Re: Autoprop vs Maxprop and specs of each


Jose Venegas
 

Great story and inspiring to all of us close to making the jump.  I will have plenty of questions about your trip after I watch your videos.

As to the prop, there are three main differences between the Auto and the Max, all affecting performance (at least in theory). 
 I) Autoprop has beautifully shaped blades with an angle of attack that varies radially and becomes smaller as you move away from the shaft. This "twist" is key to increase efficiency and avoid cavitation as it accommodates the changes in relative velocity of the blade with respect to the water that increases with the radius.   In contrast, the blades of the Max prop have no twit; they are flat.  Of course, this reduces the drag when the engine is off compared with the Autoprop.  
2) Autoprop has blades that adjust the angle of attack depending on the speed of the boat.  This maintains thrust during motor sailing even as the wind speed increases.  The auto prop blades have a constant angle of attack and, at constant RPM, the thrust decreases as the speed of the boat increases.  In fact, at a certain boat speed, the thurst becomes zero as the prop just cuts through the water.
3) Depending on where it happens, growth on an Autoprop blade is prone to create much more imbalance and vibration than one equivalent on the Max.

 Although these are theoretical reasons based on fluid mechanics, I don't know the extent to which they affect the fuel consumption or boat speed since I have not made the measurement.  I think the increased drag of the Autoprop during sailing should not be noticeable but the difference in efficiency may be more critical in long passages if motoring or motor-sailing is called for.   So, personally, I prefer the Autoprop for our boat because we tend to motor or motor-sail more than I used to in my previous coastal sailing boat.

Jose Venegas
Ipanema SM 2K 178
Boston









 

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