Pontoon pinging is a little different

Allan Tarvid

December 02, 2008 at 3:19 pm  | Mobile Reader | Pring this storyPrint 

Installing two homemade extensions between this transducer and its mount allows the transducer to run deep enough to get below the extreme turbulence present behind a pontoon, and deliver detailed readings up to the boat’s top speed.
Installing two homemade extensions between this transducer and its mount allows the transducer to run deep enough to get below the extreme turbulence present behind a pontoon, and deliver detailed readings up to the boat’s top speed.
Photo By Allan Tarvid
A friend showed me his new pontoon boat, and I was impressed until I saw how a sonar transducer was mounted on the back of a pontoon. I remarked that it probably wouldn’t work above idle speed, and he confirmed that as soon as he throttled up above fast idle the fish finder lost its reading.

A transducer must have a constant flow of water across its face, free of turbulence and bubble streams. This transducer was mounted with its bottom higher than the bottom of the pontoon, guaranteeing that it would be buried in turbulence during acceleration, and would lift out of the water at cruising speed.

I looked at the transducer mounting plate welded to the back of the pontoon and the Lowrance Skimmer transducer’s mounting bracket, and could see no way to mount the transducer low enough to reach undisturbed water.

Obviously, some shade-tree engineering was in order. A trip to the hardware store with a spare transducer and mount in hand got me started. For about $15, I bought a piece of aluminum stock 1-inch wide by 1/8-inch thick by 4-feet long and enough stainless steel bolts, nuts and washers to construct a mount extension.

I cut two 8-inch lengths of the aluminum stock, and rounded their sharp corners and edges with a file. Next, I drilled a hole at one end of each bar sized for a bolt that would go through the Skimmer’s factory stainless steel mounting bracket and a smaller hole at each bar’s other end sized for a bolt that would fit through the transducer’s adjustment pivot point.

Returning to the boat, I loosely assembled everything on the existing factory Skimmer bracket. I discarded the plastic adjustment washers that normally fit in the bracket’s transducer mounting bolt holes so my larger diameter bolt would fit. I placed a flat washer on the bolt, then slid one of the homemade extension arms on, followed by a sharp star washer, and then slipped the bolt through the bracket’s left mounting hole.

Next, I installed enough washers on the bolt to fill the space between the bracket and the other extension arm. After sliding on that second extension arm, I added another star washer, and slipped the bolt through the bracket’s remaining hole.

Finally, I added another flat washer and a lock nut. The idea was to install a star washer between the homemade bars and the mounting bracket so they would stay put and then add enough washers as spacers so everything could be cinched up tight.

Installing the transducer between the other ends of the arms was simpler. I just put a star washer between each arm and the transducer’s plastic body, and added flat washers as needed.

I set the transducer to ride about 3 inches below the pontoon, and angled it slightly nose-up so that when I sighted down the transducer’s belt line (the line where the upper and lower halves of its plastic body meet) my line of sight hit the bottom of the pontoon about three-quarters of the way to the pontoon’s nose.

I was careful not to over-tighten the lower bolt, which might break the transducer’s plastic housing.

My buddy’s unit is a Lowrance LMS522 iGPS, and I downloaded the latest software update for it before the installation, and it’s a good thing I did. We put the boat on the water, and discovered that lowering the transducer increased the speed at which the unit would work, but it still lost its picture before the boat reached cruising speed.

I considered putting the boat back on the trailer so I could lower the transducer’s running depth a bit more to try to get below the turbulence, but I decided to try installing the software update first. With the update installed, we watched the screen anxiously as my buddy eased the throttle forward and grinned when we got a great picture all the way to the boat’s top speed.

So if someone you know has trouble getting sonar to work on a pontoon boat, show him this column. He needs to get his transducer below the washing-machine-strength turbulence at the back of his pontoon and make sure his unit is running on the latest software available for it.






View other articles written Allan Tarvid