foil area for sailing yachts

Discussion in 'Hydrodynamics and Aerodynamics' started by Rich_Hamer, Sep 10, 2014.

  1. Rich_Hamer
    Joined: Nov 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    Rich_Hamer Junior Member

    In Principles of Yacht Design, Larsson & Eliasson suggest keel and rudder area should be a % of sail area - in practice using this rule of thumb tends to produce foils with what appears a generous area by modern standards. Also this ratio doesn't take into account operating speed, I.e a faster operating speed meaning a reduced area due to increased lift.

    I'd be really greatful if anyone could offer a more refined approach to foil area determination in sailing yachts?
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Using rules of thumb will get you close, but only testing (modeled or actual) will focus down the various considerations in terms of area, foil selection, plan form, etc.
     
  3. Rich_Hamer
    Joined: Nov 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    Rich_Hamer Junior Member

    Thanks for the reply, what form of modeling would you suggest? And what peramiters would you use to determine whether the area was of optimum area?
     
  4. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 140, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    size is a complex issue. if you are trying to optimize high speed performance, than smaller keels and rudders would have less drag and still supply adaquate control. but at lower speeds, or when pointing high into the wind, they will be too small and you will not be able to hold coarse.

    so it just depends on the operating envalope of your SOR. Aircraft are the same way, aerobats need to have controls large enough for stalled speeds, and even when falling backwards, to maintiain flight controls during stutnts. but an large commercial aircraft will have two sets of controls, tiny ones for high speed crusing, and larger ones for landing and take off, and even then they rarely get below about 90 mph when they land.

    same prinipals apply for a sail boat. so there is no "perfect" size, it is good for low speeds, and too much drag at high speed, or low drag at high speeds, and not enough control at low. You could solve this of course by have keels and rudders that you can adjust the size as you sail, optimizing it for each of the conditions.
     
  5. johnhazel
    Joined: Jun 2008
    Posts: 250
    Likes: 5, Points: 18, Legacy Rep: 60
    Location: Michigan

    johnhazel Senior Member

    Search for sailing polars of boats similar to your design. From that polar take the speed of the hull through the water, for example at the point of sail for best VMG windward, then estimate the sail side force, (F_side =Cl 1/2 rho V^2 area), just assume all the force is lateral for going upwind and you wont be too far off. BTW use apparent wind speed for this calculation. Now you have side force needed and velocity through the water. From there you can size the keel fin.
     
  6. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 247, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    What are the modern standards you are referring to?
     
  7. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    There are many different approaches depending on your goals and budjet

    1. Copy a good known design

    2. Use a VPP and test different foil parameters. You would want to have one that uses foil parameters and calculates leeway from them.

    3. Use CFD and tank tests to derive foil (and hull) parameters at different leeway, heel and velocity and then apply them to VPP

    4. Use polars from a similar boat and calculate sail forces from e.g. http://www.wb-sails.fi/Portals/209338/news/SailPowerCalc/SailPowerCalc.htm Then design foils for some reasonable leeway.

    All of these methods need a compromise between different points of sail. On a run foil area is just drag without a function. During start, after a tack or holding a line in traffic, you would like to have more foil area than on optimum beat etc.

    Also different foil sections may need different leeways. E.g. laminar foils only work well with a very small leeway.
     
  8. Rich_Hamer
    Joined: Nov 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    Rich_Hamer Junior Member

    Superb replies, many thanks for the pointers! Really helpful and very much appreciated.

    When referring to "modern standards" I was inferring that current cruising yacht designs seemingly use narrow chord, foils with less plan form area than was common 15-20 years ago.
     
  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Simply put, each sectional, plan form, etc. choice you make for any particular design, will be governed by it's ability to add or fulfill it's role in the SOR. Hypotheticals are not really practical, except in a very general sense, which is what you're receiving here. There are so many variables and considerations that are at work, that little more can be offered without some detail about the yacht and it's ability to meet it's SOR.
     
  10. philSweet
    Joined: May 2008
    Posts: 2,350
    Likes: 210, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If you compare old and new boats of equal displacement, not equal accommodations, then there may not be as big a shift as you imagine. Foil area is related to sail area; and sail area is related to displacement. Cruisers have been getting lighter for a given size. Lighter more so than faster.

    As far as steady conditions are concerned, the keel size and shape is related to righting moment, rig size, and how much heel you want to endure. But the real issues stem from maneuvering and gust response and seakeeping. These get harder to manage as speed under sail gets slower, hence you turn the key and start the motor, or if not, you need a bigger keel and rudder.

    So you have two trends - lighter hulls with greater windage and improved auxiliary propulsion. Lighter suggests less keel. More windage suggests more keel when speed is low. Better motors mean that part two isn't really a problem.

    If you are willing to state in advance that you are going to do all close maneuvering under power and will cruise under power any time you can't maintain, say, 4.5 knots under sail, then you can reduce the foil sizes quite a bit from that used when yacht motors were not as refined as they are today.
     
  11. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    How do you define size? Same LOA, LWL or something else? For the same LOA boats have certainly not been getting lighter. For the same LWL maybe, but there are many quite heavy Cruisers.

    E.g. Hanse 345 has measured values of LOA 9.99 m, LWL 9.67 m, Displacement 7255 kg and main + jib 55 m2. Still it's keel is quite small compared to typical ones from 70's and 80's.
     
  12. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There's no doubt that sailboats, as a general rule have gotten lighter for their length, no mater which it might be LWL, LOD, LOA, etc. Proportionately, appendages have gotten smaller too, which has been a trend for a least half a century now, as techniques and material choice have permitted stronger lighter structures. The same is true of ballast/displacement ratios, SA/D ratios, D/L ratios, etc. Yes, you can find exceptions to these trends, but (again) generally, a 40' yacht has a longer LWL, typically is beamier, carries considerably more ballast and still is lighter then, say a CCA class of the 50's - 60's or even the IOR's of the 70's. Back then a 250 - 270 D/L was considered modest, if not on the lighter side for a performance oriented ocean cruiser, but today 160 - 190 would be the norm, even with the extra beam and freeboard newer hull forms have taken on. The Hanse 345 would be a good example, being a quite fat boat, having a 200 D/L and a 2.98 beam/length ratio and 6 tons on her lines. My old Lion class CCA yawl was just a few inches longer, carried only 8' 9" of beam on a 30% shorter LWL, displaced 6.3 tons and healthy 400 D/L. There's just no debate in this regard about modern craft, even if you only go back a decade, you'll see significant differences. Of course they will not be as dramatic as my 50 year comparison, but they'll still exist.
     
  13. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    Typical modern 40' Cruisers like Bavaria, Jeanneau Oceanis, Hanse, Delphia etc. have a displacement of 8-9 tons, ballast of 2-3 tons, LWL of 10-11 m and beam of 4 m. I don't know much about CCA, since we don't have them around here. Maybe S&S Swan 40 is a 70's IOR reprensantive. It's displacement is 8.6 tons, ballast 3.6 tons, LWL 8.7 m and beam 3.3 m. So not heavier (for its LOA) and still clearly more ballast. Much shorter LWL and much narrower.

    I don't think there are many older cruising boats, which has only 25-35% ballast ratio, which is typical for modern cruisers.

    Different generations of Hallberg-Rassy might be an interesting comparison:

    1975 HR-41: Displacement 9900 kg, ballast 3800 kg, LWL ?, beam 3.6 m

    1980 HR-42: Displacement 11 500kg, ballast 4500 kg, LWL ?, beam 3.78 m

    1991 HR-42: Displacement 12 500 kg, ballast 4700 kg, LWL 10.35 m, beam 3.95 m

    1991 HR-39: Dipslacement 10 000 kg, ballast 4200 kg, LWL 9.85 m, beam 3.76 m

    Current HR-40: Displacement 10 000 kg, ballast 4100 kg, LWL 11.04 m, beam 3.82 m.

    For HR, there seems to be no trend for displacement nor for ballast. LWL gets longer and there is a slight increse in beam.
     
  14. philSweet
    Joined: May 2008
    Posts: 2,350
    Likes: 210, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Okay, Joakim, but how have the foil areas, sail area, and hp changed on HR's boats?

    The point I was trying to make was that we needed to better define which old boats we were comparing to which new boats. That will effect the strength of the trend and its probable causes. Are we just going to compare a slice of the market, or are we going to compare boats with similar SOR's?
     

  15. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    HR boats have had quite the same SOR. Sail area is about the same (75-89 m2 for the models I listed, no clear trend). I don't have any foil area details, but visually the current model has much smaller foil area compared to older models.
     
Loading...
Similar Threads
  1. RumnCoke
    Replies:
    21
    Views:
    2,710
  2. TyphoonUGent
    Replies:
    3
    Views:
    274
  3. Dave_S
    Replies:
    3
    Views:
    394
  4. revintage
    Replies:
    3
    Views:
    1,346
  5. revintage
    Replies:
    11
    Views:
    1,107
  6. Niclas Vestman
    Replies:
    25
    Views:
    1,629
  7. Iridian
    Replies:
    11
    Views:
    1,258
  8. Slingshot
    Replies:
    16
    Views:
    2,390
  9. jmf11
    Replies:
    3
    Views:
    970
  10. Kenneth Dodd
    Replies:
    26
    Views:
    2,340
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.