230 GBP is pretty expensive. Almost as much as the MHz drives?
The jet 2 from MHZ is £100 more so £200 more for the 2 units needed.
I think 230 per drive for Marks is a pretty good price for what you get!
Do not trust these specs, there are numerous faults in them.
Trust the physical explanations.
Also i would like to precise this:
Kv (like i write it to express K(onstant) suffix V(oltage)) has to be given in SI Units rad/s/V to get the reciprocal Km in Nm/A. Of course, losses and load conditions are not taken into account.
What makes a well-designed engine can be very different, depending on its intended use.
Sometimes the weight per torque plays a very decisive role, this would result in very short outer rotors with a very large diameter, largely hollow inside.
But we can’t afford that under water.
However, the formula T ~ D * D * L allows us to compare compact inner and outer rotors. It has nothing to do with the winding itself, but with the ability to conduct the magnetic field over a certain area. This area is multiplied by the lever arm. This gives the relationship T proportional to D * D * L. The ability to conduct the magnetic field through the air gap is limited by the magnetic field strength inside the corresponding iron parts of the rotor and stator and is a physical limitation of the iron, the saturation. If it gets too strong (1 Tesla could be seen as a limit), the field will stray (or short cut) too much due to saturation and the efficiency will go down dramatical.
All these and many more effects have to be considered to build or choose the most compact and powerful motor for our/your special needs.
If you want to use an inrunner with gear, always have a look at the corresponding ungeared outrunner solution to make a comparison for cost, efficiency and weight. I get 3.2kW out of a simple APS 6384 PG 100Kv with 1kg weight, being very well cooled, so i can run it at minimum 80A motor current (RMS) continuously. It is performing far away from its structural ability because of rather low RPM, but for the torque and current it is always at the limit.
This leads us back to the original title of this thread i do not want to miss:
If you try to dimension everything to run at a dutycycle of 100%, you will loose a lot of efficiency on the prop side. You need a very small propeller to get rid of the need to limit the current, so if the prop would be overloaded by low vessel speed, it just keeps spinning with very high RPM at full throttle, cavitating or at least thrusting with low efficiency.
So lets make a conclusion: You have a typical machine, and if somewhere a part is too weak you either can overdimension it or you protect it. Protection by sensing and control is successful design. Electrical Fuse instead of mechanical overload clutch. Current limitation instead of fuse. Current limitation instead of clutch. Current limitation instead of small propellers. Current Limitation instead of low pitch propellers.
All this current limitation implies that the switching losses can be accepted from different viewpoints like maximum power, efficiency, cooling, etc.
The switching losses are proportional to U * I * f. In an ideal world, f would drop down to the motors frequency much lower than usual pwm frequencies from 5-40kHz to realize sensorless control by phase current sensing shunts added to voltage measurement. So these losses would go partly away. Unfortunately we would need sensors to get away from the need to pwm.
So last conclusion: The question is qualified, the answer is complex. A lot of clever people will work on the optimization of physical and electrical systems in the future. Lets give them some hints. Add some links please. You put me on this lane together.
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hey guys , great thread here , and thanks to @aethyr for his explanation about the need to take the full advantage of the motor by using gear boxes. this is true as the need for rpm is less than the motor max high speed point. most motors are “high speed” instead of “high torque” or more designed in that way
I have then an idea to remove the gear box and hear me out 
Taking the sss 360kv or 500kv and using the max voltage we will get the max speed of 45000rpm , then we can design a propeller to suit this speed which will be smaller than propeller using a lower rpm value. here I am then transferring power via voltage and speed at the max point. the motor will draw no more than is max current , here 90-100A.
it is also advantageous as no need to parallel batteries ans can use 12c 10000mah ones. but more series has to be used in series to reach 24-36S
what do you guys think ?
There will be cavitation above a certain speed of the prop, that’s why people use propellers between 120 and 150mm diameter @ around 3000 to 5000 rpm.
The propeller pitch is a form of gearing. A higher pitch results in lower RPM, but higher current/thrust per revolution. A lower pitch results in higher RPM but less current/thrust per revolution. However, as @sat_be said, there’s a limit to max RPM because of cavitation.
Now a jet drive can tolerate higher RPM on its impellers, but roughly 20% less efficient than a propeller. However since our gearing options for efoils are limited, we don’t use the full power of the motor in a prop drive, often missing out on 50% or more of available power. So if going jet drive allows for full utilization of the motor, despite its 20% drawback, it’s net more efficient.
I think people are trusting the specifications on these motors way too much.
There is no way a SSS 360KV motor will live a very long life at 45k RPM.
The nsk 6900 bearings they use are only rated for 32k RPM.
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thanks guys, I understand we need to limit the rpm, and to really get rid of the gearbox we would need motor with a low kv like 50-100kv or motors with a very high current ratings.
so I am thinking to have the rpm around 6000 - 12000rpm , broad range for now ,
I am running the equation of power and thrust and prop disc area to calculate what my system would be like
there is even a calculator on www.vicprop.com
what is the power I should target to ride comfortably at 25km/h?
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Thanks to everyone contributing, nice when people spends efforts and time to teach us that don’t have the knowledge!
Still a few things puzzles me:
- Electric motors are said to have superior torque, still in this application we struggles to get same performance that of same kw combustion engines(outboards) of both 2 and 4 stroke, and it seams that the torque is mainly the issue?
- some here says we need gearing. There is a propulsion system called Azipod, used even on icebrakers. That is direct drive… and super high torque application. Off course totally different type of vehicle, but still no gear, no one there saying they are loosing maximal capacity of engine with direct drive?
Just wondering, can it be so that as most of the motors we uses are originally developed for RC purposes, and because of that they are not that great for our purposes?
Combustion engine outboards use gears As I said, if you have a 12kw motor that revs high in RPM, then by not using those RPMs, you’re missing full power. A formula 1 engine generates 800hp but is tiny, around 1.6L. It generates those 800hp using its 20k RPM. Meaning it has low torque at the engine shaft, but because it revs so high, the car uses gears to convert that low torque to high torque. But if you DON’T use gearing to take advantage of all those RPM, then yeah, the raw torque at the shaft is low, because its a 1.6L engine. Each rotation of that tiny engine generates a small amount of torque.
The hobby BLDC motors we use are designed for small r/c airplanes, boats, helis, all which need super high rpm. But our real world e-vehicles don’t. So we’re basically using high reving formula 1 engines, but not using full RPM and thus hurting our potential torque output.
Regarding azipod:
You don’t necessarily need gearing if you use the full RPM and current and voltage capabilities of the motor and don’t need extra speed beyond the max RPM of that motor. And remember, the propeller pitch is gearing too, there is, in essence, a single gear on the azipod, just not in the traditional sense.
Unlike us, they can design/build their motors exactly to the specifications they need. I guarantee you if their motor is a 1000V motor that can handle 2k amps, they’re using nearly every single volt and amp of the motor, along with the optimal propeller pitch to achieve max efficiency.
What would u pick, 20% efficient system that drives u around on 50A or 95% efficient system that drives u around on 100A and why? If there are no cooling and longetivity issues. Just an example.
I built a few race engine for some Subaru , and it is not that easy to rev at high rpm , problem to lubrificate crankshaft , big work on the head cylinder to fill with air … It is a choice to make just like the efficiency and cavitation with a propeller , heat and Breaking with gearbox …
And all of that in the name of power ! Because if you want speed with a surfboard just add a Foil 
Again , its interesting that the opinions are so divided.Not a single thruth i guess. I know, on the general level the marine environment rather well, but this e stuff basically not at all… One thing i know is that for any application that i am aware of, from outboard to ice breaker, you always leave a huge margin when its about boats/ ships. What ever You have on land, its downgraded when used on sea. Period, still stuff goes apart…The never ending uphill (water resistance) just take its toll. On paper can see the 50K rpm making some sense, in reality its just the opposite of what has been done in real life application at sea.
However, still not saying Aethyr is anyhow wrong, its just a different path!
1 thing is what would be best to use, another is what u can use and what currently works until something better comes
thanks guys for this chat , I have found at alien power system a 100kv. 56200 inrunner , so with a 3x6S series battery I would rev a 6660rpm wish which is then ok to directly connect to a a prop.
I really want to remove the gearbox as it add complexity. It is also a good mental exercise to see where the limits are. someone on the forum used this motor already in a direct drive config. should be good. ?
Nothing I’m saying is wrong 
I’m simply saying that not utilizing the full RPM powerband of a motor is not using the full power of the motor, nothing more. Not that you HAVE to a have a high rpm motor. Plus the larger a motor is, the harder it is to run at high RPM.
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Hey. I already ordered this motor and will do some static tests. With a good sized prop this motor should work fine.
I will post some pictures when I have it at home.
I am not familiar with tailor made propellers, but outboard propellers typically run(on the shaft) 2500-3000 rpm. In racing boats even more than 3500 rpm. With 10S you would probably bee good for 3500 ish? To run a standard yamaha 5 hp prop the problem is Torgue to run that size. A 5hp combustion outboard has probably 6-7 N.m plus 2.1 reduction. So to swing it You need probably 12-14 N.m How much torgue your motor have?
To run 6600 rpm You need to reduce diameter and pitch. I have seen from here that some have been running the smaller (about 3hp) engine prop with 5 pitch. That can work for You.
56114, 780kv 10:1 Getriebe 7,8x8 Prop und rund 30km/ h an 12s und 45 A
Vers.
6384 Outrunner 120kv, 7x5 Prop, 12s und 40A, 30km/h
ESC in beiden Anwendungen 130A Seaking HV
Beide Antriebe laufen sehr gut,
Mit Getriebe bescheinigt aber schneller.
Gruss Frank
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