Space between mizzen mast and mainsail

Discussion in 'Hydrodynamics and Aerodynamics' started by sanantonnio, Dec 7, 2017.

  1. sanantonnio
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    sanantonnio New Member

    Good afternoon

    I currently trying to design a ketch rig. but I am have no idea of what has to be the space between mizzen mast and the mainsail.

    It is obvious that they have to be as as far as possible. This way there are no interverences between both aerodynamics and air flows. We also agree that if the gap is to small the mizzen sail become just useless windward.

    How about the minimum space in between required before having a useless mizzen ?
    Is their any kind of rules, to have an ruth idea or solve the problem for project in early stage ?

    Thanks in Advance
     
  2. gonzo
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    gonzo Senior Member

    They don't need to be as far as possible. If you want upwind performance, then a ketch is a poor choice. However, they are easy to balance and self steer compared to sloops. Once you bear off a few degrees, there is not too much interference or backwinding. The minimum space, if the main has a boom, is just enough for the boom to pass without hitting.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Canting the mizzen to windward, will improve a ketch of traditional proportions to windward. You still will not get as high as a sloop, but better then a ketch without a canting mizzen. This is a bit of a contrivance, as the choice of this rig, wouldn't be for the best windward abilities. This rig is a more casual sailing rig, though some can have more performance attributes, there's still the pointing ability thing to consider. The ketch has much to offer, to those not rounding buoys each weekend, as Gonzo has mentioned. Simply put, you can get a few more degrees into the wind, with clever design and rig complication, though to most these few extra degrees, just can by justified compaired to cost, convenience and the handiness of the rig in general.
     
  4. sanantonnio
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    sanantonnio New Member

    Thanks for your replys

    I understand that a sloop rig will always point better windward than a ketch rig. But as you also mentioned ketch rig give advantages regarding handiness and flexibility in different weather. Project regard a globetroter vessel more than a regatta one.

    In the meatime there are quite some articles mentioning useless mizzen sail passed 60 degree to the apparent winde. Even so I am not looking for to the best performance. I am still looking for proper performance windward. So I was wondering if there was any calculation / proportion rules to follow to have an efficiente mizzen.

    I am sorry but a don't understand what you mean
     
  5. gonzo
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    gonzo Senior Member

    I like ketches in that you can simply lower the main to reduce sail area and the boat will remain well balanced.
     
  6. rwatson
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    rwatson Senior Member

    I understand what you are saying from experience.

    The further back from the mainmast, the better the mizzen will generate forward thrust while beating to windward, because the flow around the front of the mizzen will be less affected by the backflow off the mainsail.

    Sails are a real compromise of aerodynamic efficiency, and it doesnt take much to destroy the lift component on a relatively small sail, hidden behind a mast, at high windward angles.

    BUT

    That has implications for the balance of the whole boat.

    You will need to decide whether the dubious advantage of two masts is worth sacrificing windward performance.
    The idea of liking the ability to lower the mainsail to "balance" the boat is strange to me. There is such a thing as reefing.
     
  7. PAR
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    PAR Yacht Designer/Builder

    I use the "jib and jigger" method frequently, as quick moving thunderstorms roll through and it's a fast and easy way to decrease area, while still having a well balanced boat.
     
  8. rwatson
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    rwatson Senior Member

    Sounds horrible, having to re-raise the main after a storm rolls through.

    If your'e expecting sudden squalls, a triple reef and a number three or four jib saves having to fight flogging mainsail booms every two or three hours, and will get your "there" faster.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Dousing the main is fast and easy. The boom doesn't flog, unless you just leave the sheet loose, which isn't wise. This is a tried and true technique for fast moving squalls, though certainly not for carrying on in heavy weather. These storms roll through very quickly, usually over 40 MPH, often well over 50. Dousing my main is a 60% reduction in area, which is significant and dramatically lowers the CE too.
     
  10. rwatson
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    rwatson Senior Member

    Maybe so - its putting it up after that I was complaining about, especially if it is a bit gusty.
     
  11. gonzo
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    gonzo Senior Member

    How do you reef without lowering the main? Also, setting a deeply reefed main and a storm jib unbalances the boat horribly. It won't point at all.
     
  12. PAR
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    PAR Yacht Designer/Builder

    After these storms roll though the weather is typically calm as hell.
     
  13. rwatson
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    rwatson Senior Member

    That's just a false assertion. Properly setup boats have their different size jibs to balance a reefed mains, so a 3rd reef and a number 3 jib will often be a balanced rig. I don't know of many boats where 3 reefs and a storm jib leads to "unbalance". Also, if you have roller reefing you can "fine tune" the foresail size. Is it a co-incidence that racing boats are ALL sloops ? No!

    Yeah, you have to lower the main a bit to reef it, but it can stay that way all day.

    Yeah, it might be calm after a gust, but then it might not, and no-one has denied the obvious thing, raising a main is pain.
     
  14. gonzo
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    gonzo Senior Member

    The reason the boat gets unbalanced, is that the center of effort of the sails gets shifted forward considerably.
     

  15. TANSL
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    TANSL Senior Member

    Gonzo's comment, as on many occasions has made me think. Do you believe, the experts, that the cases that I have drawn represents a "considerable" movement of the center of effort of the sails, that could unbalance the ship horribly? Snap47.jpg
    Perhaps this example is totally far from reality. I do not know but it can give an indication of what this "problem" really represents for a sailing boat.
     
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