Determining motor(s) size for water craft

Discussion in 'General Electronics Chat' started by RogueRose, Apr 19, 2016.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
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    I'm trying to figure out how motor size will effect the speed of a personal motor craft. I have looked at trolling motors and they have a very wide range of thrust outputs and most seem to be under 1hp from what I have seen.
    Obviously the craft will be battery powered but I don't think power will be much of an issue as there is plenty of room for custom lithium packs.

    One thing I need to make a note of is that the craft will probably run at about 1/3 power for most of the time but it will need to have some short bursts of full power for 20-30 seconds and possibly 5-8 seconds of a "turbo" or maybe over-voltage if that can be possible. I know the over-voltage is potentially dangerous, especially for prolonged periods. If done it would be something like 15-18v running through a 12v motor or 30v through a 24v.

    I'm trying to figure out if it would be better to have 3-4 motors of 500-750 watts each or one that is the sum of the wattages. What I am most confused about is motor size. I find it really odd that some cordless drills are rated at 820 watts and an older motor I have rated at 640 watts (1/6hp rated) is about 20+ lbs.

    I'm either going to use one motor (pancake type or standard axial) with one large impeller or an impeller on each of the smaller (much more work probably.

    If anyone can explain the difference in motor size vs wattage and if those drill motors really put out 820 watts - 1.13 hp (745watt/hp)

    Also are there any types of motors that might be especially beneficial for this type of application (like using an impact motor setup - or something equally as unique.)
     
  2. MrSoftware

    Member

    Oct 29, 2013
    497
    122
    Are you just playing around, or trying to design something specific?

    Before you select a motor, select your hull design. To oversimplify, for high speeds a planing hull is best, but it will be inefficient at displacement speeds. For lower speeds a displacement hull will be most efficient. So based on your desired cruise speeds choose a hull style. From there you can estimate what your power requirement will be, and that will help you choose a motor. Prop design will also be significant, but that too will be driven by your desired cruise speed and hull style.

    Side note: electric motor manufacturers lie through their teeth about rated output. I've got a 2HP electric trolling motor produced by minn kota, a respected brand, and it isn't even in the same league as a 2.3HP gas Honda motor, on the same hull. On my 13' aluminum boat I get maybe 5mph from the electric motor, but the gas motor almost gets the boat on plane. So do your research carefully when selecting an electric motor.
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    Ditto the comments by MrSoftware. Few topics generate more conversation between boating enthusiasts than prop pitch, diameter, blade count and so on. That's just the prop. The optimum always comes down to - "it depends". The best prop depends on how you use the boat, where you want best performance (hole shot acceleration versus top end speed) the rpm the engine runs at and a host of other factors.

    In other words, a boat is a system with all the parts - including the hull - interacting. You have to optimize the powertrain as part of the system. It can't be done in isolation.
     
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  4. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
    4
    I'm trying to design something specific which requires electric motor(2). I am having a difficult time deciding what type of motor to use - AC or DC. I'm trying to determine how I can run multiple motors on one drive shaft. It is difficult to find a motor of the size, power and price to fit my needs while I could easily place multiple smaller motors (like those from large cordless power tools, especially drills), I could easily fit 6-12 in the space provided (not ideal to have so many, not sure it is possible..). If multiple motors are used they will be the same model.

    The plan for the motors is to turn a small jetski/waverunner impeller or some similar pump.

    I am also trying to decide what type of motor to use, Permanent magnet or universal (I'm reading some pages on that now). It seems that corded drills (universal motors I guess) like some 1/2" makita's I've used have a TON more torque than battery powered drills. As all manufacturers seems to rate drills differently I'm not sure how to accurately compare them. Higher torque vs slower speed but at = speeds which has more power.?

    I would guess that a universal motor would require more energy as the electromagent needs to be powered but IDK how much that requires. Battery life isn't too much of an issue but getting the most power from the motors is.
     
  5. Picbuster

    Member

    Dec 2, 2013
    373
    50
    Mr Software is correct.
    Moving water by pump or prop is creating a lost of grip.
    Speed boat mass and pitch and rpm are all linked. Each boat has a waterline and a maximum speed associated with it.
    However; all calculation will fly out the window when a boat goes in planes /glide. Colossal power increase needed to move from waterline speed to planes/glide.
     
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  6. wayneh

    Expert

    Sep 9, 2010
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    I think that will be a poor choice. I have a small jet boat, with two jets that each resemble a jetski propulsion system. My props turn at up to 7000rpm, and that requires an 85hp motor to accomplish.

    You're not going to have even a tenth of that much power. I suspect your prop will need to be designed for low speed and low rpm and will resemble a trolling motor prop. I'm no expert, but I don't think a jet has much appeal for this application.
     
  7. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
    4

    I've seen the prop I'm planning to use being turned by a mid-power had drill and it did a very good job delivering what was needed. I have no doubt that the prop will be sufficient for at least a prototype.

    Since I'm not planning on running the motor for extended periods of time I'm considering using a modifies engine starter as they seem to some in many different sizes and they seem to put out a lot of power but I need to research this more.
     
  8. MrSoftware

    Member

    Oct 29, 2013
    497
    122
    A pump, such as a jet ski pump, will give you roughly 1/2 the efficiency of an open propeller. Your motor will have to deliver significantly more energy to get equivalent thrust. It's a poor choice if you're searching for more power or more efficiency. Jet pumps are good for shallow water or water with obstacles (rocks, etc..) since there's nothing exposed to make contact, and when safety is a concern such as with a jet ski.

    A car starter motor will certainly give you power, won't be good for long run times. They will self destruct relatively quickly if used for long duty cycles.

    I think DC will be a better choice, AC would require an inverter which would cost efficiency. Electric drills have a gearbox, keep that in mind when trying to decide which drill to borrow a motor from. There are tons of electric boat motor videos on youtube, check those out and see what has worked well and what hasn't. Also check out the motors used in production electric motorcycles. A similar style motor might suit your application.

    Edit: 3-phase AC might be OK, similar to what they use on flying toys. You will need a 3-phase speed controller though.
     
    Last edited: Apr 21, 2016
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