Help me design the "Poorqeedo", another efficient electric boat

Discussion in 'Boat Design' started by mental_boy, Jun 2, 2010.

  1. Jeremy Harris
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    Jeremy Harris Senior Member

    The first part of this paragraph is OK, but the conclusion that you can go faster at displacement (i.e. pre-onset of significant wavemaking) speeds with short hulls isn't quite right.

    The approximate, rule of thumb, formula for maximum displacement speed is around 1.4 x sq rt of waterline length, which indicates that increasing the length increases the speed at which wavemaking drag becomes prevalent.

    Partially true, in as much as a long thin hull has to have greater stiffness in the longitudinal axis. However, wider, shorter designs need to be stiffened more in the transverse and normal axes and are subject to higher torsional loads, so need increased torsional stiffness. Nothing is truly simple when it comes to structural compromises!


    I agree wholeheartedly, the sort of low powered craft in this thread, and my own "efficient electric boat" are really only suitable for sheltered waters as they stand. There's no reason why they couldn't be made more seaworthy, though. Offshore canoeists seem to do OK with only around 100 watts or so of propulsive power, and the power available from the sails of a small dinghy isn't much greater than the power levels being looked at here.

    Jeremy
     
  2. portacruise
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    portacruise Senior Member

    Good post Jeremy. One thing I wonder about on offshore canoeists is whether the 100 watts is much higher due to the skill level these individuals have. Their experience allows them to instantaneously correct their output for maximum impact. At least I have seen such things on rivers where "reading" the boils of water has allowed seemingly impossible propulsion against high currents. Almost like fish, which are able to feel the path of least resistance and adjust output with short bursts at 4X say, when needed, then coast when eddies favor....

    Porta

     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    Remember you will have to steer this thing: the longer the hull and flatter the bottom the harder that will become. Design is about compromise ...

    Canoists get by with very little power but it is used efficiently (even more so for Rick's leg-propelled creations) and they are only pushing their own weight through the water, not a crew of 4 plus batteries etc.

    My opinions on hull length - I did not do the math so that's all they are - recognise that at a given speed, skin friction increases with area whereas wave resistance drops as length (and therefore area) increases: somewhere there should be a length yielding minimum overall resistance, and the law of diminishing returns will apply as this length is approached. The weight and size of the craft have an impact of course, as does the limited power available. Rick has done the math for us and has provided a shape for an ideal hull of 6m / 20' length, but as always, design is about compromise. It looks like a fairly straightforward build, but might prove difficult to store and handle, both on the truck and in the water. It's all about what the customer wants in the end and the finished product will be an interesting study in its own right. Assuming that is the minimum resistance length and shape, my own inclination would be to trim the length down until the resistance starts to rise a little for practicality.
     
  4. Jeremy Harris
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    Jeremy Harris Senior Member

    I agree about length making it slower to respond to a given yaw moment, as it would increase the polar moment of inertia in yaw and increase yaw resistance from the greater wetted area fore and aft of the C of G. However, wouldn't a flat bottom actually give a shallower draft and so reduce lateral and yaw resistance to some extent?

    I'd have thought that a long vee bottom hull, or any shape that increases the depth of immersion of the hull, would tend to make lateral and yaw resistance greater and that anything that tended to reduce the depth of immersion (for a given displacement) would reduce lateral and yaw resistance.

    I suppose there is some effect from the slightly reduced lateral Cd of the inclined hull side on a vee or round/elliptical hull, but would the small Cd reduction have a greater effect than the deeper draft?

    I'm not qualified to answer and am purely speculating based on my own aerodynamics background.

    Jeremy
     
  5. mental_boy
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    mental_boy Junior Member

    Ok, it sounds like we're really on the edge for power, so I'll have to find a way to get 72v to the motors (With 72v max power will be 806w). I'd definitely like to be able to maintain 5 knots in most conditions and it sounds like that won't be possible with only 36v. I'll also need to see whats available for smaller traction batteries since I'd rather not carry 264lb of batteries around.

    I'm really not concerned about the hulls breaking. I was inspecting a fiberglass hobie 18 yesterday and it looked fairly flimsy compared to what I'm planning, especially where the cross beams connect to the hull. The hobie also endures much higher stresses from the rig and from the high speeds under sail.

    I'm also aware that steering the boat will be difficult. If I want to make tight turns I'll just reverse one motor.

    It's interesting that the monohull requires 1/3 less power. If I switched to a monohull I'd loose the pontoon boat/party barge aspect of the design, and that's just not worth the power savings to me (I'd rather just go slower =)

    Rick, thanks for the drawing of the lowest drag hard chine hull. Interesting that it's essentially flat on the bottom, I wouldn't have guessed that would give good performance. Which end is the front of the hull? Also, it seems like it would be a little narrow for my application, with the maximum designed weight of 1150lbs/522kg, the hull would sink ~75% deeper, right? What program are you using to model the hulls and calculate drag?

    Also, does anyone know how suitable these APC 16x16 propellers are for pushing around this kind of load? And how would we design for rpm? I know the props have 16" pitch, but there is obviously going to be some slippage. At 5kts and no slippage rpm would be 380. 10% slippage = 420rpm, 20% slippage = 475 rpm. No idea what to expect....

    As for the motor I think I can figure out what the rpm would be for a given wattage under load. I'm thinking I'll apply 72v to the motor, and bring the amperage up by applying friction to the output shaft. When the power input (accounting for the efficiency of the motor) reaches the expected power required I'll see what rpm the motor is doing.
     
  6. mental_boy
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    mental_boy Junior Member

    Also, How about building the hull with a perfectly flat bottom, plumb bow and stern and straight sides? Sort of like an extrusion. Each bulkhead would be perfectly rectangular, not trapezoidal. Not optimal, but very easy to build. After looking at Rick's hull I imagine this wouldn't be so bad performance wise.

    On the inside corner between the bottom of the hull and the sides of the hull there could be a fat strip of pine allowing me to make a generous roundover with the router (followed by a layer of glass of course). I know this could be done on any hull, but it's much easier when the angle is 90 degrees for the full length of the boat.
     
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    These motors are permanent magnet DC motors, so will have a voltage/rpm constant, Kv. If you can find a way to measure rpm then you can apply a known voltage (say 12V) and then work out the value of Kv (in rpm per volt). Once you know the value for Kv, then you can use it to estimate the rpm at any other voltage.

    When loaded the motor will slow down slightly, as the voltage generating the magnetic field will drop slightly due to the resistance of the windings and the current flowing through them, but this drop should be quite small, no more than about 10 to 15% of the no load rpm.

    Once you know the value of Kv, you can work out another useful constant, the torque constant, Kt. This is simply calculated by dividing 9.5478 by Kv, giving an answer in N-m of torque per amp of current through the motor.

    Knowing these two numbers, together with the motor maximum current rating and maximum structural rpm limit, will allow you to work out the best combination of voltage, current and reduction drive ratio to give the performance you're looking for.

    Here's a worked example, for a motor with a Kv of 100, a required maximum propeller rpm of 1000 and a required maximum power of 500 watts.

    First, let's use a 48V battery. This gives a maximum motor rpm of 48(V) x 100 (Kv) = 4800rpm. The reduction ratio required is therefore 4800 (motor rpm) / 1000 (prop rpm) = 4.8:1. The motor Kt will be 9.5478 / 100 = 0.095478 N-m per amp. For 500 watts input the motor will draw 500 (W)/ 48 (V) = 10.4 amps and will deliver a torque at the motor shaft of 10.4 x 0.095478 = 0.993 N-m.

    Now, let's look at the same motor running on 72V. The max motor rpm will now be 72 x 100 = 7200rpm. The reduction ratio will need to be 7.2:1 to maintain the 1000rpm at the prop and the current for 500 watts will drop to 6.94 amps. Torque at the motor shaft will drop to 0.663 N-m.

    If you want the motor shaft torque in lbs-ft then just multiply the N-m figure by 1.35582. Propeller torque will just be the motor torque multiplied by the reduction ratio.

    These rough calculations don't take account of losses, but will get you close enough to be able to decide on the best combination of voltage and reduction ratio etc.

    Jeremy
     
  8. mental_boy
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    mental_boy Junior Member

    The manufacturer's spec for KV is 38v/1000rpm or 26.3 rpm/v. So 2000 rpm with no load, and 1750 rpm assuming a 12.5% drop under load.

    Using your number Kt = 9.55/(26.3 rpm/V) = 0.36 Nm/A

    At 72V and 403 watts per motor the current will be 5.6A (the continuous rating of the motor). 5.6A * 0.36Nm/A = 2.0Nm of torque.

    For the reduction I still need to chose a target speed and know more about my power requirements and prop efficiency. I think....

    If I choose a reduction for 5kts, and the motor has a surplus of power in calm conditions then it seems like i will be better off in adverse conditions than if I shoot for top speed, right? Kinda like being in a lower gear all the time?
     
  9. portacruise
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    portacruise Senior Member

    The 16X16 has been used successfully in at least 1 HPB powertrain, Bob Stuarts spinfin. http://microship.com/bobstuart/spinfin.html The range of HP can go all the way to 1HP peak, though there will most likely be some flexing at that output. The beauty of using RC props is that there is a continuous graduation of sizes and pitches and they are dirt cheap. If you do get significant losses due to flexing, carbon fiber may be available from other makers at higher prices. Probably anything within the range of 400 to 1000 rpm will work out. You can then work out the best size and pitch using McDenny's process, after a rough calculation. http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html

    Hope this helps.

    Porta


     
  10. portacruise
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    portacruise Senior Member

    The props are fairly efficient, all things being considered as indicated by McDenny experiments. You can do a little? better by Rick's hand make method.

    If your motors are the brushed type you have quite a bit of latitude with voltage/power so long as you don't exceed design amps and/or rpm at which armature wire flys off. I wouldn't go above previous mentioned design 7200 rpm, and probably your motor will last much longer at 3600 rpm. The gear train also will run cooler continuously at 3600.

    For brushed PM motors, usually the lower rpm and voltage and the higher the torque (diameter) the better, as these can be overvolted to run a higher efficiencies.

    Hope this helps.

    Porta

     
  11. Guest625101138

    Guest625101138 Previous Member

    To keep it simple.

    1. The hull shape I have shown will displace 407kg when fully immersed. The two hulls will displace 814kg (1790lb).

    2. The power required to drive the cat at 6kts is 414W. With two motors it will be 207W each.

    3. I have not handled the 16 X 16 APC props but I know they are suitable for pedal power, which can easily exceed 207W. The force will be 50N on each prop. That is say 11lbf for the metric impaired. A simple test is to load the hub with a string carrying an 11lb weight with the blade supported flat either side at 75% of total radius. This will be roughly the flexing you will get in operation.

    4. The hard chine hull is for ease of construction. Making it with rounded chine will make little difference maybe 3% less drag. The slab sided hull with vertical sides and pointy ends will add about 10% drag. It is probably easier to build with curved flared sides than slab sides.

    5. The 16 X 16 props will not have much slip. 6kts is very close to 3m/s so a pitch of 0.4m will need to spin at 7.5rps or 450rpm.

    Rick W
     
  12. Motivator-1
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    Motivator-1 Junior Member

    Parallel thread

    Hello again to all of you guys that have begun to help me with my project in another thread here "Another Attempt at an Electric Catamaran". I kind of see this thread going in the same way as mine, and I was reluctant to post here as well, and have my project spread around three threads. I have seen the obvious push to take Catamaran attempts into Monohull ones by a numbe of you, and for some good reasons, but for us Die-Hard Caamaran guys that insist on the advantages of this Boat format, can we start a thread like "Efficient Electric Catamaran Design" and focus explicitly on Electric Catamaran Design and the advantages associated. I think this could become a lengthly and very informative thread that will attract a good deal of attention.

    I do want to keep moving forward with my thread, but without posting in several areas. I truly believe that there are a lot of guys like me out there that insist on the Catamaran Concept.

    What do you guys think? If someone can assemble what we already have into an Electric Catamaran compilation, that would be great.
    John
     
  13. Guest625101138

    Guest625101138 Previous Member

    I have attached the prop analysis for the APC 16X16 prop operating at 3m/s, say 6kts.

    They will require 177W on the shaft.

    If your motor and controller efficiency can achieve 85% then the battery power used for each motor will be 208W.

    Rick W
     

    Attached Files:

  14. mental_boy
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    mental_boy Junior Member

    Thanks!

    I was just thinking before you posted this that the reduction would be on the order of 4:1. If I use a 16x4 prop I could eliminate the reduction and get the same speed, right? The difference would be 4x higher drag on the props blades and the elimination of the cog belt reduction which is probably on the order of 90% efficient. I'd be willing to take a small hit on efficiency to avoid a belt reduction, assuming there aren't other problems with this approach.

    Do you know what the difference in wattage would be and If the prop would be suitable for that rpm? I'd try to make some sense of your attachment but it would be difficult from the device I'm using at the moment.


     

  15. Guest625101138

    Guest625101138 Previous Member

    To give an idea of the design space for a boat displacing 400kg and operating at 6kts the lowest possible drag for the different configurations are:

    Monohull
    Power 179W
    lwl 9.7m
    bwl 480mm
    Draft 207mm
    Section shape - almost semi-circular

    Catamaran with hulls equally loaded
    Power 248W
    lwl 9.4m
    hull bwl 340mm
    draft 170mm
    Section shape - almost semi-circular

    Catamaran with square chine
    Power 265W
    lwl 8.3m
    bwl 298mm
    draft 155mm
    Section shape - rectangular

    With the 6m length limit on the hard chine catamaran the power is 290W. A lot higher than the lowest drag monohull but not too far from the lowest drag catamaran.
     
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