Understanding gearing

[toolsinbox]

I am trying to understand why there are different gear setups for different engines with the alpha drives. More over why one setup would not funtion properly with a given engine. How does this relate to the bravo drives. Are they also geared differently for different engines?

Thanks for taking the time to weigh in.

 

[maxxman04]

Re: Understanding gearing

gearing is not brand or model specific. gearing is motor specific. a 4 cylinder motor cannot produce as much torque as a v8, in stock form anyway. if you take a full size truck, put a 4 cylinder in it, it will take a long time to hit freeway speed. now, if you put a v8 in same truck, with same gearing, that truck will hit freeway speed alot faster, because the v8 produces more torque at lower rpms. if you take that same truck, but change the gears, with 4 cylinder, it may not get to freeway speed as fast, or keep up with the v8, but it will reach a higher speed faster, if the gears are right. a v8 boat motor will work with 4 cylinder gears but not for long. it will take off quick, but will reach top speed very fast because the gears "run out", so you end up not being able to use the motor's full power. now, a 4 cylinder will work with v8 gears, but it will strain the small motor too much, and will never reach a cruising plane, as the motor will be doing all it can to just move the boat. hope this helps and isn't too confusing.

 

[TilliamWe]

Re: Understanding gearing

The gearing is matched to the engine/boat combination, in a way that will allow the engine to reach its maximum wide open throttle RPMs, while using a prop that is 17" to 23" in pitch. Don't know why on the props, except to say that probably 85% of all boats out there have anywhere from a 17 to 23 pitch prop on them.
So on identical 18' bowriders, a 135hp 4cyl might need a 2.0 ratio drive, in order to reach its 4800rpm limit, with a 19p prop. But a 190hp 4.3 V6 can turn the taller 1.81 ratio, and still hit its 4800 rpm range, with the same prop. If it had the 2.0, it would over rev.

Bravos have different gear ratios available, and they are matched to engine/boat combos as well.

 

[muc]

Re: Understanding gearing

YEAH, WHAT HE SAID and
I think it?s really about being able to use the same size props.
Good Luck Muc

 

[Don S]

Re: Understanding gearing

It's actually all about the props. They must be in the 17" to 21 or 23" pitch range to be most effecient in all ranges. Over 23, and you will loose out on hole shot, along with not having enough engine to turn it. Under 17" pitch, and you have no top end speed. You would have plenty of RPM, just no speed.
You could probably change diameters a LOT to compensate, but in some cases, they wouldn't fit between the cavitation plate and the prop shaft. In others the prop would be so small in diameter that it would be unusable.
Then you have the engines. They have a max rpm range, all are very similar, in the 4200 to 4800 range (with some variation on both ends) for best torque and engine life.
The only way to make the two things work together at the most efficient ranges is with gear ratio changes.

 

[maxxman04]

Re: Understanding gearing

even i thank you Don. didn't think about the prop. learn somethin all the time.

 

[toolsinbox]

Re: Understanding gearing

Thats very clear and understandable. Thanks for the imput.
Now , with respect to props, clearly the pitch is easy to understand but where I get lost is the 4 blade props, 5 blade chopper type and the reg 3 blade. What are the differences? What are the signs to look for that you are using the wrong prop?
Also in my case with a freshly rebult 454 what prop is the best to use when breaking it in? I bought an intire alpha outdrive, arriving this comming week, everything but the trim pump, Its comes complete with a stock merc prop. I will post pics of all three props I have then as the two I have right now dont have any pitch markings on them, for your input as to what I should be using.
Thanks.

 

[Don S]

Re: Understanding gearing

Now , with respect to props, clearly the pitch is easy to understand but where I get lost is the 4 blade props, 5 blade chopper type and the reg 3 blade. What are the differences?

Now that you understand the why's of gear ratios, now you have to repost over in the prop forum for Prop Information.

 

[toolsinbox]

Re: Understanding gearing

Thanks very kindly Don. I will do just that.