Trolling Motor Performance

(click on any photo for a larger view)

The Secret to Trolling in a Straight Line

The Hobie Float Cat does not troll (or row) easily in a straight line. This is not a design issue, but a consequence of boating physics: the short "water-line length" of the Float Cat just doesn't provide enough keel/rudder effect to offset the torque effects of the trolling motor's propeller. The solution turns out to be very easy and effective: just drop the oars into the water when running your trolling motor! The oars will provide enough keel/rudder effect to keep the boat moving in the direction that the trolling motor is aimed. The photo to the left shows the position of the oars when dropped into the water aiming toward the rear of the boat.

Trolling Speed/Distance and Battery Capacity

The trolling motor that I use is a Minn-Kota Endura 30. It has a 30" shaft and a maximum thrust of 30 pounds. The control stick extends an extra 6", and offers 5 forward speeds and 3 reverse speeds. The motor is rated at a maximum load current of 30 amps.

I bought my deep cycle battery at Walmart for about $45. It is a "Group 24" size battery, weighing 51 pounds. It is rated at 75 ampere-hours, with a reserve capacity of 100 minutes. The "ampere-hour" rating is amps*hours where the hours value is 20, and the "reserve capacity" is the time that the battery can sustain a load of 25 amps. These two values essentially define the discharge curve for the battery. You get what you pay for: Group-24 batteries costing 3 times the price can be found with ratings of 80AH, and 160 minute reserve capacity.

The two battery rating values can be used to solve the battery capacity equation, I^N * T = C, for the two unknowns, N and C, where: I is the current, T is the time, N is an emperical factor, and C is the "theoretical AH capacity." Once N and C are known, you can compute the actual capacity in hours for any particular current draw in amps.

I put an ammeter on the battery while trolling, and logged the actual current drawn by the motor at each of its 5 forward speeds. If you don't have a high-rating ammeter, but you do have a digital voltmeter that can measure millivolts, then the voltage in millivolts across a 7-1/2" length of 12 gauge solid copper wire equals the current in amps flowing through the wire. I used just such a "home-brew" current shunt to make my measurements.

I used the tracking feature of my GPS to record a series of runs across the lake at each forward speed, and then calculated the effective speed of the boat from the GPS tracks.

I put all this data together into an Excel/2000 spreadsheet, which you are free to download and use. It takes the two battery rating values as inputs, and produces a table like the following one:

Control

Amps

Hours

MPH

Miles

80% Miles

1

7.2

8.5

1.6

13.6

10.9

2

9.3

6.1

2.0

12.2

9.7

3

12.5

4.1

2.4

9.9

7.9

4

15.3

3.2

2.8

8.9

7.1

5

29.8

1.3

3.5

4.6

3.7

So, the bottom line is: the first column is the motor speed control setting, and the last column tells me how far I can expect to troll before my battery is dead! Your mileage may vary :-)

It is worth noticing two things: (a) average rowing speed, without getting a coronary, is about 2 MPH, so you can cover alot more water with the motor, and (b) the current drain doubles between speed settings 4 and 5, but the speed of the boat over water does not even come close to doubling. Hence, with this boat/motor combination, speed setting 5 is very inefficient.

Deep-Cycle Battery Charging:

After frying countless motorcycle batteries using supposedly "automatic" automotive-style chargers, I finally researched the subject of charging on the web. The bottom line: the only way to properly charge a lead-acid battery is by using a microcomputer controlled "3-phase" charger. You can do even better with a 4-phase charger, but they are way too expensive for this application. The charger I bought, about $80 from West Marine, is a 6-amp, 3-phase charger -- ideally matched to my deep-cycle battery.

By the way, the three phases are:

  1. Bulk Charge (constant current ~ 6 amps) to about 80% capacity (voltage rises to about ~ 14.4).
  2. Acceptance Charge (constant voltage ~ 14.2) to 100% capacity (almost no more current flows).
  3. Float Charge (constant voltage ~ 13.3) to maintain full charge indefinitely.

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