Re: Up to what speed should you fishfinder function?
How fast can a car go?<br /><br />Its really about the same sort of question and just like the one you asked, it has no answer. There is a maximum speed the boat could go, in a specific depth of water, at which it could not function, but that is purely a matter of the speed of sound in water and the boat's ability to outpace the returning echo from an outgoing ping produced by the transducer. I have calculated that speed a couple of times and the absolute number is very high, much higher than any of our boats will go. The deeper the water the slower the absolute speed you can read. So the problem of a failed reading is not one of boat speed, its something else.<br /><br />As Boatits said, the problem lies in the mounting or in the hull configuration, but in each case it is related to the flow of water over the face of the transducer or the hull below where the transducer is located if the thing happens to be in-hull mounted.<br /><br />Because of its incompressability water is a very good medium for sound to travel through. Air, on the other hand, provides a much less dense medium for the transmission of sound waves. So if you have constant contact between the transducer face and the water you get the best sound transmissio characteristics. On the other hand if you introduce air into the water that is flowing over the face of the transducer or if for some reason a bubble forms over the face of the transducer then the ability of the transducer to put out pings into the water or to receive echos back from the bottom is greatly reduced.<br /><br />So, where to bubbles come from? Well, as Boatist said, from rivits, from strakes, from thru hulls ahead of the transducer, and sometimes just from a poorly designed hull that foams up the water a lot. That is the hull side of things. Then there is the design of the transducer itself. Some of them are simply shapped poorly and create their own bubble-mess as they are pulled through the water. Of course there is the mountying style of the transducer to contend with as well.<br /><br />While we often think of transducers as commin in basically three styles; In-Hull, Thru-Hull, and Trnasom Mount, in fact the actual transducer element that is used in each of the three is often the same between models made or sold by any given fish finder manufacturer, and many times between manufacturers as well. As you may already know a single company, Airmar, makes the vast majority of transducers used on small boats all over the world - and most of those used on larger boats too for that matter. There are a few off the wall companys that make their own, folks like Humminbird and Lowrance, but in fact Airmar even makes transducers that can be used with those company's machines if you want to buy them. My point is that inside the transducer the same elements and circuitry are used and so the performance of one is very much like the other. The outside cases vary a bit and that is often the cause of speed reading problems, but the transducer itself is reall probably the same no matter who made it, within reason.<br /><br />Of course where and how the transducer is mounted matters a lot. If the transducer is mounted so that its face is always covered with a smooth flow of water and it is also pointed straight down, or slightly forward of down, it is going to work at any speed you can go. That is why true thru-hull transducers work so well. They are mounted on the outside of the hull, on its bottom, and they enjoy constant immersion. On the other hand if you mount a transducer to the transom the first thing you run into is a little space between the back of the boat and the face of the transducer and you have some sorts of bracket holding everything in place, and some sort of adjusting scheme to allow you to compensate for the shape of the transom and all of that stuff adds up to bubble makers when you get going at some speed or another. That simply means that with a transom mount at some speed you will get bubbles and when that happens the transducer will quit working.<br /><br />The third method of mounting a transducer is to put it inside the hull. With this system the sound passes through the hull material and into the water as does the return echo. If the flow of water is smooth and not filled with air directly under the mounting location these transducers will operate well. Of course there is a basic problem with this sort of mounting. That is that there is power loss passing through the glass and some difusion of the concentration of the outgoing signal (roughly equal to an incresase in the much-misunderstood "cone angle") but it is of little importance.<br /><br />So basically the bottom line is that in-hulls will work at the fastest speeds, thru-hulls are next best, and transom mounts are the worst when it comes to speed. Its all a matter of mounting of course and what you have to do is find a place with clean water to get one to work well. After that its a matter of adjustment to make sure its shooting straight down, or slightly forward of straight down, when the boat is at its running attitude.<br /><br />As to the absolute question of speed, well that vaires from one hull to another and transducer type. As as example I've gone through several different transducers on our boat along with several different fish finders. The fish finder I use (Furuno 582L) is probably one of the the more sensitive small boat fish finder made and it has very sophisticated software for interpreting return signals. It also has plenty of power so in any water up to a couple of thousand feet it is not limited in its ability to read on that front. <br /><br />For about a year I used a standard thru hull trasducer with it, the Furuno model 520ST-MSD, which is a bronze depth/speed/temperature unit. I used it with a standard fairing block and a great deal of attention was paid to getting it exactly level when it was mounted. That transducer/fish finder combination was good for speeds up to about 30 knots on nice days, less when it was bumpy, considerably less when we were getting our eye-teeth knocked out. I removed that transducer last winter and glassed the hole it passed through shut. I then ground out a section of the glass in my hull's bottom and reglassed it to insure that there were no voids in my glasswork and mounted a new in-hull transducer (Airmar M-260) in essentially the same spot as the old thru-hull had been mounted. This mounting was right on the center line of our 23' boat, situated about 3~4 feet forward of the transom. That mounting location corresponds with the smoothest flow of water under my hull. That transducer will not loose bottom because of any speed my boat is capable of going. My maximum speed is 38 knots (43.7 mph). That is not to say that the fish finder will not loose bottom at lower speed. If we are in water greater than about 1,500~2,000 feet it will loose bottom at lower speed, but that is a function of power, not transducer type or placement.<br /><br />So, does that help answer your question? The speed you read is a matter of transducer type, mounting location in terms of a smooth flow of water over its face, quality of mounting in terms of being pointed straight down, and finally the relationship of depth to speed.<br /><br />By the way, if you are interested in one of the high performance transducers for a small fish finder the M-260, and its thru-hull cousin the B-260, are as good as it gets. They are quite large and they have a maximum input power of 1 kW, but if you want the ultimate in fish finder performace they are the transducer you want. They are available to connect to many different makes and models of fish finder. Be prepared for sticker shock though. The B-260, which is the thru-hull model is expensive and you will want to use the high speed fairing block with it. In the in-hull, M-260, version it will come with a mounting tank that you will cut to match the interior of your hull and then seal in place with 5200. The tank is filled with mineral oil and the transducer is supspended within the tank. This outfit isn't cheap by any stretch of the immagination, and in fact costs about $100 more than the bronze thru-hull version. For the money you get to look at the bottom at any speed you want to go and you get to do it with a clairity unmatched by any other transducer. Each of these two transducers uses a single large element (about 50% larger than the standard transducer) for its high frequency operation and 7 smaller elements for high frequency. This is in comparison to a single smaller element doing dual duty for standard transducers.<br /><br />Thom<br /><br />Oh, just for comparison, just to give you an idea of the size, here is a picture of the M-260 and its mounting tank next to a standard thru-hull transducer. The 260 is the big black blob, the mounting tank is the yellow thing. That stainless steel plate is what holds the transducer in place at the top of the tank. In the foreground (because its closer to the camera it looks a lot larger than it really is) is the standard bronze thru-hull:<br /><br />
