Question on trolling motor's mechanism

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velcroak@hotmail.com - 19 Jun 2007 15:21 GMT

Folks,

I am a total novice in this field. Please pardon me if my questions
sound stupid.
Trolling motors typically have three main parts. The propeller
assembly, shaft, and a device attached to the handle for navigation.

My questions are as following:

1. Where is the actual drive motor residing? Is it in the propeller
assembly itself or is it in the device attached to the handle, and the
power is some how being transferred to the propeller using some kind
of gear mechanism? My guess is that the motor is connected to the
propeller itself.

2. I am looking for a used DC Motor controller. I intend to buy a
small trolling motor with small (9-11 lbs torque). Can you advise,
what specifications should the controll have (Amps). Will a 15 AMP
controller do?

3. I am sure controllers for marine applications are much more robust
and built to last. Can you please suggest a good source to buy a used
controller.

Thanks so much.

Ash

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R Swarts - 19 Jun 2007 15:56 GMT

> Folks,
>
[quoted text clipped - 10 lines]
> of gear mechanism? My guess is that the motor is connected to the
> propeller itself.

Most trolling motors consist of a direct drive permanent magnet motor in
the lower housing of the motor where water helps keep it cool.
Conventional motors may also have resistors or "speed coils" in the
lower unit that are placed in series with the motor by the controller in
the upper unit to diminish the thrust and, to some extent, the current.
The controller in this case is simply a switch. The prop is connected
directly to the shaft of the motor.

> 2. I am looking for a used DC Motor controller. I intend to buy a
> small trolling motor with small (9-11 lbs torque). Can you advise,
> what specifications should the controll have (Amps). Will a 15 AMP
> controller do?

As a generalization, most 12 volt trolling motors draw around one amp
per pound thrust. A 15 amp controller should do for an 10 pound thrust
motor. However, unless you plan to spend a lot of time at less than full
thrust, a pulse width modulating electronic controller is scarcely
worth the time or effort.

> 3. I am sure controllers for marine applications are much more robust
> and built to last. Can you please suggest a good source to buy a used
> controller.

Used controllers, I don't know. But new ones are available from
suppliers of parts for scooters, mopeds, etc for considerably under $100.

> Thanks so much.
>
> Ash

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velcroak@hotmail.com - 19 Jun 2007 17:14 GMT

Mr. Swarts,

Thanks so much for your valuable input.

Ash

> velcr...@hotmail.com wrote:
> > Folks,
[quoted text clipped - 43 lines]
>
> - Show quoted text -

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Brian Whatcott - 20 Jun 2007 01:36 GMT

>Folks,

>1. Where is the actual drive motor residing?...
>
[quoted text clipped - 10 lines]
>
>Ash

1) The motor sits in the lower unit.

2) Torque is a unit of force times its tangential distance from a
pivot.
Instead, let's see what current 11 lb of force would take.
Power is easily worked from force times speed, but interestingly,
troll motor makers specify thrust (force) of a fixed troll motor.
We will suppose this moves water at 3 mph.

11 lb = 11/2.2 X 9.8 newtons = 49N (in SI units)
3mph = 3 X 0.45 meters/second = 1.3 m/s
So force X distance = power:
And 49 X 1.3 watts = 64 watts (at 100% efficiency)
That would be 6 amps thereabouts at 12 volts.
Troll motors are often full on, or through a big dropper resistor
or off About 1 ohm at 20 watts rating for example for about half
speed.

Brian Whatcott Altus OK

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Boat Forum / Building / June 2007

Question on trolling motor's mechanism