Prop Selection for Altitude

[CBO]

I have searched all over and I have come up short. Can someone lead me to data that shows what size props to use in relationship to the altitude. The boat is mainly is used at 2100 feet and I currently have the F5 props.
Thank you all for your help, Jeff

[Big Block Power]

What rpm are you getting at wot?

[bruceb58]

What kind of WOT RPM are you getting now and what is your current altitude?

What model engine do you have?

[cmattj]

What is your Maximum RPM in the tachometer with the throttle all the way forward?
( see how I changed it up ).

In all seriousness your Elevation does play a major role , and Cobalt uses different outdrive gearing in the build for low and high elevation. This is done by the dealership when they order the boat for its final location.
Either sea level or 4,000 feet
.
So first you need your build or share engine size… drive ratio ( Volvo has it in the side stamped).

You most likely are just going to do a prop change and the Cobalt owners manual won’t work as the ratio in the drive is also prop changed as well.

So get back to us with above information in your set up and we can go from there.

As my experience at Firebird raceway was a .3 tenth difference in Drag race index ET. .The altitude calculations will show your down 10-15% horsepower ( 300hp is now 260 ish) so a F4 would make sense if your finding the F5 sluggish . F3 for people and water sport.

[bruceb58]

As my experience at Firebird raceway was a .3 tenth difference in Drag race index ET. .The altitude calculations will show your down 10-15% horsepower ( 300hp is now 260 ish) so a F4 would make sense if your finding the F5 sluggish . F3 for people and water sport.

General rule of thumb is 3% power loss per 1000 feet....which is approximately the amount of oxygen percent you lose every 1000 feet.

[bruceb58]

Another general rule of thumb....every 1000 feet, you drop the pitch 1/2" so for 2000 feet that is 1". Only problem is that the difference between an F4 and an F5 is 2" so if you are the high side of your WOT RPM, you may not need to change. Conversely, if you are the low side, you may need to drop to an F4.

My boat is at 6,200 feet. a 3" reduction in pitch was perfect for my boat from sea level...no gear change necessary. I have a mercruiser so was able to get props in 1" increments.

[cmattj]

As my experience at Firebird raceway was a .3 tenth difference in Drag race index ET. .The altitude calculations will show your down 10-15% horsepower ( 300hp is now 260 ish) so a F4 would make sense if your finding the F5 sluggish . F3 for people and water sport.

General rule of thumb is 3% power loss per 1000 feet....which is approximately the amount of oxygen percent you lose every 1000 feet.

Yes calculators are good to use Bruce but they lack true Density altitude measurements that include water grains and humidity to factor in. We saw this in Boise Idaho so my cuff thinking is more than your calculation.
And not knowing what the OP even has it could be worse if it’s torque is falling off.
Maybe he’ll chime in this week

[CBO]

Thank you all for the replies, looks like I need a little more information. As for the WOT, I have not done that yet in this boat. I have gone up to 4500 rpm, does anyone know what the top end should be as well as is there a governor like a car?
OSXI 5.7
DPS-B with 1.95 Gears and F5 Props

[cmattj]

Thank you all for the replies, looks like I need a little more information. As for the WOT, I have not done that yet in this boat. I have gone up to 4500 rpm, does anyone know what the top end should be as well as is there a governor like a car?
OSXI 5.7
DPS-B with 1.95 Gears and F5 Props

Thanks for the update.
That combo as it is “i “. ( osxi ) is rated 320hp and yes has a limiter of 5200 rpm.

So you may be getting good cruising speed but struggling to find 5100 ish rpm.

If you find that rpm easy after 10-20 seconds of full throttle you are fine.
If not and it’s struggling to climb you need a lower prop number ( F4 ) as discussed.

Engine heat and straining will add mechanical woes. You want to be able to touch the top end and keep less strain.

Maybe on your next outing you can give us that report along with a phone app “ GPS “ speed reading.
Oh and we love pictures!

[bruceb58]

Thank you all for the replies, looks like I need a little more information. As for the WOT, I have not done that yet in this boat. I have gone up to 4500 rpm, does anyone know what the top end should be as well as is there a governor like a car?
OSXI 5.7
DPS-B with 1.95 Gears and F5 Props

Your max engine RPM is either 5000RPM or 5200RPM. Did you get an owners manual with your boat/engine? If you don't, you can enter your serial number and get one.

[CBO]

Went out today and with two people in the boat it gets to 5200 rpm fairly quick. Don’t think I will be needing to change to the f4 props. Thank you all for the help, I definitely have a better understanding about prop selection now.

[bruceb58]

Went out today and with two people in the boat it gets to 5200 rpm fairly quick. Don’t think I will be needing to change to the f4 props. Thank you all for the help, I definitely have a better understanding about prop selection now.

Nope...you don't need to change props...you are prefect.