i need to buy a prop for my graupner electro vee kit. I will be running a 775 motor on 12 or 14 cells. I know i want carbon fiber but dont know what size. I was thinking a 42mm???? thanks

I would say start with the 42mm prop and prop up from there. You probably will be able to go as high as 45mm.
Remember to keep checking the motor, ESC and battery temp.

My boat is a AS1 from Schink and has a 775 type motor with a 45 prop on 12 cells and has only brushcooling , bigger then 45 is to much and will not give a big increasement in speed above a 45 prop but it will produce a lot more heat , i tried a 50 prop just for checking it out , stick to a 45 prop as max at least i do .

I also tried a 39 and a 42 and the different is that a 45 will excellerate faster that the two but that may differ size and weight of the boat .



Michael ,

the torque of a 775 with 16,8v is something like 6x more than a regular 540. imo, it should be able to spin a much larger prop than only 45mm .....

Offcourse it can but the heat builts up much quicker using a larger prop , today i ran my boat with a 50mm prop on 14cells with no can cooling only brushed cooled and i wil put a can cooling on it very soon it whas very very very hot even the battery were hot .

So i your boat isnt bigger then say 30" stay with a 42 maybe 45 prop it will do just fine .

Tomorrow i will put in my boat a graupner 700bb turbo 8,4v and the 775 will make way for this one and hope this is a better motor then the 775 .

Michael ,

yes the batteries can be way to hot... iknow :P in my old jet stream the batteries was melting down after every run
i had twin 37 000 rpm k-speed motors and a 3-blade octura metal prop.

how to cool the batteries?

If your batteries are overheating your motor should be also. Try a smaller prop. A 2 blade prop will give more top end speed and less resistance and less heat to the motor and batteries. A three blade prop has more take off speed but less top end due to the resistance of a prop being in the water all the time and this resistance can cause heat build up on the motor and batteries especially on take off.

i mean... any good ideas to cool the batteries without water using? i was thinking about to mount the batteries in the top and use the wind like cooling, but then the boat should be very instable... maybe to make some cooling holes and use the wind anyway and then mount some heat sinks on the batteri pack

You are missing the point, your batteries should not be getting too hot. If you can hold your finger on them for more than 30 seconds without it burning your finger then it should be fine. Any hotter than that and you have a set-up problem that needs to be addressed. Some people do use air scoops to help cool the air on the inside of the boat to help cool the motor, but if your batteries are overheating then that is another story.

Best performance comes from batteries that are hot. Not overheating* I throw my batteries in my boat right off the charger so they already are warm, providing optimum performance fast.

On 12 cells, I would go with anything from 42-45mm

Ryan