Does lifting the keel going downwind makes noticeable difference?

Discussion in 'Boat Design' started by pironiero, Nov 3, 2020.

  1. gonzo
    Joined: Aug 2002
    Posts: 15,773
    Likes: 1,200, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Writing all in caps means you are screaming at a member that actually gave you good information. That is really rude. Further, there is no perfect hull or conditions. Hulls are designed to be as optimal as possible for certain conditions. You need to do the work and provide which conditions those are. If you only want downwind performance, a bulb keel may be unnecessary. A planing multihull with no keel at all is probably a good choice.
     
  2. pironiero
    Joined: Apr 2020
    Posts: 219
    Likes: 16, Points: 18
    Location: Estonia

    pironiero Senior Member

    Yes, I'm sorry for that but he gave me good information about other topic, I asked about reducing the wetted area by lifting the keel and he started to tell about other stuff, I got annoyed too fast because I thought that he is playing with me...

    Also, you and many others here talking about overall performance and i just wanted to know about reducing wetted area by lifting the keel, going downwind in hypothetical perfect conditions and hypothetical perfect hull
     
  3. pironiero
    Joined: Apr 2020
    Posts: 219
    Likes: 16, Points: 18
    Location: Estonia

    pironiero Senior Member

    I'm so sorry, I believe you misunderstood me, i was talking about daggerboard lifting keel, not swinging one... like in Seaward 46RK and whatnot
     
  4. gonzo
    Joined: Aug 2002
    Posts: 15,773
    Likes: 1,200, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Reducing wetted area will reduce hull resistance. However, raising the CG may make the boat capsize. The answer is that it depends on the overall design and conditions. Maybe you can start by defining what a perfect hull is. I would think that it would be one that attains high speeds at any point of sail with the keel up or down.
     
  5. pironiero
    Joined: Apr 2020
    Posts: 219
    Likes: 16, Points: 18
    Location: Estonia

    pironiero Senior Member

    Alright, lets take lift 40 hull with carbon mast and synthetic rigging
     
  6. Will Gilmore
    Joined: Aug 2017
    Posts: 709
    Likes: 294, Points: 63
    Location: Littleton, nh

    Will Gilmore Senior Member

    In the case of running dead down wind, raising the keel may actually keep a boat from stuffing the bow. A bulb keel that is too deep would represent lowering the resistance, thereby creating a much longer lever arm between the driving force and the resisting forces.

    -Will (Dragonfly)
     
    Last edited: Nov 5, 2020
  7. BlueBell
    Joined: May 2017
    Posts: 1,977
    Likes: 586, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . . .

    What do you mean by "noticeable".
    On a GPS (Global Positioning System) there is a good chance you'll see a difference.
    Do the math: wetted surface area, linear drag vs speed ratio.
    Lift the keel and tell us if you "notice" a difference.
    Let us know your experience, hypothetically speaking of course..
     
  8. gonzo
    Joined: Aug 2002
    Posts: 15,773
    Likes: 1,200, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    What is a lift 40? I couldn't find a reference to those boats.
     
  9. pironiero
    Joined: Apr 2020
    Posts: 219
    Likes: 16, Points: 18
    Location: Estonia

    pironiero Senior Member

     
  10. Dolfiman
    Joined: Aug 2017
    Posts: 1,443
    Likes: 626, Points: 113
    Location: France

    Dolfiman Senior Member

    In a VPP approach , to lift the keel has 4 consequences :
    - to increase the induced drag (within a VPP, that means a poor lateral resistance)
    - to reduce the heeling arm ,
    - to reduce the righting moment available,
    - to decrease the wetted surface,
    As you can see, 2 are in defavor of speed and 2 are in favor. As it was already said, it depends of the sailboat input data and of course of the true wind angle considered. Side to speed, there is also a consequence on heel angle.

    The Lift 40 has no lifting keel to my knowledge, it is the design winner of the last Route du Rhum in Class 40 with Yoann Richomme as skipper.
     
  11. bajansailor
    Joined: Oct 2007
    Posts: 2,663
    Likes: 989, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Just a little story about a lifting keel boat....
    Some pals of mine purchased a 42' Maracuja sailing yacht in France 31 years ago, and sailed her out to the Caribbean - she is similar to the yacht shown in this link.
    MARACUJA 42-1993 https://www.ayc-yachtbroker.com/maracuja-42-0
    She has a 'swing keel', and she also has a trimming daggerboard between the keel and the rudder.
    When they left the Canaries, they set twin genoas, boomed out wing and wing, wound the keel up, probably did some fine tuning with the daggerboard, and surfed their way across the Atlantic in 14 days, averaging a steady 8 knots / 200 miles a day.
     
    BlueBell likes this.
  12. MST3000
    Joined: Mar 2022
    Posts: 6
    Likes: 0, Points: 1
    Location: Caribbean

    MST3000 Junior Member

    Yes lifting a daggerboard type keel, vastly improves performance, drag goes down!

    Also, broaching is really, really slow!
     
  13. Will Gilmore
    Joined: Aug 2017
    Posts: 709
    Likes: 294, Points: 63
    Location: Littleton, nh

    Will Gilmore Senior Member

    Why is broaching slower?
     
  14. wet feet
    Joined: Nov 2004
    Posts: 867
    Likes: 175, Points: 43, Legacy Rep: 124
    Location: East Anglia,England

    wet feet Senior Member

    If you consider the topic in purely racing terms,having the boat seriously heeled and pointing in the wrong direction won't get you to the finish as quickly as sailing with the maximum attainable performance.The actual process of broaching may or may not be taking a similar time with the keel position altered.
     

  15. MST3000
    Joined: Mar 2022
    Posts: 6
    Likes: 0, Points: 1
    Location: Caribbean

    MST3000 Junior Member

    The way I see it, as speed goes from single digit numbers upwind to double digit numbers downwind, the required keel area to maintain "normal" leeway will also go way down (to around 1/3 to 1/4 area?). More importantly at double digit speeds, the induced drag associated with the lift of the keel will become equal to or greater than all of the drag(s) of the hull combined. (Not to mention the large rudder angle needed to counter this lift). Thus in the lifted keel arrangement, with the induced drag going way down, this should have the largest effect on speed.

    Also as the speed of the boat increases closer to windspeed downwind, the required righting moment will also go way down. eg the lack of winches for the spinnaker sheets on Melges24 etc. vs say a J24 which remains at single digit speeds downwind. So I'm suggesting a M24 planning downwind at speed, has less spinnaker sheet load than a J24 in similar wind but not planning downwind. Thus if a boat is able to greatly increase speed by lifting its keel, then it will also experience a reduction in spinnaker sheet load and hence a reduction in required righting-moment.

    It also needs saying; lifting the keel on a J24 or similar hull-form (L/disp) will have little effect, but I'm assuming no one wants to do that anyway.
     
Loading...
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.