Need help with electric boat modification planning, rough estimation on engine and battery capacity

Discussion in 'Boat Design' started by xellz, Jul 2, 2017.

  1. xellz
    Joined: Jul 2017
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    xellz Senior Member

    I see now that Sonic 70 sterndrive has elecrtic motor option of up to 25kW. But don't see pricing, seems have to request.
     
  2. BertKu
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    BertKu Senior Member

    First at all Xellz, my apology to go slightly of your subject, but as it is important to you, please allow me to answer the critics.. You all are right and also Ocean Volt. First at all, one of the reason why they moving to high voltage AC motors is to avoid hundreds of claims from potential deaths when 300 or 400 Volt DC system are used. There will be some users who will open their system and will get cramped at those high DC voltage and most likely die. You can touch 220 Volt or 380 Volt Ac and you most likely you will survive, due to the fact that AC goes through the zero point. Secondly there are much more high voltage components available for AC, such as Triacs etc, than High Voltage P type MOSfets used for brushless motors. Also the high currents means thicker copper cabling then AC and more energy losses, if the motor is far from the batteries and control system.

    The reason why I think that Ocean Volt has a point. They probably compare a 45 hp diesel running at 300 revolutions instead of 1700 revolutions, which normally most diesel is producing the optimum torque and power. It is just a marketing trick and is only true under certain conditions. Why the hell do they sell 20 000 Milli candela's LED's, while it actual is only 20 Candela's. But 20.000 sounds more.

    I will try to get Ocean Volt to answer how they have compared and concluded this statement. Whether I will be successful, remains to be seen.
    Bert
     
  3. Rumars
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    Rumars Senior Member

    Bert you are right with all the motives for AC, but you omited the primary reason, and that is rpm range. Smaller boats have smaller props, and smaller props need rpm's to develop the power. It's exactly like in a car, the wheel diameter is given, so the rpm's govern speed. DC motors are limited in direct drive applications. Or to be correct, since DC motors don't actually exist, mechanically commutated AC motors are limited.

    A high speed diesel is idling at 300rpm and not producing any significant power. Marketing relies on the fact that displacement hulls actually need very little power to move. Traditional calculations for sailboats used a figure of 1HP per metric ton to achieve hull speed on flat water. Any additional power is for adverse conditions like current, waves and wind. Since battery capacity is limited usually the owners don't power against adverse conditions for long, and everybody is happy.
     
  4. BertKu
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    BertKu Senior Member

    Hi Rumars,
    I have a different opinion. You need torque to turn the propeller and not power. I am able to turn a 17 inch blade in water with only 0,2 horsepower. To move the boat fast I need to speed up the power (horse power) while the toque stays the same. That is logical , because a brushless DC motor has magnets and the current is pulsed at 10 Khz, and the width of the pulse is 1/100 of a time period and then 99/100 nothing. Thus the torque is there,, because of the high current and my propeller moves, but the boat goes slowly. To let me make the boat go faster, the current stays the same, but the period is moved to let say 50/50 and now the boat is moving faster forward. There is no diesel nor petrol engine which can do that, except using a slip coupling, and that is very inefficient. Thus my conclusion is that Ocean Volt has a point, that a 15 Kw motor can be at certain conditions equivalent to a 45 hp Diesel engine.
    Bert
     
  5. Rumars
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    Rumars Senior Member

    Bert you are saying exactly what I said. Power does not exist pe se, it can not be measured, it is only calculated. Torque and rpm can be measured. When you say "speed up the power" you mean rpm, or applying the same amount of torque more often in a given time. That is exctly what a VFD does, and you explained. The internal combustion engine does the same thing only different and usually not so smooth (there are engine configurations capable of it but not commonly used) and it also varies torque at the same time. You would have that if the VFD would not only modify frequency but also amps.
     
  6. kerosene
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    kerosene Senior Member

    Power exists just like acceleration exists even though you need time and speed change to calculate it.
    Power is work done in cerain time. It can be expressed in many ways but that is the essence of it.
    P=VF (velocity * force) this is equivalent to the rate of turning * torque.
    Put a rope through a pulley really high, like 30-50 ft. Tie reasonably heavy weight to it, let's say 50lbs. It is quite easy to pull that weight up. Now try doing it fast. Quite a different task.

    Ocean Volt's claim is nonsense and a purposeful insincere twist. Really high torque at low speed doesn't happen on prop axle.

    If they said "can twist axle with weed and rope tangled to it like a 45hp and push your boat like a 15hp (or 20hp if shaft power was 15kw)." They would be right.
     
  7. BertKu
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    BertKu Senior Member

    Kerosene, I expected you to support your countrymen. Many people don't understand brushles motors. Your torque is the same at 1 rpm or 3000 rpm for the same motor. The only difference is that you modify the time period from 0.01% to 100%. Thus turning the speed up from 1 % to 100% is no sweat. and is not the same as what you quoted. All what happen is that your average Kw usage is moved up from low to full Kw usage. But during the 0.01% the torque is there while in your example your going from zero to fast is moving the torque up from 0 to fast with your body and you may damage your muzzles. Bert

    P.S. actual I may be wrong. The torque at 1 rpm may be substantial higher than at 3000 rpm. This is because the inductance in the winding's are not yet build up and only the Ohmic resistance plays a roll. In the brushless motor that is extreme low. normally in the order of milli Ohms. The initial current therefore at 1 rpm maybe higher, while the magnetism from the magnets stays the same, this then resulting in higher torque for that small initial period of time, before the KW is turned up. .
     
    Last edited: Jul 15, 2017
  8. kerosene
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    kerosene Senior Member

    Honda VTR sp2 / RC 51 (motorcycle) power curve below . Torque is not flat from 4,000 to 11,000 rpm but not far from it.
    Power peaks at 11,000 rpm.
    Do you think the machine accelerates and pulls the hardest at 4000 or 11000 rpm or more or less the same as you go through the range?

    [​IMG]

    And as far as supporting my countrymen - I support good stuff regardless of origin and like many have some nostalgia and nationalistic pride to things from the homeland. That doesn't make me not call nonsense on nonsense.
    I even believe that their product is likely really good - just that claims of superior horses is bogus.

    I think the likely truth is that diesels on sailboats are rarely used at full power. The typical usage is at fraction of max power for many and when e-motor can deliver that (with other positives like low noise) the user doesn't care or notice that he/she doesn't have the reserve power they used to.

    I get your point of zero to full instantly and being able to deliver high torque at low rpm. My point is that it is not really relevant in normal use. Responsive control is nice but high torque is only needed when the prop r vs go up. I can spin large (not huge...) prop slowly by hand.
     
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  9. xellz
    Joined: Jul 2017
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    xellz Senior Member

    Started a new thread in electric propulsion sub forum, no point to throw everything in one thread. A lot of information to think about, but the more i learn, the more i'm determined to follow through this project, with or without financial support. I'm planning on building hull myself, it will save me large chunk of finances and will allow to get hull i'm looking for. Not an easy task, but doable. With some help from friends 1-1.5years for building a 28-30ft catamaran sound reasonable, this time is good, because i can practice and get used to handling boats in any kind of weather with someone experienced on board.

    But one thing for sure, once i got the taste of ocean, there is no returning, i'll do anything to stay close :)
     
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  10. BertKu
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    BertKu Senior Member

    Xellz, I wish you lots of success. Pleased, that some of us have been able to give you broad advice. Yes, you can make your own hull, one advice, try to get an affordable plan. It will be much easier. Bert
     
  11. BertKu
    Joined: May 2009
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    BertKu Senior Member

    Kerosene, I see it different. I would have picked up the phone and ask them in your own language, what test has they done to come up with such a statement. Only then to comment negatively or positively. I personally belief in giving the other party the benefit of the doubt. For the rest I agree with you, but most brushless motors, you can overload them for a short period of time. Indeed if you need more power, all what you have to do is order a slightly more powerful motor. However the consuming power depends on the controller and is absolute minimal in battery usage as a diesel which has to run at a certain speed to be efficient. That is not the case with brusless motors. The speed is dictating the amount of power used from the battery. But it seems that the OP got his answers he needed to make the optimum decision. Bert
     

  12. xellz
    Joined: Jul 2017
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    xellz Senior Member

    Yes. this place sure helped out to get better idea on the project. There isn't much data available on full electric yet. Price drop on energy storage in last few years sure helps a lot, before it was more like luxury toy, than anything else.
    I might have to settle for hybrid propulsion, i.e. lower battery bank and diesel generator, but main goal still will be accomplished. Little to no engine noise for a while or during fishing time and greatly reduced maintenance time and cost. And i think fun factor is also important to keep working and pushing forward. I'll be posting about problems that surfaced during building and real use data when it will be available. Might help next person with electric boat in mind :)
     
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