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:cool:

[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:cool:

[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?:eek: my co' be hangin on to the canard, so the Torque don't drive the hull over:eek: :cool:

[Mark75H]

um,ware dose your co-pilot sit?:eek: my co' be hangin on to the canard, so the Torque don't drive the hull over:eek: :cool:
Well then it should be an even race because you predict it will take me "days" to get on plane :cool: :) :)

[RichardKCMo]

Small wheel turn fast ,Big wheel turn slow. So simple, torque. This has to be done in the range of the h.p. unit used.?
RichardF

[Skoontz]

That wheel diamter sure would have been a good thing to try for the old V-4 75, 80, and 90 HP OMC's. The prop diameter was small and dad always said if we could add an inch or so some really decent speeds might have been achieved with the stock gearcases.

[Tim Chance]

Sam - Your earlier post about prop diameter and pitch as applied to different gear ratios makes a lot of sense, it's something I never thought of. Your recollection on when Mercury went to the 16:21's makes me think back to my early days with an Alky Hot Rod. I got beat on a regular basis by Alky Merc's with 1:1's. So, I adapted a 1:1 Konig to the Champion, found a small prop that worked and still got beat by the Merc's (Not to mention the deflector Konig B's and the Anzani's).

And now all I am is older, not one bit smarter. With my Super E I plan on running a 14:15 Konig (or maybe it's a 13:14 or 15:16, I can't remember, I just know it is one tooth off, not 1:1). If I end up getting a Bass unit I plan on 16:17. Anyhow, someone told me at one time that a lower unit that is not a 1:1 has better wear that a 1:1 as all the gears mesh each other, not just the same tooth-to-tooth over-and-over??

I'd like to throw one more thing into that gear ration prop size discussion. we had two A Konigs that were nearly identical. One had 12:15's and a big prop; the other 1:1's and a little prop. They were pretty much the same speed around the race course. The one big difference is the 12:15's would work with either expansion chambers or megaphones, but the 1:1's would not work with chambers, it would only work with megaphones.

So, I'm wondering did the gear ratio change for the Mercury allow it to gain the advantage of the tuned pipe in the lower unit? I had a stock 20-H conversion that I ran a little, only with 16:21's I wonder if it would have done anything with 1:1's?

I sold that 20-H about 1973 for $250.00. Seeing what they go for on e-bay, I wish I'd had a crystal ball.

[Mark75H]

Tim I had to think about your 2 motors and I think the difference was most likely that the expansion chamber motor had a narrower power band and the megaphone motor had a wider power band. The 1:1 prop may have had proportionally more pitch that the narrower power band expansion chamber could pull.

As far as the toilet bowl mid section and the 16:21's ... I've done the math over and over with the rpm/gear ratio/speed numbers - I don't think the 16:21's do anything beyond allowing you to use a larger diameter prop that helps with acceleration

The toilet bowl on the other hand is very important to the 20H to add power. The original leg just plain kills the 20H's power; originally Merc's research intended to make a mid that just didn't have the original leg's power robbing effect ... no plan at all to make a tuned exhaust - out of frustration they came up with the toilet bowl and just built it to get the job done. Some times ya just gotta "Get'er done!" :)

[Tomtall]

This may make no sense but I have had it proven to me by a very respected mod racer of many years. Not only gear ratio affects the punch and top speed but the gears diameter makes a differance also. If you can make the gears live on a smaller diameter gear set of the same ratio, they will outpull a competetor out of the corners running a larger diameter gear of the same ratio. I've seen this bring a peaky mod engine from a pooch to rocket with no other changes.:cool:

[David Mason]

Tom, you are on track with that science.

Most people don't stop and think about that, they only think ratio's.

One VERY important lesson I have learned over the years. Never trust a theory that is on paper, or in a book. To many times I have been told that won't work based on this and that formula. Well, me being a hard headed lad, I tried it anyways. Sometimes it works, sometimes it don't. One thing I believe, is that just because it works on Snowmobiles, does mean it will work on an outboard or motorcycle. And the flip side is true also. While principles are the same, the variables are very much different. Weight distribution, different drag coefficients, can change performance power curve enough to make or break the theory.

[16andy]

Thats the funny thing about theory, no matter how much someone knows or doesn't know it seems the biggest factor conserning who solves the problem faster is luck. Theory is often applied to reality, while reality should be applied to theory. It seems like a risky approch, but after a lot of time is spent testing based on a theory, this is often the end result.

But that doesn't make it any less fun to discuss:) .

[Fast Fred]

how about a bit O' history, the CC FR
http://img303.imageshack.us/img303/1371/motorbignt0.th.gif (http://img303.imageshack.us/my.php?image=motorbignt0.gif)
Known not to be all that out of the hole, why?, cuzz it's got a 1 to 1.

change the foot and the mid to 1.85's and she Leaves like a Dragsta :eek:
:cool:http://img216.imageshack.us/img216/6528/buff4ep4.th.jpg (http://img216.imageshack.us/my.php?image=buff4ep4.jpg)

[Mark75H]

how about a bit O' history, the CC FR
Known not to be all that out of the hole, why?, cuzz it's got a 1 to 1.

change the foot and the mid to 1.85's and she Leaves like a Dragsta

Ratio is only one part of the set up that makes a boat acellerate. Like I said, if you doubt it, watch me blast out of the pits with my 1:1 set up for short courses ... 0 to 50 in about 5 seconds, maybe less ... change my set up to long course style, just a couple inches change in pitch, more rake angle, about half an inch higher on the transom, tilt the motor out a little and now my 1:1 set up is "not all that out of the hole" just like the CC FR - now it might take me 25-30 seconds to get to 50 mph

Fred, Jon Wright (156+ mph in a quarter, I think the time was 8 and some change) and the other top drag racers all use 1:1, why? because it is only one ingredient of set up ...

[Fast Fred]

My Hull tends to crab on a dead stop launch, it would be in your best interest to take the right lane.:eek:

[Fast Fred]

well if you get that 1 to 1 out of reverse, i am not hard to find.