The Saltwater Wireless SW55 and SW71 are all offered with Digital Variable Speed. The Digital variable speed monitors your motor's amp draw, heat dissipation, output and other vital parameters. The result is a motor that self-regulates - a cooler running motor with an extended service life. It is a self-contained, sealed module protected in the motor mount that brings all vital motor components together in one protected location, eliminating outdated analog circuit boards and related componentry found in many variable speed motors on the market. You’ll find, the Digital Motors provides plenty of thrust and the Pulse Width Modulation (PWM) variable speed circuit that is managed by the Digital processor your trolling time on the water will be extended by 3X over a traditional 5-speed motor. Shaft Length Selection "Rule of Thumb" for foot-control trolling motors. A basic rule of thumb for determining the proper trolling motor shaft for a boat and the water conditions is to measure the distance from the boat's waterline to the mounting platform (boat gunnels) and add 24 inches and then round to the closest shaft length available. 21’ Bay Boat Example: Let's assume the boat when loaded for fishing has a measurement of 34" from the water line to the gunnels. As a rule of thumb we add 24" to this number which equals a total shaft length of 58". Now we round to the nearest shaft length based on the water most often fished and your style of fishing. In the case of a saltwater boat it will probably be used in big water and would work best with a 60" shaft motor to make sure the prop does not blow-out in rough water and a 24-volt, 75 lb. thrust motor. The SW75 60” model would be the best fit for this boat. | 
