Posted by jason on March 08, 2002 at 10:52:03:
In Reply to: Recommendations for depth & fishfinder for my boat? posted by Dominatebull on March 07, 2002 at 23:32:30:
When it comes to fish finders, or depth finders, it is VERY important to be familiar with sonar theory of operation, and to learn to 'read' the sonar properly.
The term 'Fish Finder', is very misleading, and in my humble opinion, nothing more than a marketting ploy. Many think that the fish finder will show them a plot or a sonar 'picture' of the bottom, weather it's a reef, rock, or wreck. The truth can not be further from that.
Theory of operation: Sonar pulses are transmitted, which reflects off of objects under the trnasducer, and return an echo. The sonar can tell about 2 things from this 'single point' of data: 1) Time it took for the signal to return, and 2) Signal strength.
The elapsed time will provide 'depth' indication of the object, while the signal strength will provide some info regarding the 'rigidity' or the density of the object.
The sonar will then 'map' that point, and move on to the next, so width-wise, what you see on the screen is not a picture, instead, it's a history of single sonar returns.
In another words, sonar (at least the ones us divers/fisherman buy) DOES NOT 'scan' the bottom. Therefore, for example, lets assume that your boat was over a 90 feet of 'sandy' bottom, and there was a big rock or structure standing 20 feet tall in the middle of it.
You navigate your boat over the 'rock' and stop. As you were pulling over the rock, you would notice the depth would change from 90 to 70 feet on your sonar screen.
Standing still on top of the rock, and viewing the sonar screen would only yield 'flat line', at 70 feet of depth, with NO HINT, WHATSOEVER of a big rock in the middle of the sandy bottom.
This is when your less informed friends will start telling you that your 'fish-finder' is a peace of @**!@, and that you should get a good one.....
How about the little fish symbols? Well, whenever there is a suspended particle between 'the bottom' and the surface, the 'computer' will display fish symbols. A floating log, for example, 10 feet below the surface will cause the 'fish alarm' to go off, and will be displayed as a 'fish symbol'.
In short, to get an 'idea' of what's down there, you must view the screen, when in motion. The sonar and target have to be in relative motion for a 'representation' of the bottom. You would get to the general area, cruise around slowly while viewing the sonar display, until you see a hump/bottom structure of some kind. It should be easier in a sandy bottom, since there will be less 'rocky', and any structure will be easier to distinguish.
Furthermore, based on the signal strength, the display will give you an idea of the 'composition' of the bottom (hard, soft, mud, etc). By the way, when you do find the wreck, don't expect it to even remotely resemble a 'ship wreck' on your screen.
Something else to think about is the 'cone angle'. Typically, on an average, the 'area' of the bottom that you see on the screen, is roughly half of the depth of the object.
In other words, if you are in 100 feet of water, picture a cone, with it's tip at the bottom of the trasducer, openning up all the way to the bottom.
You are looking at a 50 foot diameter circle on the bottom. If a fish swims under your boat, 50 feet below the surface, it must be withing the 25ft circle of the cone to be detected.
For this reason, in shallow water, the sonar becomes a lot less effective. In 20 feet of water, the bottom circle is only about 10 feet in diameter.
When choosing a sonar/fish finder, there are several things to consider. One is power to 'reach' where you will be looking. Since I assume you want to dive the wreck, I don't think you will be going deeper than 500 or 600 feet!!!!, so power-wise, most units will cover it.
Perhaps the MOST important feature of a sonar is 'Vertical' resolution (horizontal, again, is just a 'history', and does not matter much. The more the horizontal resolution/pixels, the longer the history).
Vertical resolution is important because they can help paint a better picture of what you are looking at.
If the unit has 120 vertical pixels of resolution, and you are in 60 feet of water, each pixel will represent half-foot vertical distance (6 inches) as there are 2 pixels per foot.
A better unit, with 240 vertical resolution, under the same circumstances, will have 4 pixels per foot, or a 3 inch resolution.
That said, I like the Garmin Blue 240 pixels, as well as Lowrance X-85 (also 240, I believe). However, my old 'el cheapo' Hummingbird 100 SX model on my previous boat, had served me pretty good!(128 pixels, if I remember correctly), but was not as 'up to the task' as a higher resolution one.
Sorry for the long post... Got carried away. Hope this is of some help.