Are electric horses really bigger?

Discussion in 'Electric Propulsion' started by DennisRB, Apr 9, 2016.

  1. kerosene
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    kerosene Senior Member

    Torqeedo is kind honest in their 5.6kW claim (electric, gearing and prop losses accounted for) - they are trying to play to their strengths.
    What is special on torqeedo is the planetary reduction gear so they can spin slower propeller thus giving better propulsion efficiency at slow speeds for a slow boat. If you want to go 10 knotts you will likely run out of pitch and lose badly to the out of the mill gas OB.
    If the comparison would be with a high thrust OB torqeedo would likely lose for 10-15hp gas OB even at slower speed work. I am guessing 10kw continuous draw equalling about 10hp at shaft.

    only truly honest comparison is shaft hp to shaft HP and if you have magical propeller with higher efficiency then explain that out separately.
     
  2. DennisRB
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    DennisRB Senior Member

    A boating magazine did a test of small outboards gas vs electric. The gas motors destroyed the electric in performance. The claims were totally unfounded for the electric motors. They could not even match them kw for kw ket alone match gas obs with more power. This is obvious to anyone with any understanding of what power is.
     
  3. terhohalme
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    terhohalme BEng Boat Technology

    One problem with the Torqeedo output power 5.6 kW is and that is propulsion efficiency. It will change a lot depending on boat speed, propeller pitch and diameter. They should have balls to tell the maximum continuous shaft power.
     
  4. Stumble
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    Stumble Senior Member

    It doesn't matter to them, because the 10kw input is the same as a 25hp output gas motor. So why get bogged down with detail and honesty?


    Ok, I am going to sit in the corner for a bit and cool off. I despise the way electric outboards are marketed, and really wish just one of them would be honest about their performance.
     
  5. DennisRB
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    DennisRB Senior Member

    The issue with using torque for a benchmark is that torque has no relevance in regards to time/speed. Motors are ALL about speed. They move boats through the water at a required speed. Therefore torque is useless.

    Take the following example. A charter plate fishing boat with 3 x 300hp outboards is to engage in a pull test against an opponent to see what has more "thrust". The opponent is a strong dock cleat. In the test the dock cleat proved even with the 900hp! Neither was able to advance so it was a draw. We can now say the dock cleat is equal to 900hp! Well we can in electric motor advertising theory anyway.
     
  6. jet14
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    jet14 New Member

    Hi D, 11 pages of answers; I've not found if anyone else responded with this information so if someone dis, I apologize.

    It depends...
    All electric drives have low efficiency at low powers and efficiency climbs as the power output increases --> if its a well designed system. So you want to use your electric system close to full power to maintain the best motor efficiency. For example the EPcarry has a peak 47% efficiency its maximum 220 watts. It's made for dinks so that power point makes sense and anything above 180 watts is in the groove. The Torqeedo has similar efficiency at 500 watts. If you have a Torqeedo you'll want to use it at close to that power level so use it on larger dinks and daysailers. Gasoline motors don't want to be used at full power for cruising so you'll need a larger engine rating for the same cruise speed. So that's one element. But still a 47% overall electric motor efficiency would indicate that's not all.

    Another element is the propeller and gearing design. Most outboards and auxiliary inboards are way under-propped; not for the engine but for the boat. For displacement boats there is a direct correlation between propeller disk area and overall efficiency- larger prop diameter, higher efficiency. Higher pitch also equals higher efficiency. Good e-motor manufacturers have figured this out. Ray electric was the first to launch a big-propped high pitch electric right after the war and their efficiency is not far off of the new electrics way back then! But almost no gasoline motor manufacturers have figured that out (except for Seagull). Gasoline engines are characterized by the industry by Hp measured at the prop. so there's no common basis in claims for them to change, and to swing a larger higher pitch prop would require them to increase the gear ratio in their lower units to match engine torque-power needs. So they haven't. Their props are way to small, too low in pitch and have way too much surface area to be efficient (too much vortex creation and too much back-pressure). Comparing an EPcarry prop to a BF2.3 prop- you're gaining at least 200% in efficiency. More like 300% comparing to a trolling motor prop by my measurements.

    Add it up and arguably good electrics can be lower rated than gasoline units and if its a well designed system 3:1 is accurate. Of course if you do an inboard electric swap and don't change the prop or adjust the gearing, you'll need to add more electric power than 1/3 an equivalent ICE rating to compensate.

    You're used to horsepower right now, and its likely you've never had a horse on your boat. Over time, people will also become comfortable with electric systems. The new way is actually easier but we have no collective experience with it yet. It's happening- just not done yet.

    Joe
     
  7. kerosene
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    kerosene Senior Member

    Joe, you are making claims that are not exactly correct. 1st for power of motor you should be comparing motor output from shaft as that is truly the one thing that can be compared accurately.

    As far as props go. You cannot say that all props are over or under pitched. Retail outboards standard props are a compromise in many ways - they are made to work from shelf with many kind of boats and in varied speed ranges. Your claim that large higher pitch prop would always be better is a simplification and not always correct.

    You cannot take an e-powerline with well matched prop and compare it to a traditional powerline with poorly matched prop. And correct prop depends on the boat - so its not accurate either to just claim that electric motors come with a well matched prop.
    Also mind you that if you over-prop a trolling motor you will cook the motor with higher POWER required to spin it. On the side you lose the false argument of the weaker e-motor beating bigger gas motor as those are based on bollard pull numbers which will drop with the higher pitch prop.
    The ep carry you promote is doing exactly the same false argumenting we are annoyed about here.
    The statement above is as much nonsense as your claim of 200-300% efficiency boost. 300% boost in efficiency would mean going from 30% efficient prop to 90% efficient prop. Can you show me a practical 90% efficient prop?
    As the honda's output is at the shaft its hard to imagine any other kind of losses than the prop. When the e-motor's 240 W is battery draw! You will get 200W to the shaft on a good day yet claim that it is a match to gas powered 736 W.
    Ignorance or marketing lies?

    Or put it the other way round. You claim total efficiency of 47%. So the small EP Carry will take 240W from battery and turn it to roughly 120W of pure propulsive power - the other 120W goes into electric motor and controller losses and prop inefficiency. Fair enough, I totally buy that.
    Now for the 1hp Honda to to perform equally badly would mean that the 736W on shaft turns into 120W of propulsive power. That would equal prop efficiency of 16% on the Honda. Are you seriously claiming that?

    Joe, (is that Joe Grez?) I am not doubting your product. I believe that your motor and battery combo are good and a really practical solution for many. I also believe that your prop is more efficient than typical OB prop (might not shed weeds as well but not a problem in your typical use profile).

    BUT it is a 0.25hp e-motor with an efficient prop for small and light low speed boats.

    Nothing wrong with that. Claiming that it universally matches 1hp outboard is nonsense.

    edit: clarity, typos and grammar
     
    Last edited: Oct 24, 2017
  8. kerosene
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    kerosene Senior Member

    This is quote from Ross Lillistone
    "Here are two Youtube clips - firstly using the 2hp Honda, with which I achieved a consistent 7.9 knots measured by GPS with my weight aboard, and 7.1 knots with my wife joining me (she only weighs 60kg/132lbs. The 9.8 Tohatsu gave 15.5 knots (17.8 mph or 28.6 kph) with two heavy men aboard plus gear."

    I am still waiting for 2hp equivalent e-motor pushing any craft over 7 knotts...


    from here:
    Ross Lillistone Wooden Boats: Fleet - Videos of performance with Two Different Motors http://rosslillistonewoodenboat.blogspot.fi/2015/11/fleet-videos-of-performance-with-two.html

    video here:
     
    Last edited: Oct 24, 2017
  9. Joakim
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    Joakim Senior Member

    Honda has, according to EP Carry site, 7.25x4.75 propeller. I run some sample calculations on that with a propeller software based on Wageningen B-Screw Series. The propeller efficiency at 3 knots 65 N thurst (100 W propulsive power) is 50%, 3.5 knots 111N (200 W) is 47%, 4 knots 146 N (300W) 44%. The latter one is already clearly beyond the cababilities of EP Carry, but about what Torqeedo 500 W can do. Full power at 5 knots resulted in 300 N and 44% and at 4 knots 38% and 320 N.

    I don't know can the actual propeller in Honda reach these values, but they are reachable at 7.25x4.75 size (3 blade 75% EAR).

    Torqeedo has a 11" propeller, again according to EP Carry. Similar calculations with 11x7, 3 blade 35% EAR give 63% at 100 W, 62% at 200 W and 62% at 300 W. This is reasonable with the 56% overall efficiency claim made by Torqeedo. The 300 N at 5 knots would give 60% and 320 N at 4 knots 54%, thus 1200-1300 W propeller shaft power would be needed for the same thrust Honda is giving with 1700 W.

    So what is the efficiency of the EP Carry propeller in the above cases? What is it's diameter and pitch. You could rather easily get to 75% and even to 90% for a design allowing only very low power (like EP Carry), but having cavitation problems at "normal" powers. But that can't be even 100% better than 50 or 47% Honda has at such a low powers.

    Small propellers of outboards are not even close as bad as the electric outboard manufacturers claim. That can easily be noted from the higher top speeds of outboards that should be outperformed by the electrical ones. E.g. this Honda will outperform in top speed a 1 kW electrical power outboard, which is claimed to be 3 HP equivalent and most 2.5-3.5 HP will outperform 2 kW electrical power outboard, which is claimed to be 5-6 HP equivalent.
     
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  10. gonzo
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    gonzo Senior Member

    #3 is absolutely wrong. Torque can be changed through gearing. Power = work/time
     
  11. jet14
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    jet14 New Member

    Hello K, I agree with most of what you say. Some higher power electrics can drive faster boats, but my post was about displacement boat propulsion. As speeds increase to semi disp and planing levels, the smaller prop becomes more efficient than at lower speeds. And this supports your statement that gas engine props are a compromise at slower speeds. My measurements do indeed place our prop at 4 knots and 120 watts input in the 80% range of efficiency if you deduct controller, motor appendage and gearing losses from the easier to measure overall efficiency. And I also measured smaller traditional ob props at 25-35% efficiency at these slower speeds. I'd guess Torqeedo and Ray are also in the 80% range. So since gasoline outboards measure their output power at the prop shaft and don't include prop efficiency in any way, for these displacement applications, you would be looking at a multiplier north of 2x right there. The second point I made relates to efficiency curve against power output. Torqeedo used to post an efficiency curve showing their overall system efficiency increasing to a max point near the motor's max. input power. Clearly that's where you want to use the product most of the time to take advantage of the efficiency. Our EPcarry is no different except that its efficiency peak occurs at 1/2 the power level of the smallest Torqeedo making it appropriate for smaller boats. But gasoline motors must be used for cruise at a power level somewhat under the rated peak power. People disagree on how much, but for the same cruise speed, electric should be power rated at cruise and gasoline at some higher power point. I think where we got off trim is in the final conclusion. I do believe my statements are honest and lead people to the right choices for displacement boats only. But I agree that these things are not helpful to some who drives their boat to 7 knots. The electric boat industry is new. And in my view, the advantages of electric are currently tied to displacement applications where real distinctive advantages are easiest to realize without a NASA budget. But batteries are becoming cheaper, safer and better packaged for marine. It won't be long.
     

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    Last edited: Oct 25, 2017
  12. gonzo
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    gonzo Senior Member

    Horsepower is out of style in most of the world. Internal combustion engines are rated in Watts, so the comparison is very easy. If you compare an internal combustion engine with a propeller that is not optimized for the target speed, you are cheating. It is very easy to measure shaft power. There is no way you can prove that inputing that power with an electric motor requires less power than if the power is from any other source. That is simply sales hype.
     
  13. Joakim
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    Joakim Senior Member

    So did you get that 25-35% efficiency at the same thrust and speed as the 80% for your propeller? How did you measure that?

    OB propellers can have quite poor efficiency at very low speeds and high power, but are not that bad, if speed is a bit higher or power is low. As my calculations showed, the Honda 2.3 propeller could have 38% efficiency at full power at 4 knots. That could be in top part of the 25-35% range for the actual propeller. But at low powers of EP Carry, it should be closer to 50% depending on speed.

    Torqeedo propellers do not have 80% efficiency. They claim 56% overall efficiency (without specifying the actual conditions), which means 60-65% propeller efficiency and 85-90% combined efficiency for the motor and the controller. At the same time they claim only 27% propeller efficiency for a 5 HP OB. These claims just don't add up with the fact that a 5 HP has much higher top speed than Torqeedo 2.0 in any vessel they could be used in.

    Epropulsion claim 90% motor efficiency and 50% overall efficiency for their 1 kW model. So less than 60% propeller efficiency despite 11.5" diameter. There are no gears.

    What do you include in the 47% overall efficiency? Battery efficiency as well? Torqeedo is certainly not including battery.
     
  14. gonzo
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    gonzo Senior Member

    The overall efficiency of an electric system, if calculated honestly, is the ratio of power input from a charger, to the actual power output.
     

  15. Barry
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    Barry Senior Member

    but at specific rpms, as the input/output efficiency changes throughout the rpm range
     
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