prop duct on a sail boat

Discussion in 'Props' started by seadago, Feb 28, 2019.

  1. seadago
    Joined: Jan 2012
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    seadago Junior Member

    Hello everyone.
    Come to the experts for some advise. I have a sailboat with a smallish engine and a badly-mannered 2-blade fixed pitch prop. The result of the combination of the two above (probably in among other factors) is that every Watt of power output from the engine counts for propulsion, particularly at higher revs, and a bad prop walk that makes steering in reverse a real PITA.
    Was wandering if installing a prop duct would help to correct either or both of those problems, apart from helping with tangle ups of debris in the prop, and potentially human safety issues as well. Details of the setup below:
    Boat: 30' LOA, 9.3' beam, long-keeled ketch, GRP
    Engine: Beta 16 HP 2-cylinder diesel rated at 3.2 k RPM, with STM gearbox with a reduction ratio of 2.1:1
    Prop: 2-blade 18X8, bronze (sizing is as recommended by engine manufacturer). In terms of choice of prop, can't fit a folding prop - not enough gap between prop hub and rudder to allow it to fold, and a feathering prop is out of my budget at the moment.
    Barn door rudder, rather large one at that, hanging from an almost vertical transom.
    Any suggestion and insight much appreciated!
    Seadago
    PD: just for context, the purpose of the exercise is to correct prop walk and increase propulsion per engine power output at higher revs. This is a sailing boat, not a motorsailer, so motoring for long distances is not a key driver. However, manoeuvring in close quarters, fighting the tide in and out of harbours and on channels, are key drivers. Thanks again.
     
  2. JamesG123
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    JamesG123 Senior Member

    A duct (or anything not fair with the hull) is going to add drag that will be detrimental when under sail. A duct probably isn't going to provide much improvement on 16hp of thrust over the drag it produces. And the prop walk is likely an artifact of the prop shaft angle (what is it BTW?) , than the interface with the rudder.

    Got any pics of offending prop?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    It may be the design per se.
    Any details of the vessel...displacement, dwg or pictures showing the location of the prop/rudder etc etc?

    EDIT
    Yippie...every time i post..i get a "watched" icon...nice to see big brother at work looking after the masses :D
     
  4. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    P.S. You can change that if you wish by clicking your username near the top right of the page and then clicking Preferences and unchecking "Automatically watch threads that you create or when you reply..." (on by default)
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  5. Ad Hoc
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    Ad Hoc Naval Architect

    What is the purpose of it?...since one receives an auto email once a reply is made?
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    Doesn't make any sense then..?
     
  6. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

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  7. seadago
    Joined: Jan 2012
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    seadago Junior Member

    Hi James.
    Thanks for your input.
    Got an old pic; attached. This is a pic of the old left-handed prop (that came withe the original Volvo MD12 fitted on the boat). The new Beta engine I installed turns the opposite way at the shaft, so had to replace prop for a right-handed one. Apart from the hand, the prop specs are the comparable. The new prop is a 14X9 RH. The boat is a Freedom 30' (G. Hoyt design), cat ketch, 82, built by Fairways Marine in the Humble, England, under licence. 26' LWL, long straight keel, with a lift centre board. A bit of a chamber pot with masts, really, but sea-kind and comfy below. Displaces just short of 6 tons. The shaft is angled from the horizontal, but not by much as per pic, and it is perpendicular to he trailing edge of the keel. In any case, the angle of the shaft is not something I can change easily.
    I understand the issue of added drag caused by a duct. Minimising drag was the main reason why I chose a 2-blade over a three-blade prop when I replaced the engine. The hull speed of the boat is only 6.4 kn, so every little helps whilst sailing.
    If a variable pitch prop could resolve the issue, I'd be prepared to bite the bullet and make the investment, but it would have to be a pretty sure thing.
    Thanks again for your views.
    Nausikaa prop picture.jpg
     
  8. gonzo
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    gonzo Senior Member

    You are describing the typical behavior of sailboats, particularly with long keels like yours. They won't steer much in reverse until they pick up speed. However, you can use prop-walk to your advantage when docking.
     

  9. seadago
    Joined: Jan 2012
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    seadago Junior Member

    hehehe.... I know! ... like reversing a trailer. It just makes manoeuvring in close quarters very laborious in any cross breeze. Will have to practice more and learn to use the walk instead of fighting it.

    Thanks to all for your input.
     
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