Are electric horses really bigger?

Thanks, Kerosene.

I recorded 3 knots@2 kW towards 12-15 m/s wind:

 

2 kW or 48V and 21 A, which is 1 kW? 1 kW would be very low power, but 2 kW is about what I would expect for a 2300 kg 7.5 m sailboat at 5.5 knots. 800 rpm at cruising speed is quite normal for any sailboat diesel.

E.g. my previous sailboat was 8.2 m 3000 kg (empty) and it had a Yanmar 1GM10 rated at 6.7 kW 3600 rpm at the crank shaft. It had a saildrive with 2.64:1 gear ratio. The top speed was 6.4 knots at 3450 rpm (1300 rpm at prop shaft). 5.5 knots was reached at 2600 rpm (980 rpm at prop shaft) with a bit less than 1 l/h consumption. This equals to 2.5-3 kW crank shaft power. The propeller was a 2 blade folding 14x8 made by Radice (also known as Volvo Penta and Allpa). This propeller is known to have below average open water efficiency.

My VPP predicts 380 N drag for my previous boat at 5.5 knots. Thus the propulsion power needed is 1080 W and the propeller had maybe just ~40% efficiency. A bit less than 40% efficiency was measured for the same propeller in size 15x9. Note that the best folding propellers had much better efficiency, up to 70%.

Note than in this propeller test the best ones get 5.5 knots at 1 l/h in a much bigger boat (Sun Odyssey 34.2 + VP MD2030). 1 l/h equals to about 3 kW.

 

Hi,
today the electric drive can outperform the combustion drive for short run times easy. For longer run time the batterie will be to big and heavy. For long run times there is the other option to go with solar cells if only small power is needed or with higher power to use a fuel cell stack where the energy is only limited by the size and coast of the bottle where you fill in the hydrogen. Electric motors can be designed for high power in a small size if you use the same technic like you know from combustion engine . So you have to cool it with liquide to make it run at high power unlimited. We can not outperform a jet engine. A short look in the model builder world will show you this. The speedrecord by combustion modellboar is about 230 km/h the record for the electric driven modelboat is 314 km/h with topspeed this year of 331 km/h. The modelairplanes the combustion motors reach 430 km/h and the electric driven modellairplanes do over 524 with last year topspeed of 580 km/h . So wy is it . The 60 size (10ccm) combustion motors have around 4,2 kw power .
With the similar power the electric run the same speed. But with todays Litium polymere cells in a spezial build the can do much higher power. The current record was run witch accelerating power of 12 kw and 8 kw duration. And todays boats run up to 18 kw acceleration power when they reach 331 km/h. So for short time the electric drive can find much higher power , .Some data of the boat drive: 8 Lipo cells 28-30 volts and 600 Amps during acceleration. The motor rev up to 70000 rpm load speed . Propeller 3 blade ,1,65“dia and 4“ pitch, all stainless highest quality steel . preform laser sintered mold. The same with modelairplanes. The only diverence of electric drive to combustion drives are the much better torque formthe smaller size without using a gear. And if you use a gear it can be a one step gear with higher efficancy . In the picture i show you a electric motor i make the drawing we designed 24 years ago for a mititary drone that has 400 kw duration power . You can place this motor inside a 10 liter water bucket . It rev with about 35000 rpm and this is history.

Happy Amps Christian

 

Sorry, I mean 41A, 48V - almost 2 kW.

Can you please describe this point?
I rely on the data of this article
Folding and feathering propeller test https://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807
and simple folding props show better results at forward move than feathering or fixed.

Prop with 70% of efficiency sound like a fable for me.

 

Obviously there is no reason why a folding propeller should have a better efficiency than a fixed one, which can have equal shape. There is a reason why feathering propellers have worse efficiency. Their blades can't be curved for equal pitch (or optimal pitch distribution) as a function of radius, since a curved blade would not feather for minimal drag nor would it work well on reverse with blade turned 180 degrees.

However there are clear differences between propellers of similar type and also how well a propeller is dimensioned for the boat and engine (e.g. in a test). The efficiency of several 2B propellers was measured in the University of Potsdam for Segeln magazine. In that test Allpa got bad efficiency. I linked it already in the earlier message, but here is the link.

This test was performed in a propeller test tunnel, not in an actual boat. The propeller efficiencies were measured in several points of operation. The shaft power needed for the theoretical top speed of the boat was measured. 7.6 kn and 1630 N. Flexofold needed only 10.6 kW for that (60% efficiency) while Allpa needed 20 kW (32% efficiency, power more than the actual engine can provide). At cruising speed of 6.4 kn the thrust needed was 548 N. Flexofold needed 2.6 kW (69% efficiency) and Allpa 4.8 kW (38% efficiency). Note how much the power demand increases with speed. The difference in top speed doesn't need to be much for a propeller to have much worse efficiency.

In other tests Allpa/Radice/old Volvo Penta has performed better. Maybe that's due to a different size? E.g. in this one it had decent top speed and fuel consumption. It may have helped in fuel consumption that Allpa was clearl overpitched in that test. It still consumes more than other folding propellers at 6 knots, but less than feathering propellers.

 

Thanks for the detailed answer, Joakim