­tively slow sweep, it’s im­prac­ti­cal to stare at the dis­play, wait­ing for a tar­get to ap­pear. In­stead most cap­tains lis­ten to the au­dio of the sonar’s “tap, tap, tap” in the back­ground.

“When a tar­get ap­pears, the au­dio changes tone,” says Steve Brad­burn, as­sis­tant prod­uct man­ager at Fu­runo USA. “The sounds be­come more of a rat­tle, alert­ing the cap­tain to check the dis­play and pos­si­bly change course to in­ter­cept the fish.” An op­tional speaker can am­plify the sound.

Sec­tor sonar sys­tems such as the Fu­runo CH-37BB of­fer the abil­ity to train faster be­cause they can be ad­justed to read big­ger sec­tor steps — as much as 45 de­grees at one time. The CH-37BB also of­fers great range, up to 6,000 feet. On the 45-de­gree set­ting, one train re­quires about seven sec­onds at the 400-foot range.

Most scan­ning sonar mod­els have a fixed fre­quency, though dual-fre­quency mod­els are also avail­able. The Ko­den KDS-6000 broad­band sonar, how­ever, en­ables you to ad­just the fre­quency from 130 to 210 khz. As a rule of thumb, the lower the fre­quency, the greater the range; higher fre­quency lim­its range but pro­vides greater de­tail. You can also ad­just the sec­tor steps from 5 to 20 de­grees on the KDS-6000. On the 20-de­gree sec­tor-step set­ting, it takes 12.2 sec­onds for one train at the 400-foot set­ting.

Omni Present

With the real-time pre­sen­ta­tion of an omni sonar such as the Fu­runo CSH-8L Mark 2, in­ter­pre­ta­tion is much more in­tu­itive be­cause there is no need to wait for the screen to re­fresh to reac­quire a tar­get, Kunz ex­plains.

“The trans­ducer is blast­ing in all direc­tions at once to give you an im­me­di­ate read­ing all around the boat,” he points out. “And that makes the re­turns on the dis­play much eas­ier to fig­ure out.

“Peo­ple want to look around, rather than see one slice at a time,” Kunz adds. Range scales for the CSH-8L Mark 2 ex­tend to more than 5,000 feet.

What size boat do you need for a scan­ning sonar? “A com­fort­able size would be boats 30 feet and up,” Brad­burn says. “The trans­ducer sea chest needs suf­fi­cient head­room be­lowdecks, and sys­tems work best on boats with sig­nif­i­cant draft so the trans­ducer op­er­ates in clean wa­ter.”

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