Practical difference between a blunt versus faired stern post

Discussion in 'Hydrodynamics and Aerodynamics' started by makobuilders, Oct 19, 2017.

  1. makobuilders
    Joined: Feb 2009
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    makobuilders Member

    My question is whether there is truly any discernable difference in drag and fuel economy between a blunt versus a faired propellor post, for a heavy displacement cruiser at 8.5 to 9.0 knots? The operable word here is "practical."
     

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

    No real practical reason to fair the post down, though some efficiency gains can be seen if it is. It depends on how much more you need. If she's faired in, the prop will eat a cleaner flow, improving efficiency, but this might be a relatively small percentage of the engine's capabilities and hardly noticeable on top speeds, which is likely more related to WL length, than prop efficiency. Typically, not a big concern on a "heavy displacement cruiser", though on a light sailboat, maybe a big difference.
     
  3. Toolate
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    Toolate Junior Member

    No science at all behind it but I have seen owners of downeast style boats in the 28-40’ range realize significant increases in speed by adding tapering wedges to the aft end of the keel but all at >displacement speeds. One owner of a 36’ boat saw almost 2kt increase in speed at WOT which is quite a bit considering WOT was 14kts. Prop blade clearance would be my concern.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I wouldn't dismiss the importance of a faired stern post so easily. The turbulent wake from the squared edge may cause vibrations and cavitation of a heavily loaded propeller, besides the already mentioned decrease in efficiency. The heavy-loading condition may happen, for example, if the prop diameter is limited and insufficient for the thrust required at max speed.
    Should that become the case, the prop vibrations might cause other mechanical issues over time (at the shaft seal, for example - but also in unsuspected remote areas of the boat) and the cavitation would almost surely cause the erosion of the propeller surface and the hull painting in the adjacent area.
     
    Last edited: Jan 10, 2018
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  5. johneck
    Joined: Nov 2011
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    johneck Senior Member

    Your question was regarding drag and efficiency, so in that regard, not a huge benefit in fairing out a 6" wide skeg at 9 kts. Base drag is on the order of 300 lbs for a squared end and likely about 1/3 of that if faired. If you are using about 200 HP to go 9 kts, this might be ~5%. I do think that you would notice the difference in vibration from the propeller, especially with a 4 bladed prop.
     

  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    John, you're math shows perfectly why, it may be a good thing to fair the post in. His goal is a 6% improvement in speed. Vibration, even at modest speeds can be felt, but more importantly will tear stuff up pretty quickly, so yeah, you'd want to make the changes necessary to smooth out the flow. Is this always done, absolutely not and many work boats and some yachts have blunt stern posts, but in all instances you can get some gains, with a good fairing job and given the performance targets, certainly an achievable goal.
     
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