Logo
FULL SITE MENU

Duo Prop Gear Ratio Change

E

evantful

New member
Howdy, I have a corroded outdrive and that I am looking to replace. It's a 2009 DPS-A 2.14 Ratio with F5 props.
The boat is a 2009 Monterey 260 SCR Cruiser, roughly 6800lbs dry with a VP 5.7GXI-K engine. WOT is roughly 4900-4950rpm. Spec'ed range is 4800-5200rpm

I have been able to find some very discounted, brand new, DPS-D 2.32 ratio outdrives.

Curious if this ratio change, with the approbate set of props to get it back in the RPM range will be a deal breaker. Thinking I would need to jump up to the FH6 prop or FH7 set? Greatly appreciate insight
 
Generally the gear ratio is specific to the inherent characteristics associated with the number of cylinders of the engine or it's technology, gas vs diesel.
 
Understood, though there seems to be a bit of variability.

Just as an exercise, if I switch from a 2.14 ratio, which was achieving 4900rpm~ WOT to the 2.32 ratio with the same props, what might the RPM increase too?
 
evantful said:
Understood, though there seems to be a bit of variability.

Just as an exercise, if I switch from a 2.14 ratio, which was achieving 4900rpm~ WOT to the 2.32 ratio with the same props, what might the RPM increase too?
Click to expand...
Not necessarily 1:1 but if you do the math backwards 4900 rpm/2.14 ratio = prop shaft speed of 2290 rpm

2290 RPM * 2.32 ratio = 5312 rpm

So yes if you were happy with previous performance (assuming you are as 4900 is essentially perfect WOT RPM) you probably want to go up in pitch a step.

As an example I repowered by current boat (21 ft BR) that was originally a 4.3/ 2bbl carb/1.81 outdrive ratio to a 5.0 260 hp, that from the factory should have come with a 1.62 outdrive ratio. Kept the 1.81 ratio and went from 19" to 21 pitch prop. works great just nips the limiter at WOT.

From that it seemed to me that the drive ratios steps are roughly equivalent to the 2" pitch steps (within reason). This allows the manufacturers to use the ratio to keep most hull/driveline set ups using the 'normal 17"-21" pitch (or at least within 15-23" range) .

Not sure how good the deal is if you have to buy a new props set (like$1500 or more?) but would be worth trying in my opinion. This is especially true if you wanted more holeshot than the original setup as the steeper ratio will move you that way.
 
Update: Dieter, based on your info and a good long conversation with a retired Mercruiser Engineer, I felt comfortable enough that it was worth a shot. I wanted to update this so anyone looking in the future would have some good info.

The Economics: I could buy a new-old stock DPS-B 2.32 outdrive for $4500 or purchase a drive that matched my current 2.14 Ratio for $9500-$10500. In both cases I would need new props, my DPS-A drive was the final drive Volvo used straight splines on, before switching to helical splines on the DPS-B and newer drives.

I ended up buying the 2.32 Ratio drive, and moved up one 'Volvo Pitch' in prop-sets. Original 2.14 Ratio Drive came with F5's Props, my new 2.32 Ratio Drive I paired with FH6 props. Props cost $2000 brand new

**I performed all the labor myself.

The Result: Took the boat out yesterday for the first time, ran it for nearly 2 hours. The results were as good as I could have hoped. No noticeable difference in capability. Climbs right out of the hole with ease, WOT RPM climbs to 4950-5000rpm. Top speed seemed unchanged. Very happy with the results.

Overall investment of $6500 for a brand new drive and props. Will sell my old drive for parts, and sell the F5 prop set. Hoping to recoup $1000-$2000

Thanks again.
 
Back
Top