How close to the wind can a boat sail?

Discussion in 'Sailboats' started by tonyr, Jun 10, 2004.

  1. tonyr
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    tonyr Junior Member

    I was taught at school many years ago that there was an effective limit of about 45 degrees to the true wind for a conventional monohull sailboat. If you try to sail above that angle, your drag vector of the hull and sails exceeds the generated thrust of the sail/s, and you stall and stop.

    In a discussion on sailing upwind in another place, the claim was made that a modern 12 metre can sail at 35 degrees to the true wind, which implies that it can tack through not the usual 90 degrees, but through some 70 degrees.

    Does anyone know of hard evidence one way or the other, say validated in tank tests, or in controlled and measured conditions on the water?

    Thanks, Tony.
  2. sailsnail
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    sailsnail Junior Member

    You don't need tank tests, just figure how all those cruisers made it back from New Zealand! Or look out your window at any Sunday morning Regatta.

  3. Guest

    Guest Guest

    There are no 12 metres that would be considered as modern performance boats. None have been built since 1987.

    The replacement for them in the America's Cup are the ACC boats, larger and lighter than the 12s and with more sail. These boats are generally tacking though less than 70 degrees and probably average something like 65 degrees. That's 32.5 degrees to the wind on each tack and probably closer on one tack than the other due to wave conditions. This is very evident when watching them match race on the computer tracking during the Cup matches.

    They are possibly the most close winded keelboats sailing at the moment.
  4. tspeer
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    tspeer Senior Member

    There's the question of how close you can sail to the true wind, and then there's the question of how close you want to sail to the true wind. The two are quite different! There's a fundamental reason why boats go upwind at near 45 degrees, and that's to maximize their velocity-made-good to windward.

    A moderate performance boat like the AC class will point well above 45 degrees because while they can sail to a comparatively small apparent wind angle, they can't actually go fast and so must opt for a high angle.

    A high performance craft will sail at comparable apparent wind angles, but will opt for a lower course. The extra speed will more than make up for the extra distance sailed. So you won't actually see these boats tacking through less than 90 degrees.

    Theory says if the boat has very high performance, such that the speed can increase freely to the point that the boat sails at the same apparent wind angle all the time, the optimum course to windward is 45 degrees plus half the apparent wind angle. Very few craft can come close to this ideal and must sail higher and slower. But you can see that 45 degrees is a fundamental number and not a coincidence.
  5. Guest

    Guest Guest

    Skiffs regularly sail at much cloer angles than 45. At times they rival the cup boats. There isn't much more high performance in monhulls than skiffs.
  6. tonyr
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    tonyr Junior Member

    "Skiffs". Do you refer to a particular design here? Which one? Can they achieve their best VMG at less than 45 degrees to the true wind?

    I thought that Tom Speer's explanation was a fair summary of the situation as I understand it - one can sail closer, but (usually) it is wiser to ease off and go faster to achieve the optimum VMG.

    Thanks, Tony.
  7. DavidG
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    DavidG Junior Member

    Whilst on this thread, any thoughts on leeway angles?
  8. Chris Krumm
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    Chris Krumm Junior Member

    Regarding leeway angle, less is better. Your underwater foils need to present a positive angle of attack with regard to the direction of travel of the boat in order to generate any side force to resist the opposing side force of the sails.

    But while lift (side force) tends to increase linearly with change in angle of attack (or leeway), drag increases approxiamately to the 2'nd power, except in a very narrow "bucket" at low angles of attack on order of 1-3 degrees. Your best ratio of lift to drag is therefore at an angle of attack or leeway angle of a few degrees. There are practical limits to this in designing a 3d foil though; i.e. you're restricted by the surface area, aspect ratio, draft, or physical properties of the material you have for the keel.

    Some boats, such as inland lake scows, use dual bilgeboards/daggerboards toed in 3-4 degrees to minimize leeway and hull drag while presenting an optimum angle of attack for the foil on the appropriate tack. Hi-aspect foils are much better at developing side force than an immersed canoe body.

    Chris Krumm
  9. tspeer
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    tspeer Senior Member

    Not necessarily, at least from a design perspective. The path through the water is determined by the lift/drag ratio. The leeway angle is arbitrary, and it determines the heading of the boat relative to the path through the water.

    For example, one can reduce the leeway to an arbitrarily small angle just by adding sufficient area to the keel. However, this will not necessarily improve performance because of the excessive wetted area. One can also reduce the leeway angle by sheeting out the sails - but that will not improve performance, either, unless the sails were over-sheeted to begin with.

    There would be nothing wrong with twenty or thirty degrees of leeway, were it not for the fact that hull and keel would produce excessive drag and little lift at those angles.

    I think you're confusing two sources of drag. The two-dimensional section may have the profile drag due to viscous effects be constant within a drag bucket. But the induced drag due to production of lift increases as the square of the lift. The induced drag is in addition to the profile drag.

    For the classic parabolic drag polar, the best ratio of lift to drag occurs when the induced drag is equal to the other sources of drag. When the other sources of drag are high, the optimum L/D occurs at high lift. Often it's the case that separation and stall sets in before reaching the maximum L/D for attached flow.

    Definitely! Induced drag is inversely proportional to the square of the depth. And the drag due to lift produced near the surface is higher than for the same lift produced away from the surface.

    The amount lift on the foil is independent of the design of the foil - it is set by the sail trim. So the point is to produce the required lift with the minimum drag. So one makes the foil deep to reduce the induced drag, and one reduces the chord to cut the wetted surface and reduce the skin friction drag.

    But reducing the chord increases the leeway angle required to achieve the lift demanded by the rig. So from a design perspective, increasing the leeway actually improves performance!
  10. redcoopers
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    redcoopers Member

    This is why AC yachts have trim tabs on the keel. This reduces leeway EXTREMELY. For those of you not acquainted with trim tabs, think of them as flaps for the keel. The trim tabs are controlled by the inner wheel on the helm of an AC yacht.

    If any of you watched the races a while ago, you'd remember that these boats would go to wind at almost 30 deg TWA while maintaining about 8 - 8.5 knots. Trim tabs are pretty impressive.

  11. DavidG
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    DavidG Junior Member

    Perhaps I should have re-phrased my question "Has any idea what leeway angles are acheived when sailing a monohull to windward?"
  12. SailDesign
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    SailDesign Old Phart! Stay upwind..

    David - the answer is whatever you want it to be :)
    With asymmetrical daggerboards and trim tabs and all that other good stuff, you can pretty much have as little leeway as you want. Even negative is possible, although there are some nasty drag penalties in that range.
    As far as "what leeway angles do conventional monohulls sail at" goes, anywhere from 2 to 5 degrees would be about right, although I'm sure Tom could put some decimals in there for youy :)
  13. Doug Lord

    Doug Lord Guest

    Don't forget CBTF

    There is another topic that goes further into this but CBTF boats can zero out leeway by using "collective" steering. The boats have a forward foil and an aft foil that develop all the lateral resistance for the boat-the canting keel strut and hull are designed NOT to have to provide any portion of the lateral resistance.
    These foils are collectively turned the same direction upwind to eliminate leeway; they are also used tactically to go sideways to windward for
    short distances.
    Three major examples of this technology have recently been launched: Genuine Risk at 90' and the two maxZ86's. Reichel -Pugh's website had excellent illustrations posted of the underwater foil arrangements on these(z86's) boats...

  14. brian eiland
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    brian eiland Senior Member

    Cruising Aspect

    Back in 1971, I was looking at this question of optium performance to windward by an ocean going CRUISING boats. I was greatly impressed with Lord Riverdale's (Arthur Robb designer) very significant experimentation with twin-keel vessels. I was also suitable impressed with the asymmetrical daggerboards on Bruce King's SORC racing boats.

    The combination of the two concepts into asymetrical twin-keels with a preset angle of attack is what I sought in my first monohull drawing:

    As I point out on the website, Contrary to the use of symmetrical twin keels as popularized by Westerly, my design utilized asymmetrical foil keels which were placed at a 3 degree angle to the fore/aft centerline of the vessel. The asymmetrical foil shape was more efficient per square foot of keel area at developing the leeway reducing forces, thus the total wetted surface area could be reduced. The 3 degree toe-in was used to avoid the vessel having to crab sideways to its intended direction in order to develop any lift at all (symmetrical foils must be driven at some angle of attack in order to develop lift). The asymmetry and the pre-set angle of attack would act to significantly reduce the 6 to 12 degree leeway experienced by conventional symmetric-keeled vessels.

    The combination of the foil shape and the skewed angle of attack was accomplished with very little increase in total frontal area projection over a traditional single fat-foil keel (ed note: remember Charlie Morgan's Out Island 41 was just begining to capture the cruising world's attention at this time). The two keels were attached to the hull such that as one became vertical upon heeling, the other assumed a more horizontal attitude and contributed to the righting stability of the vessel. Thus I had the shallow draft of the Out Ialand vessels with a significant better leeway capability.

    And to add icing to the cake I incorporated my then new mast-aft sailing rig. The mast-aft rig's absence of a conventional mainsail imposed less leeway inducing forces so that the keel's surface areas could be reduced (extra wetted surface area was always recognized as a nemesis of twin-keeled vessels). The overall lower center of effort of this sail plan allowed for less voluminous keels (less ballast required).

    Should we question the relatively poor contributions of the mainsail? It is most primary in leeway inducing. Per my site, conventional booms excessively flatten the foot of the mainsail, and are often oversheeted, contributing significantly to the leeway forces. I once had a copy of a test on a Morgan 41' Out Island ketch , where upon removing the mainsail, the boat lost only 1/2 knot of speed, but cut its leeway in half (from 11 to 6 degrees). A staysail was then rigged between the masts in place of the mainsail, and the boat regained 1 knot of speed while retaining its decreased leeway.

    Or as Tom Speer hints at above , for example, one can reduce the leeway to an arbitrarily small angle just by adding sufficient area to the keel. However, this will not necessarily improve performance because of the excessive wetted area. One can also reduce the leeway angle by sheeting out the sails - but that will not improve performance, either, unless the sails were over-sheeted to begin with. (BE notes: It very often the case that the foot of the mainsail is oversheeted, and is contributing very much to the leeway).
    Or in another posting by Tom,However, I think the important point is the huge vortex coming off the boom...... The vortex is also strongly affecting the region of the mainsail where the chord is the greatest. People usually concentrate on the vortex at the top, but the vortex at the foot may be more significant. It's worth considering how to shape the mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence.
    To which I would add Steve Dashew’s observation,
    On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.

    On our 67' ketch, Sundeer, we were able to pick up FIVE DEGREES in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.

    The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats.

    Brian added:
    Now if we look at the heights off the deck (and deckhouses) of today’s booms, particularly multihulls, we really have to look at both the inefficiencies and the leeway questions for traditional mainsails, let alone their zone incompatibilities with the headsail.

    Finally I might add, that when I drew my monohull concept up, center-cockpits aft-cabin cruisers were all the rage. But I thought it would be much nicer to have a raised center covered deckhouse, albeit a mini one, somewhat akin to the central saloon of the cruising cat, and the same idea that is now so prevalent on today’s monohull cruisers.

    I still believe the twin-keeled monohull has a whole lot of potential that’s gone unexplored.
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