Sterndrive vs Jets

Discussion in 'Jet Drives' started by JordieS, Nov 4, 2011.

  1. JordieS
    Joined: Apr 2011
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    JordieS Junior Member

    Hello everyone, I know this has been discussed very much but I haven't been able to find a clear answer.

    I have a Sterndrive boat and I also have a WaveRunner PWC (I know it's not a good comparison). People are always making claims that jet drives are more efficient than props at 25-30 knots +. I find this hard to believe though from my experience with my 2 boats and I was just wondering if jets like Hamilton etc are a whole lot more efficient than a PWC's one.

    I've compared some big boats like one from Azimut, which has surface drives and jets as an option. The jet drive has a lower speed and an extra 600L of fuel capacity IIRC.

    Thoughts/experiences anyone?

    Thanks
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Jets work better at higher speeds and 25 - 30 knots aren't higher speeds. When you start pushing 40, then you should be seriously looking at jets.
     
  3. whitepointer23

    whitepointer23 Previous Member

    jordie, we used to have a 22ft hartley with a 327 chev and 3 stage hamilton colorado which my father built. we loved the boat and the handling was brilliant but i would never accuse it of being efficient. steerage in a following sea was not good but small bilge runners fixed that. i would not want that outfit with todays fuel prices. i am talking about an old jet, i guess the new ones are a lot more efficient but i would stick with the stern drive unless i was doing shallow water work.
     
  4. Frosty

    Frosty Previous Member

    If Azimut had a jet and surface drive option but the jet was slower and carried more fuel that should tell you something.
     
  5. MechaNik
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    MechaNik Senior Member

    Even Hamilton jets concede that surface drives can be more efficient in an ideal circumstance. But jets are better at displacement changes, sea conditions changes, lower noise, less vibration, greater power density and higher service hours. That's just what I can think of now.
     
  6. JordieS
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    JordieS Junior Member

    Ok thank you everyone
     
  7. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    Well maybe that is why

    Racing hydros still use propellors ?
    Somebody with money would have switched out of the props by now.

    Thrust ??? AAAAHH :) Anything like enclosing a prop in a tube WILL increase thrust very quickly. Tugboats anyone ?:D
     
  8. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Aircraft have dumped fixed landing gear long ago but boats still stick to fixed shaft drives which cause more drag why?
     
  9. Frosty

    Frosty Previous Member

    A prop and a jet is oranges and apples .......
     
  10. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    You need to put the "25-30knot" statement in context. Since 25knots on a boat that is 5m is very different from a boat of 50m doing 25knots. Thus the Froude number is a better measure. Also, the most important aspect is the length displacement ratio.

    Short fat hulls do not perform very well with jets, not matter their speed. Whereas a high length to displacement ratio hull, ie long/slender, is ideally suited to jets. That's why most cat's have them; more efficient.

    Bottom line is, if you're in the "transition zone" (prop/jets), only you can make the decision which is best for you.
     
  11. Frosty

    Frosty Previous Member

    Ive never ever seen a power cat with jets. Im sitting on a power cat now, I pay attention to everyone I see and read up on them constantly. Never seen or heard of one.

    Your talking vast 500 ton roll on roll offs arent you?
     
  12. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    What has that got to do with it?

    You'll have to clarify your definition of a powercat. Are you referring to a generic brand name, or a known hull form? And then how does this relate to efficiencies of jets v props, since the question is about props v jets and 25-30knot claims and usages etc.


    If you're referring to a powercat as a generic brand name....er...so what??...the designer selected what s/he thought suitable. Since the assertion is that you've not seen one on a powercat ergo not suitable for any cat of said size/displacement etc. As a designer one is free to choice whatever is most suitable....doesn't mean other methods are redundant!

    If you're referring to a powercat as a known hull form, like a series 64 etc...please explain more.
     
  13. pool
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    pool Junior Member

    Here is one that springs to my mind immediately:
    Mooseboats
     
  14. Frosty

    Frosty Previous Member

    Commercial!!

    Ok --never seen a private leisure cat with jets.
     

  15. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, that surprises me for someone who says:

    when talking about Powercats (since one assumes you're talking about the brand name).

    Only took 20 seconds to find this one:
    http://www.powercats.co.uk/Powercat_15.html

    The Powercat DP15....with Kamewa FF340 water jets
     
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