Less diameter more pitch?

[stratus5]

Im running a johnson 75 small gearcase on a small boat with a 12.25 X 15 hustler prop. Getting 43mph at 5400.<br /><br />What can I expect if I switch to a 11.75 X 17 ?<br />Will I still be around the same rpm?<br />Thanks,<br />-Dan

 

[ob15]

Re: Less diameter more pitch?

RPM's will probably lower. Diameter has a much less affect than pitch.

 

[QC]

Re: Less diameter more pitch?

Dave F. is right. You should ignore the diameter except for fit reasons and concentrate on the pitch. Especially with smaller boats.

 

[TilliamWe]

Re: Less diameter more pitch?

Trailer Boats magazine, july 2005 issue. get it, read, learn it, know it, live it.<br /><br />1 inch increase in pitch will drop WOT rpm 100-200 rpm.

 

[stratus5]

Re: Less diameter more pitch?

Originally posted by Quietcat:<br /> Dave F. is right. You should ignore the diameter except for fit reasons and concentrate on the pitch. Especially with smaller boats.

Thanks, but 13" pitch gives me 6100 rpm ,and 15" pitch gives me 5200 rpm. Haven't seen a 14" available.<br />As a pilot I find it suprising that diameter isn't important.<br />I know, a boat isn't an airplane (in this case) but in the air, you want a big diameter lower pitch prop for low speed performance, and a small, high pitch prop for a light fast plane. And diameter makes a big difference. Moving a large amount slowly, or a small amount at a faster speed, like a crop duster vs a jet.<br />I have small, light boat, and a small gearcase. <br />I went from 39 @ 6100 with the 12.5 X 13, <br /> to 43 @ 5200 with the 12.25 X 15. <br />So my wot rpm is still a bit low. <br />I thought a smaller diameter would slip more at low speed, getting the motor up into the powerband sooner,and it would be easier to spin wot with less diameter, mass & surface area drag, so I could go up in pitch. <br />I was hoping for 5500~5800 rpm and since I haven't seen a 14" pitch, and I got more speed at less rpm with the 15", I thought of trying the 11.75 X 15 or 17. <br />Thanks for all your comments, I do understand all of the elements involved, just not how they apply to boat props.<br />-Dan

 

[stratus5]

Re: Less diameter more pitch?

Originally posted by TilliamWe:<br />1 inch increase in pitch will drop WOT rpm 100-200 rpm. [/QB]

So I have heard, but a 2" increase reduced my rpm 8 to 9 hundred, and a 1" increase doesn't seem to be available.<br />-Dan

 

[ob15]

Re: Less diameter more pitch?

It sounds strange that 2" pitch changed 800 rpm. Did you buy any chance go from an uncupped prop to cupped prop? What type of props is each of the ones you were using. If you went up in pitch and from a uncupped prop to a cupped prop, the rpm drop is reasonable.

 

[TilliamWe]

Re: Less diameter more pitch?

stratus, you are not wrong in any of your theories. A larger diameter prop will help low speed performance (have you ever seen a Bravo 2 drive & the size of their props?). However manufacturers do not offer a lot of different diameters for your application. <br />As far as a 14 not available, since you don't have trailer boats mag July 2005 issue. Go to their website, there are props listed on there. I don't know if you'll find one or not.<br /><br />Dave is on to something, some other factor must have changed, when you swapped props. <br /><br />Also, I do not know the recommended rom range of your engine, what is it? Closer to the 6100 area? What you also may have done when you went to the 15 was exceeded the engine's ability to turn it.<br />Example: My dad was running a 21p that turn 5200 rpm (range should have been 4600 to 5000), so he thought I'll put a 25 on, thinking he'd get about 4600rpm. Well, it only turned 4200, cause the motor didn;t reach it's peak horsepower range. Changed to a 23, and got 4900 and it ran great

 

[stratus5]

Re: Less diameter more pitch?

Originally posted by Dave F.:<br /> It sounds strange that 2" pitch changed 800 rpm. Did you buy any chance go from an uncupped prop to cupped prop? What type of props is each of the ones you were using. If you went up in pitch and from a uncupped prop to a cupped prop, the rpm drop is reasonable.

Went from the original oem prop which is cupped and has the diffuser ring on the back, to a hustler cupped prop. I was suprised at the difference.<br />Would still like to know what rpm increase I would get going from 12.25 dia. to 11.75 dia.<br />The desired rpm is 5500~5800<br /><br />-Dan

 

[stratus5]

Re: Less diameter more pitch?

Originally posted by TilliamWe:<br />[QB] stratus, you are not wrong in any of your theories. What you also may have done when you went to the 15 was exceeded the engine's ability to turn it.<br /><br />Which is why Im asking about going to a smaller diameter. I know that I can turn a big prop with low pitch, or a smaller prop with more pitch. Im just trying to narrow down the effect of diameter on rpm so I don't have to buy 2 or 3 more non-returnable props to find the right one. <br /><br />I would have bought a 14" pitch if I found one, but now Im thinking a smaller diameter will unload the engine enough to run a higher pitch.<br />So, if I go from 12.25 X 15 to a 11.75 X 15, what should my rpm increase be? <br />What about 11.75 X 17 then?<br /><br />Thanks for your replies, folks!<br /><br />-Dan

 

[ob15]

Re: Less diameter more pitch?

If they were the same type props, I would think the change wouldn't be too much to make a diffenece. With a 17 around 300-400.<br /><br />I think the big difference has to do with the props. The hustler could have a more agressive cup which would definitely lower rpm's. I am going through that now. With my 11 13/4 X 17 OEM prop (aluminum) I can hit the rev limiter (6100) and assume I can go beyond that. I just had a 11 1/2 X 19 SS cupped prop rehubbed and can only get 5650 rpm's. Hope to be able to tweak the trim and maybe motor hieght this weekend to get 5800. So I am looking at at least 500 rpm and possibly more of a change in rpm.