Electric Hull Designs

Discussion in 'Boat Design' started by matt76, Sep 22, 2007.

  1. matt76
    Joined: Jun 2007
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    matt76 Junior Member

    Would everyone agree that the canoe is the most efficient hull design?
    Could we also say that the hull design is best for electric power?
    If so, should i be looking for a multi-hull design if i want to carry more weight for the batteries?
    I'm shooting for at least 12 knots, if not more, powered by a Perm132, 72volt electric motor. Small lake usage.
    I've been reading all kinds of articles and web sites about this. Phil Bolger's hull designs are said to be good in having very little wake. He achieved this by making the curve of the bottom of the boat the same as the sides.
    Isn't that what a canoe does?
    The multi-hull designs i've seen, seem to be really flat on the sides, sharp angled. Shouldn't they be more like a canoe, round, soft curves?
    Does anyone have any examples of multi-hulls that would work for me?
     
  2. PAR
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    PAR Yacht Designer/Builder

    The is no ideal or "most efficient hull design". All are concessions in convoluted compromise and you live with the choices you make as a designer. At you target speed of 14 MPH, a canoe would be a very poor choice, unless it has an 80' LWL, at which point it would be quite efficient.

    Drawing broad generalities about hull form is exceedingly difficult, there are just too many variables to consider. Bolger's more efficient designs use a lot of solid hydrodynamic fundamentals with no easy answers to their wave making qualities, though educated understanding will go a long way toward this end. A student of yacht design would do well to study Bolger's efforts as well as many other designers who have mastered the skills as well as he.
     
  3. Quietboats
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    Quietboats Junior Member

    Matt, I've used the Perm 132 motors--for those not familiar it can produce a max of 7.22 Kw at 3480 rpm. Are you planning on running at 72 volts? That is alot of weight in batteries--450 lbs for about 20 minutes running time with that draw--which may be way more power than you actually need. Beauty of that motor is it can be run at anything from 12-72 volts so you'll have a lot of options--I set the thing up at 24 volt in order to run direct drive at displacement speeds. I'm sure you realize 12 knots is planing speed which means weight reduction is critical and now you'll have to decide where you want your efficiency--at planing speed or displacement speed--can't have both. My first reaction is you'll need to accept a slower speed or think about Lithium batteries and that means getting Bill Gates to adopt you. Good luck on your project.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Rowing shells are the most efficient hulls. If speed is your only interest then something like a rowing 8 would be suitable for 12kts. These things achieve around 12kts with about 3.5kW at the oars. So it is a matter of making an electric propulsion system with storage that weighs less than 8 big blokes. Say 800kg.

    I think you would want something a bit more robust than a rowing shell. So taking 12kts and displacement of 1000kg, the optimum boat has WL length of 16.5m and 510mm WL beam. It would take a motor of 4kW to achieve the 12kts allowing for propulsion losses, windage and rudder drag.

    Obviously you would need to keep the weight low down to keep the boat stable. It would be unlikely that you could stand up without it tipping but a couple of small outriggers sitting above the water could give ultimate stability and prevent it from rolling without adding drag. You would preferably want low profile batteries.

    I doubt that this is what you have in mind.

    If the length around the above figure is not too bad then you can improve stability to an acceptable level by adding more beam without too much increase in power.

    If a 16.5m (54ft) length is well beyond what you have in mind then you need to nominate the length and I can work out what might prove practical. At some point a cat will prove to be more economic than a monohull and this has the advantage of very good stability.

    Weight is also a factor but I feel it would be hard to build a 16.5m long boat with motor, batteries and crew that has a total weight under 1000kg.

    Such a boat would not be too silly for transport as the bow and stern could be detached for transport and placed side-by-side on a trailer.

    I have been considering something similar for solar power.

    The main thing to remember is that the most efficient hulls are long and thin. If this wasn't the case rowing shells would be a different shape.

    Rick W.
     
  5. alan white
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    alan white Senior Member

    The term "most efficient" is really somewhat subjective. For any hull, no matter what the design, the slowest speed is always the most efficient, unless one design efficiency parameter is time itself.
    This can be illustrated by the following example.
    A barge must be transported across a lake using the least total energy. There's no hurry. The distance of one mile could be covered in a day. a week, a month, or even a year.
    If the lake is always calm and has no currents. The interesting thing is that the slower the trip, the least overall energy expended. Yet SOME speed must be arbitrarily chosen because the the length of time can approach forever--- it's infinitesimal in duration. It will always take less overall energy if the time is extended, no matter what.
    This is what can become confusing to anyone talking about "most efficient".
    Time spent in a pleasure boat is subjective, though when working a boat for a living, time is money.
    After all, a trip around the lake may be entirely forward movement, ending when returning to the original launch site. Efficiency according to a person taking that trip can only be personal. No matter what his means of propulsion, his best enjoyment relative to the cost of that enjoyment is never the same as someone else's cost.
    Given that speed and time are not factors that describe efficiency for all pleasure boaters, the formula for deciding what is most efficient must first ascribe a subjectively derived speed/time figure and calculate from there.
    It is better to say, in other words, "I want to know what choices I have in terms of power consumption vs speed. Once I know what each speed will cost me in energy expense, I will decide on one ideal speed. At that ideal speed, my "personal best speed", what hull will make the absolute most of the energy supplied?
    One could also use power available as the known factor. Either way, it is a distilling process. There is no simple answer to the question, "Which hull is most efficient?". Only a lot of questions and answers, a learning curve, and a refinement of the process of achieving one's "personal best speed/power formula".
    Given all this, the term, "long and narrow" as meaning "most efficient" has to be considered a subjective statement. In another's perfect world, their personal best speed for the power supplied might be a half-knot. At a half-knot, the most efficient hull would, in fact, be quite tubby. Short and fat, it's underbody would begin to approach the theoretical perfect underbody shape that uses the theoretical least energy to move----- the semi-sphere, or half-globe. Given enough time, that is.
    This is why long canoes and kayaks are not the most efficient. They are only most efficient when greater energy is used to paddle them, that is to say, each length (and hence shape, as shorter always means fatter) has a different paddler in mind. I might do best in a 17 ft kayak with (consequently) a narrower beam and a lower prismatic coifficient (volume in the ends). You might do better with a 12 ft kayak with a fat shape and a greater prismatic coefficient. This is because to me, a constant exertion of 200 watts feels the same as the constant exertion of 100 watts to you.
    I will go faster, but each of us is perfectly mated to a boat that is best for us.
    If we exchanged boats, I would still be faster, but not nearly so much faster.
    For anyone checking my math, I'm sure you'll find it inaccurate. There ARE theoretical "least energy" figures for every hull length of a given displacement.
    My energy figures of 100 or 200 watts were just approximations.

    Alan
     
  6. matt76
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    matt76 Junior Member

    keep it coming everyone. I'm learning more and more as we go. I know how to build a canoe really well, i just don't understand much about hull design and unfortunately physics.

    Take a look at this site, he was able to achieve a little over 10mph, in a 26 foot Sneakeasy.
    http://www.psnw.com/~jmrudholm/etekoutboard4.html

    If i could match this performance in a hull with a max length of 16 foot, i would be happy. Theres even more efficient motors out there too. Cost you an arm and a leg. I think it's worth it if i could find a hull that will work well.

    I understand i'm not going to be able to get the hull to plane. But these motors can produce some good power. I don't need to be able to run for long, small lakes, 40-140 acres. And i wouldn't be running around full throttle, it would be nice to be able to throttle up, and pass a neighbor, maybe impress a lady.

    I was also thinking one of these may work.
    http://www.boatplans.dk/boat_plans.asp?id=41

    any thoughts? thanks for all the replies.
     
  7. matt76
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    matt76 Junior Member

    take a look at this one.
    http://www.stillwaterdesign.com/pages/18Solo.html

    this seems to be a good design for what i'm looking for. the only
    thing i've found close to this in plans is the 15 ft row cat from boatplansdk.

    any ideas where i could get plans like the stillwater 18?
     
  8. Guest625101138

    Guest625101138 Previous Member

    A 16ft boat and 12kts equates to planing.

    Weight is now very critical. As a first approximation the hull drag will 1/8th the total weight. So if you can make a boat with a total weight of say 400kg then the drag will be 50kg (490N). A speed of 12kts will require hull power of 4kW.

    Rick W.
     
  9. Guest625101138

    Guest625101138 Previous Member

    Correction:
    Hull power is 3.1kW and total power of 4kW with an efficient prop.
     
  10. Guest625101138

    Guest625101138 Previous Member

    The original question gave a speed of 12kts.

    Now with a length constraint of 16ft the most efficient boat would use foils. However this brings in additional complexity over a planing hull. The power for a 400kg planing boat is estimated above.

    The other option is a 16ft catamaran. The optimum 16ft catamaran with a displacement of 400kg requires hull power of 2.5kW so say motor power of 3.1kW.

    So the catamaran is an attractive option.

    So far you have provided speed of 12kts and maximum length of 16ft. You now need to think about what you want to carry and how far you want to travel on a charge.

    Rick W.
     
  11. alan white
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    alan white Senior Member

    Briggs and Stratton makes an electric motor that's super-efficient. It produces up to 15 hp at higher voltages. The motor and controller together must come in at a bit under $1000. It seems that initial cost must be measured against long term generating costs, which add up over time.
    A motor that produces that kind of power could be used at displacement speeds, but also make the leap to get up on foils, and really take off, especially if the prop could be variable pitch. From what I've heard here on the forum from others who know far more about foils, it's the power to pull the hull clear that is needed, and afterwards, less power is required.
    I think this motor could do that.

    Alan
     
  12. alan white
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    alan white Senior Member

    Rick, what kind of power do you think is required to get the 800 lb boat up on foils, and then to maintain? This question is keyed to the 15 hp ETEK motor from Briggs and Stratton.
     
  13. matt76
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    matt76 Junior Member

  14. alan white
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    alan white Senior Member

    To be fair, the comparison is between motors that are different sizes. The voltage is the same, but what matters is efficiency (wattage in vs horsepower out). Amazing power to weight ratio. As long as you don't include battery weight!
     

  15. Guest625101138

    Guest625101138 Previous Member

    Alan
    The best that I can get with foils in my human powered regime without getting fancy with flaps to control shape is a lift to drag ratio of around 30:1. This is quite an extreme foil though. I have not looked at what is possible with heavier and higher power but it should be similar and may not look as silly as I am trying to do it with 140W.

    Working on 30:1 and weight of 800lb the drag will be 27lbf. At 12kts (20fps) The power required is say 1HP at the the hull. With a good prop you would need a motor of 1kW.

    There is not much allowance here for windage and appandage drag so these aspects would need to be carefully considered.

    To put these numbers into context remember decavitator did 18.7kts with a single human engine about 600W.

    You would want the catamaran hulls to provide the initial stability and they will not create a substantial power hump. I am reasonably confident that you could achieve inherent stability but there might be some inefficiencies introduced. I have only started playing with foils and my first serious attempt was too ambitious for my current fitness level.

    Rick W.
     
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