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Thread: Eletric outboard motors

  1. #1

    Default Eletric outboard motors

    Hi all,am purchasing an electric outboard for my 16ft Apache canoe,for going back up river,and wondered if 28/30 lb thrust was adequate,thanks in advance

    Bernie

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    I have an acquaintance that used such a motor and a deep cycle battery to take an old and heavy fiberglass canoe about the Mississippi backwaters for a whole day. He claimed to have power to spare at the end of the day. I have no personal experience however.

    And a warm welcome
    Regards,
    Mike

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    For comparison purposes, I have a Minn Kota 50lb thrust and it easily powers my 16" MR Explorer with myself, wife and 2 children onboard. (this was against the flow of a tidal river)
    The time depends on which power battery you purchase but I have a 110AH and it lasts about five hours on full throttle. On full throttle it nearly rips the gunwhale of the canoe so i would imagine 30lb is plenty.

  4. #4

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    i have a 36 lbs thrust moter it easily powered my old discovery scout (no lite wieght ) up the trent

  5. #5

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    Thanks for the advice

    Bernie

  6. #6

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    Well iv'e just purchased a Yamaha 36 lb thrust motor,will let you all know how i get along with it

    Bernie

  7. #7
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    I never would have thought an electric outboard would have gotten anywhere near 5 hours of use on a single charge. I'm impressed.

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    I've done some basic estimates of power needed to propel a canoe. A reasonable figure seems to be about 45 watts or so. This equates to an electrical power input to the motor of about 70 watts when cruising gently (the difference is due to motor and prop losses). Power demand at hull maximum speed (maybe 1kt or so above cruise speed) may be around 4 or 5 times greater though.

    A lead acid battery will normally deliver about 80% of it's rated capacity for this sort of application, so a 12V 110Ah battery should be able to deliver about 1000 Wh, which equates to about 14 hours of gentle cruising, much less if more power is needed to overcome wind or current.

    Rather than use a heavy lead acid battery, I'd consider going for a 24V trolling motor and using a (more expensive) lithium battery pack. Not only will this be much lighter, but it will also deliver more usable energy. For example, a 24V lead acid battery that would give around 4 hours cruise capability would weigh around 10kg. A lithium battery weighing only 4kg would give the same cruise endurance.

    24V systems are a bit more efficient as the current is lower, giving lower resistive losses, plus the battery peukert effect losses will be lower. The downside is the cost of the battery, a 24V 12Ah LiFePO4 pack (including charger) will cost around 200 or so. A pair of 12V, 15Ah sealed lead acid batteries might cost around 60 (excluding a charger), so is a fair bit cheaper. The lithium pack will outlive the sealed lead acid batteries though, as a decent pack will give around 4 times as many charge/discharge cycles.

    Jeremy

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    Default Electric outboards

    Can anyone indicate the relationship between lbs thrust and HP. I have a Min Kota 3HP motor.

    Jim

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    Quote Originally Posted by Jeremy View Post
    I've done some basic estimates of power needed to propel a canoe. A reasonable figure seems to be about 45 watts or so. [...]
    That's about twice a much as what my body can deliver when paddling :-)

    Dirk Barends
    Last edited by canonymous; 12th-April-2009 at 10:43 AM.

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    The relationship is a bit complicated, as boat speed needs to be taken into account, as does efficiency.

    The formula is power = thrust x speed, where power is in Watts, thrust is in Newtons and speed is in Metres per Second,

    To convert units the following may be handy:

    1 lbf = 4.45 N

    1 mph = 0.447 m/S

    1 hp = 746 W

    As an example, if your canoe needs 6 lbsf of thrust to do 4 mph (a pretty typical figure) then the power needed (as power delivered to the water) will be:

    power (W) = (6 x 4.45) x (4 x 0.447) = 47.74 W = 0.064 hp

    However, there will be losses in the motor, propeller and drive system, so overall efficiency is unlikely to be better than 60%. This means that the input power from the motor may have to be as much as 79.6 W (0.107 hp).

    The thrust needed if speed is increased by just a modest increment, say to 5mph, may be much greater, as hull drag increases massively as hull speed is reached. Most small electric outboards have much higher maximum thrust ratings to enable them to give good manoeuvring capability and boat acceleration, cruise thrust is much lower than the rated figure.

    Hope this helps.

    Jeremy

    PS: Does anyone know how much sustained power a paddler can deliver? I know that a really good cyclist can sustain around 100 to 150 W, but obviously leg muscles are a lot more powerful.

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    Quote Originally Posted by Jeremy View Post
    [...]
    PS: Does anyone know how much sustained power a paddler can deliver? I know that a really good cyclist can sustain around 100 to 150 W, but obviously leg muscles are a lot more powerful.
    Quote Originally Posted by jymz View Post
    Can anyone indicate the relationship between lbs thrust and HP. I have a Min Kota 3HP motor.
    [...]
    3 lbs -- A fit and efficient paddler can overcome this drag for several hours -- 0.0325 HP @ 3.5 knots (6.5 km/h)
    [/...]
    Source : Olympic kayak drag from tank tests done in 1970's and reported by Andy Toro in Canoeing: An Olympic Sport.

    Dirk Barends
    Last edited by canonymous; 12th-April-2009 at 10:49 AM.

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    Quote Originally Posted by Jeremy View Post
    ....
    PS: Does anyone know how much sustained power a paddler can deliver? I know that a really good cyclist can sustain around 100 to 150 W, but obviously leg muscles are a lot more powerful.
    The sustained output limitation is not the muscles - it's the heart lung function.

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    Thanks for those figures, they help a great deal.

    The kayak drag seems pretty close to what I'd have expected; my figure of 6lbsf was my best guess for a typical 4.7m long two man open canoe (and may be in error, as I had to guess at wetted area).

    Interesting that 3lbsf is given as a sustained thrust output, my guess is that paddling efficiency is down around 50% or less, in terms of power input to the paddle versus effective thrust, so that means that it looks as if 50W or so is the sustained upper body power limit (happy to be proved wrong - these are just educated guesses really).

    I'm not sure that sustained paddling power output can be heart/lung limited, can it? Professional cyclists can sustain around 150 watts for several hours, which sort of indicates that the heart/lung limit must be higher (although upper body exercise may well interfere with breathing efficiency, I suppose).

    All interesting stuff when it comes to working out how little power is needed to drive a canoe though. That Olympic kayak would need a motor that delivered just 24 W to do 3.5kts, so would probably draw around 40 W from a battery. This is less than the power of one car headlight bulb - pretty small!

    Jeremy

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    Quote Originally Posted by Jeremy View Post
    [...] so that means that it looks as if 50W or so is the sustained upper body power limit (happy to be proved wrong - these are just educated guesses really).[...]
    Complicated subject (for me) but that's indeed what the other figures I have here are suggesting too, if I am interpreting/converting them correct.

    Dirk

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    Quote Originally Posted by Jeremy View Post
    ....

    Interesting that 3lbsf is given as a sustained thrust output, my guess is that paddling efficiency is down around 50% or less, in terms of power input to the paddle versus effective thrust, so that means that it looks as if 50W or so is the sustained upper body power limit (happy to be proved wrong - these are just educated guesses really).
    .....
    Jeremy
    http://www.canoeicf.com/site/canoein...mes%2F1515%2F0

    3:30 for the kilometer - that's a tad over 17 km/h.

    The hydrodynamic drag figure at three and a half knots - is hardly relevant to anything competitive.

    Real world figures also need to deal with wind drag (and it's curiously asymmetric nature) and wave or sea state influences.


    But the output level "sustainable" - depends on how long you want to sustain the output.

    A medium-large sized guy might weigh 100kg and jump a 1 meter hurdle - in half a second - approx 2Kw.
    There has long been an assumption of around 300 watts as the maximum aerobic output limited by the body's ability to supply oxygen - this crashes most famously in marathon runners at about the 20 mile mark (90 minutes to 2 hours) as the glycogen supply runs out - the next limit is that of digestion - how quickly can the digestive system supply the necessary fuel.

    Weightlifters don't try to breathe.
    Middle distance runners (< 10K) don't try to eat.
    Last edited by DougR; 12th-April-2009 at 02:26 PM.

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    17km/h! Pretty impressive.

    I'm aiming at using electric power for sedate river cruising, so 3.5 - 4kts will suit me just fine............

    Jeremy

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    Sounds sensible.

    One of the interesting points is that, on flat water, weight is almost effort free - which is of course the explanation for the original building of the canals.

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