Gear ratio 67 60hp sportfour evinrude

a70eliminator · #1 March 29th, 2009, 05:31 PM

Anyone know where to get this info I'm trying to figure out my theoretical speed with a 10x9-1/4 prop @ 4500 RPM. I have the non heavy duty case if it makes any difference.
I think I may have a saturated hull, but don't want to cut a hole in the fiberglass deck just yet, I still have to weigh the rig too.
I got 29 MPH on a slight chop yesterday at 4500, 16ft MFG Westfield Regal "V" with a flat bottom at the transom, semi-V ?

a70eliminator · #2 March 30th, 2009, 04:08 AM

ok then, anyone care to make a quess. I'm going to just mark the shaft and turn the flywheel one rev and see about what I get.

F_R · #3 March 30th, 2009, 07:59 AM

Quote:

Originally Posted by a70eliminator

ok then, anyone care to make a quess. I'm going to just mark the shaft and turn the flywheel one rev and see about what I get.

Hey, give us a chance. I just saw the question. The gear ratio is 20:23.

a70eliminator · #4 March 30th, 2009, 06:49 PM

Thanks for that info FR
Does this calculation sound right?

4500RPM x 9.250 (9-1/4P) = 41,625 inches per minute x 60 min = 2,497,500 inches per hour.
12 x 5,280 = 63,360 inches in a mile

2,497,500 inches per hour divided by 63,360 inches in a mile = 39.41 MPH

Thats for a 1:1 ratio but we have a 20:23 ratio so now what?

39.41 divided by 1.15 = 34.26 MPH - prop slip ???

Is this all right for a theoretical speed ? and how much prop slip average?

F_R · #5 March 30th, 2009, 07:16 PM

I didn't go through the numbers, but the theory sounds right. And the final result sounds about right too.

The theory of screw propulsion is sort of a myth. It just doesn't happen. As I understand it, and I'm no engineer, the prop acts like a pump, moving water to the rear. Every action has an equal and opposite reaction. The equal and opposite reaction pushes the boat forward. It is how a jet boat works too, they just moved the pump. It's related to the reason a rocket has thrust in space.

a70eliminator · #6 March 30th, 2009, 07:39 PM

Well, I found a prop vs speed calcualtor and the numbers I'm getting are right in there, I was clipping along at about 29MPH @ 4500R's and with my 20:23 ratio it worked out to 12% slip.

Just thought a 16' boat should be faster with a 60hp but I guess not because even if it was a 100hp the rpm vs pitch & speed would still be the same in theory.