T-Pain Is On A Boat: Re: [Electric Boats] RPM vs Amps

Monday, October 29, 2012

Re: [Electric Boats] RPM vs Amps

so either his hull is much more efficient or his prop shape is much more efficient or the diameter of his prop is GREATLY influencing the efficiency..... unless there is some indicator to the contrary i'd have to bet on the latter, prop diameter makes a great deal of difference

From: James Lambden <james@toolboat.com>
To: electricboats@yahoogroups.com
Sent: Sunday, October 28, 2012 7:01 PM
Subject: Re: [Electric Boats] RPM vs Amps

Hi Dan,

Welcome to the world of Electric Boating!

For PMAC (Permanent Magnet Alternating Current) and BLDC (Brushless Direct Current) motors, which are almost identical, current is torque with a constant, and rpm is voltage with a constant. When programming the controller, these constants are entered into the program.

As propeller increase in rpm, they demand exponentially more torque for rpm. So when looking at the motor, you need to look at the application to find out exactly what the curve will be. This is also dependent on the gear ratio, with lower ratios having a steeper curve than higher ratios.

For a good understanding of how these numbers all come together look at the website for our Catalina 30:

http://electricboatdesign.com/database/

These are real world numbers for our Catalina 30. Please note that to establish a baseline between all boats, all measurements have been done in zero wind, zero waves, and at either high or low tide. Furthermore, the measurements are an average of two runs in opposite directions.

Only one boat that I know of has better efficiency than our Catalina, and that is Eric's Cheoy Lee which is running a four bladed prop of slightly larger dimensions. But we are still working on breaking Eric's record.

Cheers,

James

James Lambden

Propulsion Marine

325 Chapala Street,

Santa Barbara, CA

93101

805 455 8444

jlambden Skype

http://www.propulsionmarine.com

On Oct 28, 2012, at 5:21 PM, dan.randolph wrote:

Sorry, this is probably a really basic question, but I am confused so here you go.

From what I have read here, for DC motors, as amps go up, rpms go up in a linear fashion. However in looking at a variety of specifications for motors, they all show RPMs not starting at zero, but actually very high, and then linearly going slightly down with amp increase.

Efficiency usually does a rapid rise to a high point, then a slight drop. Torque is linear rise with amps. But why do the motor spec sheets show the drop from not near zero?

Thanks for indulging a newbie.

- Dan