Foresail and mainsail interaction

Discussion in 'Sailboats' started by Ray Kay, Jul 28, 2015.

  1. Ray Kay
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    Ray Kay New Member

    I have read a lot of information including on this forum about sail aerodynamics. My questions are; 1. If there is no slot effect do overlapping headsails add anything other than friction and weight?
    2. Should I get rid of my genoas?
     
  2. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    In a nutshell no, you'll want the extra area the larger headsails offer. The slot effect was initially reasonably argued conjecture, as to why headsails where necessary. Just because testing proved the slot effect wasn't as important a consideration as once believed, doesn't mean there isn't a benefit.
     
  3. Ray Kay
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    Ray Kay New Member

    Thanks for the welcome and for this.

    My thinking is that most of the time most of any genoa (as opposed to a jib) is shielded from the wind by the main anyway. They are big and heavy and so impose weight aloft and lots of strain on the boat and rig because of the weight aloft. They also take up space and add weight when stowed.

    My main headsail is 125% roller furler and I have a full batten main. The headsail is often partly furled with the problems that brings. I often wonder if a smaller roller furler would be a better option. My boat is in the Med so wind varies quickly between next to nothing and 25kn. I can sail well if slowly in next to no wind. I avoid using the engine as much as possible. I do subscribe to "two boats its a race" and usually do well. Boat is a 1980 Wauquiez Gladiateur 33 With a new Sparcraft two spreader masthead rig.
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I tend to agree that a larger mainsails and a smaller non-overlapping jibs provides more effective and economical use of both sailcloth and manpower. I think some of the old rules where intended to benefit sailmakers and sailcloth manufacturers, and winch manufacturers.
     
  5. gonzo
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    gonzo Senior Member

    If you have a restriction in total sail area, a small jib is more efficient. Overlapping jibs work for racing because the overlapping area is not measured.
     
  6. PAR
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    PAR Yacht Designer/Builder

    The overlap also helps smooth out flow over the edges of the mast/mainsail interface. You don't need much overlap for this to occur, so maybe a 110 or 115 will do for your needs. When your boat was designed the large proportion headsail improved its rate, so mainsails got smaller and smaller comparatively, to take advantage of this rule quirk.
     
  7. gonzo
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    gonzo Senior Member

    I think that a 110 has little or no overlap when the sail sets. The curve makes the clew move forward from the theoretical aft position.
     
  8. gggGuest
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    gggGuest ...

    Err no.

    Overlapping jibs used to be considered advantageous under some rating rules and in some classes which measured only the fore triangle area. Most modern rating systems measure the entire sail area,and as a result overlapping jibs are regarded as less desirable.

    As for your specific issue. Square foot for square foot jib overlap is probably the least efficient way of adding area, and of course large overlap jibs are more awkward to handle. However when dealing with an existing boat which already has overlapping jibs you may as well keep them if you're happy using them. If you wanted to keep the same sail area and have non overlapping jibs you would need a taller mast. You probably don't want to buy a new mast right now. If buying a new sail, well, if you're mainly using the current one partly furled it probably makes sense to buy a smaller one, because it will cost a few quid less and set better.
     
  9. markstrimaran
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    markstrimaran Senior Member

    so a single large sail is better than two smaller sails?

    Hi
    Just reading and have a similar intrest. With all the knowledgeable people gathered.
    I know the answer depends on a lot of variables. So I will go with 10 mph breeze. 180 sqft of a head sail. Or 100 sqft main +80 sqft jib. On a 26ft mast.

    Thanks
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think it also depends on the weight and how easily driven then hull is. I think more easily driven hulls can have higher main to jib ratio, and less easily driven hulls may benefit more from more equal main to jib ratios. Our daughter's Yngling is 21 feet, 16 feet on the waterline, and weighs 1400 pounds with 700 pounds of that being ballast, and 200-600 pounds of crew on top of all that, typically raced with 450-550. Very easily driven.

    Mainsail: 113 ft2
    Jib: 57 ft2
    Spinnaker: 220 ft2
    [​IMG]

    I think the key is not to be constrained by a rule whereby the spinnaker size is dependant on the jib or foretriangle size. As you can see the Yngling spinnaker is quite big, and still not masthead. If I was to add a sail for non-racing it might be a flat cut assymetrical drifter on the pole set very low or to a sprit, for tight beam reaching and close reaching, particularly in light winds and a lumpy sea. In that sense under those conditions I would be going to a more equal main to headsail ratio for more power.
     
  11. CT249
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    CT249 Senior Member

    Personally I'd never go for just a headsail unless it's just for the convenience of being able to unroll a roller-furler. Many boats struggle to tack under just a headsail and the gust dynamics and heavy wind sail shape of a furling headsail are often terrible.

    However, on an old IOR type boat the mainsail is often too small to power the boat efficiently without the jib. In addition, when sailing under main alone you generally have to sheet out more and/or ease the traveller. Speaking very generally the end of the boom should be roughly above the gunwale when sailing under main (which is the way it goes in most singlehanded dinghies for similar reasons) but many people keep the main closer to the centreline and wonder why the boat won't move.

    Personally I find a short overlap jib and full main to be a wonderful combination. You lose a bit in light winds compared to a 150% genoa, but normally go better in confined waters when cruising because tacking is so much easier. It's not just that the tack itself is faster, but the fact that it's so easy you actually do it when you should!

    A lot of the reason for the big headsails on old IOR type boats was the fact that the technology of the day didn't allow for efficient fractional-rig mainsails. The masts were large section and comparatively thin walled, so bending the mast to get the correct sail shape was difficult. Sailcloth was heavy and stretchy. If you had a big foretriangle you could change to a light but stretchy genoa in light winds, and the fact that you had a heavy mainsail didn't matter as much because most of your power came from the headsail. In strong winds you could shift to a heavy-cloth flat-cut small jib, and the fact that the mainsail stretched and because too full wasn't such a problem because it was so small.

    If you only use a small headsail on an old IOR type you may be underpowered in the light stuff, but in a decent breeze a 110% jib fully unrolled is a lot more efficient than most 150% jibs rolled down to 100-120%. I switched my old IOR type to a fractional rig (the modern swept-spreader short-overlap fractional is a fantastic setup IMHO) but if I still had masthead I'd definitely go for a 100-110% jib for cruising. Then again, I like to fiddle and twiddle while cruising and that fits with the short overlap rig - you don't get the low-down grunty torque of the 150% overlap so sometimes you have to pay more attention to trim.

    EDIT - the other advantage of the 100-110% headsail is that it can sheet inside the shrouds and therefore point higher - but that may not work with the Centurion.
     
  12. SukiSolo
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    SukiSolo Senior Member

    With respect I think you meant the transom corner? Could not agree more, a common mistake from those not used to una rigs. Stuffing it with the boom on the C/L is the kiss of death for speed on a una rig......;)
    Then of course, the boat goes into irons etc etc etc.

    Agree with your other comments on jib/main etc. Years of playing with jib sizes on fractional rig dinghies has shown a small jib is best upwind and a large one better downwind especially reaching - when sail area totals are fixed. Finding the optimal takes a little time, but someone must have changed the sail plan on this type before now.
     
  13. CT249
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    CT249 Senior Member

    Yep, as you say normally having the boom at the corner of the transom works best for many boats. However, in an IOR type (like the Centurion 32 or my boat in standard form) with a short boom and narrow stern, easing the main out further so that the end is above the gunwale can be the only way to get them going under main only.

    Cheers!
     
  14. Ray Kay
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    Ray Kay New Member

    Thanks to all contributors for this really useful discussion. I think it confirms my thoughts that even with my relatively small main a non overlapping jib will work better than a larger genoa going upwind. Downwind I use an asymmetric anyway.
    Thanks again.
     

  15. PAR
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    PAR Yacht Designer/Builder

    A small overlap will improve these generation IOR upwind. 110% will do with a 115% in light air, both carry well upwind and clean up the flow over the main, particularly when sailing in the boat's groove, which will not be as tight as you might think, but will produce better speeds.
     
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