Efficient electric boat

Discussion in 'Boat Design' started by Jeremy Harris, Jun 22, 2009.

  1. portacruise
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    portacruise Senior Member

    Second looks like a short to me, but maybe something is missing or drawn wrong?

    P.

     
  2. portacruise
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    portacruise Senior Member

    Oops, not short. Too late at nite here.

    P.
     
  3. pistnbroke
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    pistnbroke I try

    As it joint two points at the same voltage its not a short ...the point is it keeps the two left or right hand pairs at the same voltage so you dont get uneven discharge. Of course connecting any batteries in parallel and leaving them is a recepie for a fast self discharge but anyone contemplating an electric boat will be well up with charging technology I would think ..
     
  4. portacruise
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    portacruise Senior Member

     
  5. pistnbroke
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    pistnbroke I try

    I just spotted this on the british canal boat forum where they are finatical about wireing there house batteries right ....just put it here for comment ...not sure if I or they would agree with you.....
     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    My LiFePO4 pack is wired as 4S, 8P (so eight cells paralleled) and it remains balanced to within about 4mV per cell over a period of several months. My electric bike LiFePO4 pack is 12S, 4P and similarly stays well balanced, even when not used for long periods. Others on the Endless Sphere forums have run much higher numbers of lithium cells in big packs without problems, too, so I'm not at all sure that there is a real limit to the number of cells in parallel with any lithium chemistry.

    The same applies to any cell chemistry that is charged with a CI/CV profile, like lead acid; in fact a 'single' lead acid cell is often already a big parallel array of cells, anyway (a quick glance inside a wet battery will confirm this). It doesn't apply to any cell chemistry that needs a CI charge profile, though, like nickel cadmium, nickel metal hydride or nickel iron. There is a severe risk of cell damage from parallel cell connection of this type of cell, because differing values of Ri will cause significantly different charging currents to flow in the paralleled cells, to the point where one or more cells may seriously over-charge.

    This can't really happen with cell chemistries that show a persistent voltage rise with state of charge (like lead acid and lithium) because the lowest Ri cell increases in terminal voltage first and so automatically self-regulates it's share of the charge current.

    Jeremy
     
  7. portacruise
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    portacruise Senior Member

    Thanks for your posting, that is a good wiring system as any, Piston. The issue is charging anything in series, because the charger cuts off at a predetermined voltage for most systems. If 2 batteries in the same parallel leg pictured happen to be of slightly lower voltage, an imbalance will occur. This historically has been remedied to some degree by periodically applying an "equalizing" overcharge to very rugged systems like lead chemistries. Lithium systems are much more delicate and cannot take much of an overcharge (or discharge for that matter) but the associated electronics to individual cells may compensate to a degree. That's what I gathered from readings, feel free to correct errors.

    I've often thought that the best battery system would be a huge single cell (eliminate series connections entirely) with electronics to up convert to whatever voltage is required. More recently I have seen papers written along these lines, as the efficiency and cost of electronics has improved tremendously.

    Porta


     
  8. portacruise
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    portacruise Senior Member

    Thanks for your posting, that is a good wiring system as any, Piston. The issue is charging anything in series, because the charger cuts off at a predetermined voltage for most systems. If 2 batteries in the same parallel leg pictured happen to be of slightly lower voltage, an imbalance will occur. This historically has been remedied to some degree by periodically applying an "equalizing" overcharge to very rugged systems like lead chemistries. Lithium systems are much more delicate and cannot take much of an overcharge (or discharge for that matter) but the associated electronics to individual cells may compensate to a degree. That's what I gathered from readings, feel free to correct errors.

    I've often thought that the best battery system would be a huge single cell (eliminate series connections entirely) with electronics to up convert to whatever voltage is required. More recently I have seen papers written along these lines, as the efficiency and cost of electronics has improved tremendously.

    Porta


     
  9. BertKu
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    BertKu Senior Member

    Maybe one day, the industry would come up with a mass produced Super Capacitor, your proposal and way of thinking suits a super capacitor concept.
    Bert
     
  10. pistnbroke
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    pistnbroke I try

    I bought an 85 lb 24v 1250 watt Min Kota style electric motor for my boat thinking I could run it on 12 v .... after tweaking the prop with an electic paint stripper it ran 32 A in the swimming pool at 12v .....on the water 12 v and 25 A gave 3.2 miles per hour with an 11 ft fiberglass dingy about 250kg ....increasing to 24v and 50 A only gave 3.8 mph for 4 times the power and reducing to 15A at 12v using the resistive controller gave 2.4knots ...which goes to show efficiency /speed /power /range is a complex business..........your "efficiency" may be another persons "disaster"...
     
  11. BertKu
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    BertKu Senior Member

    Hi Jeremy,

    The above by Pistnbroke confirms your calculations that you basically need about a 100 watts to move pleasantly over the river.

    Pistnbroke Did you try it on the sea? I love to have your input on a try at those wave conditions, what sort of power you need and speed you do with some 0,5 - 1 meter waves.
    Bert
     
  12. BertKu
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    BertKu Senior Member

    Jeremy, I am under impression that a full displacement hull will sail at half the speed with the same power then a semi displacement hull. Could it be that Pistnbroke has a full displacement hull and you have a semi displacement hull?.
    Bert
     
  13. Jeremy Harris
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    Jeremy Harris Senior Member

    Bert,

    A hull is just a three dimensional shape moving through water, the names we give them really have little to do with this.

    The power needed to drive any three dimensional through water (or air, come to that) is purely a function of total resistance, which for a boat, moving across a water/air boundary at low speed will be made up of two principal ingredients, viscous drag (which is primarily a function of wetted area) and wave making drag (which is primarily a function of waterline length and speed).

    The concept of 'displacement', 'semi-displacement' doesn't apply at these speeds, as there would be very little, if any, lift acting to reduce wetted area. Pretty much any hull will be working below the onset of significant wave making drag at these speeds, so will be firmly in displacement mode.

    As a daft example, you can make a canoe work as either a full displacement, semi-displacement or full planing hull, just by giving it enough power and increasing its speed. It won't be efficient when run at anything over the onset of significant wave making drag, but then, in terms of the purpose of this thread (a low speed, low energy consumption, design) no boat is going to be efficient at these speeds.

    The most significant difference between my hull and a typical dinghy is probably weight (my hull is very light, like a canoe), which reduces displacement and wetted area and waterline length (mine is over 17ft on the waterline, where a 12ft dinghy will probably be around 11ft or so). The onset of significant wave making drag for my hull will be around 5.5 to 6kts, the onset of wave making drag for 12ft dinghy will probably be around 4.5 to 5 kts. If the dinghy has an immersed transom then the resistance will rise a great deal - my hull has a hull shape very like a canoe, with no immersed transom.

    Jeremy
     
  14. BertKu
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    BertKu Senior Member

    Thank you Jeremy,

    This must be the final confirmation that I can throw the computerprogram PROPCALC out of the window.

    It gave for a similar boat like Pistnbroke 3.75 knots and for the same size boat semi displacement boat 5.12 knts, both with the same engine power parameters.

    Bert
     
    Last edited: Aug 2, 2010

  15. BertKu
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    BertKu Senior Member

    My apology to your countrymen Jeremy,

    I did not mean that everybody must throw it out of the window. Propcalc is probably a very good program for higher power than the 1 Hp used by me in the calculation. You warned me for it, that it was not accurate at low power. Sorry for that. Maybe I can motivate Pistnbroke to do some tests with some higher waves. The problem is that all those computer programs are probably given estimates under ideal conditions. I need something more tangible to make the right decision. Are there programs on the market which could give me that, which are affordable?.
    Bert
     
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