Are electric horses really bigger?

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

  1. DennisRB
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    DennisRB Senior Member

    It only looks like that thanks to misleading graph scaling. Did you get that graph from a electric propulsion vendor?

    The graph starts at 200 and goes to 800, this makes the difference seem "dramatic", but in reality on a boat with an 8.5 max speed (as shown) would probably cruise at 6-7K. In that region it is about 20% less thermally efficient. Still FAR less loss of efficiency than converting mechanical to electrical to mechanical again.

    Maybe down below the 4k range it might start to win out, but then you are comparing pears to apples because the system can't power the boat to 8.5K. If you compare the efficiency to much smaller diesels (fair, since the claim is you don't need that extra power) then the normal diesel system will use less.

    Its an interesting graph which shows the right oversized prop will hardly impact speed but make worthwhile differences in fuel economy. Also running your standard diesel installation at maximum thermal efficiency will obviously use MUCH more fuel in the real world (IE full speed). If the need is never there to use the full power, well your engine is too big, and a smaller one will run with 20% better thermal efficiency to reach your 6k with zero power to spare. Seems not being able to make sufficient headway into a stiff breeze and sea state, while flogging the engine for 20% less fuel consumption when its supposed to be a sailing boat is a poor choice, but that last bit is just an opinion.
     
  2. groper
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    groper Senior Member

    Unless the ship has multiple gensets - as many do.

    Run 1 for light propulsion duty and house loads, then successively start and synchronize additional gensets as the propulsion loads increase...
     
  3. DennisRB
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    DennisRB Senior Member

    Sure, but then you need to spec in larger electric motors so you just move the inefficiency to them instead. I have not done the number crunching but a quick search shows most DC motors have a similar issue to diesels, in that not surprisingly, they also have loads at which they run more efficiently than others. But without an actual efficiently curve of the motors we are talking about here I can't actually work anything out.

    What makes far more sense is to just fit an actual normal diesel instead of yet another gen set. I am fine with it being 100% normal like any boat I have even owned and having just an electric in the other hull. But for those with OCD they can fit a clutched hybrid with pancake generator so they can run with direct diesel drive or electric on both sides. These things also act like a gigantic genset too so will top your bank up very fast. That means the other genset which would usually be oversized for house loads can now be smaller, lighter and cheaper. But most logically, eliminated altogether.

    I'm just talking cruising cats here. Obviously the SOR of the vessel would dictate what might work or not.
     
  4. groper
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    groper Senior Member

    Most of the motors i have worked with have a pretty flat efficiency curve from 100% down to 50% load where it then starts to drop quite dramatically. So basically if the load is 50% or higher its generally running a reasonable efficiency- many of the good motors are pushing north of 93% in small frame sizes whereas the bigger frame motors are getting above 95%.

    Im not familiar with really big motors, like the big synchronous motors, but i beleive ive read somewhere these are getting around 97% efficiency or better...
     
  5. DennisRB
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    DennisRB Senior Member

    I guess the one redeeming feature of the tiny motors they are promoting, means you would hardly ever run them below 50%. But it could be fairly inefficient to fit large electric motors if it then tips them below the 50% operating power.Which seems fairly likely considering a regular high HP diesel runs far below 50% power when trying to conserve fuel. But this should be fairly trivial to work once you have the actual performance curves of the motor and know the hull powering requirements.

    Still, I can not see any cost, weight or performance benefit for the 2 electric motors and 2 gen sets over clutched pancake diesel, other than symmetry. The clutched version can both power under 2 electric motors (smaller cheaper more efficient ones), but it can engage for full power diesel speed and efficiency when required.
     
  6. groper
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    groper Senior Member

  7. DennisRB
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    DennisRB Senior Member

    No I haven't. Nice link! :) That gear sounds legit, and reasonable price. We all know once the marine label goes on a zero gets added.

    This is the smallest, way bigger than what I would want. I think 10-15kw would be ample when you also have an actual 30kw diesel too. You could probably even use one petrol outboard, then hack and old OB to fit one of these in there for the second.

     
  8. groper
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    groper Senior Member

    go through their website and look at the videos - theyve been invloved in building some insanely fast vehicles including a 200mph electric C6 corvette with over 700 electric horses on tap...

    im trying to locate data on the round trip efficiency of their motor and controller package as a whole from the battery...

    EDIT : seems to be around 90-92% for the motor and 95% for the inverter and 90% for the battery.
     
  9. Brian@BNE
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    Brian@BNE Senior Member

    The Twin Disc Blue Drive system looks to be a good option for a power cat. Size the engines for planing performance and run them both for 'high speed cruise'.
    http://www.twindisc.com/marine-products/propulsion/bluedrives/

    But then for displacement speeds run just one engine, feed the power generated to the other side, and still have both props turning for 'low speed or economy cruise' with equal thrust from both props.

    I imagine that there will be a modest window of engine rpm where the single diesel will not become overloaded. If money were no object then Controllable Pitch Props could be added to avoid overloading, although I suspect there is enough reserve power (gap between prop curve and max power curve) for the CPP's to not really be needed.
     
  10. terhohalme
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    terhohalme BEng Boat Technology

    I was reading these posts and this came out. It's a test curve of a new "direct drive" trolling motor. Some experts here say that it is full of wrong numbers or it is difficult to read or understand. I would say it is neither. There is just a lot of data in the same graph, but nothing "wrong" or "messy".

    Let's see, in horizontally there is Torque in Nm. All other variables are function of the torque. All variables have their own color and independent vertical scale. On the left side, the first one U(V) is an orange voltage scale and in the graph an orange curve. It only tells how good is the testing battery. More horizontal curve means smaller internal resistance.

    The second vertical scale I(A) is a purple motor current and a purple curve in the graph. It is very straight or linear as it should be: torque has a linear dependence on the motor current. The third scale is the yellow input power P1(W)=U(V)*I(A) and the yellow curve in the graph. It is also almost linear.

    The first scale on the right side of the graph is light blue rpm n(r/min) and light blue curve in the graph. RPM drops from 2250 to 1600 when the torque increases from 0 to 10.5 Nm. The next scale is turquoise shaft power P2(W) and turquoise curve in the graph. Shaft power is somewhat curved because of motor losses, the maximum power is approx 1750 W @ 10.5 Nm.

    The last scale, dark blue efficiency Eff(%) is difficult to read, but is is there. Also, the efficiency curve is in the graph. Maximum efficiency seems to be about 82% and efficiency at maximum torque and power about 73%. On the right, there is a Parameter column, that tells the rest. Just don't be confused by the Chinese texts, the symbols and variables are in English too.

    So, nothing complicated, nothing wrong, just test results. This motor is 2.35 HP (1.755 kW). No more, no less.

    Then the original question: Are electric horses really bigger? The simple answer is: No, they are not. There are no physical or engineering way to justify it otherwise. It's only in salesman's dreams. The propeller doesn't know where the power comes from. Shaft power is simply P = Q*2*Pi*rps, where Q is torque and rps is revolves per second. So, three variables, power, torque and revolves are bounded together. More power needs more torque or rpm's.

    Where does this misunderstanding comes from? Maybe it is from energy balance or total efficiency. 2.4 HP petrol engine sure burns more energy in fuel than 2.4 HP electric motor consumes electrical energy to make the same distance. But it is the input difference, not the output (shaft power) difference.
     
  11. Caroute Motor
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    Caroute Motor Junior Member

    Terho,

    Yes, you are right. Brushless motor is just more efficiency. Same output power just need less input power. We just used Torqedoo's statement, comparing with input power. If comparing with output power, 3HP is 3HP. Our motor is also little diferent with Torqedoo and Haswing. They are high RPM(maybe 10000, we didn't test) through reducer to 1500-2000 RPM. The motor life can be too long. Our motor is designed to directly shaft RPM 1500-2000 RPM. If without hit or broken during usage, our life can reach around 2000 hours. If can changing the seals and bearing in time, the life can be longer.
     
  12. Joakim
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    Joakim Senior Member

    It took a while to figure out your quote was from post #61 and you talk about comments a few pages backwards from the current one.

    There is a lot of data in this graph and it is very misleading that the two different powers (electrical and mechanical) have different scales. And you can't read the scale for efficiency.

    This motor could be marketed as 2.4 kW (electrical power at 1.755 kW mechanical power) or equivalent to 6-8 HP OB. That's the way Torqeedo does it for their 2 kW model: http://www.torqeedo.com/en/products/outboards/cruise/cruise-2.0-r/M-1230-00.html

    2 kW input power (electrical) and they claim it is equivalent to 5-6 HP.

    Here they go on claiming that a 5 HP OB has only 995 W propulsive power with 3.7 kW shaft power (27% propeller efficiency!) while 2 kW input power Torqeedo has 1112 W (56% efficiency for everything, controller, motor and propeller combined). http://www.torqeedo.com/en/technology-and-environment/performance-and-efficiency.html

    While in real life 2.5 HP OB is most often (clearly) faster than Torqeedo 2.0 kW.
     
  13. Caroute Motor
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    Caroute Motor Junior Member

    If 2.5 HP OB is output power, it should be a little quicker than Torqedoo 2.0KW for same boat. If 2.5HP is input, Torqedoo should be quicker. Our 2KW motor for 400LBS flat boat with 2 adults can reach around 5 to 6 miles.
     
  14. terhohalme
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    terhohalme BEng Boat Technology

    Jep, the efficiency scale and curve are too dark. And the power scales should be identical.

    The efficiency numbers in Parameter column are very clear though.
     
    Last edited: Mar 18, 2017

  15. Caroute Motor
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    Caroute Motor Junior Member

    Yes, right.
     
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