Originally Posted by
phillnjack
The gear ratio of your engine is 2.00 - 1 ( presuming this is not a special ratio you have on the engine)
Max revs of the engine is around 5,500 (unless this has been hotted up)
that gives you 2,750 rpms at the prop @2.1 reduction .
the prop has a 16 pitch ,
so in theory without slip this would travel 16 inches on each revolution of the prop.
Ok lets see what this realy would be in the imaginary world of no slip props.
Its very simple mathmatics to calculate theoretical speed with no slip at all.
The rpms are 5500 at the flywheel,
the gear ratio 2.00-1 so its half of engine revs =2750
so we have a pretty easy starting point.
2750 rpms per minute, x 16 inches per minute. = a speed of 44000 inches per minute
divide this by 36 to give yards and you get .= 1222.2 yards per minute.(36 inches = 1 yard)
Now 1,222.2 yards per minute x 60 to give you how far in 1 hour = 73,332 yards per hour. There is 1,760 yards in a mile
so just divide 73,332 by 1,760 to give miles per hour.= 41.66 mph.
This is only theoretical with NO SLIP AT ALL and no drag from the boat ?
Maybe you have a special race engine :cool:
****If you want a real easy way of figuring your speed at 100 per cent efficiency (you will come within a mile or two of the way you figure it)::::subtract 1 inch from the pitch of the prop, in this case it is a 16 inch prop so use the number 15 and multiply it by the rpm of the prop 2992 (assuming engine is running 5500 with a 1.85 gear ratio) result will be 44.59 mph. Not too far off when you are in a hurry(within 1 to 2 percent) and not figuring the drag. Most modern props may be called 16" but in reality, this is an average pitch measured through a specific area of the prop. It may be more than that near the hub and it may be less than that at the tips, a little "cup" also changes the resultant pitch--in other words all of this scientific **** is not worth much except to start arguements. Results are what counts. Go for that 52 mph, you may or may not get it but will have fun trying. Rod
phill..:)