The definition of high-aspect changes over time.  The more time goes by, the more sails start to stretch upwards, and the average aspect ratio climbs.  Generally, if it looks more like a wing than a traditional sail, it's high-aspect.

  No one needs high-aspect sails.  I watch some catboats with sails that are essentially square do wonderfully on broad reaches and runs, better than some mulihulls (unless they whip out a screacher).

  But there are two benefits from a high aspect ratio:

  - Better performance when reaching and beating.  This is because a substantial portion of the sails drive comes not from catching the wind on the windward side, but from the lift generated as the wind flows over the leeward side.  This happens on a relatively small portion of the sail, so a sail that's twice as tall will generate almost twice the lift, while a sail with a foot twice as wide might generate negligible additional lift.  Since your boat will be a fast, lightweight multihull (regardless of the which rig, rudders, and beams you choose), you'll have a great apparent wind, and will be able to put the sail's lift to great use.  If you want to scream upwind, high-aspect is quite useful.

  - More sail area higher up.  This will help in light winds, even downwind, when there's more wind at 40' than there is at 10'.

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  You could easily argue that it's better to have a schooner rig, lower the center of effort, and give up that sail area up high.  The taller mast will be great for performance, but will require much care with reefing (and more reef points). 

  It's the classic tradeoff when considering a boat for both racing and cruising.  You can't have the fastest and the safest and the most convenient boat.  Rob's design allows a better compromise than most (weight to windward creates more righting moment, mast to leeward may prevent a capsize).

  Going to a 14' foot would definitely give you more downwind sail area without having to break out a third sail or kite. 

  By the way, have you looked at the outleader kites?  They can provide an insane amount of sail area when going downwind, with less hassle than a traditional spinnaker.

  Have you decided against a two part mast?  If so, I won't mention it again.  If you haven't written it off, it's worth looking into.  As nonsum_pisces mentioned, you could add on another six to ten feet without much hassle, even if you stick with an una rig.

       - Mike

 

 
Gardner Pomper wrote:

Hi,

I know what a sail's aspect ratio is (Luff/Foot), and I see mentions that a high aspect ratio is more efficient than a low aspect ratio, but I cant find any real description of how high it needs to be to be considered "high aspect ratio" and how much more efficient it is.

The containerizable design I have been playing with is limited to a 34' maximum luff. I started with a 10' boom, then increased to a 12'. In an extreme case, I could increase to boom to a max of 20'. What foot length is "reasonable" for a 34' luff? Is 34:12 low aspect or high aspect.

My sail area is about 550 sq ft (for both sails combined) with a 10' luff, 650 sq ft with a 12' luff and 750 with a 14' luff; all calculated with an 80% roach. I assume the 80% figure is harder to maintain as the aspect ratio goes down.

Thanks for any feedback,

- Gardner Pomper