Squeezing more RM out of keel with controllable flap?

Discussion in 'Boat Design' started by pironiero, Jul 30, 2021.

  1. pironiero
    Joined: Apr 2020
    Posts: 208
    Likes: 13, Points: 18
    Location: saint-petersburg, Russia

    pironiero Senior Member

  2. sharpii2
    Joined: May 2004
    Posts: 2,092
    Likes: 229, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    This idea was tried in 1980's I believe by Gary Hoyt. A very deep keel was fitted that was about as deep as the boat was long. A flap was added near the bottom, that acted like an aileron on an airplane wing. I don't remember if it worked. But the draft of the keel made it quite impractical even if it did.

    A major problem with this idea is that, in order to get this dynamic righting moment, you need to push the bottom of the keel to leeward.

    This effectively cancels out the windward lift on an equal amount of keel area above the flap area.

    This is probably why the keel had to be around 3 times deeper than normal. It makes sense. Only about 1/3rd of the keel area is actually preventing leeway.
     
  3. pironiero
    Joined: Apr 2020
    Posts: 208
    Likes: 13, Points: 18
    Location: saint-petersburg, Russia

    pironiero Senior Member

    why not down?
     
  4. sharpii2
    Joined: May 2004
    Posts: 2,092
    Likes: 229, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Because, as the wind acts on the sails to create lift, it also pushes the boat sideways in a down wind direction. The amount of this it succeeds at is called leeway.

    A boat with no keel or 'board will make so much leeway that it will only be able to sail down wins.

    What the keel or 'board does is to create windward (upwind) lift to counter act leeway as it moves through the water.

    The reason sailboats heel (tilt down wind) is because the center of lift of the sails is far above the center of lift of the keel or 'board.

    Since each is trying to push the boat in an opposite direction (the sails to leeward and the keel to windward), this creates a twisting moment, which causes the boat to heel.

    Now, your original question was, as I understand it, was whether or not putting a trailing edge flap on the bottom of the keel would make the boat sail more upright.

    The answer is yes. But, in order to do this, it has to actually push the bottom of the keel to leeward to make it go down.

    Now this partially defeats the purpose of the keel. How partially it does so depends on what portion of the up and down span of the keel is subject to the flap.

    If it is half or more, then net windward lift of the keel is zero. It might as well not even be there. This is because the bottom half is trying to push the boat to leeward, and the top half is trying to push it to windward.

    The solution is to make the keel so much deeper that the portion of its span that is subject to the flap is far less than the portion that isn't.

    Suppose we make the portion of the span that is not subject to the flap twice as long as the portion that is.

    Now, 1/3rd of the keel span is trying to push the boat to leeward. Another 1/3rd is counter acting that. And the final (top) 1/3rd is pushing the boat to windward.

    Does this make sense?
     

  5. wet feet
    Joined: Nov 2004
    Posts: 773
    Likes: 143, Points: 43, Legacy Rep: 124
    Location: East Anglia,England

    wet feet Senior Member

    A lengthy post that covers almost every point from sharpii2.One thing that wasn't mentioned was that a foil pulling down on the windward side would also tend to increase the wetted surface-as indeed does the lateral component of the wind on the heeled rig.We have been able to see the vast increases in the speed of IMOCA 60's since they adopted foils that lift on the leeward side which consequently decreased the wetted surface.That seems to be a more rewarding application of additional foils and has demonstrated that good structural engineering is very necessary for protrusions that yield extra speed.
     
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.