Originally Posted by floater
well as i havnt got a bloody clue about pitch and revs etc...except that my dad once told me that the pitch was the distance that the prop would go through a sold substance in a vacum on 1 revolution
It's easy enough to understand and your dad was correct except I don't know why it would need to be in a vacuum.
With your gear ratio of 1.87, your engine will turn 1.87 times more than your propeller. So, for 5000 revs per minute (engine) your prop will spin at 2673.79rpm. (5000/1.87).
For a 12" pitch prop, it will move forward 12" for each revolution. That is a nice number because it equals 1ft forward for each revolution. We've just calculated that your prop is spinning at 2673.79 revolutions in one minute, so that must also be the distance the prop will move forward in one minute measured in feet. Since there are 5280 feet in one mile, the distance moved forward is 2673.79/5280 miles in one minute. In this case, by good luck, that's very close to ½mile in one minute. So, in one hour you will cover 30miles = 30mph.
Well, you would in a perfect world but the prop is not 100% efficient and also it will not screw through the water the 12" we have assumed.
My guess was that it would screw through the water approximatley 80% of its pitch and that is why I said it would have a slip of 20%.
It doesn't actually slip nor does it screw through the water but these terms are descriptive to aid the understanding of the calculations.
Actually, it might screw through the water a bit and it might slip a bit but let's not go there just now.