Join Date: Feb 2001
Location: West Virginia
| | Through-Hull transducer mounting
OK, you asked for it:
All of the bottom machines we use on our small boats work the same way and have common parts. The machines consist of a display unit, a power supply cable, a data cable connecting the display unit to the transducer, and the transducer itself. In operation electrical power is supplied to the display unit via the power supply cable, a pulse of electrical energy is produced at the display unit, the electrical energy is transmitted to the transducer via the data cable, and the electrical energy is converted to sound at the transducer. The sound emitted by the transducer is directed down, where it comes into contact with various objects (fish and the bottom) and a return echo bounces back up. The return echo strikes the transducer where its sound is converted back into electrical energy. That returned electrical energy is carried to the display unit where it is interpreted and exhibited on the screen.
Because the speed of sound in water is more or less constant at about 4,000 feet per second (temperature and impurities in the water do effect speed, but not by much) it is possible to time the interval between the creation of an energy pulse and the return of an echo and use the time to determine the distance from the transducer of the object that caused the echo. This is how a fish finder works.
The heart of the system is in the transducer. The transducer contains one or more elements which convert electricity to sound and then sound to electrical energy. These elements are sometimes called crystals and are sometime called piezoelectric. This is a property of some materials that causes them to ring when hit with a surge of electricity and to produce a small electrical charge when they are stuck by a sound wave.
The sound emitted by a transducer will be made at a specific frequency, and some are multi frequency units. The most common frequencies used for small boat bottom machines are 50 kHz (Kilohertz) and 200 kHz. Higher frequency sound has less ability to penetrate deep water than lower frequency sound. Some transducers are single frequency but most are dual frequency and give the operator the ability to select the frequency of choice depending on the depth of water in which the unit is being used. As a general rule the units on small boats will not have enough power to penetrate much deeper than about 250 feet on the higher frequency but will work to the maximum depth their power output allows on 50 kHz. In typical units with power outputs of 300 to 500 watts RMS (Root Mean Square) the maximum depth obtainable is usually in the 700~1,000 foot range.
So, in short there is a power supply which is part of the boat’s systems. There is a display unit that is responsible for initiating electrical pulses, receiving electrical pulses, interpreting the meaning of the return pulse and displaying it. There is a data cable that moved electrical energy between the display unit and the transducer, and there is the transducer itself that goes a long way toward determining the entire system’s capabilities.
While all transducers work the same way they can vary in how they are placed on the boat and that placement can effect their operation. There are three ways a transducer can be placed on a boat, each with its advantages and disadvantages. The three ways are to mount the transducer inside the hull and allow the sound to shoot thru the hull material, to mount the transducer on the exterior of the boat, on its bottom, in direct contact with the water, and to mount the transducer on the transom of the boat with its face pointing down into the water.
Transducers that are mounted such that the face of the transducer itself is in contact with the water are generally most efficient. Transducers which are mounted such that their sound passes through the hull are generally less efficient. Hull materials and construction methods vary so much that it is impossible to tell how much efficiency is lost by passing the sound waves through the hull and I have never been able to find any information on estimates of loss. However the experience of many boaters with in hull mounted transducers has shown us that in hull mounting does not reduce power significantly and can be the best mounting method for some applications.
The most efficient transducer mounting available is a true thru hull mount. With these the transducer itself resided on the outside of the hull, pointing straight down, and it has a stem that passes through a hole in the hull. The data cable exits the stem inside the hull and is routed to the display unit. The face of the transducer itself is in intimate contact with the water for efficient energy transmission, in both directions. This style of mounting has its downsides though. First and most important to many boat owners is that the transducer is exposed to strikes by submerged objects or the bottom. For this reason thru-hulls are rarely used on small boats that spend most of their time in shallow water. Flats boats and bass boats will almost never be found using a thru-hull. A second consideration for thru-hulls, and the other two mounting methods too, is that the water that separates the face of the transducer and the target from which an echo will return must be as undisturbed as possible. Sound does not travel as well or at the same speed through air as it does through water and as a consequence air bubbles in disturbed water seriously degrades bottom machine performance. This is true no matter which of the three mounting methods is used.
The second mounting method, and the most common one, is a transom mount. The transom mounted transducer shares an important feature with the thru hull, and that is that its face is in constant contact with the water if it is well placed. The transom mounted transducer has an advantage over the thru hull in that it is rarely the lowest protrusion from the hull and so in the even of a grounding it is not likely to be harmed. It can still be struck of course but its less likely to be dragged across the bottom. The disadvantages of the transom mount are that it resides slightly back from the transducer and consequently the water reaching its face will always be disturbed to some extent and air bubbles may have been introduced into that water. Air is the death of good transducer operation. Transom mounted transducers are also susceptible to being struck by things not in the water. They are relatively unprotected and may be hit by most anything from passersby to forklifts. Also, most transom mounts are installed by screwing a mounting bracket (supplied with this type of transducer) directly into the transom along with cable ties which are often screwed into the transom as well. In every case breaching of the transom skin introduces to opportunity for water to enter the hull material. Many transoms are cored with wood and water entering the hull will eventually lead to rot and expensive repairs. Because of their susceptibility to disturbed water the transom mounted transducer is usually the least efficient mounting method available although they work well in many applications.
The third type of transducer mounting is the in hull mount. This method of installing a transducer has the unit in direct contact with the hull, attached using an adhesive. The greatest advantage this method of mounting has is that it does not disturb the water flow over its face. Consequently the transducer can be mounted in a location inside the hull that corresponds to clean water on the outside of the hull and excellent results can be obtained. There is some energy loss with this type of installation but in most applications it is negligible. However, this type of mounting can not be used unless there is a solid, continuous, and hard material suitable to the transmission of clean sound waves under it. That means that in-hull mounts cannot be used on wood boats, fiberglass boats with coring in their bottoms (rare in small boats and almost nonexistent in tailored boats) or on aluminum boats.
Who makes transducers? Some companies make their own transducers and some rely on aftermarket suppliers. The largest supplier world-wide is a company located in New Hampshire called Airmar. Airmar does not sell directly to the public but their products can be bought from a number of suppliers and one company that can be thought of as a factory outlet. That company is Gem Electronics. Gem can be reached at (843) 394-3565. They are very helpful, reasonably price their items, ship promptly, and can be trusted to give very good advice when asked.
When mounting a transducer a number of things need to be considered. Chief among them is access to the mounting location (not a problem with transom mounted transducers) and access to routing for the data cable between the transducer and the display unit. Great care must be taken to insure that the insulation of the transducer’s data cable is not compromised (cut). Mounting of either a thru hull or an in hull transducer, and particularly the in hull mount, requires working access to the location for the installer. You have to be able to reach it, not just see it. Functionally the most important consideration in transducer mounting is that it be situated in a place with smooth undisturbed water flow. The water passing over the transducer face must not be aerated because of the hull shape forward of its mounting location or because of other accessories that have been mounted forward of it. These accessories might include thru-hull fittings for water, waste expulsion, sensors, propulsion gear, or strakes. Additionally acoustics of the boat should be taken into consideration. In this regard the most common consideration is that given a choice the transducer should be mounted on the side of the boat that matches the downward movement of the boat’s propeller. This normally causes the transducer to be placed on the starboard side of the boat, or in some applications right on the centerline. Along this line, an also considering that air must not be present in the water flow, a transom mounted transducer will never work well, if at all, if it is placed directly behind the prop of an inboard boat, though the same transducer might work perfectly well along side of and forward of an outboard motor’s prop.
The actual instillation of the three types of transducers varies with type but each has some things in common with the others. None of them is difficult to mount and none is out of the range of ability for the typical do-it-yourself installer. The considerations for all three types are basically the same. Clean water flow, accessibility, an absolutely level mounting, and a strong mounting system are the keys to operation of the unit. There are specific transducers made for each mounting method. That said it should be noted that many people will use either an transducer designed for transom or thru hull mounting and use it as an in hull mount. This usually works just fine because in most cases its really the same transducer anyway, just made with different mounting hardware.
Transom Mounted Transducers:
A transom mounted transducer will be attached to the hull at the transom. In order to avoid breaching the skin of the hull it is desirable to mount a replaceable mounting pad such that it is held firmly in place and that any fasteners used are completely sealed. A small block of plywood, often about 4 or 5 inches by 6 inches or so can be encapsulated in fiberglass and attached by screws directly the the transom with liberal amounts of a high quality sealer, such as one of the 3M products to stop any opportunity for water intrusion makes for a good mounting pad. Some people use Starboard attached exclusively with 5200, but I have my own questions about the ability of 5200 to hold successfully to the plastic Starboard. I prefer mechanical fasteners or actually epoxying a pad to the hull. In either case the pad can be cut and painted to closely match the hull and not be an eyesore.
Transom mounted transducers are to be mounted such that the entire face of the device is in the water at all times. Because few boat bottoms are truly flat that means that the corner of the transducer closest to the centerline sill be just below the hull bottom but the corner against the hull but farthest from the keel or centerline will be deeper in the water. The transducer face will be as close to the hull as possible because an air space between the transducer and the hull will allow for the formation of bubbles in the water stream as it exits the boat bottom. Some people will actually use tape to smooth the flow between hull and transducer face, but I have never seen this be a long lived or acceptable instillation method.
The transom mounted transducer has to be mounted such that is it level in a port/starboard direction and such that its face if pointing slightly (just a few degrees) forward of level in a fore/aft direction. On a tailored boat the way to insure this is the case is first to level the boat. Using a 4-foot level across the gunwale or deck at the aft end of the boat for a reference use a jack to get the boat level. Jack up the trailer and use either jack stands or even short sections of 2x6 wood slipped under the trailer’s tires to get the boat level. Then, using the tongue jack on the trailer move the front of the boat up or down as necessary so that the waterline of the boat when under way is level. This second one is bit more tricky because you should try to approximate the bow high attitude of the boat when it is up and running. However, because it is desirable to have the transducer facing slightly forward of straight down it is usually OK to simply level up the boat to its static waterline, which is to say how it would sit in the water if tied to a dock.
With the boat level in two planes it is just a matter of using a smaller hand level at the transducer to get it right when installing the screws to hold it to the mounting pad. A helper is nice to have to hold the transducer exactly in place, with its face approximately a quarter inch below the bottom of the hull, while you, as the installer, drill the mounting holes. After that its just a matter of routing the data cable up from the transducer and into the boat. The cable should be supported as necessary such that it can not pull away from the hull. Plastic hold downs are used for this purpose (available at any Lowe’s or Home Depot in the Electrical Department). When installing the cable hold-downs first drill a pilot hole for the holding screw (stainless only). Install the screw and then remove it. Coat the screw threads with 5200 and reinstall it.
In Hull Transducer:
In hull mounted Shoot Thru transducers require about the same amount of effort in installing but the process is quite different. The transducer will be installed directly against the hull, with a filler material used as an adhesive, in a spot corresponding to clean water flow across the hull. The transducer will be mounted so that its face is pointing straight down, or slightly forward of straight down, when the boat is under way and it will be mounted over a spot that has good acoustic properties.
You begin mounting of an in hull by first finding a mounting spot. Find a place with you have reasonable access to the hull itself, not a false bottom, with enough room to work. One way that is recommended to do this is to use the fish finder itself as a test device. With the boat in the water you can tie a stout line to the transducer (never pick one up by its data cable) and hand the transducer over the side of the boat and put it in the water. The transducer is to be connected to the bottom machine and you note how deep the water is and the shape of the bottom as well. Then you take the transducer and place it into a plastic bag. Fill the bag with water so that the transducer is submerged and then take the bag and place it against the hull in the desired mounting position. Then look at the fish finder. Does it show the correct depth (take into account that the hull bottom will be below the water surface) and is the bottom structure as clear and crisply displayed as when the test was done with the transducer on the string? If it is you have found your mounting spot, if not look for a better mounting position. Once a suitable mounting spot has been found the work begins.
The first step, and one often omitted, is to clean up the mounting spot. Take 36 grit paper and smooth the area and remove any gunk and junk. Then use a solvent, I prefer acetone, to clean the mounting spot with a clean rag.
The transducer itself is to be mounted using an adhesive. You will have to choose what to use but the harder the final material the better its acoustic properties will be. Some installers use a relatively soft material, like silicon sealer or one of the more aggressive urethane sealers, but I think you are better served to use epoxy. If the material you decide to use is thick and will not run it can be applied directly to the hull and the transducer placed into it. In some cases a dam will have to be made around the mounting spot to contain the adhesive while it sets. Modeling clay or Play Dough can be used for a damming material in those cases. The dam is built and a pool of material is poured into it, then the transducer is set in place. In either case the installer must insure that no air bubbles are present in the adhesive. That is accomplished by careful mixing of the material if it is a 2-part substance, stirring genteelly and not folding it, or by being very careful in the placement of thicker material as it is squeezed from a tube or however applied. When the transducer is placed into the adhesive material it is imperative that the installer level the transducer, and then find a way to hold it in place while the adhesive sets. It is also very important that when setting the transducer into the adhesive that the transducer be tilted when inserted and only tilted back to level once the face if the traducer is immersed. This stops any air bubble from being formed under the transducer. If you trap air under the transducer it will not function properly no matter how good the mounting spot. Do not try to route the data cable until after the adhesive has set. To do so might well move the transducer off of level and that too will adversely effect performance.
A second method of installing shoot thru transducers involves suspending the transducer in a tank that is attached to the boat bottom. The tank is filled with a liquid, often mineral oil but sometimes water (be careful if you are in an area where freezing temperatures are encountered) and the transducer resides in the tank. Some of these tanks are fabricated by the installer and some can be purchased and cut (often made of a heavy plastic material similar to starboard) to match the hull contour. In either event it is still necessary for the transducer itself to be held in place such that it is level. The top of the tank is sealed so the fluid can’t slosh out and the data cable is routed. This is a very efficient method of mounting an in hull transducer because any air bubbles that might be introduced in the chamber are temporary and because there is intimate contact of materials all the way from the transducer face to the water itself. I suspect that very little energy is lost in one of these systems.
With the chamber systems in which a plastic chamber is purchased it is necessary to trim the container so that it lies level on the hull. The plastic chamber is then attached to the hull using a common adhesive, such as 3M’s 5200.
Earlier I had mentioned that in hull mounted transducers can not be used on boats with cored hulls. While its true that very few small boats have cored hulls, and that of boats that do have partial coring of the hull it is rarely present in the hull bottom, that doesn’t mean that yours isn’t cored. Call the manufacturer and ask. At any rate coring is not the end of any chance of using an in hull. You can simply remove the coring and reglass the mounting spot. This is a lot easier than many people imagine it to be, and doesn’t take long either. If you have coring and need to remove it here is how you go about it.
First take an angle grinder and remove the top layer of fiber glass. Use a respirator. Only remove the glass in an area a bit larger than needed for the mounting. When the glass has been removed the coring will be exposed. By hand, using a sharp chisel but no mallet or hammer, remove the coring. If it is a dense foam core it will come out easily, wood is more difficult, unless it is end grain balsa, in which case it will come out easier than the foam in many cases. Remove it completely down to the outer layer of glass. Then clean the glass with sandpaper and wash it with acetone. Then simply reglass the hole until it is once again level with the existing hull material. If the hull material is over about a half inch thick you may have to do the layup of glass into the hole in several layers. You do not want to apply epoxy so thick that it overheats when curing. Once the glass has been built up to its original level you can then continue with mounting as described above.
Thru Hull Transducers:
Mounting of thru hull transducers is more difficult than the the other two methods and requires the dreaded hole in the boat’s bottom. The transducer itself is usually shaped a bit like a small boat with a very large smoke stack (the transducer stem) sticking out of its top. The stem of the transducer passes through a hole drilled in the hull and the data cable exits from it inside the boat. The transducer is held in place by a large nut that threads onto the stem.
As with the other types of transducer the thru hull must be mounted level in the port/starboard direction and should be mounted such that when underway it points slightly forward of straight down. It must never point aft of straight down, because if it does an air bubble will form over its face when the boat is under way. Also with thru hull mounts it is necessary to inspect the hull of the boat forward of the desired mounting position to make sure you are not attempting to mount the transducer directly down stream of protrusions which would create bubbles. Also, just as with the in hull mount good access to the mounting position inside of the hull is imperative. Once you have found the mounting spot inside the boat and outside the boat measure every way you know how to make sure you are looking at the same spot inside the boat as out. Then go inside the boat with an 1/8” drill and drill through the hull. Then go under the boat and make sure the hole came out where you expected it did. If you are satisfied with the location break out the hole saw. Using the pilot hole as a guide drill the large hole from outside the boat and drill it straight up. Transducer stems vary in size but most are about 2” in diameter. Some transducers have two stems, with the second being considerably smaller than the first, often not even an inch in diameter. Drill holes as needed.
Leveling of thru hull mounted transducers is accomplished using a fairing block. The fairing block, which are generally sold by the transducer manufacturers, are made of either wood or plastic. The plastic ones are far preferable because they do not rot or crack. The block is to be cut to match the angle of the hull and then the transducer is mounted below the half of the block which was cut to match the outside of the hull. The second half of the block should then match the contour of the inside of the hull and will be used there to provide a flat surface for the large holding nut to seat to. The real trick to a good thru hull mounting is getting that fairing block cut properly.
To cut the block properly the hull have to be level to start, as with the other methods, and a special tool should be purchased to make the job easier. The special tool is an adjustable bevel, sold at Sears for around $7 as I recall. It is a simple plastic handle with an adjustable blade that can be locked in place via a large knob. You place the handle on the hull and move the blade until it is level, then lock it down. Then the bevel can be used to transfer that angle to the table of a saw, on which the block can be cut. It is a very simple process and if you feel uncomfortable doing it (the fairing blocks typically cost $20~40 for the small ones and up to about $100 for the larger ones – sometimes called ‘high speed’ blocks) yourself you may find help in a friend who does woodworking. In some cases a compound bevel is required, and this is often the case when mounting a transducer right on the centerline of the boat.
Once the block has been cut it is time to fine tune it. From under the boat, using a small hand level and a sanding block with 36 grit sand paper in it you should plan to spend an hour hand shaping the fairing block by sanding until it is a perfect match to the hull and so that its lower surface is perfectly level – do not attempt to shape it for the slightly forward pointing attitude at this time.
Once the fairing block is perfectly to shape thread the data cable from the transducer through the hole in the block, then through the hole in the hull. Go inside of the hull and thread the cable through the hole in the top half of the fairing block.
Now seal the unit in place. Beginning on the inside of the hull put a liberal amount of sealer on the lower surface of the inside fairing block. Try to keep it off of the cable. Press the block down against the hull so that its hole and the hole in the hull line up and sealer is pushed out in a bead all around the block. Now go under the boat and put a liberal coat of sealer on the top face of the fairing block and press it up to the hull, aligning the stem hole as you do so. The fairing block will generally stay in place without any problem. Finally put a layer of sealer on the top face of the transducer itself, but not on the stem. Reach up into the hole in the fairing blocks and the hull with your finger and remove excess sealer which has squeezed out into the hole. If it gets on the threads of the stem it will make removal of the unit at some date far into the future much more difficult. Now the trick. Take 2 or 4 stainless steel washers, 3/8” ones, and place them into the sealer that is on top of the transducer. Put them at the heel of the transducer but not at the edge, move them in from the edge about 1/8th to 1/4th inch. The purpose of the washers is to lift the rear edge of the transducer slightly away from the level fairing block in order to give it a very slight forward pointing attitude.
Next puts the transducer up to the fairing block, with the stem going through the hole and into the boat. Have a helper inside of the boat put the stem nut on and to run it all the way down but to only tighten it snug. This should force a little more sealer from between the joints where the top half of the block meets the hull, where the bottom half of the block meets the hull on the outside, and where the transducer meets the bottom half of the block From under the boat make sure that the transducer if pointing directly forward, and remove the excess sealer with your finger. Now stop.
Let the sealer you have used set up for a day or two (longer if you use 5200, which I do not recommend for this job) and then return to the inside of the boat and tighten down the the nut. Doing it this way avoids pressing most of the sealer out of the joints and at the same time allows the sealer to form a gasket. When you go back and retighten the nut you will be pulling the transducer into the already formed gasket. A thru hull transducer mounted this way will not leak, even if in the almost unbelievably rare circumstance where the nut ever comes loose (as a hint, its usually everything you can do to remove one of these nuts years later, let alone worry about one coming loose on its own).
And that should just about take care of it for instillation. Of course this is all general stuff and you might have specific questions. I’d be more than happy to take a stab at answering them for you, as will others I’m sure.
Now let’s just take a few moments and think about fish finders. I think it will be good to use an analogy to guide our thinking about the things. I want to use television sets as the analogy, you’ll see why quickly.
Television sets are made in differing levels of quality I suppose. Some have much better screens than others, that’s for sure, but they all seem to have pretty much the same internal architecture. It is not amazing at all to any of us that any television will work just as well if it is showing a Presidential Debate, a Spaghetti Western, a Soap Opera, the News, a good movie, or a bad movie. One thing the TV does care a lot about and what drives the quality of the picture you will see is not the guts of the TV, but it’s the screen definition and the quality of the signal source. A pair of rabbit ears on top of the set doesn’t get you much signal quality. An antenna on the roof will give you a better picture but for the most part it won’t touch cable. Of course if you want a great picture you have to go to the digital satellite signals, but they are readily available. Your fish finder is exactly like that. First consideration for a good fish finder picture is the screen quality. If you have a low definition screen no matter what you do the picture won’t be great. The next important part of the machine is its signal processing ability. Some are much better than others in this regard but there is at least one tell tale sign when you are looking at lower quality machines. Almost without exception any machine that will show ‘fish symbols’ is a machine that has inferior signal processing. That aside the real discriminator for the performance of a bottom machine is the signal it receives, and that brings us right back to the transducer.
Standard transducers, no matter which mounting style they are made for, are all generally the same. They tend to be of good quality and do good service, but that doesn’t mean that you can’t do better. Specialty transducers are sold, though you don’t see them often, and they can be selected for your specific use. Most of the ones available are made by Airmar and that company has been good enough to post on the net a catalog and other information that will allow you to select just the right one for your use. Many makers of fish finders sell their units with a transducer and the owner’s never speculate about what else might be available that would better suit their own needs. Many people would be unwilling to pay the additional money that a specialized transducer cost as well. There are some manufacturers, however, who do not automatically include a transducer of their choosing in the box. Furuno is one of them that lets you pick the signal source that best suits your needs. It is one of the reasons that they commonly give such good results. Its not because the machines are better built than anyone else’s (though they generally are) and its not because they tend to have very good high definition screens, though they generally do, and its not because of their generally better signal processing ability (which excludes trick stuff like fish symbols or enhanced fish arch drawing), its because of the quality transducers they use and the even better ones that are available to discriminating users.
There, did that help any?