TomZ

iTrader: (2)

Hey folks...

I'm slapping on a new drive next week (Alpha 1) and I need to choose the right gear ratio for the drive and then choose a new prop. I need help in deciding which way to go.

I currenly have a 1.5 ratio with a 21P crappy SS prop. I am able to get to "about" 5000K RPM but the motor has the capability of turning to about 5800 (peak torque in is the mid 4K range). Current speed with this setup is a whopping 50 MPH, and after doing some math on Go-Fast.com, I figure I've got about 18-23% slip (bad).

It was suggested to me that I raise the gear reduction. Going to a 1.65 setup (typical behind lo-po V8s and strong V6s) would give me an additional 10% increase in reduction allowing me to turn a deeper pitch prop (23P-24P). Going to a 1.81 would give a 20% reduction and allow a change of about 5 sizes (putting me at 26P or so). The idea is that I can get the motor working more in its proper RPM range.

From what I've read on the Go-Fast web site, turning a deeper pitch at a lower propshaft RPM is going to be easier on the motor and more efficient.

Anyone see anything good or bad about this (no need to mention the durability of the drive... I already know).

The boat is this...

'83 4 Winns cuddy 23.5' (hull is identical to Liberator with a different cap) 24* deadrise.

383 CID... 400 HP/450 TQ
Alpha 1 drive... low drive height (typical I guess... used 225 as a constant in the speed calculation on Go-Fast.com)

Roughly 4k pounds with quarter tank of fuel and driver.

Any help would be great!

Thanks,
Tom

bbladesprops

TomZ,

First off, sorry for the delay in response.

What rpm do you want the engine to turn at? At 5400 you would need to go down to 19P currently. You can get the engine to work in its proper range without changing g.r. just by changing propellers. Are you stuck with your "crappy" SS prop?

Anyone who knows me knows I always preach low g.r. This is because the tourque is higher. With your type of application a slightly higher pitch may also help in your slip problem. Either of the g.r. will work.

Call us if you have any other detailed questions.

TomZ

iTrader: (2)

So you think that I could run the 1.81 with a 25P or so?

The motor is cammed for an operating range of 1800 to 6000, but with the 383 it will be a couple hundred RPM lower. I think 5400 is about where I want to be.

bbladesprops

In order to get to 5400 right now you would need to go to 19P. That means your talking about 6 inches of pitch to go to the 25P. The change from 1.5 to 1.81 will not qualify you to go to 25P Your looking more at a 3" change in pitch.

That being said, if you choose to change gear ratio, use the 21P you have now to test with. We may end up with a 23 Mirage Plus with specialty work done.