Electric PWC

Discussion in 'Boat Design' started by BluSky, Sep 2, 2013.

  1. BluSky
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    BluSky Junior Member

    Hello, I would like to develop a project of a jet ski driven by batteries, electric PWC. Dimensions of such PWC are: 3,40x1,25x1,20 meters.

    The typical power of a PWC is about 150 hp (112 kW). The problem I find mainly is to achieve this power to the limitation of space, weight, flotation and stability problems, and mainly budget. I also want to set as the design basis a speed of 70-90 km/h (40-50 knots).

    So I'm considering reducing power about 35 kW (50 hp). Therefore:
    Do you think that would be enough power?
    For reaching targets marked, can be redefined impeller or propeller design, with the same power of 35 kW to reach this speed?

    Any other recomendation, comment or advice is very appreciated.

    Thank you very much.
     
  2. FAST FRED
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    FAST FRED Senior Member

    Batteries will have to get at least 2 decimal places better for this to work,

    Perhaps someday,
     
  3. Jeremy Harris
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    Jeremy Harris Senior Member

    You can certainly make an electric PWC with the performance you are looking for, but the endurance will be so short as to make it a bit impractical.

    A heavy PWC might have an empty weight of around 250kg. The fuel weight would be around 50kg max and the engine might normally weigh around 60kg or so. So, you have a weight budget of around 100 to 110kg for the electric motor, controller and battery pack.

    If you can accept only having 35kW, then the motor and controller weight is likely to be around 20kg to 30kg (if water cooled), leaving you around 80kg for the battery pack , enclosure, and battery management system. You might be able to squeeze 75kg of cells into this pack.

    If you used the very best lithium cells currently available, then this could give you a maximum battery pack capacity of about 20 kWh. The usable battery capacity would be around 70% of this, as deep cycling even lithium chemistry cells rapidly shortens their life. This means you have a usable capacity of about 14 kWh.

    Endurance is therefore going to be about 24 minutes at 35 kW.

    If you wanted to re-charge the battery pack in, say, 30 minutes, then you would need a charger capable of delivering about 30 kW, which would require a pretty big electrical connection. Here in the UK the normal domestic electricity supply will support 13 A at 240 V easily, which is only a little over 3 kW, or a maximum of 32 A via a special industrial type connection which would give about 7.5 kW.

    The quickest recharge using a domestic supply of this type is therefore going to be around 2 hours, even using a 32 A connection.

    At the moment, battery technology just doesn't allow fast, light, boats to be built that have a useful range or endurance, it is best suited to slower boats with much lower power requirements.

    If the application for this PWC was an inland lake, where there was access to fast charging, and if the battery packs were made exchangeable, then such an electric PWC might be viable. It would result in zero pollution at the point of use and much lower noise levels, so could be a benefit in an environmentally sensitive location.
     
  4. BluSky
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    BluSky Junior Member

    Jeremy, many thanks for your comments. In fact, they are very close to my estimations.

    I have a serious doubt about the needed power, I mean, if I have dimensioned the PWC with a battery of 35 kW correctly. I do not if it could not be enough for the propulsion system. I do not know if changing the pump / impeller of the PWC could reach the speed and range that I was estimated (initially 40-50 knots) for this power output.

    What is your opinion on that?

    Talking with a teacher in naval engineering in Spain (I am not naval engineer), they told me that 35 kW is very low, and the feasibility of the project can not be achieved.

    Thank you very much.
     
  5. Jeremy Harris
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    Jeremy Harris Senior Member

    35 kW is low, about the power of the early one-person jet skis that used two stroke engines of around this power.

    Water jet propulsion is generally not very efficient, and the jet pumps that current PWCs use are probably about as good as you're going to get. If there was an easy way to get more thrust for a given input power then the PWC manufacturers will have already done it, as this is a pretty performance-oriented market.

    I think that to get to 40 to 50 knots you're looking at a lot more that 35 kW, although fast water jet propulsion systems are outside my area of expertise.
     
  6. messabout
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    messabout Senior Member

    Why is it necessary to have a speed potential of 40-50 MPH? That kind of speed is irrational, it is dangerous, it assures a rough ride, it is difficult to enjoy the scenery at high speed, it arouses the ire of more sane boatmen. (please excuse the rant)

    We could build a one person PWC that will go half those speeds with 10Kw, especially if prop driven. Still a fun boat, one that will cost less, run longer, and be far more socially acceptable.
     
  7. BluSky
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    BluSky Junior Member

    Thanks Messabout. I think that the most important to start the design is to contact with pump and impeller manufacturers, in order to establish the power that is necessary for the electric motor, and so, to dimension the battery stacks. They could provide me more information about the power I would need, to optimize the design.
     
  8. Skyak
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    Skyak Senior Member

    Great practical information above leaves me to offer some wild alternatives to the impossibility of and electric equivalent of a gas PWC.

    Alternatives to batteries- there are fuel cells that are light and high powered. There are also natural gas reformers and fuel cells made to operate on nat gas. This is all more expensive and more complex than gasoline but likely would be a better solution than the big battery and charging problems -and the safety problems that haven't even been considered yet.

    Super capacitors should also be considered to address the charging and power demand problems of batteries.

    If you just wanted a vessel that would carry one person at 40 to 50mph on electric power I would say it is possible with a conventional prop hydrofoil. The big problems are with the assumption that you are going to produce a jet ski like the ones powered by gasoline. Jet skis are simply not designed for energy efficiency.
     
  9. Jeremy Harris
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    Jeremy Harris Senior Member

    Currently, super capacitors have less usable capacity than the very best batteries, so don't offer an advantage in this application (which is one dominated by energy density).

    Fuel cells can give better energy density than batteries, but they have a very low power density, so cannot deliver the required power within the weight allowance.

    The problem here is that a PWC requires a high power output more or less continuously, which means any power storage system needs both a high power density and a high energy density. At the moment this is something that hydrocarbon fuels excel at.
     
  10. BluSky
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    BluSky Junior Member

    I think that the most important is to think in the electric motor, its rpm, and so, to see what is the most optimal impeller.

    On the other hand, probably it is interesting to install a propeller in the PWC.

    Thanks for your comments!
     
  11. waikikin
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    waikikin Senior Member

    I think to hit the mark in a marketing sense that this electric vessel may be way too quiet for most PWCers, my suggestion is to invest battery power to stereo & speakers, with the option of virtual reality, options for cranking out bass heavy tunes or extreme engine noise to givE the impression of speed & power:eek:
     
  12. Jeremy Harris
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    Jeremy Harris Senior Member

    The usual starting point for propulsion systems is the resistance of the hull at the desired cruise speed. Once you have this you know the thrust that has to be delivered and this gives you the power that has to be delivered to make the craft move at that speed.

    Say you want to go at 30kts and find that at this speed the craft has a resistance of 2260N. This means the craft needs 2260 N of thrust to achieve this speed, and 2260 N of thrust at 30 kts is 35 kW of propulsive power.

    A very good propeller might be around 80% efficient, so to get this 35 kW of propulsive power you would need a motor capable of delivering about 43.75 kW.

    On the other hand, a small diameter water jet might only be around 60% efficient (may be less) and so would need about 58.33 kW of motor power.

    Propellers have been used in early PWCs (I owned a water scooter that had a prop running in a tunnel about 40 years ago). They present hazards, though, and also need to be fairly large in diameter to be effective, which then causes problems when launching from beaches etc.
     
  13. portacruise
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    portacruise Senior Member

    Thanks for your comments, Jeremy. My comments below inside your message.

    Porta

     
  14. Jeremy Harris
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    Jeremy Harris Senior Member

    I'm guessing that the 2 man hydrofoils are probably working with an input power of around 500 to 800 watts, maybe a bit more if these are super fit athletes, so perhaps a power budget of 1 kW or so might be reasonable.

    That sort of power is easily within the capability of a very light electric propulsion system, I'd have thought. If we assume that the same two man human powered hydrofoil could be a one man electric hydrofoil with the same displacement, then we have around 80 or 90kg allowable for the battery pack, motor etc.

    A 1kW motor might weigh around 1kg to 1.5kg or so, plus around 0.5kg for the controller and wiring, so perhaps 2kg of the allowable weight budget. If we allow another 5kg for the battery box and management system then we could have around 75kg or so available for cells.

    At an energy density of 260 W/kg that would give a pack capacity of around 19.5 kWh, with a usable capacity (assuming 70% max discharge) of around 13.6 kWh, plenty for a day of having fun on the water.

    Another option with this arrangement would be to have a fuel cell charging a smaller battery pack, with the battery pack only there to deliver the peak surge needed for acceleration. A 210 W methanol RV type fuel cell charger weighs around 8.2kg, and uses around 0.72kg of fuel per hour. Using five of these would just give enough continuous power and would weigh in at about 41kg. If we wanted a 4 hour endurance then the fuel tank capacity could be around 18 litres of methanol, so adding another 14.4kg, making the combined fuel and fuel cell weight around 55.4kg.

    Combined with a battery pack of, say, 10kg (about 2.6 kWh) this should work and have the advantage of being free from the need to recharge from shore power, it could be filled up just like an ordinary PWC.

    The performance would be well below conventional PWCs, but the advantages of being quiet and pollution free may well compensate to some extent. The biggest problems would be the very high cost (the 210W fuel cell units are around £5000 (~ $7700) each and there would be five of them, plus a lithium battery pack) and the practicality of launching a recovering a hydrofoil with a fairly large diameter prop.

    Interesting intellectual exercise working the sums, though.
     

  15. Skyak
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    Skyak Senior Member

    We seem to be making some progress (from NFW to theoretically possible). The major point is that today's PWC are extremely inefficient BECAUSE of the availability of cheap high power gas engines. A PWC based on electric power needs to be reconsidered and optimized based on the higher cost of electric power storage. A GRP planing hull with axial jet is a match for cheap gas engine and tank, but a carbon fiber hydrofoil with (surface piercing?) prop is a better match to expensive electric drive.

    Forget the power numbers of today's PWC, they are meaningless. You need to focus on your market and your value proposition. I like the picture of an electric hydrofoil PWC silently zipping along at 40mph in a 2 ft chop past a 200HP high end gas PWC banging, splashing, and screaming with it's jet aerating. Today's high end PWC's are capable of 40+mph, in the right conditions, but what speeds are they actually used at in real conditions? My experience with PWCs is that 40mph+ is brutal on choppy weekend waters. The other thing about gas vs electric is the mismatch in the toque/power required. When sailboats go from diesel to electric the motor power rating is about half.

    About supercaps, I meant they should be considered to address the charge and discharge peak limitations of batteries or fuel cells. Today's PWC are operated with high continuous demand (actually just open throttle) but they are wasteful and likely differ from an efficient craft where peak power will greatly enhance quality perception. An electric PWC that out accelerates high power gas models is not hard to do.

    About fuel cells -stay with gaseous fuel. Liquid is too heavy and will likely be a safety and pollution hazard (methanol is toxic on the skin). In Europe you may not know it but in the US the price of natural gas is extremely low. You might even find it worthwhile to use NGLs ethane, or butane.....to store as a liquid at reasonable pressure.

    Another consideration, don't base the price of the technology on some low volume retail -there may be a huge markup and you may even be a higher volume market the manufacturer wants to enter. The price to consider is what the volume users are paying and what your negotiated price will be. Technologies need marketing, your best price will be a deal that gets the tech's name in the news story of your sexy product.
     
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