Refer to About Downrods page for further information about fitment.
Determination of baseplate length for your transducer mounting assembly is relatively easy. Note in the photo, there is 4 inches for mounting area, and 4 inches that must sit over the edge of the boat. So subtracting 8 inches from the standard downrod lengths gives you the available distance to get over the edge: 0.25 on a 8.25 base, 4 on a 12 inch base and 7 on a 15 inch base. Other lengths are available on request so don't be shy and drop us a line if you need something specific.
Downrod length is easily determined from the waterline on the boat. We recommend 10 to 15 inches of the downrod be in the water when deployed. The minimum distance on top is 4 inches to be used as a handle (or to mount a handle). The remaining is used to get to the waterline. Typically a 48 inch downrod is more than sufficent for a DeepV boat and a 36 inch downrod is fine for most Skeeter, Nitro Z or RangerZ style boats.
If you are unsure, please give us a call and we will be happy to discuss your requirement