Ingenuity needed

[Ed Barnes]

We have a pontoon boat on which I installed a depth finder and placed the transducer on a bracket at the rear of one of the pontoons. It sets approximately 12" below the water line and is centered just below the bottom edge of the pontoon. The sonar image is good at low speed. However, as speed increases the image is more eratic and at times is completely lost due to the increased cavitation at the rear of the pontoon.
As woodworkers we have to be a bit of "out-of-the box" thinkers when it comes to project design and implementation. I'm trying to come up with an idea to solve this problem. I am thinking of mounting the transducer on a mast that could be lowered so it would be below the area of cavitation. To make this easier (and because I am a bit of a nerd :)) I thought of adding a 12VDC linear actuator to lower and raise the mast from the helm. Do you think this would work? There are a lot of very ingenious people here, I'd like to hear what kind of fix you would come up with.

Ed

[Jamie Buxton]

IRRC, there are ways to detect cavitation -- I think from the distinctive sound of the cavitation bubbles collapsing. You could hook that cavitation detection to your linear actuator so that the mast lowers automatically when there's cavitation to avoid. The mast would automatically retract when there's no cavitation, so there's no danger of running the mast and transducer into the lake bottom. You could claim big nerd points with this rig.

But why not mount the transducer someplace where there's no cavitation, like the front or the side of the boat?

[Ryan Mooney]

This may be (read: probably is) a stupid question, but if you mount it on a mast wouldn't there be cavitation around the mast/depth finder enclosure as well? Or is it the amount of cavitation that matters and you wouldn't get a sufficient amount to matter with the smaller support?

[Ed Barnes]

Ryan, no it's not a stupid question. The source of cavitation is caused by the disturbance at the rear of the pontoon tube. My goal is to be able to move the transducer below that disturbance and still be able to raise it to avoid the lake bottom in shallow water or when beaching or loading the boat.

[Lee Schierer]

What you are experiencing is not cavitation, but bubbles from the hull caught in the turbulence created by the hull. When I installed my depth finder on my 17' Chris Craft I/O, The instructions said to mount it so that the face of the transducer was 1/4-3/8" below the bottom of the hull. I have no problems with losing signal up to full speed (40 mph). I get good depth readings, but fish images are unreliable since the targets are small and the beam is moving past them pretty quickly.

Your mast idea might work, but the lateral forces as you start to move are going to be pretty significant. Try holding a stick in the water while someone else is steering the boat and you will see what I mean.

[Joe Angrisani]

.....Your mast idea might work, but the lateral forces as you start to move are going to be pretty significant. Try holding a stick in the water while someone else is steering the boat and you will see what I mean.

Sounds like it's time for a preliminary test. Attach the transponder to your boat hook, and lash the boat hook to the rear railings.

[Ed Barnes]

What you are experiencing is not cavitation, but bubbles from the hull caught in the turbulence created by the hull. When I installed my depth finder on my 17' Chris Craft I/O, The instructions said to mount it so that the face of the transducer was 1/4-3/8" below the bottom of the hull. I have no problems with losing signal up to full speed (40 mph). I get good depth readings, but fish images are unreliable since the targets are small and the beam is moving past them pretty quickly.

Your mast idea might work, but the lateral forces as you start to move are going to be pretty significant. Try holding a stick in the water while someone else is steering the boat and you will see what I mean.

Lee, that is how my transducer is mounted 1/4-3/8" below the tube. This pontoon is just a different beast than our old v-hull boat.


Sounds like it's time for a preliminary test. Attach the transponder to your boat hook, and lash the boat hook to the rear railings.

Joe, before investing in the actuator, etc. I think I will do a test as you propose. I plan to use the same mounting bracket at the bottom of the tube and I hope just a few inches of additional depth with the transducer will resolve the problem. By using the lower mount and one possibly secured to the rear of the deck I hope that the lateral forces as mentioned by Lee can be minimized. Thanks for your input guys.