Baseplate assemblies hold the downrod systems for your transducers.
Fitting a baseplate to the boat requires consideration of the following:
4 inches at the back of the baseplate to mount to the boat.
4 inches of the front of the baseplate hanging over the water.
and some span between those dimensions.
So an 8.25 inch baseplate would need to be mounted right at the rub rail of a boat or on a track system)
A 12 inch baseplate would have 4 inches of span between mounting and cup assembly.
A 15 inch baseplate would have 7 inches of span between mounting and cup (typically used with bass cat or nitro boats with a pedestal system).
An 18 inch baseplate would have 10 inches of span (typically these are used on Targa boats by Tracker.