What is more efficient?

[FCnLa]

A lower gearcase ratio (numerically) with a lower pitch prop, or a higher ratio and a higher pitch prop. And yes, I've been thinking too much.

[mercrazy]

they ran a test on this a few months back in B&W Boats magazine. can't remember the exact details, but i do remember the higher pitch props had a little more slip. they tried 3 different props and gear ratios. seems like the middle ratio won but i think the effectiveness of each prop would have more to do with it than a certain gear ratio. look up the article.

[WillyT]

I have always been told that best efficiency is where the prop pitch is about twice the diameter (a 28" pitch X 14" diameter prop, for example, is theoretically more efficient than a 34" X 14"). This is obviously not an exact parameter and is just a ballpark. In addition, you always need enough diameter to develop and use the available thrust. Obviously, the first thing to worry about is to use a prop/gear ratio combination that lets you reach the proper RPM to make maximum horsepower. You should use a gear ratio that lets you do this while using a prop that is close to the "2 to 1" parameter mentioned above. I have a friend with an Allison/ProMax combo. He switched to the 1.75/1 gear ratio (from the stock 1.87/1) and went to lower pitched prop. This got him closer to the "2 to 1" parameter. He gained several mph. I think there is as much art (black magic?) as science in this, however, and trying a particular setup on your rig is the only sure way to tell what is best in your application (if this was easy, it would not be so much fun!)

[FCnLa]

for your input. It does make sense. I guess this also depends on desired engine RPM and torque range. My old motor does not turn that high, so I'll stick with 1.86 and a 14 1/4" X 29" prop for my biggest wheel. Good day!

[WILDMAN]

A 1.87, 1.75, and a 1.62 will all be the same speed and rpm if you change the prop 2". A 28 on the 1.62, a 30 on a 1.75, and a 32 on a 1.87 will all be pretty close to the same speed at the same rpm. The higher gear and lower pitch will be the most efficient as far as speed though.

[Forkin' Crazy]

LOL!!! Transomsaver, you've been busy digging in the archives, haven't you? LMAO! :D

[T-REX]

Our ole buddy, Howard Arneson haz prolly went thru more props than most ov us have ever seen, and hiz therory iz az WillyT's...Twice tha pitch az that ov tha diameter....

Top end with geer changes are bout tha same az stepp'in up or down on Pitch(az Wildman haz said), BUT, goin from a 1:87 geer to a 2:1 geer will not be equal to juss stepp'in down 2"s ov pitch on tha bottom end/mid.....Going to a lower geer MULTIPLIES TORQUE, which shows up more on bottom end....Tha lower tha geer, tha more torque applies....A mota with a 2:33 geer will pull a 30" wheel harder out tha hole, than a 1:87 with a 26" wheel!!!....

Boat weight iz tha biggest factor az to which geer works best...Heavy boats work well with lower geers, and lighter boats work better with hi geers(with bigger CI's)....My lil Laser(980#s race ready) with a steel bore(make shift) drag mota, runs best with a 2:1 geer......With tha 2Liter, tha 2:33 and bigger diameter prop iz tha ticket!!!!......

Prop/geer/bote combos are like women...When ya finally find tha rite one, U kan't afford to play wit them!!!

[930turbo]

I found that the higher ratio & lower pitch prop generates less steering torque than the opposing option. I'll assume it is because there is less vectored energy since the blades are not as steep.

[transomsaver]

LOL!!! Transomsaver, you've been busy digging in the archives, haven't you? LMAO! :D

I was just cruisin around on the site. Musta searched farther than I thought. I didn't know it was illegal to post on threads that are older than a certain date? Should they be locked then so people can't bring 'em up? That would save the mods who deleted my posts some work. I particularly enjoyed the PWC post that I posted on, as well as some others who posted, but poof, it's gone.

[RBT]

Gearing is a multplier, 2:1 with a 20 is not the same as 1:1 with a 10 pitch prop.
For example, if your motor makes peak torque at 5000 rpm, with 2:1 gears your propshaft sees it's peak torque at 2500 rpm BUT it is double the number the powerhead actually produces. Where as the 1:1 see it's peak torque at a proppeller shaft speed of 5000 rpm, but at half the torque of a 2:1 ratio.
In the real world this means that a 2:1 will pull VERY hard for a short period of time, but fall flat on it's face, where the taller gear will not pull as hard, but will pull longer.
Consider where your motor makes peak power, then be sure that you don't operate past your peak. If you do you are operating in the over rev or the backside of your power curve.
To keep it simple, for the most part 1.87 gears are best.
FYI, our drag boat runs UNDER 5% slip and the prop is under square.

[mirage243]

I was just cruisin around on the site. Musta searched farther than I thought. I didn't know it was illegal to post on threads that are older than a certain date? Should they be locked then so people can't bring 'em up? That would save the mods who deleted my posts some work. I particularly enjoyed the PWC post that I posted on, as well as some others who posted, but poof, it's gone.

Are you sayin' that your post was removed from this thread? I wanna know who the jack ass is that removed it.

[Forkin' Crazy]

I was just cruisin around on the site. Musta searched farther than I thought. I didn't know it was illegal to post on threads that are older than a certain date? Should they be locked then so people can't bring 'em up? That would save the mods who deleted my posts some work. I particularly enjoyed the PWC post that I posted on, as well as some others who posted, but poof, it's gone.

It is absolutely not. I was just picking on you. :) I also was amused because FCnLa was my old user name before Greg changed it for me. By all means, carry on! :)

What PWC post is missing?

And if no one has said it, welcome to Scream and Fly. :)

Oh, and as far as speed and pitch, it really depends on a lot of factors. Wayne Taylor told me a faster spinning 28 would be more efficient than a slower spinning 32 because of the side thrusts created by the high blade angle. That said it depends on what set up, engine and a lot of other factors. I know a guy that is running a 1.72 gear with a 34" wheel. ;)

[Forkin' Crazy]

Are you sayin' that your post was removed from this thread? I wanna know who the jack ass is that removed it.

There is one missing on here too he dug up. I know because I posted on it busting transomsaver's chops a bit, all in fun. :)


Come on Admin, don't you have better things to do? Jeeze, talk about micro-management. :rolleyes: