Aspect ratio questions

I would say look at the wing of a Cessna and then look at the wing of a Citation Jet. Or birds, that is a duck vs a falcon.

Aspect ratio = wingspan squared/wing area.

High aspect ratio = Long and skinny

Low aspect ratio = short and fat

High aspect has lower drag and vs lift generated, ie: higher potential speed in our application. The lift created by sails is generated in the first 30% (from the luff). In a lightweight platform like a cat, that does not need a lot of force to overcome it's inertia and drag thru the water, high aspect allows for higher speeds for a given wind.

The true definition of aspect ratio is simply luff length / foot length.

OK, to answer your question relative to two sails with the same total sail area, the one with the longer luff (and thereby a shorter foot) will be the one with a higher aspect ratio.

The new (to some) Square Top type mainsails you see on most newer designs (F-16, F18, A cats, etc.) were developed because with a fixed rule to total sail area, the sail makers figured out that they can generate the most lift by having a shorter foot and longer luff. In effect they were moving the sail area from the foot up to the top, in the square head, vs. the older triangle sail designs like you would see on most boats prior to the advent of the Square Tops.

So, if you have a choice but are limited to a rule for

Total Sail Area

you would be wise to have a

high aspect ratio

type sail made. It will have a square top and look much more like a rectangle than the older triangle type sails. It will also generate more lift and less drag.

You can also keep the same luff length, same mast, and just add the square top, shorten the foot to keep the same sail area.

Thanks for the info.

The Isotope rules say it must have 185sq ft of total sail area so next year when I get new sails I'll def go for something with a high aspect ratio <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

Then rake the mast back a little to compensate for having moved the Center of Effort (CE) forward (by reducing the sail area in the aft part of the sail, increasing the sail area in the forward part of the sail).

Gary

Does the class have a specified sailmaker? You might have to put class sails on it.

I'll make sure to find out <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

It seems to be open as the pics I've seen have all shapes of sails.

BTW, there was a sail number 1100 on the banner at Carolina Sailing club - is that your boat?

^^ Not mine, the owner of that boat (Howard Alexander I believe) must have sold his old sails at some point to the guy that I got my boat from, or one of the previous owners

Ryan,

The class rules allow a maximum of 185 sq.ft. of soft sail main and jib combined. We have formulas for measuring square-top and pin-top mainsails, and jibs. However you elect to divide up the area between the sails is fine. Most of the jibs measure out at 44 sq.ft., so we typically cut the mains to 140+. There is also a maximum mast height, and the mainsail may not extend above the top of the mast.

Several members of the fleet are sewing their own sails from pre-cut pentex panels. Howard Alexander also sews them for sale through CFI (formerly IFG). Most of us are opting for a medium-aspect square-top and it seems to do fairly well. The kits come oversize so there is some leeway in how you cut the leech.

I'm the class measurer, so should you choose the do-it-yourself route, we can lay out and measure the sail before you cut it.

Regards,
Eric

There is no restriction on sailmaker or shape - just total area. If you take a look at http://www.carolinasailingclub.org/xoops/modules/news/article.php?storyid=154, you'll see a few different shapes. I believe the dacron pin-top sails were all made by North (although one could be Kelly or Schurr). All the smoke-color pentex sails were made from SailRite kits. Most were sewn by the skippers themselves.

Are you required to have a jib? Is there a limit to mast height?

Thanks for the info Eric, when the time comes for new sails (next spring) I'll be sure to get in touch with you guys <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />