Variable DIAMETER ? Prop

Discussion in 'Props' started by FAST FRED, Mar 2, 2010.

  1. FAST FRED
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    FAST FRED Senior Member

    I know the folks on this board have resources of information that are vast.

    So has anyone ever seen a variable diameter prop?

    Changing pitch is fine for minor differences in propulsion , but an engine sized for flank at 2100rpm is never going to be efficient when throttled back to 1000rpm for long range cruise.

    My simple solution ( But really pri$y) is a ZF 2 speed tranny , speed up the shaft speed at LRC to increase the load .

    The graphs say it will work ,(and its a simple purchase) but changing diameter would work even better , but I have never heard of the variable diameter concept.

    FF
     
  2. marshmat
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    marshmat Senior Member

    The only reason those transmissions are so pricey is because they're not all that common. If Hyundai or GM can build a computer controlled, five speed F/N/R gearbox to transmit 200 hp for a thousand bucks (they cost a LOT less for the OEM than they do as spare parts) and have it last a quarter million kilometres before needing any work, the only thing holding marine gear prices high is the economies-of-scale problem.

    I have no idea how one would go about making a variable diameter prop, but I'm curious to hear what everyone thinks of the idea.....
     
  3. yipster
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    yipster designer

    keeo me posted, to give it a start, a 4 blade retractable :D
     
  4. tom kane
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    tom kane Senior Member

    A trimable shaft drive gives the effect of variable propeller diameter.
     
  5. FAST FRED
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    FAST FRED Senior Member

    A trimable drive would work,

    but I'm looking at an Atkin Box Keel and reverse deadrise to get really efficient at SL 2 to 3 .

    Over that plaining makes more sense .

    I want the beachability , and the difference in power required for SL2.8 is too far from lo cruse SL.8 for an engine to be efficient.

    FF
     
  6. marshmat
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    marshmat Senior Member

    In this case- you want to use the same engine at speed/length 0.8 as at 3, and you want it properly loaded at both speeds- a two speed gearbox seems appropriate. Trimming a surface drive gives a rough approximation of "variable diameter" but, short of something like that Yellowfin drive that was being talked about a while back (a huge surface prop whose blades could individually change pitch and rake under computer control), we don't really have the technology to change the shape and size of propeller blades.
     
  7. apex1

    apex1 Guest

  8. FAST FRED
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    FAST FRED Senior Member

    The engine for a cruise at about 18K will require to be Rated at close to 140hp,, and be expected to produce at least 100 -120hp on a cont 24/7 basis. 18K

    In slow cruise 10 hp or so will be required.

    I don't believe a mere pitch change will allow a prop that absorbes 120hp 2100rpm efficiently will be anywhere efficient at 1000rpm and load the engine well.

    The pitch required would surely stall , so I'm back to a CPP and 2 speed tranny.

    CPP seems a great tool in a limited range but when the difference between cruise regimes is 10-1 it's asking for a lot.

    FF
     
  9. apex1

    apex1 Guest

    Well 10:1 is a lot (and uncommon) yes. 5:1 or 6:1 are the average application ratios.
    I.e. a Troller doing 1-2 kn trolling speed at around 100 of it´s 600hp (which is of course too much power for that speed, if there is no line to tow) and 12kn wot.

    But the discussion is very theoretic, because the common Diesel has a ratio of about 5:1 between idle and wot, so the CPP installation is able to cover that in any case, and a gearbox does´nt do it better!

    Regards
    Richard
     
  10. powerabout
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    powerabout Senior Member

    So where's the inventor with the variable diameter prop drawings?
    I can see it now, some kind of mechanism to rotate skewed blades about a pivot point, may as well make it cpp as well


    PS when people say 'efficient' do they mean the engines BSFC or nm/g?
     
  11. TollyWally
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    TollyWally Senior Member

    "when people say 'efficient' do they mean the engines BSFC or nm/g?"

    I'm guessing most people think knot/gal rather than specific fuel consumption. Like I said just guessing but in my experience bsfc is more a motorhead tuning topic.
     
  12. apex1

    apex1 Guest


    BSFC is meant, what else? It would be a bit idiotic to calculate MPG for a trolling boat (for example).
     
  13. powerabout
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    powerabout Senior Member

    I was a bit confusd by FF's post that if your engine does 2100 it wont be efficient at 1000?
     
  14. FAST FRED
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    FAST FRED Senior Member

    I was a bit confusd by FF's post that if your engine does 2100 it wont be efficient at 1000?

    Most engine mfg desire a prop that will allow the engine to reach top revs as as soon as the throttle is pulled back the load deminishes at a rapid rate.

    The mfg are interested in not having warentee problems , not in long life or your fuel use.

    If you look at a "prop graph) and compare it to the power curve of actual power at all the RPM you will see the engine is VERY underloaded at most cruise rpms.

    The CPP is great at loading say a 2100max rpm engine at 1800 or even 1600 , but there comes a point where just twisting in pitch doesn't work as the prop stalls.

    Most engines have a minimum rpm at which the can be loaded , however at low rpm the engine can stand its full (for that particular rpm) load , although most folks use the 80% rule as it is more efficient than the perhaps 15% the prop would load the engine.

    The torque peak is frequently the max efficient place for fuel/ hp.

    So the desire is to work out a way to load the engine at the torque peak and below .

    Seems a two speed tranny , shifting the shaft to a higher speed when operating at low rpm LRC is about the only way.

    Ricks concepts of a huge prop turned with a very deep gear reduction is great , till the reality of the vessels draft is cranked into the design.

    Also after about 24 inches prop costs soar !

    FF
     

  15. PetterM
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    PetterM Senior Member

    The Yellowfin VSD surface drive (www.yellowfin.com) had variable rake and variable pitch. The variable rake did alter the diameter.


     
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