Friday, August 19, 2016

The secret life of hulls and transducers

It's a great transducer. It was carefully and properly installed. The only problem is it doesn't work if the boat is moving faster than 10kts. You can't put this style transducer anywhere on this hull where it will work at speed. This is a far more common problem than you would believe and the problem is caused by the hull design,  not the transducer.

The owner of this boat is experienced and has expectations about transducer performance. Another owner of the identical boat is new to boating and just assumed transducers don't work if the boat is moving. Hull design, construction, and rigging all contribute positively or negatively to transducer performance. In my world a transducer should be able to hold bottom and mark fish targets at most boat's normal cruising speeds. For reasons we are going to learn about this is often not the case.

The problem in too many cases is the transducer is an after thought. Designers strive to create safe, stable and hydrodynamicly efficient hulls and our boat in question is all of that.

But this design never contemplated installing transducers. This transducer will work, but needs shimming out from the boat using a fairing block of sorts to clear the turbulent boundary layer created by the hull.

Here is a one minute 101 simple lesson about the boundary layer using a borrowed NASA drawing.

When a fluid like water travels over a surface like a boat hull, the closer to hull the water gets, the slower it moves and friction caused by the hull creates turbulence. This is the boundary layer. 

The smoother the surface, to a point, the thinner the layer will be but it will always exist on a boat hull. Anything on the surface of the hull from strakes to a through hull fitting can dramatically increase the thickness of the turbulent layer downstream from it. Just above, or below the boundary layer depending on your perspective the water has a smooth non-turbulent laminar flow which your transducer loves. It's a very complex subject with endless variables from hull shape to speed so this is a good place to stop.

Here are a couple of very dramatic photos of hulls underwater rarely seen by boat owners. The one above is traveling high speed. The strakes really jump out at you and this is the reason universally manufacturers say don't install transducers near these structures. On this boat you could likely install transducers on or near the keel line. 

Here is another hull, and the boundary layer turbulence is significant. I can't see a single place where a transducer could installed without a substantial fairing block to get it down into clean water.

The take away is that a hull may perform really well but can be at the same time very transducer unfriendly. What can affect transducer performance? Every part of your hull that's in the water. Some have minor impacts, others huge. Here are a few examples.

It's almost cute and seems harmless. It's a drain for an anchor locker. I suspect it's located below the water line so the locker dripping stains wouldn't be seen. 

At speed the venturi effect sucks in air from the locker through the hose and spews it down the hull. Looks good, it's nicely installed but very bad for a downstream transducer. This is just a one inch hole. Just imagine the turbulence you can get from a six or eight inch bow thruster tube in the bow.

Strakes that stop mid hull create a lot of turbulence downstream. The more abruptly it stops (squared off) the more turbulence it will generate. There was some effort to fair this one but not enough.

Hull ripples and flaws will increase the thickness of the boundary layer This is more typically seen in molds that were made by hand.

Newer robotically (CNC) cut molds are smoother and straighter. Even in this case as a mold ages and is continually repaired flaws can creep in. Look at your hull at a low viewing angle and see how straight it is. You might be surprised either way.

This is a more extreme example but the owner was complaining about poor transducer performance. It didn't take a rocket scientist to figure out what the problem was. Bottom paint that isn't smooth or is flaking and chipping off can cause a smaller version of the same problem. A clean smooth hull is more fuel efficient and transducer friendly.

All boats are compromises and trade offs have to be made. But knowing the depth is important to all boaters!

To get good transducer performance the hull design has to contemplate and have locations suitable for transducers to be installed. In order for this to happen the builder has to work with the naval architect to make sure this happens. A motor yacht may need numeric depth and the customers may want forward scan tech usable at slower speeds. If it's a quad powered "Fishing Machine" the hull may require multiple transducers of differing types and the hull design should be capable of this and work well at higher speeds. Lets take this one step further. The manufacturer should specify exactly where varying types of transducers can be installed indicated on a dimensioned drawing that comes with the boat and the speeds they can be used at.

If this is done most transducer performance problems go away. No more guessing. I have seen identical boats showing up at dealerships with completely different locations for the same transducer all with varying degrees of performance.

Builders have to think about this when the hull is rigged along with appropriate locations for through hull fixtures used  for intakes, discharges or other purposes so they don't ruin valuable transducer hull real estate. Often I see the need for both symmetry and convenience ruining ideal transducer locations. 

Here is a classic example above. Both of these water intakes could easily have been installed on the port side of the hull. The starboard  (propeller  down stroke side) could have been left free for transducer installation almost anywhere.

Transducer technology has improved in both performance and technologies used. Just a few years ago the venerable 20/200kHz transducer was the standard. Today we have CHIRP, side and down view, live sonar with steerable arrays, forward scan and others. Boats have changed over this same period. Step hulls are common and speeds overall have increased. It's clear that many designers and builders are not keeping up with the need to integrate and understand the requirements of these new transducer forms into their products and this needs to be improved.

It's tough to explain to the new boat owner that he can only have one transducer installed on his new toy that will work at cruising speed while his buddy's boat has all of the new sonar tech installed. By design this can and needs to be improved.

The two underwater hull photos were graciously provided by American Custom Yachts and are not for reuse in any form without their express written permission. All other photos are by the author. The NASA image is public domain.

Many thanks to Airmar/Gemeco for their technical feedback.


  1. This should be required reading for anyone thinking of getting a fishfinder!

  2. Excellent analysis, Bill. Our sailboat (which if it's going 10 knots means we are falling down a wave) has a fairing block for the transducer. Not my idea; the original owner's.


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