Electric Propulsion

[Adamante]

For those interested in learning more about the sizing of an electric propulsion system, here is a presentation by Dave Tether titled "Choosing Your KW motor size from EMotion Hybrids". It includes an electric conversion case study with pre- and post-conversion performance numbers for his own 47 ft catamaran.

‪Choosing Your KW motor size from EMotion Hybrids‬‏ - YouTube

[daddle]

Originally Posted by Adamante

For those interested in learning more about the sizing of an electric propulsion system, here is a presentation by Dave Tether titled "Choosing Your KW motor size from EMotion Hybrids". It includes an electric conversion case study with pre- and post-conversion performance numbers for his own 47 ft catamaran.

I suppose that convinces the faithful. He doesn't impress me. Scribbling on a whiteboard doesn't make it so. Too much "virtually", "almost", "about", "probably" for a technical presentation. And then relying on changing the definition of hull speed and switching to a far more efficient 3-bladed 18" prop. When facts fail the snake oil salesmen they always switch to testimonials. Replace 80hp of diesel with a 30hp diesel and run a 5hp airconditioner too? Too slick. That's my opinion.

[LauderBoy]

No harm in sitting back and seeing how things develop. Especially if you have a perfectly good working diesel engine already installed.

What I find really interesting though is how hard of a sell the tech is. If electric is hard to sell on sailboats, that have their sails as a means of transport, can fit solar and generators to recharge away from the "recharge station", have home "recharge stations" already setup at their docks(standard shore power plug in), have re-powers that cost about the same between electric and diesel, have very very small engines in them to start with, and the electric "experience" fits right in with the sailing concept... what does that mean for electric cars?

[mikereed100]

I enjoy this back-and-forth concerning horsepower, torque, power, kW and such, but would't it be more usefull to simply speak in terms of thrust when comparing diesel and electric? This would take all variables into account including transmission losses, shaft friction and prop size/pitch and could be done at the dock with a simple thrust guage. Just a thought.

Mike

[daddle]

Originally Posted by mikereed100

I enjoy this back-and-forth concerning horsepower, torque, power, kW and such, but would't it be more usefull to simply speak in terms of thrust when comparing diesel and electric? This would take all variables into account including transmission losses, shaft friction and prop size/pitch and could be done at the dock with a simple thrust guage. Just a thought.

Yes, that may work...with comparable and identically optimal props. Because once the torque spins the prop shaft at a some rpm (the product of which is simply power) the propeller doesn't care if it's from electricity, diesel, or galley slaves.

It would be a somewhat better test if somehow it could be made a some moderate cruising speed dragging a fixed load representing some adverse weather. That's because some props may be better at a static pull than others.

[fastcat435]

Diesel or electric need different props, the rpm range and the torque are different.
Facts the we have verified on the same model boat at the same weight are
2 x 29 hp Yanmar Diesel at a prop rpm of 1650 ( engine 3600 RPM ) speed 8.00 knots
2 x 10 Kw electric rpm (max) 1500 8.2 knots electricity consumption 23 Kw
When going to identical speed of 8 knots the electricity consumption drops to 20.5 Kw
The differences in efficiency have to do with many points.
a few I will mention below:
1. lower prop rpm
2. more efficient prop
3. Tractor arrangement ( prop forward mounted )
4. no transmission losses
5. Prop mounted away from hull
6. No alternator to be driven

10 Kw is actually available at the prop , assuming a prop efficiency of 60 % 6 Kw is used for the propulsion.

to compare the diesel and the electric propulsion the 23 Kw consumed electricity should be used so this is
20.5 Kw for the electric motors versus 42 Kw for the 2 diesels @ 29 hp each or 21 Kw each about half the energy is used to reach the same speed.

[skipmac]

Originally Posted by fastcat435

Diesel or electric need different props, the rpm range and the torque are different.
Facts the we have verified on the same model boat at the same weight are
2 x 29 hp Yanmar Diesel at a prop rpm of 1650 ( engine 3600 RPM ) speed 8.00 knots
2 x 10 Kw electric rpm (max) 1500 8.2 knots electricity consumption 23 Kw
When going to identical speed of 8 knots the electricity consumption drops to 20.5 Kw
The differences in efficiency have to do with many points.
a few I will mention below:
1. lower prop rpm
2. more efficient prop
3. Tractor arrangement ( prop forward mounted )
4. no transmission losses
5. Prop mounted away from hull
6. No alternator to be driven

10 Kw is actually available at the prop , assuming a prop efficiency of 60 % 6 Kw is used for the propulsion.

to compare the diesel and the electric propulsion the 23 Kw consumed electricity should be used so this is
20.5 Kw for the electric motors versus 42 Kw for the 2 diesels @ 29 hp each or 21 Kw each about half the energy is used to reach the same speed.

Hi Fastcat,

Thank you very much for publishing this data. The numbers are quite interesting.

Convenient coincidence for my research as your tests of 2 X 29 HP diesels adds up to 58 HP, exactly the rated HP of my single Westerbeke.

You use 20 KW electric to replace 58 HP of diesel which is about half of the straight conversion of HP to KW (58 HP = 43.27 KW).

Are these performance results based on trials in varied conditions, open water, head seas, etc or calm water? Any other details you can offer.

[fastcat435]

Hallo Skipmac

yes these performance figures are based on testing in all conditions, we have over 12000 NM of experience with the electric version and over 40.000 with the diesel powered Cat. with head seas/wind the electric motors actually do much better than the diesels, this must have to do with the slower speed needed, the electricity consumption in these conditions is also favorable.
Cats have a lot of windage and going into headwinds of force 9 was no problem.
The speed as mentioned in the thread above was done in a channel in no wind no current conditions for both. all these numbers are not fictional but testing was done in different weather conditions.
I have posted the consumption of electricity with various speeds in the thread above and this applies to 20 Kw of motor power max.

[Lee Jerry]

Alberto, Hyprdrv, fastcat435, mbianka -
Thanks for the info on the props. Not too surprisingly, it looks like conversions from diesel (two of three) have generally kept the same prop, and (the one) new design has used a ‘better’ prop. (Did I miss anyone?) Of course, as mbianka pointed out, a conversion has to work within the existing geometry (prop clearance) or incur significant expense, but there may still be room to play a little ( and there's always number of blades).

Hyprdrv,
Can you quantify a little what you mean by “upgrade”?

fastcat435,
I thought that was a pretty slick set up for a catamaran the first time I saw it. Not too practical for a monohull, though, but quite nice for a multi.

[Lee Jerry]

Originally Posted by daddle

I suppose that convinces the faithful. He doesn't impress me. Scribbling on a whiteboard doesn't make it so.

You asked for data and he gave you some (and fastcat435 gave some more in another post). Unless you witness the ‘testing’ yourself, how is he supposed to report it? Is a whiteboard less reliable (or honest) than an article in a sailing rag? (No offense meant to sailing magazines; we get several subscriptions that we enjoy.)

Originally Posted by daddle

Too much "virtually", "almost", "about", "probably" for a technical presentation.

While there were some of those, I did not notice that many. Regardless, I think this was meant to be much more of a presentation for the lay person rather than a technical presentation. (There were a whole lot of “umms” – could/should have been scripted better instead of ad lib. But that is a comment on the presentor or presentation, not the technical content.)

Originally Posted by daddle

And then relying on changing the definition of hull speed ...

Although he did mention changing what hull speed is (which would be specious), all he was really doing was choosing to operate at a slightly reduced speed on the speed vs resistance/power curve – i.e. at cruise speed. This has been previously mentioned in this thread multiple times.

Originally Posted by daddle

... and switching to a far more efficient 3-bladed 18" prop.

Actually, this is a large advantage (but not a requirement) of the electric motor, IMHO – the torque available to turn a larger, more efficient prop. It might be possible to reduce the gearing with the diesel and turn a similar prop, but I would assume (perhaps erroneously) that the ‘current arrangement’ of the diesel is already at the desired, best match.

Originally Posted by daddle

When facts fail the snake oil salesmen they always switch to testimonials. Replace 80hp of diesel with a 30hp diesel and run a 5hp airconditioner too?

See posts #159 and #163 for additional explanation on ratings.

Originally Posted by daddle

Too slick. That's my opinion.

Nowhere did he or anyone else say you get something for nothing. There are trade-offs with electric propulsion, as with everything else. These might include, but are not limited to:

• Reduced top speed (as he mentioned, but fastcat435 did not experience this)
• Reduced cruise speed (to extend range)
• Reduced range (for all electric) or larger generator required
• Reduced speed under sail (due to larger prop) or use folding/feathering prop (but then probably giving up ability to regenerate) or other fancy arrangement (like the kick-up outdrives that fastcat435 has)
• Reduced or eliminated dependence on fossil fuel and the associated cost due to ability to generate power through other means (solar, wind, regeneration, shore power)
• Increased maneuverability/response at low speed
• Quiet/quieter operation

[fastcat435]

Originally Posted by Lee Jerry

Alberto, Hyprdrv, fastcat435, mbianka -
Thanks for the info on the props. Not too surprisingly, it looks like conversions from diesel (two of three) have generally kept the same prop, and (the one) new design has used a ‘better’ prop. (Did I miss anyone?) Of course, as mbianka pointed out, a conversion has to work within the existing geometry (prop clearance) or incur significant expense, but there may still be room to play a little ( and there's always number of blades).

Hyprdrv,
Can you quantify a little what you mean by “upgrade”?

fastcat435,
I thought that was a pretty slick set up for a catamaran the first time I saw it. Not too practical for a monohull, though, but quite nice for a multi.

We have installed the setup in a monohull and the second one will be done shortly with a retractable Motogen. it works almost as good, not the tractor part since the motor is in the wake of the keel anyway, see picture.

[fastcat435]

* Reduced cruise speed (to extend range)

this is always the case , with diesel or electric

* Reduced range (for all electric) or larger generator required

not really, actually better range with the same quantity of fuel on board with the same speed ( fuel savings of 30 % + are achieved in favor of the hybrid version)

[Lee Jerry]

Originally Posted by fastcat435

We have installed the setup in a monohull and the second one will be done shortly with a retractable Motogen. it works almost as good, not the tractor part since the motor is in the wake of the keel anyway, see picture.

That's a neat setup, too. I've seen retractable outboards in a well (Henderson 30) and also retractable shaft/prop before, but they have always been on race boats. I wouldn't have thought that cruisers would care enough about the drag to offset the complexity, cost and lost interior space.

Originally Posted by fastcat435

* Reduced cruise speed (to extend range)

this is always the case , with diesel or electric

* Reduced range (for all electric) or larger generator required

not really, actually better range with the same quantity of fuel on board with the same speed ( fuel savings of 30 % + are achieved in favor of the hybrid version)

These statements were lost in translation in trying to boil many variables down to generalizations.

Yes, reduced speed will reduce fuel consumption regardless of type. I was referring to the all electric and small generator designs discussed earlier going (much) slower to increase duration. For example, see post #13 (just to pick one) which discusses "cruising" at 3.6 kt even though capable of higher speeds. A little slow for my tastes. And post #33 - 5 kt for 2 hr. Of course a larger, "replacement" generator will allow for better speed and range.

However, I find it a little hard to swallow that hybrids achieve 30% fuel savings at same speed (assuming it is a "reasonable" speed and not augmenting with alternative power sources like solar), especially when accounting for all other house loads.

Good stuff though. Glad to see people (like you) working on and promoting them.

[fastcat435]

Hallo Jerry

No to much interior space is lost, the space used is where normally the engine is located.
The fuel savings accomplished are over the whole speed range.

The reason is the higher effiency of the generator running at it optimum rpm setting and the better prop efficiency as stated above. in the presently used generator we use a volvo penta D2-40 , a 39 hp engine that runs at 70 % ( +- 2750 ) rpm

[OceanPlanet]

Originally Posted by mikereed100

I enjoy this back-and-forth concerning horsepower, torque, power, kW and such, but would't it be more usefull to simply speak in terms of thrust when comparing diesel and electric? This would take all variables into account including transmission losses, shaft friction and prop size/pitch and could be done at the dock with a simple thrust guage. Just a thought.

Mike

Except that thrust while stationary vs. at speed are two different things. Either one could be affected by prop design/pitch/etc.