High v Low aspect ratio

[BlueSkyNick]

Question from YM Jan 08 Quick Quiz:

Is an overlapping genoa described as having a high or low aspect ratio? Answer is Low.

So, at the risk of being accused of ignorance, what measurements determine the aspect ration, then? And what is the benefit of calculating it?

 

[Twister_Ken]

Height divided by width. The bigger the resulting number, the higher the aspect ratio.

 

[jimi]

you've got a high aspect outlook, mine's is low except when I'm on a ladder ..

 

[Thistle]

With a low aspect ratio, more of the sail is low down so will tend to have less heeling effect than a high aspect ratio sail of the same area. The high aspect ratio sail will be able to make more use of the - slightly - stronger and more stable air flow well above the water surface.

 

[bilbobaggins]

Many other things being equal, a relatively High A/R has a greater Lift/Drag Ratio than a relatively Low A/R.
See C.A. Marchaj's 'Aero-Hydrodynamics of Sailing Part 1.C' and 'Sailing Theory and Practice' if you have trouble getting off to sleep.....


[ QUOTE ]
....the aspect ratio AR has a profound influence on the magnitude of lift generated at a given angle of incidence...

AR = Span² / Lateral Area

....With some reservations ( aspect ratio should be considered with taper and twist ) one must agree that high aspect ratio sailplan is more efficient for round-the-buoys racing.... many people question the advantage of a high AR rig in...boats intended solely for cruising.

....Where the close-hauled course is not the most important performance feature and the maximum lift coefficient ( is )..... gaff-headed or even square sails may prove superior....

[/ QUOTE ]

Oh, dear! I hope this doesn't degenerate into an 'anchors'-type mud-slinging......


/forums/images/graemlins/smirk.gif

 

[Bajansailor]

Here is a rather fine example of fairly extreme aspect ratio - it is one of the Mini-Transat racing boats, which are restricted to an overall length of 6.5 m., and not much else in the way of rules, hence they have developed with enormous sail areas.

Bill has noted above that gaff headed or square sails can be superior - one could argue that the Mini-Transat mainsail above is a modern version of a gaff rig, and while not 'square' in the conventional sense, it is effectively rectangular rather than triangular....

 

[bilbobaggins]

The very brief quotes from Marchaj were chosen simply to encourage one to examine one's assumptions a little. What makes for 'more efficiency' in a sailplan is a hugely complex subject, and quite a long way beyond my capacity to comprehend and adequately explain

Except....

Lift versus drag features significantly. So also does twist and camber, Reynolds Number, and the relative stability or 'robustness' of a particular set-up. It's of little practical use on the water having a rig that produces lots of drive ( lift ), but only over a very narrow range of 'angles of attack' ( trim ). Otherwise, it's deeply stalled, producing drag most of the time.

Next, the generation and shedding of vortex sheets from the pointy ends of the sails generates much unwanted drag, as do batten ends and reefing lines. Fathead sails, as illustrated in Bajan's pic, are one quite effective means of reducing the sizeable 'tip losses' and managing the shape of the unstalled ( lifting ) airflow rather better. On modern aircraft, use is made of so-called NACA wiglets to help control tip losses....

....and one suspects it will not be long before something similar is seen on racing yachts.


As for the Mini 650, I want one!

 

[Bajansailor]

Very well said! The rest of us with insomnia dont have to resort to Marchaj's bibles now, so we shall just have to remain wide awake....
BTW I met Tony Marchaj once - a fascinating chap. His other well known book is 'Seaworthiness - the forgotten factor' and written in his usual (occasionally controversial) style.

Sails are basically aerofoils, rather like aeroplane wings, but unlike aeroplane wings the sails have to operate over a much broader range of angles of attack, from sailing close hauled to broad reaching and running downwind, hence one could perhaps say that they are more complex in many ways.
Aeroplane wings have to operate effectively over a relatively small range of angles of attack, with the largest angle probably being when they are taking off. Once this angle starts exceeding about 30 degrees the lift forces start to drop off, and drag forces kick in, until the wing is stalled - rather like putting a rudder hard over (rudders also work most efficiently (re Lift /drag ratio) at smaller angles of attack).
And rudders are sometimes equipped with 'fences' and or end-plates in the same fashion as the aeroplane winglets, to help reduce tip losses.

Agree with you re these Mini-Transat boats, they really are rather wicked - I would love one too!
I think they can consistently turn out over 200 miles a day when sailing hard in the trades - not bad for a 6.5m. waterline.....

 

[Skyva_2]

True for a rectangular sail, but more generally:

Height divided by mean chord

and more generally still:

Height squared divided by area

 

[jimbaerselman]

[ QUOTE ]
use is made of so-called NACA wiglets to help control tip losses....


[/ QUOTE ] I like the idea of a little 'wiglet' on the top of the mast. Sound a bit like an anti-static wick.

I guess a proper job would have to be gyro stabilised to make sure it didn't add drag when heeled . . . and then there's the extra heeling moment caused by the weight. A nice, expensive America cup project to counter those wiglets attached to the keel.

Perhaps, back to the early Spitfire wing would be desirable instead, a neat ellipse, minimising the vortex generating area, rather than relying on twist with all its attendant wetted area drag?

What was it called - wing sail? On a Cat to remove the need for gyro stabilising.

 

[Twister_Ken]

Spitfire sails - the most Spitfire like I can remember seeing are on International Canoes. And they certainly go like organic fertiliser off of a digging instrument. At least they do when upright.

 

[tross]

The other benefits of using a rectangular/ elliptical / high aspect ration sail is the "lift "is generated in the same relative position - if you think about it the " maximum draft" position on a sail relative to the mast changes as you as you move from the foot of the sail to the head. Hence, the lift moves forward as you go up the mast.