Boat Trolling Accessories: Shaft Length, Thrust, Batteries and Mounts

Shaft length is the first decision

A trolling motor that is too short ventilates the moment you hit a chop. It pulls air down to the prop, the thrust disappears and the motor whines. A motor that is too long drags, steers heavily and bangs the skeg on every stump.

Measure from the mounting surface, which is the bow deck for a bow mount or the top of the transom for a transom mount, straight down to the waterline. Add 16 inches for calm inland lakes, 20 inches for typical open water, and 24 inches or more if you fish big water with real waves. Round up to the nearest available shaft length. Extra shaft above the water costs you nothing. Too little costs you the day.

Thrust, weight and voltage

The working rule is two pounds of thrust for every 100 pounds of fully loaded boat weight, and fully loaded means the hull, the engine, the batteries, the fuel, the gear and the people. A 2,000 pound rig wants at least 40 pounds of thrust. Add 20 to 30 percent if you fish current, wind or a heavy pontoon.

Voltage follows thrust. Roughly, motors up to 55 pounds run on 12 volts and one battery. Motors in the 70 to 80 pound range run on 24 volts and two batteries. Motors at 100 pounds and above run on 36 volts and three batteries. Higher voltage is not only more thrust, it is longer run time at the same thrust because the motor draws fewer amps to do the same work.

Batteries and charging

Use deep cycle batteries, not starting batteries. A starting battery delivers a big burst and dies if you draw it down repeatedly. Group 27 and Group 31 deep cycle batteries are the common choices for trolling duty.

Flooded lead acid batteries cost the least and need water checked. Absorbed glass mat batteries cost more, charge faster and handle vibration better. Lithium iron phosphate batteries cost the most, weigh a third as much and last far longer, but need a charger rated for lithium. Never mix chemistries or ages in one bank.

Fit an on board charger sized to your bank, with one bank per battery. It is the single accessory that most extends battery life, because a battery left partly discharged over a week sulfates. Keep the trolling bank separate from the cranking battery.

Wiring and connections

Use marine grade tinned copper wire sized for the current and the run length, and put a circuit breaker within seven inches of the positive terminal. Undersized wire is the most common cause of a motor that feels weak. Seal every crimp with adhesive lined heat shrink.

Mounts, stabilizers and accessories worth buying

• Quick release mounting brackets: take the motor off in seconds for trailering and theft prevention.
• A motor stabilizer strap or arm: stops the shaft from bouncing on the trailer, which is where most trolling motors actually break.
• A bow mount plate or reinforcement: spreads the load on a thin deck so the mount does not crack the fiberglass.
• Prop and hardware kits: keep a spare prop, nut, pin and washer aboard. A weed wrapped prop is a ten minute fix with spares and a lost day without them.
• A shaft mounted transducer bracket: puts the sonar exactly where the motor is pointing.

Maintenance that actually matters

Rinse the lower unit after every salt or brackish trip. Check the prop for fishing line every few outings, because line wrapped behind the prop cuts the seal and lets water into the motor. Grease the mount pivot annually. Store batteries charged and off a concrete floor in winter. Inspect the shaft for hairline cracks at the mount after any impact.

Tell us your boat length, loaded weight, bow height above the waterline and battery setup, and we will confirm the shaft length, thrust and wiring parts before you order.