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Location: Green Bay, WI | Well, but the *angle* of the transducer beam should not depend upon the depth at all. The angle of the beam, is the angle of the beam. It's the angle the beam comes out of the transducer. The amount (volume) of water covered IS dependent upon the angle however.
For instance, if the starboard side of the transducer has a crystal with a 90-degree beam width (say 45-degrees either side of zero), and the crystal is pointed down and right at 45 degrees, then the arc scanned by that beam is from straight down under the boat (-45 degrees), to just under the surface of the water on the starboard side of the boat (+45 degrees). So in that case, an arc of 90 degrees on either side of the boat, would give you 180 degrees of water coverage--and the limits of your scan would be the bottom of the lake, or (as the beam moves out farther to the sides) the point where any sonar returns from targets are so weak that they are not strong enough to be perceived with certainty. But that's with 180 degrees of coverage...90 to the left, and 90 to the right. But I doubt that is what's going on here.
My guess is that the beam width for each half is something like 70 degrees, centered around the +/- 45 degree points on either side of the boat. So then there is an area directly under the boat being missed by the side imaging beam, and also a triangle-shaped column of water that is getting larger as the beam travels out from the transducer. So if it IS a 70-degree beam width (symmetric on either side), that means that about 10 degrees of coverage is missed under the boat, and 10 degrees are missed out to the side of the boat. Therefore:
tan(10) = height of missed water column / distance from boat
0.176 = height of missed water column / distance from boat
So at 100 feet out from the boat there is about 17.6 feet of water depth not being scanned, starting at the surface.
The importance of this is very simple--if you are scanning the basin of a deep lake (e.g.; Trout Lake in Vilas County) looking for bait fish, you need to realize that because of the beam width, you aren't going to be scanning the upper 18 feet of water 100 feet to either side of the boat. And as active basin muskies are often shallower than you might think, this becomes very important. Simply put, you might be scanning UNDER the bait fish (and any muskies!) that are sitting out there 100 feet to either side of the boat.
You're looking for a needle in a haystack out there to start with--you may as well do it knowing whether or not both eyes are open.
TB | |
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