You want the transducers on the outboard side of the prop which rotates away from that area: the area of least turbulence. I have standard transducers on the Starboard hull of my Caracal Cat, and the Tom Cat when I owned it. Both hold the bottom well at speed. Properly set up (that is with the counter rotating prop on the Starboard side--the reverse of what is done with the standard monohull), either hull will work.
The Panoptic 21 is another story. Basically these have to see "Forward"--and were primarily designed for putting on trolling motors, so you could have the transducer on the front of the boat. The "beam" can then be rotated in the direction of your casting.
The installation manual of the 21 TM (Transom Mount) states:
Poor transducer location may result in poor transducer performance or impact the handling of the boat.
The transducer should not be mounted in a location where it might be jarred when launching, hauling, or storing.
The transducer should not be mounted behind strakes, struts, fittings, water intake or discharge ports, in-hull transducers, or anything that creates air bubbles or causes the water to become turbulent. Turbulent water may interfere with the sonar beam.
The transducer must be mounted with the transducer face submerged and facing forward. The transducer shape requires a larger surface area to be submerged than a typical transom-mounted transducer. The large submerged surface area creates drag that can impact the boat's handling and top speed, ranging from a negligible drag to unacceptable drag. You may need to adjust your boat's trim tabs if the transducer is not mounted in the center of the transom. For optimal results, the transducer should be mounted as close to the center line as possible.
When operating at speeds greater than 12.9 km/hr (8 mph), the performance of the transducer is limited, the bottom return is diminished, and the depth information and alarm may not provide warnings for potential shallow depth areas.
The transducer has an effective forward range of between five and eight times the depth of the water. For example, in 3 m (10 ft.) of water, the effective forward range is between 15 and 24 m (between 50 and 80 ft.). Water conditions and bottom conditions affect the actual range.
The reality is that you are not going to "see" items such as semi submerged logs, or items on the surface. Wind chop and waves may impact the images.
The unit is limited to a multiple of the depth of the water, so in 4 feet of water, it may be 20 to 32 feet--the Tom Cat from transom to forward waterline is about 22 feet!
There is a valid reason that, although forward looking sonars have been available for the commercial and high end recreational market for a long time, but they are retractable into the hull transducers which extended a significant distance down into the water. (Westmar and Furuno were the primary units)
Using a transducer bracket which slides up and down on the transom is a great idea--but....there is a problem--the outboard bracket goes across the entire transom. One could work around that, but I wonder how satisfactory this will be? You would have to either have a remote mechanism for lowering and retrieving the bracket/Transom mount, or manually go all of the way to the motor bracket. The physics may be a problem also. The slide up and down bracket usually is on a conventional transom, where the lower part of the mount is only a few inches at the most from the lower edge of the transom. In the Tom Cat, it will be secured 20" to 25" from the outboard bracket and there will be turbulence of the bracket body, and cat hull in front of it--I suspect it may not work well.
Side scan may also be limited with a cat, unless you use two transducers--I would suggest that unless you are a die hard fisherman, that you not use this feature. The down scan should work fine.
