Re: In hull transducer
Well, here is a different version of the story, from a different perspective I guess I should say.<br /><br />There are three mounting styles for transducers. The first and simplest is the In-hull mounting where the transducer is physically inside of the hull with its sound emitting face pointed down. It may be held directly against the hull with an adhesive or it may be suspended in a tank that is filled with a liquid. The sound produced (the "ping" which actually sounds like a click) passes through the hull and into the water column and of course the return echo does the same. There is suprisingly little loss of sensitivity with these mounts and they have definite advantages. Because the water flow under the boat is undisturbed these things will generally work at any speed the boat will go. You never loose the bottom. They aren't suceptable to being struck by objects in the water, or forklifts, or clowns in parking lots.<br /><br />If you mount one of these and glue it down use the hardest setting material you can find. Normally epoxy resin works best and if you wanted to mix some aluminum oxide power in with it that wouldn't hurt a thing either. You put down a pool of the stuff and then slip the face of the transducer into it making sure no air is trapped below. Then just clamp it pointing straight down until the epoxy sets up.<br /><br />The alternative involves a mounting tank. Generally a plastic tank is cut to match the bottom and then it is held in place with a marine adhesive (3M's 5200 or similar). Once its set up the tank is filled with something like mineral oil and then the transducer is suspended in the tank by its lid. Basically anyone with a hacksaw and a caulking gun can install one.<br /><br />In the picture below you will see a bronze thru hull transducer in the foreground. Is size is out of porportion because of the photo. The big black blob is a high performance In-hull transducer. The yellow plastic thing is the tank, in this picture this one has not been cut to match the bottom yet. The tank comes cut at some angle (maybe 15 degrees or so) but you cut your own if that isn't the correct angle. You can also see the stainless steel lid in the far right of the picture. I holds the transucer above the bottom in the oil. There is also a cork liner to the tank that deadens outside sounds.<br /><br />
<br /><br />It doesn't take much room for access for one of these things either. Here is one installed where an inner liner had to be cut out a little bit to gain access:<br /><br />
<br /><br />The most common type of transducer sold by far is the transom mount. The reason these are the most common is that anyone can mount one and get it to work if they follow the instructions. The other advantage this type of mounting has is that the transducer face is in direct contact with the water. This is far and away the best situtation but unfortunately this is not the best mounting style - no matter what. The main problems with this mounting style is that there is some level of turbulence aft of the transom and that is where it will be mounted. The turbulent water detracts from the transducer's ability to work properly, particularly as the boat goes faster.<br /><br />The third mounting style is the thru-hull. With this mounting the transducer resides outside of the hull but has a stem that passes through the hull to get the data cable inside. This type requires a hole be drilled in the bottom of the hull. It is generally speaking the best transducer type to use. It has the advantage of having its face in direct contact with the water, it also has the advantage of being in genearally an undisturbed water flow. They are certainly useful with look-ahead sonars (which are rare as hen's teeth and only really work if you have thoudands of dollars invested) but thru-hulls are common as dirt on salt water boats - including trailered boats. It requires considerably more skill to mount one of these than the other two styles but its still not bad. There is a variation of this type of mount where a pocket is glassed into the bottom of the hull and it holds the transducer so its face is flush with the hull but still in direct contact with the water. You can usually have the glass work done for this type of instillation for about a grand if you don't want to do it yourself.<br /><br />Otherwise the mounting of a thru-hull will normally require a fairing block ($35 for a blank one you will have to cut). <br /><br />Here's what the transducer looks like standing alone. That stem is about 2" in diameter to give you some feeling for the size:<br /><br />
<br /><br />Notice that I have constantly refered to the transducers by their mounting style. The reason for that is that as a general rule most fish finder manufacturers or retailers will package their fish finder with either of the three types but in fact the guts of the three transducers are usually the same so any performance differences come from mounting.<br /><br />I presume you understand the operating characteristics of transducers so I won't go into that. You should know that there are high performance transducers available for most fish finders that can make a very big difference in performance. They are not inexpensive but they work. For your money you get a lower Q factor (less ringing) and generally a much tighter cone, more elements and bigger ones. <br /><br />Also you probably should know this too. The days when just about every fish finder needed a matched transducer are long gone. Today not only does one company (Airmar) make the vast majority of transducers on the market but as it turns out most of them are actually using the same 40-something mm element anyway. That means that with a switch of plugs you can change many fishfinders without having to switch transducers.<br /><br />It was that improvement in life, impedence matching to 300, 600 and 1 kW that got me to make the jump to one of the big transducers (mounted in hull). The improvement in performance was simply stunning but more imporant in my decision to buy it was that I can use it with any future fish finder I might buy up to 1 kW.
