sail aerodynamics

CT ... you have it right ...

It is the L/D of the complete vessel that maters.

The L/D of the rig is only one part of it. The L/D of the hull/foils in the water cannot be ignored. Above the water, only the sails produce lift, everything else is drag.

Putting the very best, highest L/D rig on a barge is not going to make it sail very well.

Have the fast windsurf guys started to experiment with drag reduction of the sailor? High L/D rigs with low CE are nice, then Bubba stands next to it in a baggy outfit and a PFD. How much "free" performance is to be had by experimenting with sailing suits and postures that reduce drag? The mast has to be a very small part of it.

It cracks me up to see $$$$ in new sails on a boat that has had NO attention paid to the hull and keel.

 

Isn't the real number 'increases in wind velocity SQUARED'. That's how frontal drag is calculated. I can't see how this would be any different with the mast rather than, say, a sheet of plywood sticking up with an equal frontal area to the mast.

Bob

 

This view does not account for the significant pressure-drag forces which the mast, jib, and mainsail impose on each other. Once the additional pressure drag induced by the mast is added, its contribution to the total drag increases from a velocity^2 scaling to a velocity^3 scaling.

The velocity^3 dependence also shows up when total drag is computed by summing the dissipated power over all the wetted area elements dA of the wing or sail system and its wake:

V_inf * D_profile = Sum [ rho V^3 Cdiss ] dA

Where Cdiss is the dissipation coefficient and V is the speed over the area element dA. Note the V^3 factor, and the absence of pressure forces which don't explicitly contribute to viscous power dissipation.

Cdiss is similar to the more familiar skin-friction coefficient Cf, except that it's much less weakly dependent on pressure gradients than Cf. In particular, Cdiss is strictly positive, even in separated reverse flow, so unlike Cf it also accounts for pressure drag due to separation.

 

I think an important point that's often lost is one of the functions of the jib is to reduce the drag of the mast.

In the absence of the jib, there would be a much larger separation bubble behind the mast, or just outright separated flow all the way to the leech. The favorable pressure gradient in the slot leads to quicker reattachment of the separation behind the mast and a significant reduction in the drag.

I don't believe you can sum the drag of the mast alone with the skin friction of the sails and get a very accurate estimate of the profile drag of the whole configuration.

 

I'm sure I've seen a wind tunnel test that shows the comparison of a sail vs a sail with mast ... all that is lacking the test of the mast with no sail. I doubt that mast + sail = mast with sail ... the sail would prevent the mast from shedding vortexes.

 

But couldn't the jib slow things down enough at a low enough speed to get the flow behind the mast into the re region where once detached, the flow (in any form) doesn't reattach? And at least from Gentry's streamline/speed figures (like fig 17) the amount of overlap then matters, at least in the sense that the boundary layer on the lee side of the mast is being stressed by different wind speeds at different points on the jib in the slot?

I'm not sure this is english.

Paul

Don't panic.

 

Mast-Aft Development & Justification

This is indead a great thread.

I see there is renewed technical discussion of the headsail/mainsail 'slot' interaction , and some new participants with respectable technical backgrounds.

Over the past number of years I have attempted to make a positive case for my mast-aft sailing rig configuration, both within, and aside of the technical discussions of these sail aerodynamics. I’m going to refrain from re-entering these technical discussions at this moment, and watch where they go.

Rather I will choose to do a brief review of why I pursued my alternative rig configuration based upon the real-time observable phenomena that we experience as sailors.

Because my submission is more ‘rig specific’ and not of the technical ‘sail aerodynamics’, I will make it here under the aftmast subject thread

http://www.boatdesign.net/forums/showpost.php?p=198605&postcount=98

I also thought I would add in some other materials I've collected in the past.

 

This is a great thread - many thanks to all the knowledgable contributors.
I have a challenge for anyone willing to take it up - design a NS14 rig. NS14s are very simple boats but with very free rules on the rig design. Essentially, sail area is limited to 9.3sq m (100sq ft), which can be split in any configuration provided there is only one mast and it has a maximum length of 5.5m (18ft). So you can mainsail only, jib only, 6.9/2.4 main/jib split etc, etc. The mast section must pass through a 100mm ring and over-rotating wing masts are the norm. Hull length is 4.3m (14ft), waterline beam is 1.2m (4ft) and max beam is 1.83m (6ft). I'd be fascinated to see what you come up with. For info, these are typical:

 

Yes... it seems that the mast is not just a drag-device but contributes significantly to the driving force of the sails. In a recent CFD study on Star sails, the mainsail drive was 10,2 kgf, jib drive 6,7 kgf and the mast drive 1,2 kgf (that's in the positive, forward direction for the mast as well). This was in 6 m/s apparent wind and at AWA 27 deg.

The mainsail behind the mast cuts down the drag remarkably, compared to a bare mast. The Star mast is really slim and refined, though.

 

What kind of calculation are the Star results from, Mikko? A Navier Stokes code?

 

Milgram did some stuff on mast sail interactions in the '70s.

He made a statement like "significant amounts of lift are developed by the mast, which is probably why big old fat IOR masts are never quite as bad as we expect them to be"

I can't remember whether it was tunnel or beginning of CFD applications to sails stuff.

Best wishes
Michael Storer

 

Isn't part of the problem that for most boats the heading of the boat redefines the importance of lift vs drag.

Dead downwind in a slow boat I am trying to get drag to the max. I'll get my crew to wear their pants on their head if it will help.

I have had a bit of a browse through the past few pages and it seems to be focussed on solutions to the upwind problem. For fast boats this may be acceptable approach, but the results will be less useful for slower ones.

So doesn't the problem become a whole set of optimum L/Ds for different headings? Or isn't that different apparent wind angles?

??

Best wishes
Michael Storer

 

Yes, N-S code. The model is rather elaborate, with boat hull, sails, rig and crew included. But the mesh is coarser than one would hope for, due to memory limitations on my desktop PC. You can look at more pictures from last year at http://web.mac.com/mbrummer/Mikko.mac/CFDpics.mac.html

At least according to this analysis, it would appear the mast in front of the mainsail is doing a good job in capturing some of the "suction force" at the leading edge of the sail, a problem well known for thin, plate-like airfoils.

The viscous 3D-analysis reveals many things that won't show in an inviscid (panel method) analysis, or even a 2D N-S run. Separation bubbles (that play such an important role in sail aerodynamics) appear rather as "separation vortices" than "bubbles"... Heel angle, and the hull under the jib have a major effect on separation at the tack on the leeward side at larger angles.

Also, wind gradient and the associated twist in the apparent wind (inflow) have a significant effect on the absolute forces produced by sails... it is understandable, when you think that you loose both wind speed & angle of attack in the foot part of the sails, where the most of the (triangular sails) sail area lies.

In this particular Star case, for an AWS= 6 m/s and AWA= 27 deg, you only have 4,9 m/s and 23,5 degt at 1 m above sea level. If you refer drive and heel coefficients to the apparent wind at 10 m height (as you want them for a VPP, for instance), they tend to be much smaller (some 25% if I recall) than for a uniform flow.

 

How Airplanes Fly: A Physical Description of Lift

Back in 2003 I posted a reference to these two sites explaining 'lift'. Well they no longer are reachable, but that's okay because they were substantially lacking.

I was going back thru a quick review of some of this subject, and ran across this excellent reference site submitted by Tom Speer on another forum. So I felt obligated to bring the references up to date:

...per Tom
This is an excellent article - one of the best you'll find.

How Airplanes Fly: A Physical Description of Lift
http://www.allstar.fiu.edu/aero/airflylvl3.htm

 

Brian, it appears the link isn't working.....