Better GPH under prop or over prop?

[magnoman]

I have been going crazy the last few weeks testing props for my Yami F225.

I tested a SWS 1, 15 1/4x17 that was perfect WOT rpm range 5800 and 40mph. Then when I went to purchase one they do not make the (SWS 1) any more its the (SWS II) So I got a (SWS II) same pitch 17, but the dio was 15 1/2 and it was way over proped 5200-5500,
but it did cruise nice at a lower rpm?
So I exchanged it for a (SWS II) 15 1/2 x 15 and the rpm is right in there but on the upper end 5800-6000. But I can trim bow down a little more and keep the rpm up more and the hole shot is better but lost a little top end.
But seems to me that the more rpm the more GPH (Gas!)

 

[magnoman]

Re: Better GPH under prop or over prop?

Ok just read a eary thread that explained the GPH and pitch . But how does the larger diameter effect the performance. I went from testing a SWS1 that was
15 1/4 to know a 15 1/2 (SWS II) to me the larger diameter would give it more drag and cause the motor to work harder, but that is why I had to go down in pitch I think to compensate for the large diameter??

 

[steelespike]

Re: Better GPH under prop or over prop?

I can't be too technical but to me the most speed for a given rpm will produce
the best mileage as long as you remain in the upper end of the rpm band.
Usually diameter is simply the product of the design and doesn't affect rpm results except in the extreme.I would suppose you could select a prop that produces the best cruising speed/mileage without regard to top end again be ing sure to operate within rpm range.
Best cruising mileage/speed can occur just into planing speed but I've seen it as much as 2,000 rpm above.
I think the only way to test for best mileage is with a good flow meter
and operate the motor through its entire rpm band no wake to wot.
I'm sure some designs will produce better cruise performance than top end and viceversa.Trying to think logically it would seem that larger diameter might do better at low to mid range.

 

[zaroba]

Re: Better GPH under prop or over prop?

Ok just read a eary thread that explained the GPH and pitch . But how does the larger diameter effect the performance. I went from testing a SWS1 that was
15 1/4 to know a 15 1/2 (SWS II) to me the larger diameter would give it more drag and cause the motor to work harder, but that is why I had to go down in pitch I think to compensate for the large diameter??

only way i can describe it is to look at it similar to how a fan works. a prop is basically the same thing, except it moves water instead of air. a larger diameter = larger blades which means the blades have a greater surface area and thus a greater flow rate and greater thrust. the same could be said for having 4 blades vs 2 blades. although yea, slightly more force would be required to turn it threw the water due to the higher mass. as for pitch, pretty similar, a sharper angle = a harder force applied to moving the water out of the way and thus more thrust. think of a fan with blades angled at 10 degrees vs a fan with blades angled at 45 degrees both spinning the same rpm. air would have to move faster to get out of the path of the 45 degree blades then it would for the 10 degree blades. also would be more engine power required due to a higher resistance of the water against the blades due to the higher angle.