CS22 with Volvo 250 SX — Cruising RPM vs Prop Pitch Advice?

[MnSeabalt]

Hi everyone,

I’ve got a 23' CS22 with the Volvo Penta 250 SX (V6), and I’m trying to dial in the most efficient cruising setup. Right now I’m running a 14.5 x 19 prop, and I’m seeing:

30 MPH at ~4000 RPM
23–25 MPH at 3200–3500 RPM
Guy at the marina told me that 4000 RPM is too high for extended cruising and I should really aim to keep it in the 3200–3500 range. But at that RPM, my speed feels pretty slow — I am hoping to cruise closer to 28–30 MPH, not 23–25.

He suggested going up in pitch, maybe to a 21P, to get better speed at lower RPMs and run more efficiently.

Does anyone have real-world experience with doing this?

What kind of RPM vs speed are you seeing with your setup?
Has anyone moved from a 19P to a 21P and seen noticeable improvements (or drawbacks)?
Any thoughts on fuel economy, planing, or pulling tubes/wakeboarding with a higher-pitch prop? (I dont care much about pulling)
Is this a straight forward DIY job to swap the prop?

Appreciate any input.Just trying to find that sweet spot with performance.

Thanks!

[NautiWeasel]

The prop should be based off of being able to hit the suggested WOT RPM (or just under). I'm guessing that the suggested max RPM for that engine is somewhere in the range of 4500 - 5000. That means if your boat can easily achieve that rpm range with a typical boat load of people (how many do you typically have onboard), then sure, you need to pitch up in prop. Then that will dictate cruising speed (how fast your boat goes in the suggested cruising RPM).
I'd look at your actual suggested wide open throttle RPM for your engine, and go from there. Choosing a prop on speed you want to be going is a little backwards of how to correctly choose your prop. You should choose it based on typical boat load, and being able to just hit max RPM at your wide open throttle.

[Matts19]

Your rpms sound about right for the gen5 Volvo V6 (and perhaps the gen 5 5.3 V8). Cruising rpms are about 1000 higher than what you got with the older engines. You should top out around 5800-5900 rpm trimmed out with a light load. My guess is that you have the correct pitch.

[jhnmdahl]

It looks like 5800 rpm is the top rpm for that engine, and I'll echo what others have said - you want to be able to just hit that rpm with the correct prop and a "normal" boat load. You'll likely also find that your most fuel-efficient cruising speed is perhaps a bit higher than with other engines, and 4000 rpm or maybe just a bit under sounds about right.

Note there will usually be two different peaks in a fuel efficiency curve - one at really low rpm (often right around 1000 rpm), and another lower peak at "cruising" rpm, often in the mid-3000s for rpm. Expect to get between 3 and 4 mpg with your boat/engine configuration at cruising speed.

[MnSeabalt]

The prop should be based off of being able to hit the suggested WOT RPM (or just under). I'm guessing that the suggested max RPM for that engine is somewhere in the range of 4500 - 5000. That means if your boat can easily achieve that rpm range with a typical boat load of people (how many do you typically have onboard), then sure, you need to pitch up in prop. Then that will dictate cruising speed (how fast your boat goes in the suggested cruising RPM).
I'd look at your actual suggested wide open throttle RPM for your engine, and go from there. Choosing a prop on speed you want to be going is a little backwards of how to correctly choose your prop. You should choose it based on typical boat load, and being able to just hit max RPM at your wide open throttle.

Interesting, thanks.

Your rpms sound about right for the gen5 Volvo V6 (and perhaps the gen 5 5.3 V8). Cruising rpms are about 1000 higher than what you got with the older engines. You should top out around 5800-5900 rpm trimmed out with a light load. My guess is that you have the correct pitch.

Oh wow. So the guy at the marina that told me 4000 was too high was actually Volvo mechanic. I am surprised he wouldn't have known cruising RPMs are higher with the newer engines. So it's possible even say 4200 might be max efficient?

It looks like 5800 rpm is the top rpm for that engine, and I'll echo what others have said - you want to be able to just hit that rpm with the correct prop and a "normal" boat load. You'll likely also find that your most fuel-efficient cruising speed is perhaps a bit higher than with other engines, and 4000 rpm or maybe just a bit under sounds about right.

Note there will usually be two different peaks in a fuel efficiency curve - one at really low rpm (often right around 1000 rpm), and another lower peak at "cruising" rpm, often in the mid-3000s for rpm. Expect to get between 3 and 4 mpg with your boat/engine configuration at cruising speed.

Awesome I'll take it for a test run and see how easy it is to hit 5800. How would you say I go about finding that "most fuel-efficient cruising speed"?

[Kate]

"Is this a straight forward DIY job to swap the prop?"

Very much so. I borrowed a 19 and a 21. Then took her on test drives to see the performance under various conditions.

Something is way off with the advice you're getting. The rule of thumb is that cruise rpm is about 80% of WOT rpm. If your WOT rpm is 5800, then cruise rpm should be around 4600.

[jhnmdahl]

Awesome I'll take it for a test run and see how easy it is to hit 5800. How would you say I go about finding that "most fuel-efficient cruising speed"?

That's not trivial to do - you'd have to chart out how many gallons per hour of fuel flow you have at various miles per hour (speeds), and do the math to make a chart. Boat reviewers do it all the time, but they install a fuel flow meter to measure, which costs hundreds of dollars. It's more complex for fuel-injected engines with a fuel return line - you have to account for returned fuel as well. Garmin makes a cheap, basic one called the GFS10 that interfaces with some of their GPS units, but it's probably not worthwhile for most boaters.

The difference also won't be huge - expect a small change, like 3.6 mpg at 3900 and 3.4 mpg at 3500. Being close is probably close enough.

[MnSeabalt]

"Is this a straight forward DIY job to swap the prop?"

Very much so. I borrowed a 19 and a 21. Then took her on test drives to see the performance under various conditions.

Something is way off with the advice you're getting. The rule of thumb is that cruise rpm is about 80% of WOT rpm. If your WOT rpm is 5800, then cruise rpm should be around 4600.

Maybe the correct answer is somewhere in the middle, so like 4200 RPM is the sweet spot?

Awesome I'll take it for a test run and see how easy it is to hit 5800. How would you say I go about finding that "most fuel-efficient cruising speed"?

That's not trivial to do - you'd have to chart out how many gallons per hour of fuel flow you have at various miles per hour (speeds), and do the math to make a chart. Boat reviewers do it all the time, but they install a fuel flow meter to measure, which costs hundreds of dollars. It's more complex for fuel-injected engines with a fuel return line - you have to account for returned fuel as well. Garmin makes a cheap, basic one called the GFS10 that interfaces with some of their GPS units, but it's probably not worthwhile for most boaters.

The difference also won't be huge - expect a small change, like 3.6 mpg at 3900 and 3.4 mpg at 3500. Being close is probably close enough.

Makes sense. Getting close/having a good idea of what it is, is definitely good enough.

[Kate]

"so like 4200 RPM is the sweet spot?"

That is about 75% of the WOT rpm, which is in the range.

The wild card here is that you have a V6. And I don't know if that WOT/cruise rpm ratio is the same for V6's.

[jhnmdahl]

Cruising rpm isn't very well defined, and can be whatever you want. If you're targeting rpm that gives the greatest fuel efficiency on plane, it can depend on the engine peak rpm, number of cylinders, engine output relative to the boat size, and a bunch of other factors. On a smaller boat with a big engine, most fuel efficient rpm may be lower, whereas it may be a fair bit higher on a bigger boat with a smaller engine despite the same peak rpm.

One way to find or estimate the most efficient cruising rpm if you don't have a fuel flow gauge is to look at sites like boattest.com or other boat reviews that show fuel efficiency curves for a wide variety of boats and engines - find your engine on a similar boat, and you'll be very close.

Here's one example on the other end of the scale - a Chapparal 307 with twin Volvo Penta 300hp V8s. The longest range and best fuel efficiency on plane (again, ignoring the second rpm and range peak near idle) are both found between 2500-3000 rpm, which most would consider ideal cruising speed - between 50 and 60% of peak rpm.

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