A bit of theory

[16andy]

After reading through this site I finally signed up.
This is a question I've always wondered.
I'll use a unrealistic analogy to make this easier to understand.

Say you have 2 boats with identical setups (we'll use an outboard boat) with the exception of one has a gear ratio of 1 to 1 and a prop of 12" pitch, and the other has a 2 to 1 with a 24" pitch prop. How would their performance differ, if at all?

Now I know because being a HUGE pitch difference could really effect the out come, but just pretend its a closer comperison. I play with "little" outboards, which don't have alternet gear ratios available, and I don't know tipical ratios for engines with this capability.

Just throw out any ideas you have, no matter how trivial it seems.

Andy

[Fast Fred]

the 2to1 would get on plain DAYS before the 1to1

[16andy]

why is that?

[Fast Fred]

the lower gear makes mo Torque at lower rpms.

5.13 rear gear, smokes the tires at will.
2.83 rear gear, be lucky to get it to chirp off the line, but it'll out top end the 5.13's

[16andy]

I thought pitch would make up for it. With one prop traveling 24" per revolution, while the othere traviels 12", but spinning twice as fast.

guess an equivelt to cars would be like changing the tire diameter, (big tires low gearing vs. small tires higher gearing). I believe larger dia. will have a larger contact patch (generally) and therefore more traction, but it also has the issue of increased inertia to overcome.

As for the props the only thing I can think of would be the 24" may cause more torque steer and posibly increased prop walk if it is surfaceing. (because 24" is closer to being a padle wheel)

[Mark75H]

Fred, I challenge you to a drag race ... I have a 1:1 set up that can slam just about anyone , I'd have serious doubts about a 2:1 being significantly quicker if at all, but you are on the right track.

Pitch does make up for rpm, but there are other factors.


I puzzled for a long time the reason Mercury supplied a gear change to the Mark 20H racing motors back in 1959. Champion brand motors had been kicking their butts and one of the differences was the 17:19 gears in the Champions ... along with the appropriate differences in prop pitch ... and diameter. All I could see for a long while was the Merc's 30% decrease in prop rpm and 30% increase in pitch.

The additional elements, blade area and more importantly increased diameter is probably what makes the acceleration increase that can be seen with reduction ratios. It took me a long time to realize that for optimum performance these have to change in harmony with prop rpm and pitch.

Going back to the 1959 Merc update ... what I didn't see for a long time was the change in diameter that went with the gear ratio and pitch change.

Without the reduction in rotational speed (prop rpm) the increased diameter can overload the motor and reduce acceleration - because blade tip speed needs to be optimum, but not above optimum. Once you go too fast on blade tip speed the motor will be overloaded and loose acceleration even if you don't have too much pitch.

[Skoontz]

Cool theroy Sam. Let me toss this one at you.

The blade tip theory holds true to lawn mowers as well. There has been this quest to cover more ground without widening the mower deck, so, it was thought that when you increase blade tip speed, it would cover the additonal ground speed. Wrong...It just ended up lugging the mowers down and tearing grass rather than cutting it. You're saying the same thing is happening as you increase prop blade diameter which makes perfect sense. Weather it is grass that blade cuts, or water it uses to propel a boat, load is basically load, and, there is a slip factor with a mower similar to what a boat motor produces. I realize one is the ground and grass, the other is water, but I see some faint relativity here.

[fyremanbill]

Probably comes down to finding a ratio that let you run the most efficient prop. The overall drive ratio being the same, the only other factor I can think of would be the power required to accelerate the prop. Would take a lot more power to spin the prop from 0 to 10,000 RPM in 8 seconds than 0 to 5000 in the same period of time.

[Mark75H]

The reduced pitch and diameter let the prop spin up just as quickly as a prop only winding up half as fast.


The mower blade thing is similar but on props you also can't run too high tip speed because excessive speed on the tip will cause water to boil causing drag and reducing thrust - not the more common "ventilation" that is incorrectly called "cavitation", but real cavitation

[Fast Fred]

Fred, I challenge you to a drag race

um,ware dose your co-pilot sit? my co' be hangin on to the canard, so the Torque don't drive the hull over