Ben
My manual says the same thing, then I red this in the MVVS forum.
You should try to prop for about 6000 rpm static. That can be achieved ith 24x12 up to 26x8-10
For three bladed props, 22 x10-12.
The large 26" prop should be of the narrow blade type.
I turned a Menz-S 24x12 at 6050 rpm using a Krumscheid 50-70cc pipe without tuning the length for best rpm.
Look at the power curves on my web site and you wll see what rpm are best suited.
I hope this helps.
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Best regards,
Pe Reivers, www.mvvs.nl , Mejzlik, 3W. Mail can be sent using the web page.