Aspect Ratio vs Foil Area

Discussion in 'Sailboats' started by Johny Blaze, Apr 3, 2020.

  1. Johny Blaze
    Joined: Apr 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Australia

    Johny Blaze New Member

    Hi All,
    I currently have a bulb keel that has a foil size of 1100mm deep by 375mm chord. It is a Naca 0012 foil.

    It travels upwind at approx 5kts.

    I’m wanting to increase draft to increase righting moment. I’m wanting to increase the foil length to 1400mm. Can I decrease the total foil area as presumably the higher aspect foil will be more efficient?

    Thanks for any advice.
    Cheers
    JB
     
  2. tspeer
    Joined: Feb 2002
    Posts: 2,285
    Likes: 252, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I wouldn't decrease the foil area. The foil still has to produce the same side force, because the side force is dictated by the sail rig. The longer foil will have less drag due to side force, but decreasing the area will make it more prone to stalling out at low speed.

    The other consideration is thickness. Keeping the same physical thickness and planform area will mean increasing the thickness ratio to 15.3%, and you might need to make it thicker to handle the additional bending loads. If you decrease the chord even more, the thickness ratio could get excessively large.
     
  3. Johny Blaze
    Joined: Apr 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Australia

    Johny Blaze New Member

    Thank you for your response.
    I’ll stick with the same total area as you suggest.

    I was planning to keep the same 12% thickness.

    Cheers
    JB
     
  4. tspeer
    Joined: Feb 2002
    Posts: 2,285
    Likes: 252, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    If you extend the span from 1100 mm to 1400 mm and keep the same planform area, your chord will be 295 mm. If the keel has a 12% thickness ratio, its physical thickness will shrink from 45 mm to 35.4 mm. Given the increased righting moment, are you still confident of the structural integrity of the thinner blade? That's a big increase in the stress on the keel.
     
  5. Johny Blaze
    Joined: Apr 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Australia

    Johny Blaze New Member

    Hi,
    Yeah, it’s just a day sailer with a relatively small bulb (100kg)
    I’ve checked the engineering and mechanically it will be fine.
    The keel will have a steel spar.

    Thanks
     

  6. Jasper51
    Joined: May 2020
    Posts: 1
    Likes: 1, Points: 1
    Location: Biscay gulf

    Jasper51 New Member

    Hi all, I´m brand new here.

    I´ve been reading the forum quite often and finally registered as I have a doubt about rudders.

    Correct analysys about thickness reduction but: By makink the keel deeper, the bulb´s weigh may be reduced for the same righting moment (as arm is now longer)?

    So, an analysys should be done -IMO-: Redrawing keel for same lateral area and righting moment. This will give a thinner chord. THEN checking keel´s wing resistance for new conditions... after this, maybe a second turn or new decissions may be requiered ...? (maybe reducing flex moment and righting moment to keep same cord/ length ratio ? maybe increasing chord ?

    (fast personal view...) Cheers,
     
    DogCavalry likes this.
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