"To start with you are definat
"To start with you are definately "under-propped" you have a max rpms of 5500 with that particular motor - you are lucky that have not damaged the motor.
And to answer your question you need to do some math [img][/img]
Assuming your tach is reading correctly you need to move up about 5" in pitch to at least 26.
However, simply trying to achieve greater speed is a little more complex than simply changing your pitch.
The pitch increase to 26 would (technically) give you 5 more inches of forward motion for every revolution of the prop.
However, since you would be bringing the rpms of the motor down (to where they should be, about 5400), you would get 1000 less revs at the crank which (after the gears) would translate into about 535 less prop revolutions per minute.
To figure (roughly) what kind of speed difference you can expect (if slippage stays the same) we can do the math.
Currently - 50 mph @ 6400 rpm with 21 pitch.
To calculate your "slippage" we divide your rpms by your gears to determine prop shaft rpms = 3422 - then multiply that by the theoretical distance the prop should travel in an hour (21" (pitch) x 3422 (rpms) x 60 (minutes to get inches per hour) divided by 12 to turn it into feet then divided by 5280 (feet in a mile) to figure out your "pre-slip" speed - which in this case equals 68.05 mph.
Since you are only seeing 50 mph that means you are "slipping" 18 mph or 26.5%
If all stays equal, a 26 pitch should result in (26" x 2887rpm x 60 div 12 div 5280) 71.08 mph reduced by the 26.5% slip for a (possible) top end speed of 52 mph.
So while you can probably save your motor from an early grave by properly "propping" it, you are unlikely to achieve the 60 mph you are looking for...."
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)