Talk:Genoa (sail)

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A genoa (pronounced like the city, or as jenny) is a type of large jib-sail used on bermuda rigged craft, commonly the single-masted sloop and twin-masted yawl, less frequently on a ketch. Its large surface area increases the speed of the craft; however in high wind conditions a smaller storm jib is usually substituted.

The term genoa is often used somewhat interchangeably with jib, but technically there is a clear delineation. A jib is only as large as the foretriangle, which is the triangular area formed by the mast, deck or bowsprit, and forestay. A genoa is larger, with the leech going past the mast and overlapping the mainsail. Genoas are categorized by the percentage of overlap. This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular"). A 150% genoa would have an LP 50% larger than the foretriangle length. Sail racing classes often specify a limit to genoa size. On International Offshore Rule boats, different classes of genoa have 150%, 130%, and 98% overlap, and so on. Under Performance Handicap Racing Fleet rules most boats are allowed 155% genoas without a penalty.

Genoas do add more sail area to a craft, but at the expense of more difficult handling. It is harder to tack a genoa than a jib, since the overlapping area can become tangled with the mast unless carefully tended during the tack. Genoas are very popular in some racing classes, since they count only the foretriangle area when calculating foresail size; a genoa allows a significant increase in actual sail area within the calculated sail area. In boats where sail restrictions are not applicable, genoas of 200% overlap can be found, although those over 150% are not often seen, since the additional area is shadowed by the mainsail and generates diminishing returns in terms of power per actual sail area.

The gennaker is a fairly new type of sail, and is a hybrid between a genoa and an asymmetrical spinnaker. Gennakers are even larger than genoas (200% overlaps are not uncommon). They have a much greater camber than a genoa, for generating larger amounts of lift when broad reaching and running, but the large camber results in poor performance when close hauled, making the gennaker a downwind only sail. Spinnakers, on the other hand, are strictly for running, and will collapse if used to reach.

Comment:

The phrase in the article above introducing the luff perpendicular is not clear to me.

"This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular")."

A line is a distance between two points. The clew is a point, but the luff is a line. One must define the point on the luff to use.

The word "perpendicular" describes a relation of 90°. A line cannot be described as a perpendicular in isolation. A perpendicular line is necessarily in relation (90°) to something else which has at least two dimensions (a line or a plane). Only once the relation has been described can one refer to the perpendicular alone.

If the editor of this article understands the LP, he can easily make the necessary clarifications. If not, I would be happy to research this point and do it myself. I can't do it right now because I'm in Paris and my yachting books are in Saint-Tropez.

I happened to read the article because I was hoping to learn the history of the genoa. I'm a specialist of racing sailboats of the class called the 6 metre International Rule. In our oral tradition, the genoa was invented by one of our illustrious owners of the 1920's, the Swedish diplomat Sven Salèn. He was apparently posted to Genoa when he developed it.

I was looking for information on the genoese contribution to this invention. The Web site of the Seawanhaka Corinthian Yacht Club of Oyster Bay, Long Island U.S.A. says that the first genoa was introduced to the U.S. during the 6 metre Scandinavian Gold Cup competition of 1927. It says that it was introduced by "an Italian boat". During this competition, Sven Salèn represented Sweden on May Be and there was also an Italian entry named Mati (now Fissa).

If anyone was in a position to introduce the genoa to the U.S. in 1927, it would have been Sven Salèn on his Swedish boat. Perhaps, Mati had also adopted the genoa. Perhaps the Seawanhaka confused Salèn's consular posting at Genoa with his Swedish boat - or perhaps they automatically ascribed this genoese novelty to the genoese boat Mati.

I'll ask them if they can sort it out.

Basil Carmody basil@carmody.ws Paris & Saint-Tropez, France

I have added a History section, with a brief summary of Manferd Curry's development of the "Genoa jib". Photos from his book or other sources of "the Swedish boat" and his dinghies (in particular the one of Aero) would be nice if anyone has time to add them. GilesW 12:46, 10 August 2007 (UTC)

I deleted the section that said the spinnaker was strictly for a run. Spinnakers can be used on reaches; if the pole is pulled far enough forward you can practically sail a beam reach.

Contents 1 Proposed title change 2 Aerodynamics 3 Venturi effect 4 Gennakers

[edit] Proposed title change

As mentioned in the History paragraph, the inventor of this sail referred to it as the "overlapping jib". It came to be known as the "Genoa jib" as explained. It is now usually referred to as the "jenny". I suggest changing the title from "Genoa (sail)" to "Genoa jib (jenny)" or something like that.

GilesW 11:37, 10 August 2007 (UTC)

[edit] Aerodynamics

The purpose of the large overlap is not only to provide additional unmeasured sail area in some classes of racing yachts and dinghies, its enhanced slot or venturi effect can also improve the overall aerodynamics of the rig.

The purpose of setting the sail as low as possible, "sweeping the deck", is to increase the aerodynamic efficiency of the sail by reducing the ammount of air escaping under the foot of the sail, from the high pressure (windward) side to the low pressure (leeward) side.

GilesW 11:40, 10 August 2007 (UTC)

Right, the slot is important, because it allows the sail combination to operate at a higher angle of attack, delivering more lift without stalling (see slats for the wing analogue). However, according to Marchaj (if memory serves), in classes where overlap is not calculated as part of sail area, large overlaps are found--up to 200% genoas; in classes where only actual sail area matters, you find little or no overlap. This seems to indicate that while the advantages of the slot effect are less than the advantages of using that sail area somewhere else--a masthead rather than fractional rig, or more area in the main. scot 16:52, 10 August 2007 (UTC)

Agreed. Curry did not discuss the best way to apportion a fixed total sail area. However I think that fractional rigs are preferred to masthead rigs these days, and masthead rigs usually use overlapping jibs, probably for good reasons. I suspect that an overlapping jib helps to reattach airflow to the lee of the mainsail behind a relatively thick mast, that might not happen with a non-overlapping jib (with such a mast).

I hear a lot about mast bending with respect to modern rigs, and I suspect this may have much to do with the preference for fractional rigs--I think that having the forestay at about the 3/4 point lets you put a precise amount of curve into the mast by playing with forestay and backstay tension, and thus control the flatness of the main with more range and precision than just using a cunningham and vang. The overlapping jib would help reattach flow by keeping flow from separating, but this is most needed at the top of the main, where the mast thickness is much greater compared to the length of the main. I do clearly recall Marchaj's experiment where he cut the tip off a triangular sail, and lost no lift, because the flow up at the tip was always stalled. This is addressed, when possible, with lots of roach up top and full length battens, and I think it's the new Volvo 70 class that I've seen with a giant headboard/batten to square off the top of the sail--puts me in mind of a short, horizontal gaff. scot 14:49, 17 August 2007 (UTC)

Incidentally Curry said that to take full advantage of overlapping jibs it is necessary to use a fully battened mainsail to improve the venturi effect, as he did on his dinghies. I suspect that subsequent developments in mast and sail aerodynamics may well affect his findings. GilesW 12:21, 12 August 2007 (UTC)

I think the reason for the fully battened main there is that you can easily get airflow off the trailing edge of the genoa pushing into the main, putting a bubble in the sail. I think Marchaj called this "backwinding the main" or something like that, and it can result in stalling the airflow over the lee side of the main. You're just trying your best to get me to go dig through all the boxes of books in my garage, and at a time when it's 103F outside... scot 14:49, 17 August 2007 (UTC)

Agreed. Curry explains all that. Try to borrow or buy a copy of his book. Although some of his explanations are mistaken, it's fascinating to read in the light of modern knowledge. He was closer to the truth than some of his sucessors, it seems.

As for sail sweeping the deck, that is the endplate effect, which has the effect of doubling the apparent aspect ratio of the sail, giving a higher lift/drag ratio since it's only trailing vortices from one end.

Agreed. However is not the aspect ratio doubling effect a rough approximation, that is rather esoteric for the body of an article of this nature? (Could be in a box etc). The pressure-loss explanation is not wrong and is more easily understood by the lay reader. This makes it easy to explain why setting a jenny a foot or two off the deck above a roller reefer reduces the efficiency of the sail significantly, as does the compromised aerofoil section of reefable jibs, though those losses can be offset by the advantage of being able to optimise the sail area quickly as the wind changes. GilesW 12:21, 12 August 2007 (UTC)

Well, the way the endplate works is to prevent pressure loss, which prevents a vortex from forming on that end of the sail, which cuts the vortex drag in half, so really they're the same explanation. scot 14:49, 17 August 2007 (UTC)

A non-overlapping headsail would have a higher aspect ratio, and thus a higher L/D ratio, anyway. I'd dig up some more solid data for this, but I've recnetly moved and my copy of Sail Performance is still buried in one of the dozen boxes of books in the garage. scot 16:52, 10 August 2007 (UTC)

Presumably the aspect ratio (AR) of the rig plan as a whole is more significant than that of individual sails, so that the low AR of the jenny has little effect on overall AR and hence L/D. The practical effect of changing sails from a No1 (jenny) to No2 (jib) when the wind increases is to "keep the boat on its feet", by reducing both L and D, but keeping L/D much the same. GilesW 11:26, 17 August 2007 (UTC)

I'm not sure of that, and I don't think I've seen any studies on it. The reason a high aspect ratio sail has lower drag is that the tip vortex size is related to the chord of the foil; a 1 foot chord foil will have the same size tip vortices no matter whether it's 1 foot or 100 feet span. Now if this relationship is linear (as I suspect it is) then a single 100 by 100 foot sail will generate 1 vortex 100 times the energy of a 1 by 100 foot sail, and and 100 of those 1 by 100 foot sails will thus have the same total drag as the 100 by 100 foot sail. That would then indicate that the sum of the drag would be the sum of the drags of the aspect ratios times the relative sail area of each sail, which should be close to the aspect ratio of the rig as a whole. scot 14:49, 17 August 2007 (UTC)

Rig AR is addressed in Rig Theory AYRS 113, ISSN 0144-1396, which refers to Marchaj et al. GilesW 18:24, 21 August 2007 (UTC)

[edit] Venturi effect

I cannot find the term "venturi effect" in Curry's book. I only used it because there was no WP entry for "slot effect". Naughty me! I will correct it.

Curry did claim accelerated airflow behind the overlapping jib, when used with a fully battened main. He also showed correctly that the airflow close behind a main with a jib is faster than the (stalled) airflow behind the same main at the same AoA with no jib.

Curry was adamant about their advantages over soft mainsails because they allow a narrower slot and a more efficient slot effect. He likened soft mainsails to umbrellas without ribs. I cannot find anything in Marchaj AHOS 2nd ed about fully battened mains. It is fascinating to study Curry's results in the light of modern knowledge, he was surprisingly close to the truth, and spotted something that Marchaj seems to have missed & I had to learn the hard way: when looking for power, mainsails are operated on the ragged edge of the stall. However Currys boats were evidently so much better than the opposition that he sometimes drew the wrong conclusions. He did not mention two-boat tuning.

I think this is the subject of another article. GilesW 17:27, 21 August 2007 (UTC)

[edit] Gennakers

I suggest replacing specific angles with qualitative descriptions, e.g.

"Flat cut gennakers can generally be used closer to the wind than spinnakers. However when running before the wind, the main sail blocks the wind of gennaker, whereas the conventional spinnaker's movable pole allows it to be kept full." Or something like that. GilesW 11:15, 30 August 2007 (UTC)