Originally posted by crazyhorse
Maybe Im asking a silly question but does the weight of the boat make any difference and the water line length and the waterline beam are needed to work out the theoretical speed. of the boat?
Cant see any of that on the formula thingy,its brill though,just not sure whether its accurate?
Crazyhorse, I see from your quote you are having a bit of a job grasping this. It really is just simple maths and common sense, but the answer is FACT.
People used to use the term single or double screw when talking about props, thats because they are a screw. The "pitch" is the theoretical distance a prop moves in one revolution. This is how it works.
Max revs (per minute) is 6,000 divided by ratio 2.33 =2575. (here is where the magic 1056 bit is used , 36 x 1760 / 60 = 1056) 2575 is the amount of inches it would travel in one minute, divide by 36 is the amount of yards, divide by 1760 is the miles, then you multiply by 60, this is to make the calculation into an hour, end result is max speed in mph = 48.77. Now this is theoretical, you have to subtract slip. Lets call this the S factor. Now I know what the S factor is for my boats, because I know all the variables, revs, pitch, ratio, speed. If you don't have the boat, you can't work this out. The S factor is obviously determined by hull design, weight, drag waterline length and many other thing, most of which a Naval Architect will claim to know, but you will have to wait till you have your boat in the water to find out if he was correct, or if he has overcharged you!!