Make a hole centered on the tip of your prop and shaft and then insert a screw.
The esc watches for voltage under load which is lower than on no load.
In our case our big lipo battery voltage dropped down under the esc limit even if they had only 20% discharge. Then the motor was limited to 30%
As virus said your prop might be slipping
I think it is a way to test the Yamaha propeller 
A prop slipping would be practically impossible as set screws were screwed INTO the shaft, not just set to a flat spot. Same thing with the motor and gearbox, we made it so there is no chance of slipping.
I want to try the Yamaha propeller, but it is 10mm and my shaft is 8mm! 
Your propeller is 100% full…why print in PLA and not in PETG or ABS which can be more rigid, no?
I can try to print in PETG, I have some of that laying around
Just print an adapter 8 to 10 in pla or petg it is the easy way
The propeller shown (Kaplan type) is not appropriate for use without duct. In operation there is a pressure difference between the sides of the blades which is related to the driving force. The wide blade ends of this type cause a strong flow of water around the blade ends that means a short cut of the pressure difference. The Kaplan propeller types are meant to be used in ducts. In this case the small gap between blade ends and duct prevents a short cut largely.
I took the duct off because folks were saying it was hurting performance. Are you saying I should add the duct on or should I print a different prop with a different shape?
Should be able to fly with and with duct , after succesfull tests , optimized shape design safety and running time , it is hard to do everything the first time… Low rpm , enough torque , i would some pitch
Ok the plot thickens! I’m thinking now it isn’t the batteries or the prop. Check this out when I tried to test out the motor this evening:
I’m guessing water in the pod? Any other ideas?
You may have blown the ESC. Were you running water cooling?
Maybe make a video with the full startup sequence.
You could strip the system back and test separately if you have extra components.
In my opinion you should add the duct for performance and safety reasons. Sure the duct will add some drag also, but the gain shold be much greater, because your minimise the losses of your propller.
Metal prop and larger wing. If you can’t use a larger wing add a washer under your front (back wing) bolt to lean the leading edge of the wing down, that slows you down a tad but adds lift. you need about 1500-1700sq cm of front wing. That is tiny looking and your batteries seem to be taxed right away as a result.
The duct will only add a bit of low speed performance. It definitely won’t stop him getting up on the board.
Plenty of ductless setups with the exact same config already running on the forum.
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I have the same foil and it gets up at a very low speed, so I don’t think the wing size is an issue.
This is one of the largest foil wings you can buy. I highly doubt it’s the surface area considering it’s bigger than all commercially available wings.
I might have blown the ESC. I had the seaking on an aluminum plate that on the bottom of the board that makes direct contact with the water. Seeing as I never got off the water during my 5 minutes of testing, it was always in contact with the water. But I was giving it a lot of throttle so it’s quite possible. I have another ESC on the way this week.
That wing looks very similar to the liquid force foil fish, i used it a few time without any problem to fly. There are thicker/bigger wings creating even more lift, but that’s not needed and limits top speed.