Trolling Motor

Moonwalker031Moonwalker031 Club Members Posts: 9
I am looking for a Trolling Motor for my boat (20ft) but the ones I found are quite big/heavy/expensive since they use a DC brushed motor.

So I was thinking if it is possible to use a brushless motor to do the same job. The specs of the motor they use is 12V at 50A. My reasoning is that a brushless motor is much smaller and can deliver more power since it has 3 cables.

I have very little experience though on brushless motors so all help is welcome.Moonwalker0312016-04-28 16:47:26


  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    "My reasoning is that a brushless motor is much smaller and can deliver more power since it has 3 cables."

    3 cables doesn't mean anything as far as how much power can be delivered. 2 thicker cables could easily deliver more than 3 thinner ones. Brushless however are supposed to be more efficient. Buggered if I know how you'd be best setting up how it connects into things though.

    That's a pretty big lump of a boat to be trying to push around with an electric trolling motor. You'd need a lot of battery power to keep running that for any length of time. I have an 18 foot half cabin that has a 115hp Mariner for the main motor and I have a 10hp Honda 4 stroke for a backup/trolling motor. For just trolling I think I could manage with a little light 2 hp Mariner I used to have and it would run forever on a cup full of fuel. If it's strictly for trolling lures I'd be looking more at something like it, or maybe slightly bigger unless noise is a big problem.
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    I know that a 2 HP motor could be enough for trolling 2-4knts. That equates to 1.5kW. So would this motor give me same power -

  • __Evan__Evan Club Members Posts: 1,122
    The problem with brushless motors is that they don't like going slow. The one you've linked is 3180kv. This means that on 12V, it'll try to spin at 38160RPM. Under load it'll do a bit less than that, but you're probably still looking at over 30,000RPM. This is fine for RC boats (where the aim is to spin a 2" propeller ridiculously fast) but not good for low-speed, high-endurance.

    For comparison, "full speed" for a normal trolling motor appears to be closer to 1500RPM.

    To use that motor you'd need a 20:1 gearbox that can handle 2hp going through it. This will be a challenge.

    The other option would be one of the big, low-speed aircraft motors. Something around the 150kv mark should be able to run at 1500RPM on 12V quite happily. It'll require forced-air cooling and proper sealing of the propeller shaft, but it does avoid needing a gearbox.
  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    "The other option would be one of the big, low-speed aircraft motors. Something around the 150kv mark should be able to run at 1500RPM on 12V quite happily. It'll require forced-air cooling and proper sealing of the propeller shaft, but it does avoid needing a gearbox."

    With that the only practical option I can see is that you'd then have to have a prop shaft permanently stuck through the hull, seals (as you mentioned), bearings and mountings, a support and coupling from motor to shaft and who knows what other problems, then you still wind up with a lot of modifications to the boat, a great very heavy stack of expensive deep cycle lead acid batteries and quite a limited run time. Sorry, don't mean to sound negative. It's just I've spent a fair amount of time trying to think of a practical setup just to propel my little kayak along, let alone a whopping 20 foot, or even my 18 foot boat along and all I keep getting is a large headache   image . I know it 'can' be done, but it's trying to find the balance between running time, power and weight and delivering that power to the water in a safe practical way that burns out my tiny brain.
  • SootySooty Christchurch NZClub Members Posts: 6,110 ✭✭✭
    Why not just buy an electric trolling outboard motor complete. They have been around for 30 years and are quite inexpensive.

    The least complicated DIY method would be to build a direct drive electric propulsion unit similar to the ones the Asians use on their river & canal boats with the bloody great 6 cylinder car engine driving a long crankshaft extension and prop sticking out over the stern.Sooty2016-04-30 10:23:33
  • WTWUKWTWUK CornwallClub Members Posts: 13,605 ✭✭✭
    Stick a Chevy V8 with a Rootes blower on there ... yeeeehaaaaa!!! heh heh!!!
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    @sawdust - my aim is to put the whole motor assembly in water - probably fitted on top of the outboard motor like the minkota.

    @Evan - So what is the speed range control of a brushless motor? Why is that they don't like going slow - means they are inefficient at slow speed?

    @Sooty - As I said the "cheap" trolling motors are quite big/heavy. The best ones seems to be the minnkota but they are very expensive.

    So would a large prop turning at slow speed produce more torque than a small prop turning at high speed?
  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    "@Sooty - As I said the "cheap" trolling motors are quite big/heavy. The best ones seems to be the minnkota but they are very expensive. "

    Most of the ones I've seen have been very light, even the more powerful 24volt ones, as in one finger pick up, you can get many of them very cheaply new of fleabay and certainly much lighter than what you're likely to wind up patching together. Besides, when it comes to weight, the batteries are going to be by far the major source of weight. As for light weight, again that little 2hp Mariner only weighed about 12 pounds and instead of masses of heavy batteries weighing a hell of a lot, a lousy 5 litre container of 2 stroke mix weighing a few more pounds would keep that going for a full day.
  • __Evan__Evan Club Members Posts: 1,122
    @Moonwalker031 - there are two limitations on low-speed operation.

    Brushless motors require active control. You don't just feed them power and watch them spin; you have to be feeding power into one wire, taking power out of another wire, and carefully watching the third one. When the voltage on the third one passes through 0V, you change which wires are being used for what job (in a carefully defined 6-step sequence). The problem is that the voltage on the third wire depends on how fast the motor is turning. If it's turning slowly, then the voltage is so tiny that it's undetectable (at least compared to the electrical noise from driving the motor). This makes control impossible, and is why quadrotors have an "idle" mode where the motor are spinning slow enough to avoid flight but fast enough to allow reliable control. Generally once you get past maybe 500 - 1000RPM the motor controller will be getting a decent signal. Some of the motors designed for cars have dedicated hall-effect sensors, which eliminate this problem - but they tend to be designed for very low voltages and high speeds. They also require special sensored ESCs.

    The other limitation is power. Power is torque times speed. Power is also voltage times current. Both are equal, give or take a bit (mechanical power will be a bit lower than electrical power due to motor/ESC inefficiencies). To a decent approximation, voltage controls speed and current controls torque. You can run the motor slowly by cutting the voltage down, but the current (and therefore torque) limit remains the same. Say you want to run that 3180KV motor at 1000RPM. That'll require a voltage of around 0.3V. Assuming that you feed it the absolute maximum current of 97A (which you probably shouldn't, at least not for any prolonged period) this produces a power of 29.1W (0.039HP). It'll actually be a bit more than that (because you'll run a higher voltage to compensate for the drag from the propeller) but you're not going to get anywhere near 2hp.

    Compare to something like a 150KV motor running at 7V and 100A. This would produce 700W, which is getting up to around 1hp - but this would not be a good use of it. Ideally you want to hit both the maximum current (limited by the motor) and maximum voltage (limited by the battery in this case; the big motors can all handle 25V or more) simultaneously at full throttle. Therefore you'd fit a smaller propeller so that at 7V it only used maybe 50A (350W), but at the full 12V it went up to 100A (1200W, a bit under 2hp).

    Technically you can do this with the 3180KV motor too, but I suspect you'll end up with a propeller about an inch across spinning at 30,000RPM. This might produce adequate thrust, but it'll have horrible efficiency.__Evan2016-05-01 16:36:18
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    Thanks Evan for the detailed explanation. That was needed for me to understand more about BLDC motors.

    I was looking at ESCs and most say that can go down to 25% power on the motor so maybe I can go up to 300KV without much problems.

    However the 150KV motors I found are quite big and expensive.

    So can you post some links to 150Kv - 300kv motors with at least 500W at 12V. I need to decide if this is the way to go or else go back to DC brushed motors.
  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    "at least 500W at 12V."

    That's only about two thirds of a horsepower. Not knowing precise details of speeds and such that you require that may be all you need but it would be pretty slow, even for trolling.
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    Ok the actual application is not trolling. I am putting 2 motors to use them to keep boat at same spot during fishing. So I would have up to 1000W with 2 motors.
  • WTWUKWTWUK CornwallClub Members Posts: 13,605 ✭✭✭
    Would be better with four motors and a multirotor control board with GPS like an APM comtroller. That would do the trick! Keep you well and truly on the spot. Of course, the power motors you would be needing will depend upon the length/weight of the boat, strength of currents in the water and windspeed/direction above water. Sounds like you are attempting to achieve 'station-keeping' and that is quite a complex task.
  • __Evan__Evan Club Members Posts: 1,122
    I don't think there's any such thing as a cheap, small 150KV motor. The only reason for having a really low-KV motor is for running very high voltages (which means you can get 3000W+ without needing 300A wires and batteries). As a result, they're all big and powerful. Still cheap for the power they produce, but not particularly cheap in absolute terms.

    This looks like it'd do the job reasonably well, although it's out of stock - and still not all that cheap.
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    @Evan - Thanks that motor looks promising enough. But since its max voltage is 37V would it run ok at 12V? And is there a specification about how slow can the motor run?

    @WTWUK - Why would it be better with 4 motors? To be able to use smaller motors or to have better control? I think of building the controller myself as I like experimenting on electronics :)
  • __Evan__Evan Club Members Posts: 1,122
    @Moonwalker031 - it'll be fine at 12V. The downside is that you don't get anywhere near the maximum power, because maximum power occurs when you have maximum voltage and maximum current. At 12V instead of 37V, with the 90A maximum current, it should manage 1080W. Realistically, expect more like 700 - 800W continuous, or about 1hp.

    The minimum speed for the motor depend on all sorts of things - load, sensitivity of the ESC, length of the wires, etc. I'd think that a 190KV motor should be fine down to a few hundred RPM, maybe all the way down to under 100RPM.

    Regarding four motors - I'm guessing that it's because that gives you one propeller pointing in each direction. You can just generate negative thrust by running the motor backwards, but most propellers are pretty inefficient when spinning backwards and aircraft ESCs don't do reverse. Having four motors (or even three) means that for any direction you want to go, you can run one or two motors forward.
  • DaithiDaithi Belfast,IrelandClub Members Posts: 8,708 ✭✭✭
    Of course a simple way of staying in one spot would be to drop a heavy hook with a rope attached over the side - oh wait, somebody else thought of that: it's called an 'anchor' image
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    @Daithi - I fish in 400m depth - not feasible to anchor!

    @Evan - I was thinking more about two motor at the back on each end. However I do require reversing of the motors. Does the ESC/motor have problems working in reverse?

    About motor - I might have about 2m of cable between motor and ESC - would that be a problem?

    And if the motor is designed for 90A at 37V would that mean that at 12V it could only take 29A (ie 0.4ohm resistance)
  • airwaveairwave Club Members Posts: 3,563
    no you would be good up to the 90 amps.the wire size makes it the max not the resistance.wattage goes down at lower voltage.airwave2016-05-04 17:13:53
  • __Evan__Evan Club Members Posts: 1,122
    @Moonwalker031 - as airwave has said, you can still have 90A at 12V. Voltage and current are totally separate in this case.

    The motor will have no trouble running in reverse; they're completely symmetrical. However, most (if not all) aircraft ESCs are not designed to be able to change direction quickly (you can reverse the direction, but it's a configuration setting so it takes a few minutes to change). I'm not sure about boat ESCs, but I suspect they're the same. Car ESCs do have reverse, but many of those can't even handle 12V.

    2m of cable may be a problem, no matter where it is (between battery and ESC or between ESC and motor). You'll probably be losing a fair bit of power there, and the longer wires cause interesting voltage spikes and drops as load changes. If at all possible, I'd suggest locating the batteries and ESCs as close to the motors as possible.

  • airwaveairwave Club Members Posts: 3,563
    some boat ESC do REV. do not know how fast or if any have 90 amp ratings??
  • DaithiDaithi Belfast,IrelandClub Members Posts: 8,708 ✭✭✭
    90 Amps at 12 Volts = 1,080 Watts

    90 Amps at 37 Volts = 3,330 Watts

    Reducing the voltage on a brushless motor will produce less power and lower rpm. HK do a 90 Amp water-cooled ESC (see here) but whether or not the 12 Volt suply will produce sufficient power to do what you want is another matter

    However, if you expect the current draw to be 90 Amps, it would be better to use a higher rated ESC, like this 120 Amp oneDaithi2016-05-06 20:09:39
  • swervynswervyn Club Members Posts: 1,313
    Variable speed, reversible, battery operated outboard motor right

  • Moonwalker031Moonwalker031 Club Members Posts: 9
    I've been very busy lately but now I am back on this project.

    When a motor specifies the cell count, does it mean that it does not work properly with lower voltages?
  • WTWUKWTWUK CornwallClub Members Posts: 13,605 ✭✭✭
    It will work, but will have lower RPM and torque than if run with the relevant 'correct' cell count.

    In theory, motors will run at any voltage but are limited by generated heat. I regularly run a motor that is rated for 4S maximum with a 6S battery. But it has a smaller diameter lower pitch prop than it would have on 4S, so does not excede its rated 'Watts'.WTWUK2016-06-19 16:20:01
  • Moonwalker031Moonwalker031 Club Members Posts: 9
    Ok thanks for that.

    So how could I calculate the correct prop? I need a much larger prop than used on RC boats.

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