Aimar Tilted SS164 Question

[Out-Station]

Grandmarlin's thread on fitting that airmar ss164 tranny prompted me to ask a question that has troubling me for a bit.

I am considering fitting an airmar 1kw ss164 through hull 12 degree tilted element into my 15 degree deadrise platey ( i run a furuno 585). Reason being to fish deep, 300m plus for blue-eye etc. Now given the deadrise is 15 degrees but the tranny offset is 12 degrees, the tranny will always be shooting 3 degrees off centre. It may not seem allot but i calulated the effect of this when fishing in 400 meters.

1. In 400m the beam would be shooting 21 meters off centre to the side
2. In 400m the 6 degree narrow cone 200khz beam would be 42 meters in diameter so the offset would be half this cone
3. In 400m the 22 degree wide cone 50khz beam would be 150 meters wide so the offset wouldn't be as noticable.

Given the boats rocking around anyway etc etc do you recon that the 3 degree offset from perfect would be noticabe in pinpointing fish and dropping in on them in these big depths??

Any ideas from those with experience would be great.

Cheer's, Scott

[Captain Seaweed]

good pots, I had the same q's maybe throw this one at Grand Marlin?

[Captain Seaweed]

I think you will find the transducer he is mounting is a 20' tilted transducer not 1 but still thhere is 2 ' difference, Are your maths correct , What is Petes response?


Marty

[Captain Seaweed]

Just had a thought. I would like to know the actual maths for that one. I had a seafarer which I installed a 20' tilted element on to a pre drilled transdcucer area. A few questions regarding this and error.

If I have to install a transducer what effect has 1 or 5 degrees of tilt. I mean is the transducer at the same degrees is it accurate or not with boat roll and sea conditions. Does this impact transducer angle or is this insignificant? I understand the area interpreted is a cone shape and not a equal scan line. What r ur thoughts?
Marty

[Out-Station]

Marty, just google "right angle triangle calculator", just puch the scenarios into any of the calulators and away you go, trust me the numbers are right. I'll chuck the calcs up here if i get chance in the next few days. The airmar ss164 is available in 3 versions, 0,12 and 20 degree offsets, the 12 is the closest to my deadrise so this is what i'm looking at. In say 50 meters the offset is only a couple of meters to the side but my current 600w is fine for that depth.

Other thing to do is for me to use a 20 degree tranny, rotate the tranny on the bottom of the hull untill the elements are sitting at 15 degrees and put up with the beam shooting either behind or in front of the boat, still the same prob though of beem not shooting straight down under the hull.

Hopefully Grandmarlin jumps on here with some advice.

Cheer's, Scott

[Grand_Marlin]

good pots, I had the same q's maybe throw this one at Grand Marlin?

Why Me??? I only fit the stuff

Ok, a few basics first up...

A transducer throws out a cone shape beam (like an upside down funnel)
It receives a return signal that is bounced off the bottom.
A hard bottom will return a stronger signal than a soft bottom.
A shallower depth will return a signal faster than a deeper depth, given that the hardness is the same.
The depth sounder screen will display the strongest and / or first signal returned, no matter where this comes from in the cone - as it turns out this is very rarely straight under the boat. (play with Lowrance Structure Scan to find this out)

Therefore with a boat pitching and rolling, the transducer is never ever going to be shooting exactly where we think it is, or would like it to be.

Therfore, calm days and slow speeds (4 - 6 knots) are the perfect paramaters for sounding.
Accurate sounding at high speed in deep water is somewhat of a fallacy.

If you are over the shelf in 400m you will definitely be running 22 degree at 50KHz in this sort of depth.

The footprint is indeed 155m across
Maths is triginometry. Tan@=op/ad
Tan(11) = x / 400
0.1943 = x / 400
x = 0.1943 * 400
x = 77.7 m (half beam footprint)
beam footprint = 77.7 * 2
= 155.4m
(why are you lot doing this to me at this time of night on a Friday

Anyway - the returned beam can be from anywhere within this 155m area, with the boat rocking & rolling gives a huge area that it is trying to intrerpret.

.... makes the 3 degree error in the deadrise of your hull seem rather insignificant now.

Airmar rate the transducers to suit deadrise angles a few degrees either way for exactly the same reason.

Cheers

Pete

[Out-Station]

Pete, thanks for your thoughts. I suspected that would be the case, bottom coverage is a pretty variable science at best, my boat could easily have a 3 degree list today and me not notice, just put the sinker pot on the other side and all good!!!

Cheer's, Scott