Are electric horses really bigger?

Hello J, Thanks for your feedback and questions. This is really helpful to me. First, to answer your question, our 47% efficiency is measured as thrust x speed divided by input watts and it has a tolerance of +- 5%. My measurements are in a dynamic test tank where speeds and thrust are measured simultaneously and of course input power is also measured at the same time. I also have a thrust-measurement mount to the side of a boat so that I can measure speed using GPS and thrust. This setup is useful for creating full-power thrust-speed curves. The energy into the charger, inefficiencies in charging a LiFePO4 battery are not included because you're not charging while running and so it seems irrelevant in use. After all do you measure the BTU content of fuel in the HP rating of a gas outboard? But charging efficiency is relevant to folks with a mother ship who charge aboard. For them I can say that the energy out of the battery is about 80% of the energy from your house bank (if you use a 150 watt inverter and our charger). I measured that too.

I started this thread to clarify the misconception that E outboards are dishonest about their statements of equivalency because I am interested in being up-front and finding the best way to communicate performance in a way that folks can accept. I frankly cringe at people asking me what lb thrust or hp it provides. Because the rpm x torque vs thrust x speed characteristics of electrics are so different from gas. I have been in this game long enough to formulate an understanding of equivalency but now that I've had this opportunity to bounce it off you guys, I learned that the word "equivalent" comes across as misleading. That's an important learning for me and I hope that other manufacturers are listening too.

What I can still honestly say is that the input power rating for a well- designed electric outboard can be lower because the electric does not need to be over-rated to the same degree as gas outboards, and that the propeller efficiency is much higher , at least at displacement speeds. Now, I am still left with a need to figure out how to say that in an acceptable manner.

I always felt that the best way to communicate is to tell people how fast and how long the EP Carry will run on your boat and under what conditions. So here is a short list of actual speeds to make the point. I am surprised that other manufacturers have not produced a database like this for their motors too. Note that boats over 600 lb generally do not achieve hull speed with our motor. But for normally loaded boats under 13 ft, it generally exceeds hull speed so that you have reserve for weather and waves... Does this seem a more honest communication approach? J

 

Joe, you didn't answer my questions about the propeller efficiency measurements. With 47% overall efficiency and 80% propeller efficiency the efficiency from electrical input to propeller shaft power is only 59%. There should be plenty of room for improvement! Epropulsion and Torqeedo are in 80-90% range for that and have around 60% propeller efficiency.

Gasoline OB's have only the propeller efficiency between their claimed power and propulsive power. So you should compare your overall efficiency to their propeller efficiency at the same thrust and speed. Your full power is about 120 W propulsive power. How much shaft power does Honda 2.3 need for the same propulsive power? Even with the lowest propeller efficiency of 25% mentioned so far that would be only 480 W, which equals to 0.65 HP. I would say the propeller efficiency is close to 50% at that low power so the equivalent shaft power is 240 W (0,34 HP).

Since electrical OB's seem to always give electrical input as their power, they need much higher propeller efficiency to be comparable with equal powered gasoline OB's, which always give their propeller shaft power. In all the tests between electrical and gasoline OB's I have seen the top speeds of equal power (e.g 2 kW and 2.5 HP) have been about the sameor the gasoline one has been faster. Those tests include several different dinghies, sailboats etc. with a top speed range from 4 knots to 20 knots (10 HP).

Then most small OB's have considerably more shaft power than they claim, which makes the equivalence even worse for the electrical ones. For EP Carry there is no even closely equivalent gasoline OB, since the smallest one made in this century is 2 HP, which is far more powerfull.

 

I doesn't seem like an accurate or honest approach. The reserve power needed is the same regardless of power source. It is not true that gas outboards need to be over-rated any more than an electric outboard. That is sales hype and not backed by any engineering calculation or collected data.

 

Now, Jet14, it's fair that you present a list of your achievements. Please excuse me for not wading through the complete mass of previous
notes here, but what was the total maximum input power to the controller (i.e. battery output Power) during those tests? There seems to be a lot of confusion involving propeller efficiency, that has nothing to do with the original question. Provided the propeller is driven at the same rotational speed and pushing the same hull at the same speed, the propeller will produce the same thrust and efficiency (ie consume the same power), whatever mechanism is turning the shaft.

Considering the affinity laws for propellers and pumps, and using the conventional correction for propulsive speed, the propeller input power varies in proportion to about rpm^k, where "k" is 2,5 to 2,75 depending on hull shape et c.. Strictly for pumps and waterjets the exponent is 3,0, because the "pumping system" is determining the inflow, not the hull moving through the water.

So, if we take, as Joakim did, a 2 hp gas OB, say a Parsun or Yamaha. The nominal rpm at engine shaft at 2 hp output is 4500 rpm. The gear ratio is 2,08:1 and the 3- blade standard propeller is 7,25" x 5". With these figures we get a reasonable operating point at a forward speed of 7 knots (thrust is ~221 N and propeller efficiency ~50%, wake factor 0,9).

Let us compare this engine at 3,25 knots and an engine power output of 0,2 hp (=150W). At this speed and power requirement, the rpm's will be ~2310 on the engine shaft. Thrust at this operating point is 54 N and propeller efficiency is again ~50%. In order to make fair comparisons, you must compare apples with apples; if your "propulsion unit" has an operating envelop in the 3-to-4 knot range for small vessels, then you can't compare with another "propulsion unit", designed for a completely different operating situation, no matter what kind of energy it is using! What matters to the person paying the bills is energy consumption for a given transport work.

The thread opens with a question. The natural answer is a counterquestion: how could the propeller sense what mechanism is turning the shaft??

 

The answer to the original question is too simple to be interesting and thus the discussion has been more about different propellers and different claims of efficiencies. All the electric OB (or even IB) suppliers seem to claim that 1 kW electrical input somehow equals 2-3 HP shaft power of a gasoline or diesel engine. Mostly they seem to base that on very poor propeller efficiency of the OB they are compering to. I have never seen any real measurement data to back up this claim, except for the bollard pull data, which indeed shows clearly higher values for electrical OB's at the same power. Bollard pull is not a very interesting value and it seems that electrical OB's loose their high thrust very rapidly with increasing speed while gasoline ones may even have higher thrust at speed than at bollard pull. Bollard pull is easy to test and good marketing material for electrical OB's. Maybe they are even optimized to give high bollard pull?

Here is one quite recent test. Unfortunately it is in Swedish, but you can read the numbers. Honda 2.3 reached a top speed of 4.6 knots and Torqeedo 1003 4.4 in a Suzumar 265 inflatable. If it has a 2.6 LWL, the "hull speed" would be 3.9 knots so both were pushing the boat against the high resistance hump. Now let's do some estimations. The drag increases very rapidly at those speeds. I would estimate that the Honda had at least 30% more propulsive power to reach 0.2 knots higher top speed. Torqeedo claims 1003 to be equivalent to 3 HP and have 480 W propulsive power. Adding 30% to that we can estimate Honda to have 624 W propulsive power and with 2.3 HP that would result to 37% propeller efficiency. With a propeller calculator I get 45% for that operating point (264 N, 4.6 knots, no wake factor, 7.25x4.75 3 blade. 75% EAR). So clearly 1003 was not equivalent to even to a 2.3 HP Honda and propeller efficiency was not below 30% as it would be necessary for the claims to be anywhere close to reality.

In antoher test Honda 2.3 was measured to consume 0.87 l/h at full power. That suggests that it does not have excessive power. Unfortunately I haven't seen power measurements for it. I do have measurements for Honda 5 (Finnish magazine Vene, 2001). It had only 4.35 HP peak power and consumed 1.77 l/h. Assuming the same hp/l/h the 2.3 would have 2.14 HP and the propeller efficiencyy would be 40%.

 

Why would a gas outboard need to be "over-rated"? I've seen lots of users of gas outboards on dinghies who outside of no-wake zones run at or close to wide open throttle.

 

Why also a charger, than you must add a complete oil refinery to it also. Bert

 

Gonzo, although I agree with most of what you have said, but we should be realistic. I love brushless motors. I make my own controllers. I even have re-winded one of my motors. However I have my controller powered from a fixed 12 Volt power source and whether I draw 40 Ampere from 12 or 24 or 36 Volt. My controller consumes only 160 milli Ampere and that is at 12 Volt 1,92 watt. while my motor current is anything from 10 to 45 Ampere i.e. 1,62 Kw or lower. the percentage is thus anything from 0.001185 percent to 1,5%. Also the magnets of a brushless motor does not need any energy, all what is left over is the energy consumed in the MOSfets and that could be at 45 Ampere (The motor can go to 90 Ampere, but I don't want to work with 72 Volt DC) 6 x 2 watt = 12 watt We talking about peanuts in relation of the energy provided for the propeller. Look electrics is coming, faster than we think. The Netherlands , special Friesland province has made a law for zero emission by 2020. They will only allow basically electric driven boats into the Fries harbours. To overate an electric motor does not cost that much more in comparison to bigger outboard or inboard diesel engines. Bert

 

As a matter of fact, when I pay for fuel, I am paying for the cost of the refinery. If you don't include the cost of electricity to charge the battery, with the losses incurred, it is not an honest comparison.

 

Agree with you, but then it gets complicated, I have 2 bearings and some winding's and magnets, what can go wrong in a diesel or gas engine? everything. Then the maintenance cost for an electric motor is lower. Bert

 

I agree that the maintenance is cheaper. It is relatively simple to do an economic analysis, which gives the cost of ownership throughout the life of the product.

 

Electric motors are a compromise in their design. A small diameter makes higher rpm speed, but less torque than a larger diameter motor. It's the short lever vs the long lever.
The limitations of the larger diameter motor is because it requires more poles and is then limited by the speed the electron magnets can alternate polarity. The limitation is in the alloy available. You need POWER to force the polarity change.
If this can be overcome you can have a fast large diameter motor with huge torque.

 

Hi everyone,
I have read all thread, and still have a question:
why my electric motor at 48V21A pushes my boat 5.5 knots at calm water and 3 knots towards 12-15 m/s wind and river current? It's just 2 kW, It shouldn't show such results.
Specs:
Motor GoldenMotor 3 kW HPM3000 48V version.
Battery hand-made lithium polymer 13s2p, cells used 37Ah, cell middle voltage - 3.7V. 48V battery middle voltage.
Controller Kelly KLS6018H
Hand-made belt reduction 3 to 1
Prop 12" x 7" x 2 blades, at 2 kW it revs 800 RPM. Located under the hull, connected to reductor via long shaft through hull.
Boat weight 2300 kg, length 7.5 meters, wooden cruise sailboat.

Speed checked by phone with GPS and by Chartplotter Lowrance mark 5

I have no doubt that electric horses are same as ICE, just wondering because of results. So far, I tend to believe this is because of prop size and slow RPM. Any ideas?

 

Yes. Slower larger prop with good putch will be more efficient at lower speeds.
Small hp outboard is a compromise to (kinda) fit to many uses.

 

Old ICE setups were low consumption too

Horse Power per ton? http://forum.woodenboat.com/showthread.php?118203-Horse-Power-per-ton