unstayed mizzen

Discussion in 'Boat Design' started by whitepointer23, Jan 24, 2015.

  1. whitepointer23

    whitepointer23 Previous Member

    Probably a silly question but has anyone ever come across a conventional stayed ketch rig that has had the mizzen converted to an unstayed rig. While I was looking at my mizzen this morning I thought how good it would be to get rid of the standing rigging which I have to get around all the time and its a pia. But I love the ketch setup apart from that clutter.
     
  2. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Yes, I designed such a rig. Actually, it started out as a completely unstayed rig for a conventional ketch design (jib. main, mizzen), a 49'er designed by Craig Walters. I was asked to do the rig, but with the masts built of S-2 glass instead of carbon fiber. S-2 glass isn't near as stiff as carbon fiber, but it is stiffer than E-glass (most common type in boat building). The main mast turned out to be way too flexible, and so we ended up putting shrouds and spreaders on it to stiffen it up. But the mizzen mast was fine and we left the stays and shrouds off. The boat sailed well. Unfortunately, I don't have pictures readily to hand to show.

    Eric
     
  3. whitepointer23

    whitepointer23 Previous Member

    Thanks eric. Thats interesting to know. Because my boat is only 35 ft the mizzen stays are in the exact place you want to go to the cabin. I thought an unstayed rig would be great. Still have the ketch versatility without the obstructions.
     
  4. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    An unstayed mast will need to extend below the deck a sufficient distance to resist the moment from the sail. Also the mast partners will need to be strong enough for the side force. The side force at the deck will be greater with an unstayed mast than with a stayed mast. This is not complicated to figure out for someone familiar with statics and simple vector analysis.
     
  5. whitepointer23

    whitepointer23 Previous Member

    The mizzen has about 4ft from the deck to the base at the keel.
     
  6. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    That should likely be enough. A bury depth of 10% of mast height is a good rule of thumb to go by.

    Eric
     
  7. whitepointer23

    whitepointer23 Previous Member

    How do I work out the size an unstayed mast needs to be. Is there a formula based on sail area.
     
  8. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    No, there is no simple formula. It is an engineering problem with inputs that include the righting moment of the boat, the sail area and the center of sail area above the underwater lateral plane of the hull, the material of the mast, and the size and cross-sectional shape of the mast, as well as the mast taper, if any. From these you solve equations that lead to determining the wall thickness of the mast all along its length. That then tells you the material quantities and the weight. Then you have to do a drawing of the mast to show a builder how to build it. If it is a composite mast, then the laminate schedule has to be created with the drawings to show how to lay it up. Fittings and their attachments also have to be determined, so that involves creating an assembly drawing to show how to build or attach the fittings so that everything fits on the boat. So it is an engineering and design process that determines how big to make the mast and how to build it.

    Eric
     
  9. whitepointer23

    whitepointer23 Previous Member

    Thanks eric. Sounds like a huge challenge to get the engineering side right. If I could find a boat close to mine in dimensions with an unstayed rig would be ideal. At least it would give me a starting point.
     
  10. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    How much area is on your mizzen now, does it carry a mule and the physical dimensions of the current mizzen stick?
     
  11. whitepointer23

    whitepointer23 Previous Member

    I will measure it up tomorrow par . Whats a mule. Is it the staysail between mizzen and mainsail. If so I don't have 1.
     
  12. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    A mizzen staysail can frequently be as large or larger than the mainsail.

    Great help in a heavy cruiser in light airs.

    Easy to set as its as light as a spinnaker , but might be exciting with an unstayed mast.
     
  13. whitepointer23

    whitepointer23 Previous Member

    Mizzen sail is 22 feet . Boom is 10 feet. Would that make the sail around 120 sq ft.
     
  14. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    A mizzen stay-sail is set flying and is tacked to windward at the rail just abeam of the main mast.

    The hoist is to the top of the mizzen and it is sheeted to the end of the mizzen boom.

    BIG!! area

    But light , easy to set or strike.

    Yes it is a PIA to tack , but its mostly a reaching ,running sail , not for windward work.

    A Mule is hoisted to the main mast head , tacked to the mizzen base and sheeted to the mizzen head.

    Far far smaller , but no work when tacking.
     

  15. whitepointer23

    whitepointer23 Previous Member

    Thanks fred.
     
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