Is an asym actually faster that a sym at various points of sail?

[Foolish Muse]

Is an asym chute actually faster than a sym at various points of sail, such as 90 apparent, 120 apparent, 145 apparent? Assume the same boat and appropriately sized sail. I've heard that because of the better shape of the luff on an asym, that there is better flow and greater lift that drives the boat forward faster. Is this actually true?

 

[flaming]

Is an asym chute actually faster than a sym at various points of sail, such as 90 apparent, 120 apparent, 145 apparent? Assume the same boat and appropriately sized sail. I've heard that because of the better shape of the luff on an asym, that there is better flow and greater lift that drives the boat forward faster. Is this actually true?

In a word, yes.

That doesn't mean that for short course racing (where you might expect to gybe a lot) that the A-sail is faster from the windward mark to the leeward mark on all boats. Boats that have small gybe angles lose almost nothing will a well executed Sym gybe, they lose a fair bit gybing an A-sail. And the gains from an A-sail speed wise are small unless your boat can plane.

However, offshore where gybing is much less frequent, A-sails are gaining a lot of traction, even on boats that have traditional poles. For example the JPK1010 that won the fastnet overall in 2013 has an all A-sail inventory, despite using a conventional pole setup.

 

[Foolish Muse]

Thanks. I'm only thinking about straight line speed here, not taking gybing into account. So at 90 apparent or 120 apparent, just how much faster would an asym be just because of the shape of the luff and flow of wind?

 

[Ingwe]

You also need to bear in mind that the asym kite for any boat will have a larger area than the sym it would use eg on my sf3200 the asym kites are 92m and the sym is 82m - either option gives pretty much the same rating under irc as long as you go bowsprit only with the asym option. So at any sort of slightly reaching angle you will have a significantly larger projected area with the asym kite whereas dead downwind you can probably get a larger projected area with the sym.

 

[savageseadog]

I came across a situation once on an offshore race where an asymmetric wasn't faster. The wind shifted gradually from a reach to a dead run as we got nearer the mark. The symmetric boats shifted their poles back and we made the mark without a gybe. The asymmetric boats ended up too far to windward.

 

[DFL1010]

I came across a situation once on an offshore race where an asymmetric wasn't faster. The wind shifted gradually from a reach to a dead run as we got nearer the mark. The symmetric boats shifted their poles back and we made the mark without a gybe. The asymmetric boats ended up too far to windward.

And yet there was the time where the wind went from broad reach to one point fwd of the beam. All the sym boats were spinning out or pointing downwind of the mark, and those with nice flat A sails were cracking on.

Or, going DDW along a narrow channel - hold one gybe for ages vs gybing every five seconds.


In other words, of course there are times when an S kite works out faster. But that wasn't the question asked.


p.s. if you fly an A sail off the pole, then you can still shift the pole back.

 

[Foolish Muse]

From what I'm reading on line, the change to PHRF handicap rating is tiny (if at all) for moving from a sym to an asym of the same size. This whole question arose because of an article in 48 North Magazine in which the author, a sail maker himself, made the statement "The A-sails are just more efficient because the air enters the luff and exits out the back nicely through an open twisted leech. " Later in the article he says "and induce more lift than symmetricals thanks to that even entry/exit flow across the sail. This results in less turbulence within the sail edges."

I'm questioning if this is really correct. So yes, I do understand that asyms might be better at very tight reaches and worse at deep runs. But what I'm wondering is if there is an actual better aerodynamic air flow (more lift) with an asym at a beam reach 90 deg. or a broad reach 120 deg. Is it just a better wing?

You can find the article on page 46 here: https://48north.com/wp-content/uploads/the-magazine-download/2016/48-north-april-2016.pdf

If you happen to read the article, I really question his statement about the asym being better for the Vic/Maui and Pac Cup races from North America to Hawaii. Both of these races have very long sections of deep down wind sailing, with 20 knot and higher steady winds. Boats with asyms need to zig-zag down this course adding considerably to their distance. With the high winds boats with syms can go a straight line at high speed.

 

[bbg]

No clear answer. I would say it depends on the way it is cut, the way it is set, the AWA etc.

 

[savageseadog]

In other words, of course there are times when an S kite works out faster. But that wasn't the question asked.

Mea culpa, I was looking at it holistically.

 

[Birdseye]

Is an asym chute actually faster than a sym at various points of sail, such as 90 apparent, 120 apparent, 145 apparent? Assume the same boat and appropriately sized sail. I've heard that because of the better shape of the luff on an asym, that there is better flow and greater lift that drives the boat forward faster. Is this actually true?

Its an interesting question and one I dont know the answer to. Its not just an issue of the shape of the luff anyway - the luffs of the two sails are in different locations on any tack other than as hard on the wind as you can take a sym. And it can never be a question of just the one sail anyway because you are always using both sails in conjunction with a main sail. In effect with the sym you are always part goosewinged. And finally there are issues of boat design because its udoubtedly true that you can get more out of an asymetric in a boat that is fast enough to bring the wind well forward - not only that but you dont necessarily even sail the same courses do you. Tacking downwind can make sense with an asymetric but not with a symetric.

I dont think that the question is answerable in the terms you have put it, which suggests that the sailmaker is wrong as you suspect

 

[Triassic]

I think the answer is that it very much depends on your boat. As Flaming said in post 2 if the hull will let it take advantage of the higher speeds an AS is capable of generating then it will be faster, however if it isn't then better to stick with the SS and go straight to the destination. Remember tacking downwind at 30 degrees off true only adds 10% to the distance, so on most of the boats we are talking about you only need an extra knot to make it worthwhile.

In some conditions my F27 will almost double it's speed when sailed properly off the wind rather than on a straight run, it's a complete no brainer!

 

[lw395]

Basically, an A-sail only has to work one way around.
The S-kite has to work both ways, so the shape is more compromised.

The issue of poles vs bowsprits clouds that sometimes, in dinghies there are definitely times when a conventional pole/kite set up has advantages over a sprit/asy dinghy, but on average a look at PY's shows you that for a similar size and weight of boat the asy has a big edge.
I know asy's tend to be bigger sails, but that's part of the point, it's a more efficient sail so a given crew weight can carry a bigger one.
In yachts which don't plane and don't surf much, a big symmetrical bag that will drag you dead downwind will probably be just as effective as an A-sail in that scenario.

 

[mrming]

Basically, an A-sail only has to work one way around.
The S-kite has to work both ways, so the shape is more compromised.

The issue of poles vs bowsprits clouds that sometimes, in dinghies there are definitely times when a conventional pole/kite set up has advantages over a sprit/asy dinghy, but on average a look at PY's shows you that for a similar size and weight of boat the asy has a big edge.
I know asy's tend to be bigger sails, but that's part of the point, it's a more efficient sail so a given crew weight can carry a bigger one.
In yachts which don't plane and don't surf much, a big symmetrical bag that will drag you dead downwind will probably be just as effective as an A-sail in that scenario.

My experience in a small, quick but ultimately non planing yacht is that the a-sail is a great tool for a long reaching leg but it's definitely not faster than a sym kite in a windward leeward situation. You can sail the angles all day but it's not worth it.

 

[Birdseye]

I think the answer is that it very much depends on your boat. As Flaming said in post 2 if the hull will let it take advantage of the higher speeds an AS is capable of generating then it will be faster, however if it isn't then better to stick with the SS and go straight to the destination. Remember tacking downwind at 30 degrees off true only adds 10% to the distance, so on most of the boats we are talking about you only need an extra knot to make it worthwhile.

In some conditions my F27 will almost double it's speed when sailed properly off the wind rather than on a straight run, it's a complete no brainer!

The speed increase needed is 15.5% at 30 degrees ( 1/cos30) and 30 degrees can be marginal in terms of shadowing from the main. At 40 degrees its 30%. Your boat is far from typical in doubling its speed.

Not all boats are fractional too. With a masthead rig the sym is likely to be as big as an asymetric.

 

[lw395]

My experience in a small, quick but ultimately non planing yacht is that the a-sail is a great tool for a long reaching leg but it's definitely not faster than a sym kite in a windward leeward situation. You can sail the angles all day but it's not worth it.

Are you talking A-sail on a sprit or A sail on a pole?
Either way I don't disagree on a dead run.

Also depends on wind strength.
In 8 knots of breeze, 6 knots of boat speed can bring the apparent wind on to the beam and still be going somewhere useful.
In 18knots of breeze, the apparent isn't going forwards unless you have a skiff.

 

[mrming]

Are you talking A-sail on a sprit or A sail on a pole?
Either way I don't disagree on a dead run.

Also depends on wind strength.
In 8 knots of breeze, 6 knots of boat speed can bring the apparent wind on to the beam and still be going somewhere useful.
In 18knots of breeze, the apparent isn't going forwards unless you have a skiff.

A-sail on a sprit with a projection of about 1m. I should add that where we race is a tidal estuary and the a-sail angle usually takes you out into adverse current. Otherwise yes it might be worth trying in lighter weather. For now I've had it taken off the IRC cert and we're just sticking to the sym kite.

 

[Triassic]

The speed increase needed is 15.5% at 30 degrees ( 1/cos30) and 30 degrees can be marginal in terms of shadowing from the main. At 40 degrees its 30%. Your boat is far from typical in doubling its speed.

Not all boats are fractional too. With a masthead rig the sym is likely to be as big as an asymetric.

I knew there were some maths involved somewhere.... With regard to shadowing the main there is no way you would sail with the apparent 30 degrees off the stern, (150 off) we try to keep it between 90 and 100 unless the wind really picks up in which case we may go to 110. This keeps the true at about 150 or more, and as you say that adds about 15% to the distance, which is nothing given the speed gains involved.

 

[Triassic]

ps: if you really want to blow your mind around apparent wind and the angles you can generate have a look at the AC72 boats. I seem to remember that the difference in their apparent between beating and running is only 8 degrees, and they're going so fast downwind that when they "gybe" the wind actually comes around the bows effectively tacking the sail.

 

[savageseadog]

It's interesting how a-sails have improved in a relatively short space of time. I have a circa 15 year old a-sail in good condition which is absolutely no match to a two year old one. The old a-sail although cut differently to a symmetric kite is nearer to a symmetric spinnaker than a modern a-sail which is more like a jib or genoa shape but deeper, more shape. The old one only works through a very narrow band of wind angle.

 

[lw395]

ps: if you really want to blow your mind around apparent wind and the angles you can generate have a look at the AC72 boats. I seem to remember that the difference in their apparent between beating and running is only 8 degrees, and they're going so fast downwind that when they "gybe" the wind actually comes around the bows effectively tacking the sail.

Ice yachts and land yachts more so!