Does 25% Larger mainsail = 25% more power?

Discussion in 'Multihulls' started by jedkins, Mar 31, 2009.

  1. jedkins
    Joined: Jul 2006
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    jedkins Junior Member


    I have a 6m Kurt Hughes designed tri with a 9 m mast with mainsail of 16.5 sq mtr. (Luff is 8.2 m and foot is 2.6 m) The boat is under powered for racing and barely buries an ama upwind even in 25 knots of wind with the main fully sheeted in. And helm remains very light.

    I am considering increasing the main sail size by abt 25% which can be accomplished by extending the foot and having a larger roach and by also having a big wide flat power top.

    The mast /rigging is strong enough but would this 25% extra sail area give me 25% more POWER.? Would I be wasting my money? Any expert advice from someone who truely understands the dynamics of how sails work would be very welcome.
  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    The relationship between sail surface and aerodynamic forces generated by the sail is not linear if the geometric similarity is not maintained when comparing two different sail designs. That's because other important properties are modified by extending the sail towards the leech - like aspect ratio and twist, for example.
    Maybe you could try playing a bit with this nice applet:
  3. catsketcher
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    catsketcher Senior Member

    Lower aspect helps downwind

    Going longer on the foot rarely helps upwind performance. As your boat will spend an awfully large amount of its time with the apprent forward of the beam this would not be a great way to go. You will also increase weather helm and your light helm may end up as lots of weather helm. On top of this the tacking performance will probably drop.

    It is probably best to give Kurt Hughes a line and ask him. He will probably recommend you go larger overall and keep the rig balanced or leave the thing alone so he doesn't get publicity from capsizing tris - 6 metres is very small and as stability is non linear is much more likely to capsize than a 7 or 8 metre tri. The factor of safety for your boat is probably higher than for a larger design.


    Phil Thompson
  4. bill broome
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    bill broome Senior Member

    you bury the ama in 25 kts, and you want more sail?

    you've got enough. but in lighter air, maybe not- can you hang out a bigger jib? a mast head genny?

    mb a longer foot would help a little, by reducing twist and holding the top to work.
  5. tspeer
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    tspeer Senior Member

    Do you have a screecher? They can be used as an oversized jib upwind in light air. They can be very effective in under 6 kt of boatspeed.

    You might consider a taller rig for racing. The taller rig will have less drag due to lift, and it would be a way to add area that can be reefed back to the original dimensions as the wind picks up. Plus, it will retain the balance of the existing rig.
  6. terhohalme
    Joined: Jun 2003
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    terhohalme BEng Boat Technology

    Look at multihull ratings

    A little late but...

    Multihull rating systems give good information about performance of multihulls:

    - multi2000:

    - texel: se faire jauger anglais.pdf

    The principle of mulltihull rating R is:

    R = L^0.3 * SA^0.4 / D^0.3 , where

    L is the length of waterline or hull
    SA is the sail area
    D is displacement

    So if you increase your sail area by 25 %, your speed will increase by 1.25^0.4 or about 9 % assumed your multihull has enough righting moment.
  7. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    Everyone is always referring to bigger sails. More sails but watch the mast loads.

    How about something that wil pick the boat up instead of trying to sink it. Lift means less drag and more speed.

    How about using a kite as an extra sail. It will make lift, you can adjust the lifting position to where it gives the best stability and you're not limited in size other than what you can handle.

    More wind is higher up too. Then if you want even more how about a second kite on the same line that sits below the first, kind of like adding a second engine in tandem.

    No I don't know either, but it is possible.
  8. tspeer
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    tspeer Senior Member

    Not necessarily. There's a drag penalty for the lift, too, and it can be substantial. And the drag due to lift is inversely proportional to the square of the apparent wind speed, so if you're not sailing fast the drag due to lift is even worse.
  9. Fanie
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    Fanie Fanie

    Hi Tom, you'r right, there are many variables.

    I watched a small cat screetching down the shore on one of our dams a while back, it was actually going really fast. The only problem this guy had was when his leward hull burried a certain amount it looks like he dropped the anchor out. If he could have added some lift on his rig...

    I experienced the same on a windrider. At 8 kn it buries the one amas completely and the center hull goes under as well, the spray over the windscreen makes it difficult to breathe. Adding a jib that makes lift greatly changed it, 12kn and you can still breathe.

  10. Stumble
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    Stumble Senior Member

    On almost all sailboats if you are serious about performance increases it is better to increase the mast height than increase the boom. This will give a higher profile sail resulting in a much more significant speed gain than a longer foot. You also want to increase the roach of the main as much as possible to again increase the aspect ratio of the sail. Though this may be limited by standing rigging (though a kicker is an option in this case).

    Either way you are looking at doing some pretty major changes to the boat and sinking a lot of cash into trying to redesign a rig. Instead you may just want to look for a boat with performance numbers closer to what you are looking for.
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