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Single mast vs. multiple masts

Discussion in 'Yacht Designers Discussion' started by Chapstick, Jun 23, 2013.

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  1. Matts

    Matts New Member

    Joined:
    Jan 7, 2011
    Messages:
    9
    Location:
    Stockholm, Sweden
    ONE small thing about the advantage of a ketch-rig is the risk of breaking a boom in high seas when broaching - which eventually happens..

    Twice for this guy...
    When the former owner of my present boat was to commission an new one he decided for a ketch, because then he could say with just headsails and a mizzen, with the main boom tied amidship.

    And of course the smaller loads - distributed.

    /Matts
  2. brian eiland

    brian eiland Senior Member

    Joined:
    Jul 28, 2004
    Messages:
    2,853
    Location:
    St Augustine, Fl and Thailand
    Just a few generalities here. The multimast rig is going to have more aero-drag force, so its windward ability is compromised...but ever so slightly. If one makes an honest accounting of the time he spends beating hard into the wind verses the other points of sail, particularly aboard a CrUISING vessel, he will often go for the 'split rig'. ...breaking those big sail areas of the sloop down into more managable sizes.

    A very popular way to sail a ketch in a reduced sail manner, is to reef the mainsail, or douse it totally, and then sail under headsail and mizzen sail. Boat can be made to 'balance' very well with just a little tweaking of the mizzen sail after the headsail is set to perform at its best depending on the course selected.
  3. brian eiland

    brian eiland Senior Member

    Joined:
    Jul 28, 2004
    Messages:
    2,853
    Location:
    St Augustine, Fl and Thailand
    Aft-mast Origination & Justification

    ...maybe a bit more info than you were looking for, but here is the text of a posting I made on another forum a few years ago...:rolleyes:

    Attached Files:

  4. Opcn

    Opcn Senior Member

    Joined:
    Mar 15, 2009
    Messages:
    478
    Location:
    Nordland (near Port Townsend), WA, USA
    The maltese falcon has a steel hull, and unstayed masts. M5 (Ex. Mirabella V) has stayed masts and a composite hull. Because the unstayed mast has to take all the loads itself and cannot apply them as tension forces to the shrouds (lines leading to the mast, holding it up) it must be much stouter and therefor heavier.

    Two things to be aware of are center of effort, and aspect ratio. The center of effort is how far up the mast the center of the forces from the wind is located. The more sail you have up high the higher the center of effort, and the more leverage the heeling force has to push your boat sideways (which hides your sail from the wind and reduces the amount of power that you generate, oh and can capsize you if it is strong enough). Part of why sails are reefed in a storm is to lower the center of effort, and reduce the leverage from the heeling force. A series of shorter masts will have lower centers of effort and you get less heeling in a beam reach (sailing perpendicular to the wind).

    Next there is aspect ratio to consider. You might think that one broad low sail is better, because then you get one mast with all the sail low and it gives you a low center of effort. This is partially true. However, with a broad short sail some high pressure wind spills over the top of the sail and then curves down around the back side. It forms a vortex, with the low pressure zone in the middle of a spiral of wind. This takes energy that you would otherwise be capturing to sail. The way to reduce this is with a high aspect ratio sail. A sail that is long and slender. You generate lift over the entire length of the sail, but you only generate the vortex at the top of it. A long slender sail has lots of room to generate lift, and then a narrow top so it only generates a small vortex.

    If you look at the wings of birds and compare a pheasant to an albatross you see a big difference in the aspect ratio. An albatross spends most of its time in the air, and so it needs to keep the vortexes to a minimum. It uses a long slender wing to generate lots of lift and very little vortex; the long slender wing moves the center of lift (equivalent to the center of effort) away from the body and makes it difficult to flap. A pheasant needs its wings for short bursts of acceleration to escape predators, but otherwise spends almost all of its time on the ground. The ability to flap it wings and generate that lift is more important than the ability to stay aloft for a long period of time on little energy, so the pheasant doesn't worry about vortexes (strictly speaking the albatross doesn't worry per say either).

    Multiple low sails are a compromise solution, giving you more sail area down low and less aloft, while still maintaining that aspect ratio on each sail. It costs more to build because you need to have more rigging. It takes up more deck space, and the sails can interfere with each other. But it's easier to go with less heeling force.