Same prop but different rpms and speed

[89retta]

I have a turning point 4 blade 21 pitch. On one lake I can get 4800 rpms and 50mph. Then I go to my regular lake and can only get 4100 rpms and 44mph. First lake is 900 feet above sea level and 2nd lake is 2200 feet above sea level. Could it be the difference in elevation? Thanks

 

[alldodge]

Don't see 1300 feet being the issue. This is a strange one, is it constant every time and does the rpm go back up when your back at the 900 foot lake?

 

[Teamster]

Elevation could certainly effect it.

 

[Teamster]

[No message]

 

[89retta]

Yes rpms go back up. Water is also cleaner and clear at the first lake.

 

[ondarvr]

Yes, 2200 ft will affect HP. The lake being clearer and cleaner would have no effect.

 

[alldodge]

Well guess so, but have never seen that much with only 1300 feet, 2500 feet more yes and more so as it goes higher. I would suggest running your but at the 900 lake for a couple days then pull the plugs. My though is your running on the edge of lean now, and going higher is making it show up more.

 

[Mischief Managed]

That's a 22.5% drop in produced HP. 1300 feet in altitude gain would result in a 3.9% loss in HP and torque, if the air and fuel mixture remained correct. If the mixture got richer (If the boat has a carburetor it would get richer), HP and torque would drop even further, perhaps as much as another 5%. If the torque drops enough that the engine can't spin that prop to peak HP RPM, it could explain the remaining 12-14 %. I am certain it would be faster if you could prop it for 4800 RPM at altitude.

 

[ondarvr]

My though is your running on the edge of lean now, and going higher is making it show up more.

Altitude makes it run richer, so if it was lean at 900 it would be closer to correct 2,200, but would still be down on power.


We do 99% of our boating here at sea level, so when we go higher we notice it a great deal, I can detect a difference at 1,500' or so, if we go higher we will switch props or live with it if we only fish there short term.

Not all boats/motors are affected in the same way, some deal with it better than others, so there are no carved in stone rules as to how each motor will be affected.

 

[Sea Rider]

Need to bump up + 700 revs. A 19 pitch would be much better when running at 220 ft of elevation. But impossible to tell if will achieve + 400 wot revs going 2 less in pitch or more, worst at altitude. A water trial will tell for sure. If still not at 4800 revs, will need less pitch.

Happy Boating.

 

[89retta]

I have a 4.3 GXI Volvo. So it is fuel injected. Sometimes I switch to a 17 pitch 4 blade at the second lake when doing water sports. With that prop I will hit the rev limiter with a light load.

 

[Texasmark]

Weather is a factor. If you are at elevation, besides being thinner, it may also be dryer which would add to your woes. That's where turbos come into play....they cram the cylinder with air, don't rely on the piston suction and environment.