5m Ocean cruiser twin bulb keel

Discussion in 'Sailboats' started by HelgeS, Mar 20, 2018.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,648
    Likes: 236, Points: 63, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    I second the problem with the mast and keels too far aft.
    For one thing, your mast is running right through the already cramped accommodation.
    The draggy stern is also an obvious problem

    Thirdly, having those bulbs on the daggerboard is a bit of an unnecessary design style. If you have to have external weights, at least dont have them sticking out in the front to pickup seaweed and rope etc. Twin.png
     
  2. gonzo
    Joined: Aug 2002
    Posts: 13,117
    Likes: 292, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    It looks like a powerboat with a sail. I think that the overall shape is not conducive to pointing well. If you treat it like a motorsailor, then it would be OK. Motor right into the wind.
     
  3. HelgeS
    Joined: Mar 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Norway

    HelgeS Junior Member

  4. rwatson
    Joined: Aug 2007
    Posts: 5,648
    Likes: 236, Points: 63, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    It has three things for the OP to consider.

    No stern drag
    Proper relationship of mast and keels to the rudder position, and no mast thru the cabin
    No leading bulb to pick up ropes/seaweed
     
  5. rwatson
    Joined: Aug 2007
    Posts: 5,648
    Likes: 236, Points: 63, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    That third photo show the boat being made to round up because the mast and keel are too far back.
     
  6. HelgeS
    Joined: Mar 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Norway

    HelgeS Junior Member

    Small boats are sensitive to cargo and packing. I think the transom comes well up, maybe. Downwind is planned planing. Seaweed is not so much around here in Norway, but can be easily removed underway. I would like to have the bulbs in balance with the haul line attachement for easy mounting, hitting the holes with bolts. I would like to have the keels inline with the sailing course and inline with the side curvature of the boat. Something like this, maybe a bit more forward. 10-17cm.
    [​IMG]
    [​IMG]
     
  7. HelgeS
    Joined: Mar 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Norway

    HelgeS Junior Member

    Starting to make 2 x 70kg bulbkeels. For the best NACA profile I wonder if 15% or 20% makes much different drag? Max speed will be 10-12kn.
    upload_2018-10-8_9-44-35.png
    This is made with 20% width. Wetted surface is lower than the 15%width.
     
  8. tspeer
    Joined: Feb 2002
    Posts: 2,175
    Likes: 143, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    You might want to take the coordinates to the 3/2 power. Instead of y(x), use y_3d(x)=sqrt(abs( y(x) )) * y(x) /sqrt(ymax). That will give you a shape based on the 2D airfoil section that is more suited for use as an axisymmetric bulb.
     
  9. BlueBell
    Joined: May 2017
    Posts: 474
    Likes: 40, Points: 28
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I have read repeatedly that 6:1 is an optimum l/b ratio for truly submerged bulbs.
    Is this generally correct?
     
  10. sharpii2
    Joined: May 2004
    Posts: 1,812
    Likes: 80, Points: 48, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I still think your keels are too far aft.

    With single keelboats, the keel can be somewhat aft and may well need to be. This is because, as the boat heels, the drag caused by the lift the keel is creating tends to be to windward. The more the boat heels the truer this is. This causes the boat to want to round up into the wind.
    With leeboard or twin keel boats, the leeward foil is creating the most lift and, therefore the most drag. This causes the boat to want to turn away from the wind. With leeboard boats, with only the leeward board operating, the situation is worse than with twin-keel boats. This is because the windward keel is creating some lift and therefore drag too, but considerably less than the leeward one. For this reason, Leeboard boats and twin keel ones tend to have their maximum Beam further forward, so the boards/keels can be further forward too.

    I see three possible solutions to this problem.

    1.) Change the hull design by moving the maximum Beam forward. This would solve the problem. It would also increase the interior of the boat. It would increase its sail carrying power too. But it would also mean a more blunt bow. This bow would not be so good for going to windward in a chop but would be far better for sailing downwind, as it would be far less likely to stuff into the back of a wave.
    2.) Keep the maximum Beam and the keels where they are, but add a cutwater to the Bow. This cutwater adds a third lifting surface which moves effective Center of Lateral Area (CLA) further forward. The exact size and shape of this cutwater can be determined by experimenting with the model.
    3.) Keep the maximum Beam and the keels where they are, but add a centerboard to the bow. This would have the same effect as the cutwater but would be retractable. This could be a huge advantage when sailing downwind for long periods of time. The boat will tend to want to stay sailing downwind, which would put less strain on the auto-pilot or other self-steering mechanisms.
     

  11. tspeer
    Joined: Feb 2002
    Posts: 2,175
    Likes: 143, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    It depends on what your assumptions are regarding flow in the boundary layer.
     
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.