High Altitude Propeller

[ebykowsky]

Looking for bass boats online, I found one in Denver with a "high altitude prop". Does the density of the water at high altitude change enough to warrant a prop with a different blade angle, or is this based on the fact that engine air intake changes with altitude, and without a "mixture" setting on boats, there's a need to change the prop RPM in order to compensate for a lower oxygen intake by the engine?

 

[Capt. Geoffrey Thorpe]

Probably a lower pitch prop to compensate for the reduced power available. But, that's just a guess.

 

[gismo]

Probably a lower pitch prop to compensate for the reduced power available. But, that's just a guess.

Good guess. Boat propellers (virtually all are fixed pitch) have to be sized to match the engine's HP/RPM/Torque curves with the boat's drag/speed curve. There's some room to optimize for max speed and efficiency vs low end thrust (e.g. for pulling skiers) but the 15-20% reduction in power at 5000-6000 ft can require a similar reduction in pitch for the desired optimum.

To the OP, water density does not vary with altitude (at least not until you get so high that it boils). Temperature does affect water density but nowhere near as much as it does air (or any gas). And water has the fairly unique property that it's density actually goes down as the temperature drops (when the temp is near freezing). If that weren't true most bodies of water wouldn't freeze in the winter.

 

[ebykowsky]

And water has the fairly unique property that it's density actually goes down as the temperature drops (when the temp is near freezing). If that weren't true most bodies of water wouldn't freeze in the winter.

Point well taken, but the last statement is false; water increases in density as temperature approaches freezing, and is most dense around 4*C. Unless you're talking about ice, in which case I don't think a boat prop will do much

Edit: Unless you're talking about density decreasing as temp decreases in that 4*C to 0*C window.

 

[gismo]

Edit: Unless you're talking about density decreasing as temp decreases in that 4*C to 0*C window.

Exactly. That's why I said "when near freezing".

 

[Graueradler]

Cavitation will start earlier because of the lower air pressure. I expect that the prop addresses reduced HP but it might also do something to reduce cavitation

 

[poadeleted20]

Usually, those lower pitch props are called "climb" props rather than "high altitude" props, and this contrasts with a higher pitched "cruise" prop. For the Grumman AA-5/5A, the prop pitches are 57 inches for "climb" and 59 inches for "cruise". As noted above, the lower pitch has lower rotational drag and allows the engine to spin faster at full throttle, creating more power. The downside is that since it spins faster, you'll hit redline sooner in cruise, and may be limited to significantly less than 75% power at medium altitudes, which limits your top cruise speed at normal cruise power.

 

[kyleb]

Looking for bass boats online, I found one in Denver with a "high altitude prop". Does the density of the water at high altitude change enough to warrant a prop with a different blade angle, or is this based on the fact that engine air intake changes with altitude, and without a "mixture" setting on boats, there's a need to change the prop RPM in order to compensate for a lower oxygen intake by the engine?

The air is thinner so the engine makes less power, requiring a lower pitched prop so the engine can deliver the appropriate RPM.

 

[Sac Arrow]

Cavitation will start earlier because of the lower air pressure. I expect that the prop addresses reduced HP but it might also do something to reduce cavitation

Very true. That's an issue when designing pumped systems.

 

[Norman]

Usually, those lower pitch props are called "climb" props rather than "high altitude" props, and this contrasts with a higher pitched "cruise" prop. For the Grumman AA-5/5A, the prop pitches are 57 inches for "climb" and 59 inches for "cruise". As noted above, the lower pitch has lower rotational drag and allows the engine to spin faster at full throttle, creating more power. The downside is that since it spins faster, you'll hit redline sooner in cruise, and may be limited to significantly less than 75% power at medium altitudes, which limits your top cruise speed at normal cruise power.

Did we read the first post?

 

[EdFred]

Did we read the first post?

No, he just likes to Romit.

 

[poadeleted20]

Did we read the first post?

Sorry -- I thought this was an aviation web site -- silly me.

 

[EdFred]

Sorry -- I thought this was an aviation web site -- silly me.

Hangar Talk: Open forum for discussion of any topic you like, aviation related or otherwise.

Maybe you'd like to chime in on the BBQ rant thread and explain how you've been flying for 40+ years in there.

 

[tmyers]

wonder if that high altitude prop comes with chrome muffler bearings too


Sent from my Nexus 7 using Tapatalk

 

[Captain]

Sorry -- I thought this was an aviation web site -- silly me.

It is. And this is Hangar Talk.

Hangar Talk Open forum for discussion of any topic you like, aviation related or otherwise (but no spin zone material, see below).

Virtually all topics in this forum are permitted - so long as they are discussed in a civil manner.

 

[VanDy]

wonder if that high altitude prop comes with chrome muffler bearings too


Sent from my Nexus 7 using Tapatalk

Just get an aeromatic, and be done with it!


-VanDy

 

[Sac Arrow]

Hangar Talk: Open forum for discussion of any topic you like, aviation related or otherwise.

Maybe you'd like to chime in on the BBQ rant thread and explain how you've been flying for 40+ years in there.

I think he'd cite the FARs to prove whether or not carnitas is BBQ.

 

[gismo]

Cavitation will start earlier because of the lower air pressure. I expect that the prop addresses reduced HP but it might also do something to reduce cavitation

Actually, IIRC cavitation in a liquid is more likely when the PP of dissolved gasses is higher, which would lead to the opposite conclusion.

 

[Henning]

Looking for bass boats online, I found one in Denver with a "high altitude prop". Does the density of the water at high altitude change enough to warrant a prop with a different blade angle, or is this based on the fact that engine air intake changes with altitude, and without a "mixture" setting on boats, there's a need to change the prop RPM in order to compensate for a lower oxygen intake by the engine?

No, the "high altitude prop" is just pitched down a few inches to deal with the reduced power.

 

[gismo]

Very true. That's an issue when designing pumped systems.

I think that's true for pumps (especially pumps that lift liquids above the supply), not so much for boat props. With a pump, the ambient pressure on the supply increases the pressure on the suction side of the pump (which would be zero if there wasn't any ambient pressure on the supply).