How does gear change affect need for new prop pitch?
 

HI all,

If i changed from a 1.50:1 to a 1.32:1.

What would happen to my performance If i did not change the prop?

If I did change the prop which way would i go?

Thank you in advance...

Re: How does gear change affect need for new prop pitch?
 

Depends on what pitch and engine power you were running to start with. Generally, though, if you could reach say, 5200 RPM before, and you keep the same prop, the motor might not be able to turn up into the powerband, and you might be stuck at 4000RPM, way out of the power.

If you are combining this gear change with a big power upgrade, like a supercharger, you might be able to start off using the same prop. If you assume for a moment that you will run roughly the same propller slip as you go faster, you'd be increasing your boat speed by 14%, which, using some rough calculations should require a 30-40% increase in power. So, if you are putting a blower on a HP500 to get 650 or 700 HP, this will work. Without a massive increase in power, you'd better start off by dropping the prop pitch by 3 or 4 inches. By going to a taller ratio, you should end up with a setup which has less numerical propeller slip, but potentally worse efficiency due to faster blade speeds. Most likely, the boat will not accelerate as well with the taller ratio. Guidance I've been given and experienced says to get the prop pitch up near 30" or better and adjust the gears as necessary to run it. I wouldn't run 1.32:1 unless you are running well over 100 mph.

Just my thoughts...

Re: How does gear change affect need for new prop pitch?
 

I just saw another post of yours and realized that you are running an Alpha Drive. No way will any Alpha application benefit from a 1.32 ratio, as you could never put sufficient power into it without breaking the drive.

Re: How does gear change affect need for new prop pitch?
 

why you changing gears?

Did you blow up a 1.5 and find a good deal on a 1.32? That's the only reason that would make sense.

Re: How does gear change affect need for new prop pitch?
 

I did find a good deal but first I was reading Alpha Info @ Sterndrives.com They say this is the gear ratio you would want to keep the drive live with more power. This was more of a preventive move if anything. I'm looking forward to a good summer with the least amount of problems.

Is it heat that destroys the drives or just sheer torque?

Re: How does gear change affect need for new prop pitch?
 

Yes. and Yes.

All mechanical things have an ultimate limit before damage occurs. Any time you exceed this limit, you damage it. In the case of a continuous stress application, if you reach that limit under power, then the resultant damage will progress almost instantly into complete failure.

If you stay below that ultimate limit, but are still above the continuous use limit, then over time, with heat and wear, eventual damage will occur with the same results.

If you keep the heat out of the lube, then the lube maintains its lubricating properties longer. This results in more time before wear or fatigue induced failure occurs.

If you push more than 375 ft-lbs of torque into an Alpha it WILL fail prematurely.