Self tacking jib versus overlapping.

Discussion in 'Sailboats' started by Omer, Oct 14, 2005.

  1. Omer
    Joined: Oct 2005
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    Location: istanbul Turkey

    Omer New Member

    Hi Everybody.
    i am a new member from istanbul turkey.
    i am currently trying to design a sailboat which will be built
    in wood-epoxy. Overall length will be around 37 foot.
    i plan to cruise shorthanded on the agean coast and ocassionally
    participate in races. Therefore we are talking about a boat which
    will be a cross between a racer-cruiser and a short handed cruiser.
    My question is about performance difference between self tacking 100 %
    jib versus a more conventional overlapping genoa.
    for the same boat and for the same sail area what would be the gains
    and losses in terms of both speed and IRC rating.
    i am particularly interested in beating performance as downwind sails
    take most care of downwind legs.
    will a bigger main and smaller jib much inferior to smaller main and genoa
    combination?
    My D/L target is around 175 and my SA/D target is around 20. if this
    contributes to the equation.
    Many thanks.
     
  2. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Performance wise there's no doubt in racing circles that a non-overlapping or minimally overlapping jib is the best option in terms of horsepower per square metre. The only real advantage of the big genoas is that it means that you can hang more sail on the same spars.
     
  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    In many classes the overlappng area is "free", that is not measured. Look at the Scandinavian 30 square meters for example, the nomnal area is 30m2, but they have huge genoas, so the real area is maybe 40m2.
     
  4. FAST FRED
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    FAST FRED Senior Member

    The disadvantage with a self tacking sail is there is usually a jib boom attached .
    These become ankle bashers if you work the foredeck anchoring ect.
    There is sometimes also a traveler , to trip on too.

    The Dutch came up with a simple solution , they sew a heavy eyelet in the foot of the sail before the mast. And use a line on deck thru the eyelet to work as a traveler.

    When short tacking the sail IS self tending , at least the luff and most of it.
    Only the part from the eyelet to the clew needs to be hauled tight for full performance. Usually just the tight luff and most of the area is needed in a river.

    FAST FRED
     
  5. Tony H
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    Tony H New Member

    I can understand how area-for-area performance (‘horsepower per square metre’) is maximised by using a non-overlapping jib - the projected area of the sails is as large as possible.

    But in designing a boat with a free choice of rig-design, we are looking for maximum thrust for a given heeling moment (not side-force as is often quoted). It seems to me that an overlapping jib might well permit a larger total area and more thrust than the alternative non-overlapping version with the same heeling moment; I’ve found no test results which shed light on this, however (like I said, people have usually concerned themselves with side-force, neglecting the height of the centre of pressure/heeling moment).

    Other factors to consider:

    A non-overlapping/barely overlapping jib seems to make de-powering by flattening and easing (reducing the incidence angle of) the mainsail more effective. Thus a larger total area might be used than at first seems possible when considering the fully-sheeted-in condition.

    The precise trim of non- or barely overlapping jibs seems to me (and this is just a personal guess after sailing very many dinghy classes - maybe others have different ideas) to be more critical for the efficiency of the rig as a whole than is the case with overlapping jibs. Thus overlapping jibs seem to give a rig which is more tractable when, for example, sheeting in quickly to get off a start-line, or when the crew is lazy or inexperienced.
     
  6. Omer
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    Omer New Member

    Many thanks for your replies.
    i understand that you are all in agreement that for the same total sail area
    non overlapping jib is perhaps a more efficient one if only due to the fact that it allows a bigger main provided that the boat can carry higher center of pressure.
    So i visualise it like this. The same two boats rigged with these alternatives
    when beating into the same wind, the non overlapping jib version would
    heel more but generate more thrust and overtake the one with the overlapping jib. Perhaps this would be the case to the point where the wind speed increases such that the excessive heel begins to slow the faster boat.
    After rounding the windward mark, and eliminating downwind sails, i guess the boat with a non overlapping jib would still be advantageos due to its bigger main, as jibs do little work downwind.
    So i am arriving at the point that the reason why you should ever have an overlapping jib is, a-your boat is too tender to carry a decent sail area
    in its most efficient form, b-since the overlapping area is free in terms of rating calculations you actually carry more sail but look as though less in terms of ratings. Hence you probably have a lower rating than the one with a non overlapping jib.
    This is all too well in terms of comparisons. all other things being equal you either go for more thrust per square meter, heel more, trim more carefully and beat the other boat on water and hopefully expect to win after rating corrections.
    But i wonder what the reality is. What i mean by that is how fast one boat actually can be under any given circumstances and what are the resulting rating differences are?
    The rating rule applicable in my part of the world is IRC. As far as i know
    irc rating formulae is a secret one and does not allow designers to optimise
    boats according to the rule. Probably the parameters also get tweaked from year to year as well.
    So if this formula is aimed at making speed predictions as accurate as possible, how on earth it can favour one rig to another accurately enough based on sail area or (if this is still the case) giving overlapping area for free. Therefore, my case is, still at the end of the day we seem to have very little knowledge or data to compare the two rigs in real situations, and less knowledge and data still as regards what happens to these same boats in terms of rating calculations.
    This amazes me where there are very little stones unturned in performance analysis of sailboats these days.
    Perhaps some of you out there can shed more light into this problem.
    Especially any knowledge regarding how irc treats sail area in its calculations would be most welcome.
     
  7. FAST FRED
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    FAST FRED Senior Member

    "This amazes me where there are very little stones unturned in performance analysis of sailboats these days."

    Thats because the analysis was done 3 or 4 decades ago.

    Look in "Offshore" or "Further Offshore" for comparisons of sloops and cutters with varing mainsail sizes.

    FAST FRED
     
  8. CT 249
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    CT 249 Senior Member

    I think you'll find that there's been a lot of recent analysis, it's just that it's been done on a proprietary basis. For example, Bottin & Carkeek have done sisters with overlapping and short-foot headsails and have studied the differences closely. That's just one case, there are lots of others.

    Would a non overlapping jib boat heel more? Wouldn't the higher efficiency (lower drag) of the higher-aspect jib cancel out the higher C of E to some extent, or does it not do it enough to compensate for the taller jib?

    Talking to Marcellino Bottin (a few years back) he was worried that the short-foot version Synergy 40 may suffer coming off start lines and in other down-speed situations. Other smart guys like Grant Simmer note that the short overlap rig demands Code Zero or jib top sails be carried as well for coastal racing, becxause the short overlap is poor in close reaches (I don't know whether it's the poor lift/drag ratio at wider Angles of Attack, or just a problem with excessive twist). Bob Fraser (Fraser Doyle) and others note that the short-overlap rig needs a lot of tweaking of the D2s etc, whereas a runnerless long-overlap rig needs less tweaking.

    Maybe you should try asking the question on Sailing Anarchy (carefully) because there are quite a few British guys there who have re-configured many boats under IRC.

    IRC seem pretty tolerant of most lengths of overlaps as far as I can see but I'm no expert. The LPS is measured, unlike IOR where is was just assumed to be 150% of J.
     
  9. usa2
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    usa2 Senior Member

    self tacking jibs sometimes allow you to sail closer to the wind than a genoa.
     

  10. Paul Scott
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    Paul Scott Senior Member

    Just came across this old thread. One arguement in favor of overlap is Gentry's- in his example, if the leech of the jib is at the point of maximum flow speed of the main, the Kutta condition applying to the leech of the jib
    would be beneficial to the flow over the lee side of the jib, as the flow would be less stressed after the point of max flow, because it wouldn't have to slow down as much as it does when non overlapped.

    What I don't know is how important this is to the drive of the system. The aft part of the main would be affected too, as the flow still has to slow down to the ambient air speed, but whether the depth of the turbulence back there is deeper, or more shallow, as a result is something I don't understand completeley at this point.

    Paul
     
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