sail aerodynamics

Thanks Larry,
I just haven't had the time to get back to this subject thread to the extent I would like.

 

Sail design for really dumb people (such as myself)

Hi:

I have been reading some of this thread and much of what has been said here is beyound me.

One thing I have noticed conspiciously absent in this thread is the contribution matterials science has made to rig design.

In the jolly olde days, before 1x19 steel cable was available, jibs really werent useful for much other than light air sails and ballance. The reason being was that the luff line would stretch and the sail would bag and sag. Even the cloth at the time was no great shakes.

For these reasons, low aspect rigs, that were not so dependent on foil shape, a concept that was not even understood at the time, ruled.

Once 1x19 rigging was available along with high strength cotton, it was possible to make really effective working jibs. Even then, they stayed small.

Thier size was then limited this time by the hulls inability to hold the now much stronger luff wire tight. The hull would just sag in the mddle under the tension and the luff would still go slack.

By then, the need for high lift wings for notoriously under powered airplanes became a major issue. The concept of air foil design was becoming common currency amongst the scientifically literate for the first time. amongst these people, of course, were sail boat designers.

My understanding of why a high aspect ratio wing or sail is more effective than a low aspect one is that, in physics, momentum is conserved, but kinetic energy is not. So moving a lot (of air molecules) a little is far better than moving a little a lot. If I have a sail that is 20 ft high and 10ft, wide it is going to redirect a swath of air nearly equal to its height and of indeterminate depth. Since changing the direction of anything involves acceleration, you are actually accelerating the wind molecules, giving them kinetic energy as well as momentum. The problem is that you only get to count the momentum, and the more you accelerate the molecules, the more energy you waste.

So if my proposed rig is now 10ft high and 20ft wide, I am now accelerating a much smaller swath of molecules to a much higher kinetic energy level. Because the formula for momentum is mass times velocity and the formula for kinetic energy is mass times (velocity squared), my new sail is going to to have half the 'lift/drag ratio' of the old sail. And that is not even counting vortises.

Now, if I am dealing with crappy sail matterials and inside ballast, I will probably have to go with a compromise between high aspect ratio efficiency and low aspect ratio lower center of area. I might even have to go with energy robbing sail over laps. The extra sail is no where near as effective as with no over lap, but it is much more effective than no extra sail at all.

A triangular sail, by the way, is a good cheap way to get a higher effective aspect ratio and still keep the virtical center of area within reason. One way of figuring aspect ratio is hieght squared divided by area. So a 1 to 1 trianangle is going to approach a 2 to 1 aspect ratio with nothing but a turning block at the top. Even if the top 25% of the luff is doing nothing but agitating the air.

A really interesting web sight to go to in regards to sail design is pdracer.com.
This crowd is the only one I know of in yachting that brags about the cheapness of thier boats. The hulls are cloely alike below the waterline but the sail rigs can be anything the skipper chooses to put up. Most of the sails are made of 'poly tarps' by inexpert sail makers. It is instructive to see how the trianguler sails do against the rectanguler ones in thier races.

I think they win every time.

Bob

 

[Quote by Sharpii2)

A really interesting web sight to go to in regards to sail design is pdracer.com.
This crowd is the only one I know of in yachting that brags about the cheapness of thier boats. The hulls are cloely alike below the waterline but the sail rigs can be anything the skipper chooses to put up. Most of the sails are made of 'poly tarps' by inexpert sail makers. It is instructive to see how the trianguler sails do against the rectanguler ones in thier races.

I think they win every time.

Bob[/QUOTE]

Great web site, and excellent concept as well. It seems to have captured a lot of interest, and boats are being built. Interesting is the rule allowing freedom of sail area and rig design. It is too early in my opinion to predict what will eventually be the best allround. Sometimes extra sail area if it is easily controlled overcomes efficiency in rig sophistication especially in light and moderate air.

 

frosh:

glad you checked the sight out. It is interesting to watch the class evolve. It seems to be moving toward sail areas of 80 sf+. Triangular sails seem to have the advantage here. they have had it since the class started, and they still have it. The boats are so beamy and can be allowed to heel so little that it is possible to put rediculously long booms on them. an austrailian put an 85sft jib headed sail with a sprit boom on his and got it to move out at almost six kts! the boom was longer than the boat and the mast was over twice as long.

It would be interesting to design a 10ft x 10ft sprit sail rig and see how it does. But, most likely a very large setee or boomed lateen may be more effective.

Don't get me wrong, I have a real soft spot for rectangular sails. All my boat designs have them. But race results are race results. I have yet to hear of a rectangular rig beating out a triangular one in this fleet. But there is a first time for everything, eh.

One of my biggest frustrations with this class is that the sail area is unlimited.

My cc312 design, if it ever becomes a class (see "My own design" thread) will not have that idiosyncrasy. It will be limited to 4.7 sm or 50sft and no hiking will be allowed. Sail matterial will be restricted to 4mil visquine or poly tarp. No wire rigging will be allowed but polypropylene stays and shrouds will be ok. The hulls would all be pretty much the same and they will have mininmum weights.

Wouldn't this be an excellent labratory to test sail design theories?

Bob

 

PDracer quickest sail?

If owners ever decide to get really serious about racing and winning you would want about three different rigs like Aussie skiffs that allow various sail areas. Maybe 50 to 90 sq. ft. I personally like Wharrams "Tiki" style wingsail being efficient, aerodynamic (with the luff sleeve and no stays) and allows a lot of sail area for a moderate boom length. I am sure that on a short hull with almost no directional stability long booms are difficult to sail with once the breeze comes up. This was my experience on low volume short sailboards. High aspect sails for a given area were much more user friendly.
This link shows a photo of the wingsail; I would think that a boomed version would be better allround.
http://www.wharram.com/tiki_photos/tiki.shtml

 

That photo sort of reminds one of the 'fat-headed mains' of today.

Appears to be a boomless sail as well, which I utilized on the Firefly trimaran, and the virtues of which have been discussed elsewhere in this forum

 

Bob (aka sharpii2),

"...energy robbing sail overlaps. The extra sail is no where as effective as with no overlap..." Does this argue against using overlap to create higher AR for a sloop rig? Anywere I can find more info about overlap dynamics? (that is vs no overlap?) I've read Bethwaite (assertions only it seems) and Marchaj (???)



As long as I'm talking about overlap, anyone seen David Pugh's reply to Guillermo Gafaell about (dare I type the phrase?) "The Slot" in Letters to the Editor of the June 2006 issue of Yachting World? (Is DP on this forum?) If DP is right about the air accelerating (after stagnating at the entrance to the slot) "as it approaches the leach of the genoa, reducing pressure and re-establishing the suction that produces the mainsail's drive", does this have design repurcussions, like where the leach of the genoa ideally ends relative to the maximum chord of the main, and does it change where you would want the point of maximum camber on the main for that particular genoa?

Paul

 

Paul:

I wouldn't know.

I guess it has to do with the sectional shape of the mast. I suppose that if you have a 'dirty', square section mast, the overlap may actually help the situation by forcing the wind around the mast and thereby suppressing the inevidible eddies.

With a 'cleaner' mast section, by the same logic, the same overlap may actually detract performance in comparison to the same sail area with no over lap, because the air molecules are experiencing friction on both sides of 'the slot'.

Many of the arguements I have read in the thread remind me of arguements I have heard in political discourse. The principles and arguements by themselves may be very sound. As long as the implicit assumptions are in place.

The trouble is, in the real world, they often aren't.

Yes, I agree that a very high aspect ratio eliptical planform single sail behind a perfectly clean mast may be the best sail for a given sail area. But you would never see one on my boat. Why? because, in order to have one, I need expensive, high tech matterials, for the mast, and a very deep, snaggy, bulb keel, or even a canting keel, to hold it up. This implies, not only expensive building costs, but high berthing and upkeep costs as well.

There may be a time in my life when such things may not be an issue. But I don't anticipate that happening any time soon.

Bob

 

Overlap Dynamics

Overlap dynamics: Have a look at back thru Postings # 63,64,65,66 of this subject thread.
http://boatdesign.net/forums/showpost.php?p=40634&postcount=63
http://boatdesign.net/forums/showpost.php?p=40634&postcount=64
http://boatdesign.net/forums/showpost.php?p=40634&postcount=65
http://boatdesign.net/forums/showpost.php?p=40634&postcount=66

And then an excerpt from here, "We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important."

 

Brian

Thanks. The direction is greatly appreciated.

As I strive to become a dedicated 'aero amateur', I realize more and more that my father was right when he said "work is the bane of the leisure class."

Paul

 

Twin Headsails?

Not a bad looking twin headsail deployment.

(BTW, the catamaran Orange II just completed a record run across the Atlantic during which time it also set two new 24hr records as well)

 

Hi Brian, a couple of quick questions --- forgive me if this is not the correct place to post these Q's, but it's all about sailing isn't it !? -----
where is the centre of effort of the sails, in relation to the lateral centre of resistance of under water sections ?.
I was told it should be ahead of , but wouldn't that cause the yacht to fall off if hit by a big gust ? , & then it'd get knocked flat. If not, why not.

I love your rig [ aft mast] and am planning to build a bipod version -- make that a "wishbone" as you called it, as I want the top section to telescope in to stop forward overhang when down to traverse rivers & the like --for my 50' yacht [ hull & decks right now ] I'm sorry Brian, I'd love to be able to pay you to design it for me, only I'm talking real budget here !!,so I'll just do my best [ as a general engineer & rigger ] as it is, most of my land lubber mates still think I'm dreaming. Well they see the hull, see I've built the 7x20m x6.5high workshop in the last 3 months by myself , but will only "believe it when I see it !" regarding the rig , as it's unheard of round here.
I've been " dreaming" on paper of late drawing how I'll get the rig to raise & lower and drawing different keel options & or foil shaped assemetrical lee boards.
She's shoal draft, 1m, but I'm not happy with the huge designed centre board taking up , or should I say , dividing the sallon into two smaller cabins.
So I've been looking at lee boards or ----
reducing the centreboard case by 2 thirds and useing a daggerboard type retractable keel with a small wing & balasting it [ the original design called for all balast to be in the shoal bilge & the centreboard had no balast ]

I have asked the question re the placement of lee boards and Mike Johns rightfully said it'd be crutial [ damm, must download that spellchecker one day] that the placement be right or I'd have an expensive disaster on my hands .

Anyway I said most of above so you could see why I wanted to know where the centre of effort of the sails should be in relation to the underwater profile ,
I'm thinking I may have to move my "new" keel aft somewhat.
I'd really like to post some lines / drawings of my boat -- I'lll try to, & if it works, I'll get my drawings scaned so I can post them as I'd really like input from all you experienced lot.

 

There is normally a certain ‘lead’ distance factor to be incorporated in the design in order to balance out the turning moments produced by the sideways forces of the sailing rig verses the leeway resistance forces of the underwater shapes (centerboards, dagger boards, whatever). This lead distance has to be more substantial on a monohull as there is heeling involved that further displaces these force centers from one another.

The force centers on the sails are usually taken to be at the geometric center of the sail, even though this is not real accurate. And the underwater centers are similarly taken at the geometric centers as well, so one error cancels the other.

In light airs the ‘balance’ is not as critical. The lead distance is usually based on the ‘substantial’ heel angle of 20 degrees that might be experienced under full sail, under heavier conditions, or even gusty conditions; all where more balance is desirable.

Non-heeling cruising catamarans might require no lead distances, while smaller tris might average out at 5 degrees.

I would strongly recommend NO telescoping sections.

Why not simplify and use twin fixed keels. But 1m in a 50 foot boat….wishful thinking.

 

the penny drops

Thanks Brian, I understand now. To put it another way [ how I see/understand it ] as the mono hull heels it puts it's " shoulder" to the sea, that is, the forward , roundy section, which would, of course turn the hull [ round up ] windward naturally. So to balance that tendency, the centre of effort needs must be forward of the CLR , right !?.
Regarding the 1m draft being " wishfull thinking" , sounded a bit harsh mate !!
But hey , you might well have given me a good name for said vessel !!! as like most people I want my cake & eat it too - meaning shoal draft, preformance, etc etc.
Could you elaborate on why definately NO to a telescopeing top section of a wishbone rig !? --- or is it that you think/feel it unatainable due to ??.

I won't assume you think I'm an idiot who couldn't engineer one sufficiently, or that it's just wishfull thinking ,but rather that it's too difficult to try to explain why not .
I've read every post you've put up regarding your aft mast rig as I think it exelent[ I have printed of a copy of your rig & it's stuck on my office wall !], and seen how, like me, you've been studying to identify the loads/ stresses on on rigs, so as to be able, one imagines, to build such a rig .
There are several advantages,[ perhaps only for me !?] I believe, for being able to safely lower one's rig aspect/ height while afloat.Beter stability, less windage, a better "fit" when main rig lowered, still able to be sailed while getting under some obstacles --- the scows who dropped their rigs & " shot " the bridges on the thames !!, wow, that's a classy act . [ engineless of coarse ] and probably more I can't think of [ yep, no doubt lots of dis advantages also ]

 

Take a 50ft boat, figure what the RM is. Then figure what the loading on the join in the mast will be. Once the loads are known, see how heavy the telescoping mast would have to be. Try to design a system that would allow the mast to telescope reliably and figure out how much that will weigh.

How are you going to deal with halyards? the electronics at the masthead?

I'll wager that a telescoping mast will be 3-4 times the weight of a standard mast.

If you want to be able to reduce the rig height easily, build a gaff rig.