T-Pain Is On A Boat: Re: [Electric Boats] Optimizing for Efficiency Increases Performance and Range

Saturday, March 15, 2014

Re: [Electric Boats] Optimizing for Efficiency Increases Performance and Range

IMHO, Many of the numbered items are not universally correct, or are somewhat misleading.

1. Only if the motors are identically manufactured.
The comparison may or may not be right between different motors.

2., 3. Sometimes, but not necessarily.

5.This is not necessarily correct.
A water cooled motor of brand x may be very inefficient.
Nevertheless the water will keep it cool.
Efficiency is one thing and external cooling is another.

6. The 2 items are not related.
Motor efficiency depends on what range it is built and good for.

7. No.
Only if they are the same reduction from the same manufacturer of the same materials and tolerances.
This is not necessarily the case.

9. Not necessarily.
Dripless shaft seal may use near zero power, as they are a form of hydrodynamic bearing.
Hydrodynamic bearings are nearly 100% efficient, and last forever.

In practice, the matter is different and will depend on installer (how tight they are) and everyhting else.
Dripless seals may be desirable for other reasons, but they may or may not be efficient.

10. Battery lossses decrease with lower current.
Its not quite the same thing.

The calculation in and of itself is very good.
It also illustrates very well how tolerances stack up or in this case how inefficiencies mount up.
I question some numbers - particularly the bearing efficiencies.
I suspect that bearing efficiencies are much higher.

Also, grease vs oil is a much harder question than mentioned.

I use grease for high speed spindles, with bearings that are about 10x better than anything ever seen in a boat (for 50 mm shaft size, 7210ACDUP-P2).
Upto about 6000 rpm grease is the best option for many reasons-
The correct high speed grease is Kluber Isoflex NBU15 (for upto dN of 700.000 ie 70 mm diameter x 10.000 rpm).
I doubt any boat shaft in the universe runs at 10.000 rpm.

Both the grease and the use of it are recommended by Timken and SKF and NSK.
In practical terms, the conclusion is possibly correct.
Using "waterproof heavy marine grease" may or may not be efficient. It might very well be efficient (or not). Its a complex issue, internally.

On 14/03/2014 02:03, James Lambden wrote:

I have been following some of the threads lately with much interest which leads me to this contribution. I'm looking forward to your comments and edits after which it will be published to my website. May as well get the group to chime in and make it perfect first. Here goes:

SYNOPSIS

1. Torque is more efficient at transfering power than rpm - A larger motor running at lower rpm will have less rpm related (friction) losses in the motor bearings, seals and speed reduction system.

2. A larger motor will run cooler than a smaller motor for any given power level and will be consequently more efficient

3. A double stator motor has over twice the power of a single stator motor.

4. A double stator motor can make the same amount of power as a single stator motor at less than half the rpm. Less rpm in the motor will reduce rpm in the speed reduction system and further contribute to higher efficiency.

5. A water cooled motor will run at a lower temperature with higher efficiency and higher output than an air cooled motor

6. A motor geared to make hull speed will be less efficient than a motor geared to go at a lower boat speed ( 90 % of hull speed) because for any given rpm the motor will be running on more current and less voltage. Voltage is always more efficient than current.

7. Lower speed reduction ratios will be more efficient than higher speed reduction ratios because the speed reduction system is doing less work. Lower speed reduction has a smaller drive pulley so the propeller can be closer to the hull and more horizontal so more of the force is pushing the boat forward rather than up. The exception to this rule is planetary gears which work at higher efficiencies at higher gear ratios.

9. Propeller slip determines propeller efficiency. More slip, less efficiency. Larger diameter, higher pitch propellers turning slower are more efficient. Propeller efficiency varies with rpm and propellers get less efficient the faster they turn.

9. Dripless shaft seals are more efficient than packing glands

10. Batteries run more efficiently when using less power. Higher efficiency in the drivetrain requires less current from the batteries so total system efficiency increases even more.

11. Lithium batteries are the most efficient followed by Odyssey TPPL Batteries, followed by AGM or Gel. Flooded lead acid batteries are so inefficient and dangerous they don't belong on an electric boat.