Charging Up to Hybrid

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  • 13 Dec 2019 23:26
    Reply # 8270429 on 8265784
    Anonymous wrote:

    PS: Just for the record; what is the ‘burst’ power of the electric engine?

    "Burst power" (in HP) does not make a lot of sense with an electric motor in the way it might with an IC. They do have momentary torque which is pretty much equivalent. In theory, an electric motor has maximum torque at 0RPM which then falls away with speed such that a constant HP is kept to maximum speed. In reality, The maximum torque is set by the maximum current the windings can handle without burning up which is controlled by a mixture of winding size and cooling available (the same motor may be rated twice as high with water cooling instead of air cooling). As a result most electric motors have a large portion of their speed range where current and therefore torque are constant (normally about half the speed range). In the case of the project in question, the motor is expected to spend it's life within this constant torque range.

    In this case, the continuous current draw is rated at 125 Amps. The motor can stand 420 amps for up to 60 seconds.

    I am sure there are some bright minds who are going "great, 1 HP is the same as 750-ish watts and we know the voltage so we can convert to HP". Actually no. The equation for HP can be set by using torque and speed or mass over distance. So if the motor torque is 15 Nm but the rpm is 0 then HP is 0 as well. So if the motor is constant torque till 3000rpm then the only time it makes sense to equate electrical power to hp is at 3000rpm. Of course torque in the case of turning a prop from idle to starting to turn is more important... try starting your IC motor with the transmission engaged (and the prop in the water of course). Many cargo ships use electric motors as the transmission for this reason (most trains too). In general, IC engines are poorly matched to low speed, high torque loads while electric motors do very well for this use.

    But wait! there's more... The unit in question on the quiet torque web site lists the motor's maximum torque as 35Nm which does not match up with either the continuous current rating (15Nm) or the max for  60 seconds rating (50Nm). What gives? Well, at 35Nm the motor should be able to run for more than 60 seconds, so maybe a bit of fudge factor to save the motor... I don't think so. These maximums are rated in Amps and so far we have only talked about the motor itself. There is another component involved which also has limits. The controller.

    The controller takes DC current in and converts it to three phase (for this motor) at the frequency that matches the motors rpm (or a multiple thereof). Motor controllers do this with pass elements which even run in full on (minimum resistance) still have some voltage drop which means they dissipate heat. So the maximum current they can pass is also based on their ability to stay cool enough to not burn up. In this case, the controller limits the amount of current maximum to 290Amps. So burst power electrically, is 13920watts at 48v and continuous is 6000watts at 48v. So burst power is about 200% with whatever method you use for calculating that.

    Of note: While the electric motor is able to produce maximum torque at 0 rpm, in a boat this does not happen for very long because as soon as the prop starts to turn torque goes down too. Everything changes faster than it can be calculated at any one speed.

    I have tried using some of the prop calculators on the web to see what size/pitch prop would make sense for an electric motor. They don't work very well because they assume 0 torque at 0 rpm. The actual prop calculation is done with torque not HP but they calculate torque from HP and assume a torque curve from an IC engine... which is wrong. The prop as calculated this way has a lower size and pitch in order to be usable at low rpms (without stalling the engine) where an IC engine has little torque. (while the electric has full torque)

    Probably way more than you wanted to know... sorry.

    Last modified: 13 Dec 2019 23:31 | Anonymous member
  • 13 Dec 2019 16:48
    Reply # 8266733 on 8133077

    For anyone interested in looking into electric drive, there's a useful site at:

    Of particular interest to me, with a boat that doesn't suit outboard motors very well, be they petrol or electric, there's a comprehensive guide to pod motors.

    And how about an Amazing new electric boat motor based on fish fins!

  • 13 Dec 2019 14:49
    Reply # 8265784 on 8133077
    Anonymous member (Administrator)

    now I have re-read your initial posting of this thread, and I am about to turn more positive to you conversion project.

    Your plans have changed from long-distance to local or coastal cruising. The matter of battery range is therefore not so important, and with that Honda generator in cold, hidden in the cockpit locker, the battery bank needs not be that big (2-5 kWh?).
    I checked the Honda  EU2200i generator. It can even be made to run synchronously with a second Honda, if you later find you could use more power.
    The lovely thing with this Honda is that, besides being good, it is light enough (21kg) to bring ashore for service, repair or replacing it. Much cheaper than if you get trouble with a diesel.

    With this setup, there is less need to complicate things by turning your ship it into a complete solar plant. After all, you have a car. If you let your Honda generator burn up to 10% of what the car burns in a year, you could well end up with 100-200NM range based on the Honda power alone. I suggest you keep it simple for the first summer. That will soon show you your needs.

    However, the greenest way of propulsion is sailing. Therefore I stress the need for keeping the rig sorted out so hoisting sail doesn’t possess a big obstacle. I also keep saying that cambered junk-sails are the greenest of all, since they let you move upwind both effortlessly and fast enough.

    I apologise if my earlier postings have sounded discouraging, and I wish you good luck!


    PS: Just for the record; what is the ‘burst’ power of the electric engine?

    PPS: That lovely young couple on Youtube surely was resourceful and not shy of doing some hard work! Being on the wrong side of 60, I prefer to cut corners and rather lower my ambitions. I am not quite the Houdini that I used to be...

    Last modified: 13 Dec 2019 16:39 | Anonymous member (Administrator)
  • 13 Dec 2019 12:45
    Reply # 8264856 on 8264358
    Check out the video series (I linked to the first video, I think there are three in total) and/or read here:

    This is an excellent link. I strongly suggest reading this page before posting declarative statements based on hasty assumptions.  It will answer many of your questions, better than I. 

    Concerning the philosophy behind the author's choice to go to electric, I like their "mission statement":

    We have found that having a solid plan and the patience to wait for the right weather and tide is the key. - After all, we own a sailboat. They are inherently slow. We are not in a rush. We love the idea of being self-sufficient. But, for those who sail with schedules, are short on time, lack the patience to sit out a wind hole, feel the need to power their boat to hull speed, enjoy maintaining a diesel engine or are just set in their ways, then an electric motor probably isn’t the right choice

  • 13 Dec 2019 11:29
    Reply # 8264358 on 8133077

    Yeah that's indeed an interesting project. I don't remember the story completely but they bought an identical engine (used) to replace the broken one that came with the boat but for some reason it didn't work out so they went electric instead. They bought a a used electric motor from a forklift for practically no money and then setup everything together themselves, with a bunch of trial and error.

    Check out the video series (I linked to the first video, I think there are three in total) and/or read here:

  • 13 Dec 2019 03:13
    Reply # 8260912 on 8260735

    Did you maybe mean Uma and Sailing Uma?

    Thanks for the correction.
  • 13 Dec 2019 02:49
    Reply # 8260735 on 8249322
    Anonymous wrote:
    I think you summed up the reason for the difference in prices very well. I'm not knowledgeable enough in electrical matters to presume I could assemble a system by myself, like the owner of Suma. See the blog, "Sailing Suma"

    Did you maybe mean Uma and Sailing Uma?

  • 13 Dec 2019 02:35
    Reply # 8260650 on 8133077

    Going by the data from the calculations in sheet the electric motor seller provided Jim (the OP) on page 3, that hypothetical battery bank of 1917 Ah would be ~400 Ah at 48 volts, which would/should give around 35 NM at 4 knots speed and 50 NM with "reduced speed" (3.3 knots). On par with dinosaur juice. This with 9500 lbs displacement which is considerably more than you (Arne) and David are working with.

    There are solar panel and electric motor efficiency numbers available on the internet, the former being I think around 20% and the latter at 85-95%?

    No arguments here that the issue with electric motors is energy storage, that's the downside, albeit one of the only ones. By combining solar, wind and regeneration from the prop it's not that crazy anymore, given that after the initial investment (and future upkeep) of mainly batteries, those are free.

    And let me also say that yes, if your intent is to motor for 12+ hours for whatever reason then forget about electric motors, I'm talking about sailors here. With a 2 hour range you already have a lot of flexibility. Pretty much the same as with a yuloh or sculling oar, except it's faster and you won't be out of breath when you run out of juice.

    All of this said I'm still leaning towards keeping my diesel engine, because the electric alternative is quite expensive upfront and I'm also somewhat lazy and not that zealous about saving the planet... With any boat designed from scratch the largest combustion engine I'd like to have would be an outboard, something like the 6 hp Tohatsu often mentioned here. But now I have what I have, so...

  • 12 Dec 2019 23:28
    Reply # 8259370 on 8133077

    That kind of thinking is also why most of the world still runs on petrol...

    Edit: Before a flame war is started, yes, pleasure sailors have quite a small impact in the grand scheme, was referring to the world at large seeing the planet go to shit before their eyes but not doing anything because of stock prices and cost calculations.

    Last modified: 12 Dec 2019 23:33 | Anonymous member
  • 12 Dec 2019 20:13
    Reply # 8257987 on 8133077
    Anonymous member (Administrator)

    A 4-stroke petrol engine runs with around 25% efficiency (..give or take quite a bit...). That means that a litre of petrol should produce around 2.3kWh mechanical energy. A single ten-litre can therefore stores about 23kWh. In 12V battery parlance that means 1917 Ah  -  and that is with loss-free batteries (which don’t exist...)

    In plain English; ten litres of fuel fed to David’s or my Tohatsu 6, will move our boats somewhere between 30 and 50 Nautical miles, with a bit reduced speed.

    David is right that for only motoring in and out of harbour, the electric engine would be great, with access to plug-in power at the berth. However, it doesn’t pay back the extra cost of replacing the existing petrol engine with an electric one.

    The only place where I think using electric propulsion would be a practical and economical advantage, is on short-distance shuttle ferries. All other use will be based on idealism, ‘electric enthusiasm’ or special needs, such as quietness (birdwatching etc.).


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