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Old 11-06-2016, 14:28   #1
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Question Mainsail/Headsail lift?

On a beam reach for example, if a mainsail has the same square foot area as the headsail, and both are trimmed properly. Which one generates the most lift (pull), and why?
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Old 11-06-2016, 15:05   #2
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Re: Mainsail/Headsail lift?

The headsail. As it's not tucked into the dirty airflow created by the mast. Nor is it operating in the lee of another sail. Including the backwash & dirty air created by same. Plus, it's also upwind of your shrouds, & crap air created by other deck mounted items. Like say, dodgers.
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Old 11-06-2016, 15:11   #3
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Re: Mainsail/Headsail lift?

If the wind is strong enough and you have a long boom and low aspect mainsail/rig, the mainsail will generate lots of lift and weather helm.

So it would be the mainsail on certain boats.
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Old 13-06-2016, 15:28   #4
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Re: Mainsail/Headsail lift?

Thanks for the replies, I can see the merit of them and would agree that for the reasons listed, the main would have the most lift. I also noted that no one has included the battens as contributing to the mainsails advantage. If we keep the circumstances the same, and remove the battens from the equation, would the mainsail still have the advantage? Also, is it the length of the boom that gives the advantage or just that the foot is longer with a longer boom? Thanks again, Sarge.
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Old 13-06-2016, 15:54   #5
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Re: Mainsail/Headsail lift?

The main always generates more lift per square foot compared to a jib. It has a higher aspect ratio, a better air foil shape, and its leading edge is verticle.

You can't just remove the battens by the way, it would have to be a completely different sail. My main without battens would be so droopy I doubt you could get it to set correctly at all, a bit like asking who is taller, me or Michael Jordan if you take all the bones out... I would guess him because his puddle of formlessness started taller, but it depends on how the goo shifts.
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Old 13-06-2016, 18:34   #6
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Re: Mainsail/Headsail lift?

. Which one generates the most lift (pull), and why?[/QUOTE]

The two sails work together. The mainsail causes a circular effect that makes more air travel behind the headsail then there would be without it. It also causes a lift or change in the entry angle of air coming into the head sail. The most lift or pull from the two sails working together comes from the headsail.

If you were to test pull from the main sail all by itself compared to the headsail all by itself there would be lots of variables between different boats, rigs, and wind speeds.

I would suggest reading "The Art and Science of Sails" by Tom Whiten former president of North Sails
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Old 13-06-2016, 21:00   #7
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Re: Mainsail/Headsail lift?

Quote:
Originally Posted by Sarge17 View Post
On a beam reach for example, if a mainsail has the same square foot area as the headsail, and both are trimmed properly. Which one generates the most lift (pull), and why?
On my boat, it most definitely depends on the strength of the wind. The stronger the wind, the more the power moves aft (i.e., more power in mainsail). But what I love about my boat (Pacific Seacraft 37) is that I can easily balance it in any wind conditions so that it tracks straight and steers easily.
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Old 14-06-2016, 03:04   #8
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Re: Mainsail/Headsail lift?

I'm still in the camp of the jib being the HP leader. As when on a beam reach, on most boats up to 50', I can pull in the main by hand (sans tackle advantage), & I can't do as much with a jib of the same size.

Also, it's a RARE boat which has a main that has a higher aspect ration than the iib. For the highest effective aspect ratio of a Bermudan sail is 3.6:1. And on masthead boats, this generally dictates the spar height (on a racer). With the mast's height being 3.6 x J
Thus, with that mast height, the luff of the main is many feet shorter than the jib's luff length. So that those standing in the cockpit don't have to duck as the boom swings across. Which results in the main having a shorter luff, & thus, a lower aspect ratio.

And on fractional boats, the ratios are closer. But still, the jib's operating in clean air as compared to the main.

The, if you go to a jib with battens, it's performance climbs geometrically higher, due to it's enhanced shape. Plus, sometimes you can add a bit of extra area to it's leech as well.

There are a huge number of factors which play into this question, so there's no one answer. But from many thousands of miles of racing, I know that on most points of sail with an upwind component, it's easier to have compromised/poor mainsail trim. Which turns it into a brake. And that doesn't happen nearly so much, or so easily, with a jib. When both sails are operating at the same angles to the wind.

Plus, how can the main be more efficient when it's operating in a continual header generated both by it's position behind the spar, & behind the jib?
As on windward type points of sail, when well trimmed, the first several feet of the main are right on the edge of continually luffing. And ideally, you carry a small patch of luffed sail (if not full battened) for most of the main's height.


Witness the French IACC boat in '92 that literally cut a hole in the leading edge of the main, about 2' long, fore & aft, for 2/3 of the sail's luff. So that they could put the cut out cloth (measured area) onto the leech of the sail, where it could do some good.
Yes, to some degree it was a PR stunt, as they were doing poorly by that point in the tournaments. But they wouldn't have purposefully done it if it were slow. Even on the downwind legs.
And it's not as if the main's on those boats were inefficient, shape wise. We had LOTS of tuning controls for the main's on them. Enough to overwhelm most sailors, including racers, up to the expert level.

Ever sail on a boat that has battens costing $30K per set? With sometimes as many as 5 different sets for each sail. To be chosen for the wind/weather conditions of the day.
And batten tuning/tensioning hardware with built in micrometers, for easy resetting of Exact batten tension. Whenever the sail's trimmer wanted/needed such adjusted. Including when actually sailing.
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Old 14-06-2016, 08:53   #9
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Re: Mainsail/Headsail lift?

Uncivilized, you toss out a bunch of ratios that have nothing to do with reality.

My sail has a 5:1 aspect ratio, plenty of others exceed the 3.61:1 you suggest. And most racers these days do not have masthead rigs at all, they have primarily moved to fractional rigs, because modern rules limit or penalize total square footage of sail no matter how it's broken up, so a smart designer moves the sail area to the main.

I am not going to deconstruct your post line by line, there is too much wrong there. But when designers have freedom over where to put sail area it will always go into the main first. It doesn't matter if the boat is designed for off wind reaching, or w/l on a short course. The main is always the more efficient sail per sq foot.

Once mainsail area is maxed, usually by hitting the maximum mast height, then jobs or thrown on as an afterthought. It's better than not having anything there, but a unirig (no headsail) is a far more efficient rig for the same sq foot of sail than a Bermuda.
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Old 14-06-2016, 10:43   #10
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Re: Mainsail/Headsail lift?

Stumble I have the utmost respect for your knowledge and post, but your multi hull bias is getting the better of you.

Fractional rigs such in light air. That's why mast head rig J29 s are preferred 10/1 over the fractional rig

Larger mains are easier to handle then large head sails. Fractional rigs are easier to control as far as mast shape and changing gears.

The Op asked about two sails with equal sail area.

Let's take the argument to the exstream. If we take two equal sized americas cup style wing sails and put one in front of the other and trim them properly, the forward wing will have more pull than the aft wing.

Just like if we put two lasers side by side, the safe leeward boat gets a lift and sails out from under the windward boat every time.
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Old 14-06-2016, 12:14   #11
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Re: Mainsail/Headsail lift?

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Originally Posted by toddedger View Post
Stumble I have the utmost respect for your knowledge and post, but your multi hull bias is getting the better of you.

Fractional rigs such in light air. That's why mast head rig J29 s are preferred 10/1 over the fractional rig

Larger mains are easier to handle then large head sails. Fractional rigs are easier to control as far as mast shape and changing gears.

The Op asked about two sails with equal sail area.

Let's take the argument to the exstream. If we take two equal sized americas cup style wing sails and put one in front of the other and trim them properly, the forward wing will have more pull than the aft wing.

Just like if we put two lasers side by side, the safe leeward boat gets a lift and sails out from under the windward boat every time.
It has nothing to do with multihulls it has to do with basic aerodynamics. The most efficient shape for an airfoil is perpendicular (or very close) the the fluid you are moving thru. The incident angle of the headstay causes a substantial amount of drag that is not overcome by any additional lift generated. The very shape of the sail itself is the limiting factor.

Even worse the forestay has a cantenary both to leeward and back, further adding drag and reducing lift.

Finally, large overlapping jibs are almost the worst possible shape for an air foil. Their low aspect rations provide a huge amount of drag for minimal amounts of power.


The reason that many sailboats have masthead rigs is because of old rating rules that heavily penalized mainsail area, while favoring large jibs. It's the same reason why small Ketch rigs came into vogue for a weekend or so 50 years ago, or massively long over hangs became popular. All of these were greatly favored by rating rules and so racers moved to them in order to maximize their gains under rating rules. But they were never designed based upon ideal engineering.

it is instructive to look at what happens when rating rules are simplified. Under rules that:

1) penalize main sail area - jibs get bigger
2) provide no adjustment for sails aft of the main - ketches come into vogue
3) restrict total sail area - boats move to unirig (just a large main sail)
4) provide a maximum height of the mast and maximum sail area - first maximize main area, then use the rest on jib.

The only rule where designers increase the size of the jib is when mainsail area is heavily penalized.

Since from about the 1930's under CCA thru the early 80's under IOR main sail size was heavily penalized it wouldn't suprize me that you find a lot of older boats with large jibs and small mainsails. But that doesn't mean the sail area is used efficiently for sailing it just means they were used efficiently under the rules of the day.

And fractional rigs do not suck in light air. Underpowered rigs suck in light air. If you have any doubt, just take a look at the AC multihulls that are foiling in 8kn of breeze and doing 25kn in 12. They have a vestigial fractional jib at best and a massive wing sail.

On the other hand short of putting on a new mast there are limited ways to increase the mainsail area, while adding a larger and larger jib is relatively easy. So once the mast is set then you are back to 1) maximize main area, 2) add as much as you need to the jib. But this is to correct for an under powered boat, not best blank sheet design.
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Old 14-06-2016, 12:31   #12
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Re: Mainsail/Headsail lift?

No battens means roach-less main; you aren't going to point very high.
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Old 14-06-2016, 12:44   #13
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Re: Mainsail/Headsail lift?

My boat sails far better under jib alone than just the main. This is partly due to the jib having almost 50% more area but the performance is more than 50% better. With jib alone I can nearly match performance with other boats carrying full sail while main alone she just tubs along slowly.

My .02
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Old 14-06-2016, 12:49   #14
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Re: Mainsail/Headsail lift?

There are some misunderstandings here about the effect of aspect ratio. A high aspect ratio sail will have the highest lift coefficient (a measure of lift per square foot) at small angles of attack, ie beating. As the angle of attack increases, ie falling off the wind, the lift coefficient of the high aspect ratio sail quickly degrades. At large angles of attack, as in reaching, a low aspect ratio sail has a much higher lift coefficient. See Marchaj "Sail Performance" for lift coefficient measurements on sails with different aspect ratios. Low aspect ratios are much more powerful when off the wind.

This is why gaff rigs do so well off the wind.
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Old 14-06-2016, 13:20   #15
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Re: Mainsail/Headsail lift?

Pauls,

Thanks to modern sail handeling equipment like travelers, sheets, and boom vangs we can alter the angle of attack of the sail to the wind substantially. Certainly within the range of the OP's question out to 90 degrees AWA.

Marchaj Falls apart in reference to modern high aspect square headed and fully battened sails because they really weren't an option at the time. And many of his conclusions have to be suspect since he was such a huge proponent of the Crab Claw rig. He was so sure of it, and Marchaj has such a great reputation, GunBoats even tried a CC rig on a small catamaran (I think) which was mostly an underwhelming disaster.
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