Main / jibe size relationship?

Discussion in 'Sailboats' started by mholguin, May 25, 2005.

  1. mholguin
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    mholguin Junior Member

    Prior trends tend to enforce huge genoas with fairly small, low roached mainsails.

    Currently it seems tha large roached mainsails, coupled with "blade" jibs are preferred for racing boats.

    Question is, How large should the mainsail be compared with the jib?
     
  2. John Perry
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    John Perry Senior Member

    For maximum efficiency it is probably best to have no jib, ie. the ratio of jib are to mainsail area should be zero.

    However, to maximise power for a given mast height there is a case for multi sail rigs. Such rigs can also have structual advantages and offer greater flexibility in operation. The height of the jib is often less than that of the mainsail and this is probably advantageous to compensate for mainsail twist and in some designs to control mast bend but it does tend to also make for a smaller jib area. On the other hand, the single spreader masthead sloop is undoughtably a simple robust rig for smallish yachts. The mainsail tends to be a more controllable sail than the jib and that is one reason for going for a larger mainsail and smaller jib. Also the modern fully battened mainsail is a more effective sail than older mainsails and that may have lead to larger mainsails at the expense of the jibs. Difficulties in maintaining sail shape in jibs which have small foot dimensions compared with height limit how far one can go in the direction of tall blade jibs. So overall there are factors favouring large and small jibs and you can find successful boats which are fairly extreme in both these directions but I agree that the modern trend has been towards smaller jibs of high aspect ratio.
     
  3. SeaDrive
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    SeaDrive Senior Member

    My crew and I were looking at a Hunter 28.8 on the dock nearby. I commented that it was more of a big jib/small main boat than I liked. She replied that the big sail was on the roller furler, and the smaller sail was the one that had to be furled by hand. So, it depends on the point of view.

    One point that is often overlooked is that a genoa cannot be properly sheeted to the deck for a broad reach. For best efficiency, it should be sheeted to an adjustable length pole for anything but a beat or close reach. Racers don't notice because they are flying a spinnaker while reaching.

    The common variations are driven by racing rules.
     
  4. mholguin
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    mholguin Junior Member

    Thanks for the replies.

    Any hints on percentages?
     
  5. asathor
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    asathor Senior Member

    I have been sailing my Freedom w/o Jib for 18 years. For cruising that is just fine, but now that I cover longer distances in open water sailing between islands it is much more important for me to be able to point higher. Theoretically either sail can point similarly but with a big mast in front the mainsail will not point high as efficiently as a jib with a thin entry. I have heard repeated claims of some of the larger Freedoms with a blade jib tacking at 80 degrees - they can probably do that but it will be a lot slower than at 90.
    You can see a polar plot for a Soverel 33 here http://www.soverel33.com/Images/Articles/Polar_40810_TW.gif Note the little boatmarks for Velocity Made Good (VMG) to windward and downwind - this boat points quite high but it appears to be at a big disadvantage downwind.

    In other words it is a tradeoff that only an expensive wingmast can hope to overcome. Basically blade jibs and big fully battened mains are easy to work and the VMG to windward can be excellent and downwind will be pure fun - if you want to race triangular circuits though you will probably need a bigger Jib. It really is neither that precise nor complicated.

    Of couse none of this matters if you mount it on a hull that doesn't match your sailing environment.
     
  6. John Perry
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    John Perry Senior Member

    Yes, when I suggested that a single sail rig was best for efficiency (Cl/Cd) I was thinking of una rigs with rotating wing masts, or pocket luff sails on small craft, or maybe even a round section mast which can rotate to keep the leeside smooth which is what I have on my own sailing dinghy. If you have a cat rig with a fat non rotating mast maybe you need a blade jib ahead of it to pre-bend the airflow to cut down the size of the separation bubble behind the mast - perhaps aerodynamicists can advise?
     
  7. Paul B

    Paul B Previous Member


    Why do you think the Sov33 is at a big disadvantage dwonwind, based on this polar?
     
  8. asathor
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    asathor Senior Member

    Spinnaker

    Because it needs a spinnaker to do it.

    Actually it may be a bad example, aside from the spinnaker issue which is pretty important to me. I made a comparison chart from 2 compatible polars on the S33 (blue) and the Tartan T10 (greeen) and it looks like the T10 points a bit better to say the least - of couse it is a racer w/o a house. The red trace is a J30.
     

    Attached Files:

  9. Paul B

    Paul B Previous Member

    Even without a kite I have to believe the Sov is pretty good down the hill. Should still be faster than almost any Freedom of similar size I think (Sov 33 PHRF is a gift at 87, Freedon 30 is 168, Freedom 38 is 132).

    I think your Sov/T10 comparison isn't accurate. The scales are off and the marks for the T10 upwind are at the tail of the line, not at the best VMG point as they are on the Sov marks. In fact the Sov will rip up a T10 upwind in light/medium conditions, point and pace. PHRF ratings are Sov 87 (should be lower) and T10 126, 39 sec/mi advantage to the Sov).

    When Dave Ullman and Dennis Connor sailed Soverel 33s they were pretty competitive against anything.
     
  10. asathor
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    asathor Senior Member

    I didn't mean to start a contest.

    Well I must apologize for picking on Soverel and I sure didn't mean to compare it directly with the Freedom (as in apples to apples) - They vere used as examples of two extreemes. If you poke around the Soverel site you can read what it takes to sail the boat competitively - apparently a lot of Gorillas swinging from vines and Geeks to instruct them. You can sail the Freedoms fast singlehanded so you see they are not really the same kind of boat. To go fast downwing in ANY BOAT with a large fully battened main all you have to do is to let the mainsheet out and start hooting and hollaring, if you have enough wind that is (remember to duck during gybes).

    This thread is about jib vs main sail size trade offs, by the way, not about Soverels per se.

    I actually meant to include the following picture but I left the other one up because it is quite amazing how well-rounded the J30 is compared to the two other. In fact to me the most impressive of the three. The charts I found shows the Soverel to point poorly and but run well (I had that wrong) or the T10 the opposite. Your pick.

    Polar plots like this do not have to be scaled accurate or look good to be read. In fact they are usually not real polar diagrams anyway but computer generated vector drawings based on some well know but nevertheless theoretical math and sometimes a few actual observations.

    Like I said before: "none of this matters if you mount it (the rig) on a hull that doesn't match your sailing environment".
     

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  11. Packeteer
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    Packeteer Junior Member

    Did anybody see the wing sail in April issue of Yachting World? Apparently it works really well.
     
  12. FAST FRED
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    FAST FRED Senior Member

    For single handing a main of 250 sq ft to 300 sq ft is about as large as one old guy can reef with speed and saefty.

    If your boat requires loads of sail , 300 will be in the main , the rest in jibs.

    Here the cutter config works best , as while one sail is being changed , the boat is not bald headed and will selfsteer or autopilot easily.

    FAST FRED
     
  13. xarax

    xarax Previous Member

  14. mholguin
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    mholguin Junior Member

    I think I didn´t ask correctly. Think of a 30' LOA, 5.5' max beam boat. Requirement is a fractional sloop with non-overlapping jib. Max SA is 480 sq feet.

    What % willyou put on the main and jib? (please don´t discuss the measures of the boat).
     

  15. FAST FRED
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    FAST FRED Senior Member

    60% in main 40% in jib.

    FAST FRED
     
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