sail aerodynamics

[tspeer]

Originally Posted by Erwan

...For a catamaran I imagine that it could be possible to use a split foot sail, combined with trampoline effect, to improve the low component of induced drag....

Here are data for a similar concept. The range of planform shapes investigated is depicted in this illustration:

[TTS]

The other issue with the a-class and jibs are the class rules. Attached is the copy of the rules. It seems to state that it is for a main sail only, or at least the measurement form is laid out in that manner. Maybe only the sail area is dictated and a-class sailors have gone for a single high aspect main sail, rather than a smaller main with small jib.

Tom

ISAF A-Class Catamaran Rules




IACA Championship Rules (Draft)



Measurement Forms (in Microsoft Excel format)



The A Class Catamaran is controlled by the rules set down by the ISAF under the "Rules for the Divisions of Catamarans and The International C Class" (Last issued May 1985) which are as follows:



1. A catamaran is defined as a two-hulled sailing boat with essentially duplicate or mirror image hulls, fixed in parallel positions.



2. Sail area shall not be more than 13.94 square metres (150 square feet) Sail area to be measured in accordance with the" IYRU Measurement & Calculation of Sail Area Instructions" (Last issued May 1985)



3 The overall length of the catamaran shall not be more than 5.49 metres (18 feet)



The length shall be measured between perpendiculars to the extremities of the hulls with the catamaran in her normal trim. The measurement shall be taken parallel to the centre line of the craft and shall exclude rudder hangings, but if the athwartships width of a rudder within 153mm (6 inches) of the bottom of the hull is more than 76mm (3 inches), the length shall be taken to the aftermost point of the rudder.



4. The extreme beam shall not be more than: 2.3 metres (7 ft 6½ inches)



The beam shall be measured at right angles to the centre line of the craft at the widest point and including all fixed or adjustable apparatus with the exception of a normally accepted trapeze or retractable seat.



5. The crew in Division "A" shall be one person (helmsperson)



6. Unballasted retractable seat or trapeze shall be allowed for the helmsperson. When in use the helmperson at all times shall have at least one foot in contact with the boat.



7. The A Division emblem shall be carried on the mainsail and shall consist of the letter A over two parallel horizontal lines over national letters and sail numbers,



Sail numbers shall be allotted by the National Authority or Class Association appointed by the National Authority. The class emblem, national letters and distinguishing numbers shall be placed as prescribed in the Yacht Racing Rules.



( RRS 77 & RRS Appendix H )



8. Hydrofoils are permitted.



The following rule was passed by ballot 28th February1998 and will take effect from 1st April 1998:



9. Minimum weight in full sailing trim shall be not less than 75 kilograms.



The following rule was passed by ballot August 15, 2001:



10. Hydrofoils shall not be permitted in ‘A’ Division. (Approved 2001)



Advertising; for all National, Continental and World Championships and events which are not restricted by the organising authority to Category A advertising status, a boat may choose to display unrestricted Category C advertising as defined by ISAF Regulation 20.

Measurement Form


All A Division Catamarans shall have a valid measurement form and for all yachts measured after 1st January 1998 it shall be on the latest style form dated 4/97. This form is composed of four separate areas that are largely self-explanatory. The following notes should assist in their understanding.



Mast & Boom Measurement Form


The purpose of this measurement is to find half the total area of the mast and any mast base fitting attached. On a straight section (ie not tapered) it is simply the length L x half the mast girth. The Measurement and Calculation of Sail Area instructions defines girth as follows:



"The girth measurement shall be taken as the distance from the centreline round the surface of the spar to the same point on the centreline. The resultant dimension shall be divided by two to give the half girth measurement."



Should mast be tapered extra measurement U1 & T need to be taken and the formulae on measurement certificate utilised. Black band measurements L1 & L2 play no part in the mast measurement at this stage.



Boom measurement is only utilized if the profile height of the boom is more than 1.5 of the width.



Hull Measurement


This area is self explanatory with only two measurements needed ie width & length.



Things to look for with overall length are:



If the width of a rudder within 153mm (6 inches) of the bottom of the hull is more than 76mm (3 inches) the length measurement needs to go to the aftermost point of the rudder.



With the width, measurement is at the widest point of the hulls, this may be at some point down the sides of the hulls, especially if hulls are angled. It may also be possible that the maximum width is at bottom of centreboards when fully down.



Sail Measurement Form


When undertaking the sail measurement the following points should be noted.



Sail to be measured on a flat surface and laid out in terms of IYRU Measurement & Calculation of Sail Area Instructions



ie. "With battens set in their pockets the sail shall be pegged out on a flat surface with just sufficient tension to remove waves or wrinkles from the edge rounds and to spread the sail, as far as possible, substantially flat. Once the sail has been pegged out in this way all the required measurements shall be taken and no alterations to the tensions shall be made."



Luff length A is the maximum distance from the head to the tack of the sail. It is taken on the inside of the boltrope, which is not included in any measurement.



Base length P is a measurement from the clew to a point at 90 degrees to A.



Measurements M, F, K, D & H are all made at 90 degrees to their respective lines. All are to be the maximum distance that can be taken.



General Calculation Form


This form brings all the measurements together and all that is required is to transpose measurements from the other forms. On this page it allows you to calculate the theoretical Black Band distance that will allow a maximum sail area of 13.94 square metres for each sail that the boat ma have. It should be noted that black bands should be on the mast and form part of the measurement requirements.



The weight of the boat and any correcting weights are also listed on this page.



The weight of the boat consists of all items associated with the boat in full sailing trim. It does not include such things as shackle keys, water bottles, spare rope etc.



The boat must be weighed in a dry condition and any weights attached to bring the boat to a minimum weight of 75 kilograms must be permanently affixed and their weight duly noted on this page.

[CT 249]

RE "The other issue with the a-class and jibs are the class rules. Attached is the copy of the rules. It seems to state that it is for a main sail only, or at least the measurement form is laid out in that manner."

There were sloop-rigged As in New Zealand in the '60s and maybe early '70s. They soon found out that they were not competitive with cat rigged cats, just like the Bs did (before the development class died) and the Cs and Ds have proven.

[PI Design]

The NS14s also have very free rules regarding the rig (total sail area 100sqft, mast height 18ft, any split of sails allowed). But the norm in this class is for a high aspect mainsail and small jib (approx 75/25 split), on a wing mast, rather than a main only. Whether this is due to the relatively low righting moment needing a lower centre of effort, or whether the mast height restriction would result in too low an aspect ratio if all the area was in the main I don't know. However, Andrew Landenberger (Tornado silver medallist, Moth World Champ, good A Class sailor) has been considering a cat rigged NS14 but isn't really involved with the class much so may not get round to realising his idea.

[dimitarp]

You can find more for sail aerodynamics at:
http://www.partenovcfd.com

[PI Design]

There doesn't seem to be anything there?

[CT 249]

Originally Posted by PI Design

The NS14s also have very free rules regarding the rig (total sail area 100sqft, mast height 18ft, any split of sails allowed). But the norm in this class is for a high aspect mainsail and small jib (approx 75/25 split), on a wing mast, rather than a main only. Whether this is due to the relatively low righting moment needing a lower centre of effort, or whether the mast height restriction would result in too low an aspect ratio if all the area was in the main I don't know. However, Andrew Landenberger (Tornado silver medallist, Moth World Champ, good A Class sailor) has been considering a cat rigged NS14 but isn't really involved with the class much so may not get round to realising his idea.

It's been tried at least once. Didn't really go. How good the attempt was, I don't know. The singlehanded version of the NS (main only) is surprisingly slow - suggested rating is similar to a Radial!

[BOATMIK]

Howdy,

Regarding cat rigs vs sloop rigs, my gut feeling is that it relates to span.

For classes with restricted sail area, the classes that have spanwise freedom (to make their masts as tall as they like) seem to favour cat rigs.

But when span is heavily restricted - either by righting moment or an absolute measurement (eg an NS14 has a limit of 18ft from the deck, however that has now been metricised) then multi sail rigs seem to be preferred.

Classes from the NS14 through to the Bembridge Redwing seem to find the jibs diminish in size but will never quite disappear.

Part too might be the advantage of two sails on a reach (if I remember Gentry's original articles on circulation properly). An A or C class cat spend much more time going upwind which has to favour a higher aspect sail, and compared to an NS they have stability to burn.

Maybe a good experiment would be to restrict NS14s to a max mast length of 10ft and see if the fastest boats are schooners, ketches or yawls.

Best wishes
Michael

[PI Design]

Landy's thoughts on the cat rigged NS14 after its first sail:
"It is very fast downwind but upwind is still unknown. It felt fast but height was an issue. I had some problems with the setup so I hope I have solved that for next weekend."

Second outing:
"I did sail a regatta on the weekend with the big main. It is a struggle upwind against the top boats but I was very fast downwind. I managed to win one heat in lighter air but generally the sloop config was clearly faster. I will try again with a flexible round mast eventually."

[astevo]

should point out that sometime boatdesign member 'phils' won an a-class "worlds" using a sloop rig in the 70's. 'Harmony' was the boats name. multi chine ply hull, precursor to the 'rhapsody' hullshape, which was very tornado like.

ive seen the cat rig ns14 a few years ago, my memory was that it was never faster, but suffered especially upwind. theory says it might be quicker off the breeze but i couldnt tell you based on th at experimental rig/boat.

[brian eiland]

I've been away from this subject thread too long. I see some things I need to review.

Meanwhile just to throw some monkey wrenches into the works, consider these unusual findings from Mother Nature:

DURHAM, N.C. -- Wind tunnel tests of scale-model humpback whale flippers have revealed that the scalloped, bumpy flipper is a more efficient wing design than is currently used by the aeronautics industry on airplanes. The tests show that bump-ridged flippers do not stall as quickly and produce more lift and less drag than comparably sized sleek flippers.

The tests were reported by biomechanicist Frank Fish of West Chester University, Pa., fluid dynamics engineer Laurens Howle of the Pratt School of Engineering at Duke University and David Miklosovic and Mark Murray at the U.S. Naval Academy. They reported their findings in the May 2004 issue of Physics of Fluids, published in advance online on March 15, 2004.

In their study, the team first created two approximately 22-inch-tall scale models of humpback pectoral flippers -- one with the characteristic bumps, called tubercles, and one without. The models were machined from thick, clear polycarbonate at Duke University. Testing was conducted in a low speed closed-circuit wind tunnel at the U.S. Naval Academy in Annapolis, Md.

The sleek flipper performance was similar to a typical airplane wing. But the tubercle flipper exhibited nearly 8 percent better lift properties, and withstood stall at a 40 percent steeper wind angle. The team was particularly surprised to discover that the flipper with tubercles produced as much as 32 percent lower drag than the sleek flipper.

“The simultaneous achievement of increased lift and reduced drag results in an increase in aerodynamic efficiency,” Howle explains.

This new understanding of humpback whale flipper aerodynamics has implications for airplane wing and underwater vehicle design. Increased lift (the upward force on an airplane wing) at higher wind angles affects how easily airplanes take off, and helps pilots slow down during landing.

Improved resistance to stall would add a new margin of safety to aircraft flight and also make planes more maneuverable. Drag -- the rearward force on an airplane wing -- affects how much fuel the airplane must consume during flight. Stall occurs when the air no longer flows smoothly over the top of the wing but separates from the top of the wing before reaching the trailing edge. When an airplane wing stalls, it dramatically loses lift while incurring an increase in drag.

As whales move through the water, the tubercles disrupt the line of pressure against the leading edge of the flippers. The row of tubercles sheer the flow of water and redirect it into the scalloped valley between each tubercle, causing swirling vortices that roll up and over the flipper to actually enhance lift properties.

“The swirling vortices inject momentum into the flow,” said Howle. “This injection of momentum keeps the flow attached to the upper surface of the wing and delays stall to higher wind angles.”

“This discovery has potential applications not only to airplane wings but also on the tips of helicopter rotors, airplane propellers and ship rudders,” said Howle.

The purpose of the tubercles on the leading edge of humpback whale flippers has been the source of speculation for some time, said Fish. “The idea they improved flipper aerodynamics was so counter to our current doctrine of fluid dynamics, no one had ever analyzed them,” he said.

http://www.pratt.duke.edu/news/?id=101

[Paul Scott]

Brian, have been looking at the picture of the model, and what shape are the bumps, concave on the side facing the lens, and convex on the other side?

[brian eiland]

I don't know for sure Paul. Maybe a little more searching on this subject will bring up other details.

This subject was brought to my attention by another party that emails me privately every once in awhile about subjects he feels may interest me. I've not even had time to fully digest it myself...just thought the forum might have some interest.

[Guillermo]

Humpback whale tubercled flippers have been around for a while in these forums. Brian, do you think they would be applicable not only to rudders and stabilizers, but also to the sails themselves?
Cheers.
Guillermo.

[Leo Lazauskas]

Originally Posted by Guillermo

Humpback whale tubercled flippers have been around for a while in these forums. Brian, do you think they would be applicable not only to rudders and stabilizers, but also to the sails themselves?
Cheers.
Guillermo.

Hi G,
Do you mean a combined mast and sail? Or a scalloped leading edge of the sail?

To me these bumps seem to behave like a combination of fences to reduce spanwise flow (and thereby increasing the effective aspect ratio of a wing) and riblets.

Maybe the Japanese are correct in doing more scientific research on whales.

Regards from Adelaide where it is now 40C!
Leo.