Re: [Amel Yacht Owners] Re: Autoprop vs Maxprop and specs of each

Colin - ex SV Island Pearl

Hi Jose

Very interesting information from you and I fully agree with all your comments. I have never had a Maxprop yet but this is the second boat on which we have had the autoprop and really like this prop for all the reasons your theory presents.

On Island Pearl II we have now done 3000 hrs on the Yanmar 75 (I absolutely love this engine too!) and 90% of those hours would have been done at revs between 1400 - 1600 at which we cruise and motor sail often, however we can actually do 9.5kts at 3400 - 3600 revs (tested both ways to ensure no current impact). I am not so sure that this speed would be possible with a fixed prop or the Max Prop or any other brand. 

This may sound ridiculous or far fetched, however I can tell you a story of how this top speed saved our lives when a large angry steel Indonesian fishing boat tried to ram and sink us in the dead of the night with just my wife and I on board, with no guns or any means of defense! The nearest yacht to us was 15nm in front, and we were probably over 100nm. from our next safe port. This boat chased us for two hours and appeared to be doing 9.3kts and was gaining on us as the seas were rough, until the wind came up hard enough that our poles out genoa and all sails up took us over 10kts. We ran like hell with all lights, radar, AIS etc off for 5 hours at this speed! In my view the autoprop is an excellent match with the 75HP Yanmar and also provides brilliant fuel consumption extending our motoring range to huge distances compared to larger motors or different props.

Best regards
Colin Streeter
SV Island Pearl II, SM#332
Cape Town, South Africa

On Thu, Oct 11, 2018 at 7:57 PM jvenegas@... [amelyachtowners] <amelyachtowners@...> wrote:

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


Colin Streeter
0411 016 445

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