Quest for speed... gear ratio/prop.
 

(Posted in the Prop forum as well)

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!

Re: Quest for speed... gear ratio/prop.
 

Your high slip number is probably a result of a poor setup/ x dimension thus changing gear ratio or props may not affect it at all. Ever think about an alpha ss with shorter x dim.? Of course it would be nice to try before you buy....but you know the real fix on that large of a boat is a bravo/shortie combo if you want it to live. just my 2cents, good luck. :drink:

Re: Quest for speed... gear ratio/prop.
 

Finding an Alpha SS is next to impossible. Maybe if I could find a lower unit I could try that at some point, but again, finding anything related to those drives is tough. You may be right though on the X-dimension. I'll need to measure it this evening. I know that the drive hangs rather low, however, the boat has a fairly deep-v and may not be that bad off. These hulls are known for being slower than similar boats in its class, but it rides very well and has a lot of control. There's always a trade-off.

I'm either going to go with a 1.65 or 1.81 behind it. With the 1.81 I think I'll be able to get away with a 25P or 26P prop, but we'll see. Whatever way I end up going I'll be sure to baby it out of the hole and keep the boat in the water.

Re: Quest for speed... gear ratio/prop.
 

What prop do you have? Has it ever been damaged and repaired? What is the blade diameter? There are lots of reasons why it could be slipping, but before you trust the slip calculation you need to have a GPS speed and a good prop that you know for sure has a pitch of "X".

As far as changing to a higher gear ratio and a higher pitch at the same time, this won't change your WOT RPM very much and you will still be lugging an engine that likely wants to RPM a bit more.

Here's the math:

21/1.5 = 14
23/1.65 = 13.94
25/1.81 = 13.81

Because the ratio of "pitch to drive ratio" hasn't changed very much, all of these combinations will give you the same speed at the same RPM.

Before you do anything, test the boat with a good SS prop. If you want to raise the engine RPM to get it into it's power range, try a 19" pitch prop. . The boat will be more responsive when you load it up with people and it may prolong the drive's life.

Re: Quest for speed... gear ratio/prop.
 

What tomcat said,

Even though higher gear/more pitch is mechanically more efficient(therefore faster) you are constrained by the limitations of the Alpha drive. In any case i'm not sure how much difference yoo will see in the 50 MPH range.

If you had some drives to try it would be an interesting experiment.

Re: Quest for speed... gear ratio/prop.
 

"I currenly have a 1.5 ratio with a 21P crappy SS prop". I agree with Mbam, get a good SS prop for a baseline. If it were me, I'd go for the 1.65. Kind of a "middle of the road" decision. Would give you a good selection of props to work with and be a little easier on the drive.

Get a good 21 with a 1.65 and get the prop slip down in the 10% range and you'll be close to 60 @ 5500 rpm. :D

By the way, I thing all the libs have a 21* deadrise, at least the newer ones were. The 80's libs may have been 22*, but none of them are 24*. Just an FYI.

Re: Quest for speed... gear ratio/prop.
 

Haven't measured the deadrise... was just told that it was 24*.

I think that the title of my post might have been a little misleading. I guess the point is that I'm trying to gain more of a mechanical advantage. My boat is odd in the fact that it could probably do better with having the drive be farther down in the water to give is more "muscle" at the transom to carry the bow (bow-heavy hull). Too bad I can't drop the drive down in the water a little more. :D

I am going with the 1.81 and I'm going to start testing with the existing 21P just to see what it does. I will probably end up with a 23P Mirage Plus to replace the older Quicksilver prop (I believe it's an older Vengance) and then I'll experiment from there. May go as high as 25P.

Re: Quest for speed... gear ratio/prop.
 

Going to the ratios above 1.5 will result in less reliability. When I had my drive redone, we went this discussion. My combi is a 383 with 433hp in a 20' searay. My builder told me the 1.63 and 1.8 ratios were used in the lower hp applications and were not as durable. I think the best way to look at it is to look at the number of teeth. One may be able to conclude that a lower tooth count will result in thicker/stronger teeth. More tooth count result in thinner/weaker teeth. One other thing we need to figure into the equation though is how many teeth are loaded at any given instant? The more teeth that are in contact, the greater the load capacity. I don't know any of the factors I just spoke about, but the 1.36 gears are the strongest, the 1.5/1.47s are the next, then I think the 1.8 are followed by the 1.63 and the the 1.9s or ratio around there. I say stay with the 1.5 and prop it correctly.

Re: Quest for speed... gear ratio/prop.
 

I have to agree with tomcat on this one. It seems to me that you're changing the drive ratio, but then the prop change cancels it out, and you're right back where you started. Also, why would you want to spin a 383 so high? Did you build it for that? Like was said, get a good 21" prop to get a baseline, and try the 19" also. I have a Liberator, and I know your hull is different. Slightly different, but different none the less. My brother has the cuddy, and he just can't get the speed, or carry the bow like I can. He also got similar slip numbers to yours. Mine are much less. I have the OMC Cobra drive, and my boat always seemed to like the Raker prop over the SST II, or the Viper. I don't know what these equate to in a merc prop, but you might want to find something similar to the OMC Raker design. That always seemed to get the bow up for me. My brother's boat seems to like the SST II. There's plenty of prop guys on this board, ask them.

Paul

Re: Quest for speed... gear ratio/prop.
 

I wouldn't change to a lower (numerically higer gear ratio). First of all the lower gears are weaker because they have less tooth contact, also putting a larger pitch prop would put more strain on the drive anyway and the lower gears would run a little hotter. THe final ratio is going to end up the same or close so it seems like a lot of work for nothing.