Gear ratio change versus pitch

Powerquest_Baby!! · 12-02-2006, 06:51 PM

This is a question Ive always wondered about. Maybe someone can shed some light on it for me. We all know that if you increase the pitch you increase your speed (assuming you can turn the prop at a sufficient RPM).
So, my question is this: if you increase your gear ratio wouldnt you then be able to turn a higher pitch propeller and therefore go faster? How would this affect your RPM? Horsepower is the number one way to go faster but does what Im saying make sense or not?

Griff · 12-03-2006, 06:19 PM

You would be able to turn a higher pitch prop, but the gear ratio change would decrease how fast it spins.

raytart · 12-03-2006, 10:27 PM

If you go the other way Let's say 1.36 ratio from a 1.50 - the prop shaft will be spinning 1 rev per every 1.36 of the motor as opposed to 1 prop shaft rev per every 1.50 revs of the motor. In theory the prop shaft will be spinning faster at the same motor rpm therefor making you go faster. The problem is that you will have to go down in prop pitch to achieve the same rpm of the motor. But if you go from a stock prop to a labbed prop - you may be able to achieve the same rpm with the same pitch prop. The boat may be sluggish getting to full speed. These are just my thoughts. I am in no way an expert on this. I have inquired about it and everyone seems to believe that the 1.50 ratio works the best. I'm going to give it a try on my boat just to see what happens. (going against what everyone says) I'll let you know how it works. But that won't be until April or May. The gear change is already done but the boat has been winterized. There is a prop slip calculator that is very useful. If I can locate it - I'll post the link.

http://www.go-fast.com/Prop_Slip_Calculator.htm

Not sure how to make it a link. Just type it in. (sorry)

Griff · 12-03-2006, 11:57 PM

1.50 ratio works the best on most factory performance boats. Formula uses/used a 1.65 on a couple models. The only reason to change from the 1.50 is if you run out prop pitch, one way or the other.

throttleup · 12-04-2006, 08:36 AM

By increasing your gear ratio (numerically) the prop will turn slower at any given rpm so it's travel through the water will be less with the same pitch.

You have to increase pitch to compensate for the slower rpm.

Speed is directly related to the horsepower for a given hull and weight.

Powerquest_Baby!! · 12-05-2006, 02:54 AM

Thanks guys! Just something Ive always wondered and never had the answer to. It makes sense

bbladesprops · 12-06-2006, 07:48 PM

Great question and great answers!

I always tell folks to stick to the lowest gear ratio that they can (highest number). It will give the best throttle response (within reason till you get into the mid to upper 30's in pitch).

Like Griff said, change gear ratios when you run out of pitch. There are other reasons like gear strength, cruising speeds etc. but, the main choice is lower gears = better performance.