Single-handed Skiff

Discussion in 'Sailboats' started by nacra5.8, Jun 17, 2012.

  1. nacra5.8
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    nacra5.8 Junior Member

    Alright, I don't have the sails with me to measure- I reverse calculated the the I14 ration of 0.35, plugged in minimum weight, and two 180 pound crew, and came up with 11 for the CE-CLR. Could very well be wrong, before I make any final decisions on anything I'll check these numbers multiple more times.
     
  2. nacra5.8
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    nacra5.8 Junior Member

    Doug, I know this is crazy, but I keep wondering about going with that full-size I14 rig. People have told me that the main problems I would have with it would be launching, retrieving, and capsizing. Today, while working on my N5.8, I thought why can't I just use the righting bag as a water anchor to hold the boat upright for me during those times? Basically, I would have 2 footstraps on the sides of the hulls (for climbing on during capsizes to get up to the board). The bag would have lines clipped to both of those straps, but normally it would be in the boat. Then, when capsized, I would unclip the windward clip (the one in the air), pass it under the boat, and then clip it back on, so that the bag was then positioned under the boat. Then I could fill it with water, right the boat, and it would be 150 pounds directly under the boat holding it lat for me to get in. Then I could unclip one side, pull it out of the water (3:1 purchase), dump out the water, and reclip it in the boat. Seems like it would work to me.
     
  3. CT 249
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    CT 249 Senior Member

    Are you going to be able to pull, empty and stow the bag with the boat stalled and sails flogging in a wind strong enough to capsize you? If it's anything like a Canoe - which is pretty stable really - there will be too much going on merely trying to keep the boat stationary with the weight inboard. That's harder than actually sailing.

    Boats like this are much more dynamic than a very heavy and stable boat like a 5.8, they don't like to be stopped.

    Can you really take the wider wingspan (compared to an MPS etc)? Every foot of beam is making crossing the boat slower, and all the way through the tack you are losing speed, which can be disastrous. I think that's the sort of beam the Aussie Bucko (built on an Aussie 14 hull) had, and it couldn't keep up with International Canoes despite being sailed by a former 14 world champ. The windage and difficulty of sailing build up very quickly.

    It all depends how much you want to sail and how much you want to crash, of course, and the level of your existing dinghy skills. I was lucky when I got into Canoes as the wife and kids were OS for 6 weeks and therefore I just trained most nights through that time. If you lack that amount of time, then making a boat like these even more radical than a Canoe or MPS or 700 could result in a lot of frustration.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ==================
    As long as you understand the downside(and it sounds like you may) don't let anybody talk you out of more power. If you can't handle it you can adjust it as necessary-something you can't do if you start with a cut down rig.
    But make damn sure you do understand the down side, righting problems etc. And Good Luck!
    PS- maybe you could add the racks that would be required to somebody's 14(temporarily) just to get a feel for it. Pay them for the privilege and it may answer all your questions inexpensively?
    PS2- I keep forgetting that you're talking SanFrancisco Bay-talk to a lot of Bay area performance sailors- think about it hard and if you're 100% convinced of your own ability go for it. (as long as you know yourself well enough to know that you can take the inevitable frustration and keep refining things without just giving up-think hard about this!)
     
  5. nacra5.8
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    nacra5.8 Junior Member

    Yeah, I've pretty much shelved the idea of a full rigged boat, I'll go with the main only setup. Any comments on the design last page?
     
  6. nacra5.8
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    nacra5.8 Junior Member

    Also, what about using balsa as a core material? I know that it can be fairly light, and its cheap. Would 6 oz glass/ quarter inch balsa/ 5 oz kevlar be an OK layup for the hull?

    Thanks guys
     
  7. nacra5.8
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    nacra5.8 Junior Member

    I've also been hearing recently that the lever arm part of the SCP/W ratio isn't very effective- something that makes sense to me. It seems to counteract all of the high-aspect rigs and foils we have been developing for decades. I'll probably just start using righting moment / weight instead from now on.
     
  8. JRD
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    JRD Senior Member

    The SCP/W ratio seems only to be a Bethwaite number. It doesnt appear in the standard texts (however nor does any craft that is capable of planing upwind). I think that it is a good comparative measure in its own right and who can argue with their design and racing sucess. To be fair they were probably the first to analyse and publish these theories on modern dinghy/skiff dynamics so some new terminology was required to illustrate their findings.
    It differs from the stability theory in that no account is taken of hull form, which is just fine if you are keeping it flat as you should.

    As with every other reference to the CE of rigs, the issue is that no one really knows where it is for any given situation. The actual centre of pressure is affected by wind velocity gradient, the sail shape and trim.

    Modern high aspect rigs can be flattened off at the top, and set to twist off in gusts. Withough doubt the CE height is moving up and down continuously as the wind varies and the rig responds.
    You also would change the centreboard height as you race with significant changes in conditions.
     
  9. nacra5.8
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    nacra5.8 Junior Member

    Yeah, while I agree with Bethwaite that the larger the arm the more the heeling force, a higher aspect ration sail also creates more power- so if you just use a smaller sail you have the same power and heeling moment, along with the other advantages of a high aspect sail. The righting moment / weight calculation is apparently what the main I14/other skiff designers (Bethwaite, Murray, Morrison) use, in contrast to bringing in the lever arm.
     
  10. nacra5.8
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    nacra5.8 Junior Member

    *Bieker not Bethwaite in the list of designers
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ============================
    Usually in design of high performance boats, that are sailed flat or nearly flat) you match the rig to your maximum designed RM. You need the calculations shown in Bethwaites formula for any boat but not necessarily his formula:
    1) Righting moment is an essential calculation and one of the most important
    in any high performance design.
    2) CE-CLR is used with SA and pressure per sq.ft. on the sail to calculate heeling moment.Heeling Moment should equal RM. Also, every boat has a maximum pressure /sq.ft.(before depowering).
    3) For comparative purposes using upwind SA, you can take the max RM of the boat and divide it by the distance ,CE-CLR, then divide that result by the sail area and you get the max pressure before depowering. For an F18 cat thats about 1.8lb. per sq.ft. . But for Crossbow it is .82 Lb per sq.ft and for the Windmill, I used to race, it is .97 lb./sq.ft. pressure before depowering.
    So, MAXRM(in ft. lb.)/ CE-CLR= load( in lb) at CE of rig; Load/ SA =lb.sq.ft. pressure max(before depowering).
     
  12. nacra5.8
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    nacra5.8 Junior Member

    yeah, I meant that as a performance comparative measurement, I'm leaning more towards throwing out the lever arm. I'm not going to disregard it in the design of the boat- just when I compare the boat to a Swift Solo, RS700, or MPS.
     
  13. nacra5.8
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    nacra5.8 Junior Member

    I'm getting 0.8 for lb.sq.ft max pressure... That means that I'll be powered up in lighter winds, than the Windmill for example, right?
     
  14. nacra5.8
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    nacra5.8 Junior Member

    Plugging that in to Taylor's formula then gives me around 16 knots for the speed at which the force is 0.8 lbs/sq ft. That's good, but maybe a bit underpowered... what would you think?
    For reference, here are the specs I'm using:
    Sail Area (upwind): 145 sq ft
    Weight: 150 pounds
    Beam: 11 feet (I'll probably shorten this, especially if I'm not fully powered up until 16 knots... probably to 9 feet. Saves weight and more exciting, but also easier to tack.)
    CE-CLR: 14 feet
     

  15. Doug Lord
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    Doug Lord Flight Ready

    ================
    What exactly is Taylors formula? The .8 means you powered up slightly earlier than a Windmill-about the same as my Crossbow fl. And thats good, if you're using a square top main, because it will have automatic gust response before you depower it.
     
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