# Effect of motor size on efficiency at fixed power

I’m trying to build a drive for a powered ocean racing kayak (surfski) and am trying to understand motor size (& weight). The short form of the question I have is: given operation at moderate fixed electrical power (150W or 300W) is there an efficiency advantage to a larger heavier motor, e.g. 6374 vs. 4074?

The details behind the question are that I have one prototype using a BlueRobotics T200 thruster, which has a 4074 sized motor, a rather small prop, all in what looks like a kort shroud. I have a second prototype which uses a Foil Drive, which is a ~6374 sized motor and their std folding prop. Driven at a specific electrical power (150W or 300W, @20V) the Foil Drive version produces vastly more thrust from standstill to at least 13km/h. I want to believe that the majority of that performance difference comes from the prop & shroud and not the motor, but I’m not sure… Any insights appreciated!

The motor are at peak efficiency at rather low load, therefore the 6374 will have an edge at higher loads. If you search my posts i have dyno plots of the 6374 motor in one post.

Second thing is that motor torque capability is roughly proportional to airgap diameter squared so there is a max torque advantage of roughly
63^2/40^2 = 2.5 times for the 6374 (these are bell diameters, i know, i don’t have the real airgap diameters at hand but you get the point)

Where the breakpoint for efficiency is hard to say. You can measure the noload current for the two motors and compare that as a start.

one thing though: you can’t compare two motors of different kV and different props fairly so if you want to compare then you’d need to test at comparable conditions (same kV, same prop)

Can you reach 13kph on 300w? This seems like a low wattage.

1 Like

Thanks for the tips! I assume you mean this plot: What's the max reasonable motor current for a 6384? - #3 by Larsb, that looks like about a 5% difference from peak efficiency to efficiency at max torque?

How much difference does the torque capability make? I’m driving both motors way below max torque at 150W-300W.

In my tests the (very) subjective feeling is that of a 2x difference in thrust. I’d like to use the 4074 motor with bigger prop an perhaps no shroud because of weight (~250g vs 900g) but I want to avoid going down that route if it’s the bigger motor that makes a lot of the difference…

Those plots are nowhere near max torque, they are deceptive, sorry. What you can get from them is that the peak efficiency is already at 1.2Nm and the max is (from memory) about 7Nm

No/yes. The motor can’t get a surfski to 13kph @300W. But m
y goal isn’t that. Surfskis are primarily for downwind surfing of waves and I’m putting together a retractable “UpwindBuddy” to assist going upwind. I have to paddle anyway to maintain balance and with the 6374 set-up I can reach 13kph with easy paddling (well, paddling at 13kph is never really easy).

Early prototype using the BlueRobotics T200 thruster:

This is a quick test set-up using the foildrive motor/prop:

But what are your goals with the drive? I don’t see that the motor choice is super critical for it?

My goals are to provide assist during long upwind paddles. I’d like to be able to go around 6.5mph (10-11kph) and have the motor provide half or 3/4 or the thrust, where the paddler is providing the rest. I’d also like to keep the power consumption in the 150-200W range due to battery pack weight considerations, e.g., <1kg providing power for close to an hour.

The BlueRobotics thruster pretty much hits a wall around 6mph and given that it can freely swivel back&up on the mount it actually does start to swivel if I try to paddle at 7mph, i.e., at 200W it’s a net drag. The foil drive shows that 150-200W is achievable, it’s just on the heavy end. I’m going to make a proper swivel mount for it so I can continue experimenting with it but I’d also like to pursue a lighter motor, unless I end up loosing too much efficiency because of that, hence my question here.

For weight context, the surfski weighs 12kg and my whole contraption with the thruster weighs 2.5-3kg and I’m hoping to bring that down another 500g.

Do you know the kV (rpm/volt) of the motors you have? The smaller motor might be lower kV and this might explain that it stops helping at lower speed than the foildrive. If so, then you will need a new propeller with higher pitch or a higher voltage to get more speed.

For your application i’d guess that the smaller motor might be better overall. You could try removing the duct and see if that gets you to a slightly higher speed with it.

1 Like

The smaller motor has 470Kv, the larger is the Foil Drive motor, which I believe is in the 130-200Kv range. I’m hoping the issue with the small motor/thruster is the tiny prop plus the massive shroud. Unfortunately, the shroud holds the motor together so I can’t just remove it for a test. I can buy the same motor with a “normal” mount and no prop or shroud. It’s not cheap (\$160) but it’s well made.

I guess a different question would be: if you were building this, what motor would you buy?

Thanks for the discussion, I very much appreciate the input!!

470kV, that’s odd. It will take a lot of current to get torque out of it and therefore prop will be designed for fast speed, low torque which equals low efficiency in prop drives.

Then i’d choose the foildrive since it’s efficiency for sure will be better.

1 Like

It is interesting to read that the Foil Drive unit designed for (light) efoiling is praised for its effectiveness in other applications like kayaking and more generally speaking, small watercraft assistance

I’m not sure I know what you mean by effectiveness… I believe I can put together a system for my needs that is lighter, cheaper, and safer than the foil drive. It just happens that a friend lent me an extra foil drive motor & prop so I could use that as a data point. We’ll see where I end up…

I’d imagine you’ve seen this before:

He’s pulling 160 amps out that poor battery! I love the simplicity tho.
Was a fascinating video back then.

LOL! No, I hadn’t! 3500W… I’m trying to use less than 1/10th. But yeah, the duct tape & bailing wire approach is cool.