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  SONAR Transducer Creates Too Much Spray

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Author Topic:   SONAR Transducer Creates Too Much Spray
Anjin posted 05-07-2012 12:03 PM ET (US)   Profile for Anjin   Send Email to Anjin  
I re-installed the transducer of a Garmin 498 on my Montauk 17 using instructions and information I got [from a prior discussion on this topic].

Originally, I installed the transducer farther out on the Starboard side of the boat and had an ongoing problem with losing the depth readings. Once the depth readings were lost the unit never seemed to recover them. Or it took a long time to recover and then it lost them again quickly.

The thread posts I read here said the traansducer should be about 13 to 14-inches from the centerline of the boat. After re-installing the transducer, the depth finder loses the depth reading much less often and quickly recovers the reading. The performance has been greatly improved.

The only problem is that now the transducer shoots up a big roostertail. The port side of the transducer is 1/8-inch below the hull as specified by the Garmin instructions. I suspect the transducer is adding a lot more drag now. Will this affect fuel consumption? Speed? Does the transducer have to be raised? What is the best way to deal with the roostertail without losing performance of the sounder? Or are transducer roostertails common and acceptable in this type of Montauk 17 transducer installation? Thanks for any info you all can provide about the roostertail.

jimh posted 05-07-2012 09:08 PM ET (US)     Profile for jimh  Send Email to jimh     
I read the thread you reference, but I don't see where it gave a particular recommendation for "13 to 14-inches."

The depth of immersion of the transducer working face into the water stream flowing under it will probably affect the mount of a spray coming off the transducer (or as you call it, the rooster tail). If your installation left some room for vertical adjustment, try moving the SONAR transducer up about 0.25-inch. Then test the new mounting position to see if the SONAR performance is still good and if the amount of spray being thrown from the transducer is reduced. If the SONAR is good, try moving even higher. Once the SONAR starts to go bad, lower the transducer 1/8-inch and see if the SONAR comes back. That should locate the position of the transducer so it gives good SONAR bottom echoes and makes the least spray.

If the spray cannot be managed well--and this probably happens with some transducer designs--you can install a spray deflector above the transducer to knock down the stream of spray coming off the transducer before it gets airborne.

Dragging the surface of the transducer through the water creates drag, reduces boat speed, and affects fuel economy, but I doubt it does so in any measurable way. On a really light boat and with a really big transducer, the transducer might act like a small trim tab.

jimh posted 05-07-2012 09:40 PM ET (US)     Profile for jimh  Send Email to jimh     
The rounded center section of the hull bottom of a MONTAUK curls upward and creates a recess just inboard of the outer sponson. As Tom Clark notes (in the linked thread mentioned above), aerated water can tend to accumulate in that recess. The aerated water creates a stream off the transom where you certainly want to avoid putting the SONAR transducer.

A SONAR transducer emits ultrasonic energy. To work effectively, the energy must be coupled right into the water column under the boat. If there is any sort of other medium between the water column and the transducer face, there is a tendency for the sound energy to be reflected off the interface between the water and the other medium. If there is a layer of airy water, this layer acts as a reflector and stops the sound energy coming off the transducer from entering the water column. This also works on the return echoes. The return echoes will end to be reflected back at the interface between the water column and the airy water layer.

Sound travels through the water without reflections. The sound reflects only when it hits some interface between the water and another material. This is why fish show up--they make the sound bounce back. The sound bounces back because the fish bodies have a different acoustic impedance than water. In really sophisticated SONAR sets you can tune the sound frequency in order to maximize the reflection from specific fish species. Different fish have different acoustic impedances, I presume. So you tune the SONAR frequency to find the peak reflectivity.

Airy water has a much different impedance than solid water. The barrier between solid water and airy water makes a very good reflector for sound. Water of different temperatures also has different sound characteristics. Sound will reflect from the border of two layers of water.

On your boat transom you just need to get the working face of the transducer into clean, not airy, water. It only has to be just into that layer, just a 1/16-inch into the layer. When you get the transducer immersion set right, the spray should be reduced.

Anjin posted 05-09-2012 08:47 PM ET (US)     Profile for Anjin  Send Email to Anjin     
Thanks for the replies Jim.

In regard to the "13 to 14 inches" distance from the centerline. You stated in the prior discussion that you had your transducer mounted 13 inches from the keel centerline. Someone else said "at least 12 inches". Anyway "13 to 14" is how I remembered it. But the sonar function works very well now. Much better than it ever worked. I could even say I am happy with it.

The only problem is, as I said, the "roostertail". I did a search on CW and found a few discussions of roostertails from transducers. Someone who sounded very knowledgable and whose theory I want to buy into says that, basically, in order to get good sonar performance there is a good chance you are going to get a roostertail from the transducer.

jimh posted 05-09-2012 08:57 PM ET (US)     Profile for jimh  Send Email to jimh     
I think the amount of spray generated by a SONAR transducer is affected by the design and shape of the transducer housing, as well as the amount of immersion. I have good results with the LOWRANCE Skimmer transducers.
carlo2533 posted 05-10-2012 07:39 AM ET (US)     Profile for carlo2533  Send Email to carlo2533     
[Seeks a picture showing a SONAR tranducer mounting on the transom of a Boston Whaler MONTAUK.]

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