So the 80100 80kv of my dreams has been on backorder since I started this project. However, the 50kv version is available and I think a couple of people have used it.
Any reason anyone can think of why I shouldn’t go with the 50kv? Presumably I’d just need a slightly larger prop?
Place an order for the 80kv motor and he will usually ship within 2 weeks. I was waiting for the restock for a long time and he never got them in stock. I placed the order online for the 80kv version and had one in 10 days. I think he is unable to keep up with the demand for now (as he gets a shipment in they are already sold).
I figured I would cancel my order if the wait became too long.
Thanks for the advice, I’ll put in an order now, we’ll see how it goes.
Order Placed! Now, about those batteries…
I ordered this motor a few days ago, will see when it arrives. Direct drives get more common it seems! 
Yes, I saw @s9tim’s direct drive thread and I thought, I’m clearly overthinking this. So I went to eBay, ordered 2 types of the same size prop he is using, ordered the 80100 and now I’m just about to go get some batteries.
It is just so much simpler, it’s faster to get going and I am sure it will be more reliable. And if not it doesn’t hurt much to replace the motor after 1-2 years - as research and development on this topic progresses we probably will want to upgrade it at some point anyway.
What batteries do you plan to get? I will use a pair of cheap LiPos for a start and later build a 18650 battery pack.
The motors are remarkably cheap, all things considered. So yes, you are totally correct that in 2 years there’ll be a new shiny thing. I will probably pot the motor in some way, I’ve been toying with the conformal coating they use on PCB’s. I’m going to test it in the tub the next few weeks with an old throwaway motor.
For batteries, again, for simplicity sake, I’m going to start with 4 of these:
If I were going to build a pack I wouldn’t use the 18650’s, I’d use these 20700’s
due to the capacity. However, at 15A you’d need to do at least 6P to get where you need to go. I was thinking 12S7P. I’m currently using them in my mountain board 10S6P.
(no association with either of these companies)
Thanks for the hint, I will look into these cells. I was planning 12S15P with Samsung 30Qs.
I’m hoping to build 2 battery packs, so one can be charging while the other is in use. As with all of these sorts of endeavors friends inevitably show up and want to try it out, so it’s handy to have an extra pack around.
I think I mathed out about 22 minutes as the ideal length of time you’d want a pack to last given reasonable cost and weight considerations.
Possible that this is a good compromise, but this is a different approach than I was after, I was aiming for around 1h of ride time. The price is in the range of these Tattu Lipos with just 22Ah. Don’t know what negative impact the weight has though.
I try to avoid doing maths on a Sunday, but let’s do some here. assuming an average of 40 Amps to keep you on foil you’d need a 40aH pack. That’s a 10P of those nice 20700’s. Now we need the 12S part, so that’s 120 cells.
with the quantity discount that’s $907. Not too bad.
I don’t happen to have a 20700 in front of me but I do have an 18650 right . . . here, and it weighs 1.2 oz. So the 20700 is roughly 120% the size of the 18650, so that’s 1.44oz per cel, times 120 = 10 pounds in cells (alone, not including welding strip, wire, fuses, etc.). Also not too bad.
So assuming I did my math right I think you’re fine. In fact looking at those numbers the Turnigy pack is not such a good deal. Almost as much money and only 22Ah. Hmmm. Looks like I might be digging the ol’ battery welder out after all.
Of course I did say something about convenience, which still matters a lot, so I’ll likely stay the course, though my second pack might be a 1 hour pack.
The thing I have to do is design ahead of time so that all packs use the same size Pelican case and fit in the same cavity on the board.
Doug thank you because I have been thinking that too but I was worried I was just getting old and lazy. I am tempted by the 50Kv so that I can know the RPM can max out as will the power before the prop cavitates in the worst way.
if you plan on using a commercial prop, you’ll likely use one designed for around 3 to 6Hp (roughly 2 to 4.5kW). Quick google search for those motors shows answers in the 5000 rpm range (i guess that’s true for any motor :))
50kv will give you 2500 rpm max, and 80 will be 4000 rpm… cavitation shouldn’t be an issue with both…
Thank you Mat and I know most outboard motors have a 2:1 reduction gearing and a peak RPM max of 7000-8000 so my finished final rpm off of the outrunner should be 3000-4000 so the 80 might actually be better?
better i dunno, but it works well for sure
50kv might give you more torque, but i’m not sure we need it…
Okay thanks Mat I am designing the power train first then adding the motor last. Someone else here did a fast switch system for the propellers which I plan on doing as well so I can just throw on a new prop like we do on boats. I figure I can always worry about too much power as I am riding around having a blast versus looking like an idiot as I plow along needing a mere 10-20% more pwoer to PLANE…NOTHING worse than that! (just got a new boat…works well used new boat and it planes great but due to the new deep v it wont pull a water skiier with the same engine now…one of the kids is very annoyed…
My backordered motor is now shipping, after 7 days 
Excellent! I should order those batteries sooner rather than later. I’ve decided to go straight to the 20700’s rather than get the LiPo packs. Even though I’ll have to build the pack myself, for only $200 more I can get 40Ah over 22Ah that the LiPo’s get me.
I also thought about saving the extra money on test LiPos and directly building the pack, but I have to distribute my expenses over some time due to my student job 