Scanning sonar mounting using Nylon thumbscrews to hold the actual mounting system to the boat must handle the pressure of a transducer in the water while relieving the excess pressure caused by a boat running into an underwater obstacle. The inertia of a boat running into something in the water, even at trolling motor speeds is very high. Without a break away system, a mounting sufficiently strong enough to stop even a light aluminum boat would be extensive (if the mounting location could take the pressures generated).
To make our transducer mounting systems more affordable and reliable, almost 10 years ago, Fishing Specialties started considering what can be done to alleviate the potential for obstacles to break scanning sonar downrod components when they are being used. After much testing we determined that the attachment method at the boat is really the best location to prevent damage to Livescope, Active Target, and Humminbird live transducers.
The next step of course was to determine what kind of pressures we must deal with (i.e. different transducers in the water while the boat is moving). A lot of testing with different lengths of downrods yielded many dents in our test boat and a water pressure number that must be held and any pressures over that number that must be released in order to prevent damage to the transducer mounting for scanning sonars. And this is how Fishing specialties determined that a couple of nylon thumbscrews at 1/4-inch diameter were sufficiently strong to withstand water pressure while yielding in the case of an emergency: running your scanning sonar into an underwater stump for example. As transducers typically have a speed over which they tend to lose resolution, we decided that a 4mph max speed when using the downrod was likely the best rate. The nylon thumbscrews hold very well at that rate with even a very long downrod into the water (ie fence mounting on pontoon boats).
Thumbscrews break away in a couple of ways: shearing off or striping the screw out of the holes. Most of the time the user can get his fingers onto the stub, but sometimes the screw breaks below the mounting plate surface. If this is the case, heat the tip of a small flat screwdriver and just melt a flat groove in the thumbscrew in order to get a bite on it and unscrew it. Stripped screws can usually just be reinstalled without difficulty.
After completing the initial designs using these thumbscrews, our customers requested a faster method of mounting the baseplate to the boat that precluded the need for thumbscrews. Thus, the Magnetic mounting line of baseplates was born. While a bit more expensive, these mountings are very fast and reliable up to about 4 mph.
Even while using the nylon thumbscrews to hold the mounting to the boat, the very fastest method to prevent any damage to your scanning sonar pole system is to simply pick the sonar pole up out of the mounting if you are about to hit something, another design requirement for all Fishing Specialties mountings!