Single-handed Skiff

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

  1. nacra5.8
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    nacra5.8 Junior Member

    Doug-
    Taylor's Formula is not actually the name of the formula I was using, I must have remembered incorrectly. But it calculates the wind pressure for a given speed, or vice versa. Here it is:
    F=.00256V^2
    F = pressure of wind in lbs / sq ft
    V = wind speed in mph (my earlier measurements said knots, I meant mph. With a 10 foot beam the boat will hit the limit in 14 knots)
     
  2. nacra5.8
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    nacra5.8 Junior Member

    Also, why is an F18 so high?
     
  3. Doug Lord
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    Doug Lord Flight Ready

    ---------------------
    You can use this but I wouldn't rely on it literally. Your top windspeed upwind may be more or less than this because the actual wind pressure depends on how high you're pointing and the apparent wind. Skenes Ekements of Yacht Design has a page with wind pressure coefficients but they're flat plate like the above formula. My opinion is that comparing max wind pressures makes more sense than trying to extrapolate the actual
    max wind speed before depowering which depends on many factors.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    =============
    I guess I got mixed up with another boat I did because 1.8 is wrong:
    1) I used .4 X the luff length(est* 28.28') plus est* 3.5' for CE-CLR=14.8'.
    *based on known mast length and est. CLR position
    2) Beam = 8.5', est hull beam 1' so CB lee hull to CG boat= 3.75'.
    3.75' x 396lb.(boat weight w/o crew)=1485 ft.lb. RM
    3) Crew Righting(2@175 on traps) arm = 3.75+4.25+3.5=11.5,
    11.5' X 350= 4025 ft.lbs RM
    4) Total RM= #2 + #3=5510ft.lb.
    5) 5510/ (CE-CLR)14.8=372.3,
    6) 372.3/ 227(upwind SA)= 1.64 lb.per sq.ft. pressure at max RM.
    Basically, this is saying that the 18 has more RM for the amount of sail its carrying vs the smaller monos which makes sense since cat is much faster and would almost always be sailing in more apparent wind.
     
  5. nacra5.8
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    nacra5.8 Junior Member

    Alright, thanks Doug. So when do you expect to start depowering with your Crossbow? I'll be somewhere around that range.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    --------------------
    Crossbow fl is .92lb/sq.ft. at the approx. point where more wind may require depowering. However, its kind of a unique case where a DSS foil provides 14% of the total RM at about 5.2 knots boatspeed upwind. I have a feeling that the boat may go faster than "hull speed" upwind which will increase RM.
     
  7. nacra5.8
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    nacra5.8 Junior Member

    I was thinking more wind-speed wise, but it sounds like it wouldn't really carry over to my boat- that's an awesome project by the way though, good luck with it.
     
  8. nacra5.8
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    nacra5.8 Junior Member

    For laminating the hull (I'll be using a male mould), could I laminate the inside of the foam before placing the foam onto the mold? Then it would be much I easier to vacuum bag etc. (I would think), and I've been seeing lately that its pretty easy to bend foam with one or even two sides pre-laminated. I'll be using 5.7 oz carbon @45/45 / 3/8" H80 Divinycell / 5.7 oz Carbon 45/45. The bottom will have an extra layer of carbon, and then a light layer of glass over the whole outer shell.
     
  9. nacra5.8
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    nacra5.8 Junior Member

    I might go to 1/4" foam too, what do you guys think?
     
  10. gggGuest
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    gggGuest ...

    You could, but you lose all the advantages of having the fibres running continuously across the join. Then you have to add cloth over the join to restore the strength, and that's a good deal of weight and expense you've added to your build - and probably enough extra time and hassle to negate any savings anyway...
    For internal bulkheads and the like, on the other hand, its well worth doing, and if they are flat you can do both sides on the bench.

    Ratios, Bethwaite's and otherwise, are IMHO most useful when you have an array of data from reasonably similar boats to your planned design, and you can make sure that you are running within a sort of basket of sanity. But compare as many as you can, because its suprisingly easy to follow one number to what seems like a logical conclusion, and find you've banged a corner too hard on design. I like to say that we all know, as sailors, if we bang the corners on every beat, we know we'll sometimes win races by a mile, but over the course of a series lose out more often than not. Much the same is true if you bang the corners too hard in design.
     
  11. JRD
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    JRD Senior Member

    There was some discussion in the ICs thread in Dinghy Anarchy (I think) a few months ago about forming foam sheets laminated on the inside only. This was to give a similar result to torturing plywood, then applying the outer skin.

    To do this though you would need to put on in one sheet (rather than strips or panels cut to match your floor shape) and your shape would be limited to a developable surface.

    I cant imagine a foam sandwhich with carbon skins being flexible enough to form a permanent curved shape around a form if you pre-laminate on both sides as suggested above (It would break first). If it is soft enough to do this, there is probably something wrong. Good for flat items like bulkheads though.

    Whats your reasoning to add an extra layer of carbon to the bottom? What would it achive that a thicker core would not? The core thickness and laminate schedule either needs to be based on some direct experience with a comparable existing build, or an engineered composite based on shape, size, speed and material properties.

    To just throw around core thickness with no reference to the panel size, bulkhead spacing and curvature is pretty meaningless really, unless you can comapre it with some existing class where these parameters are known.
     
  12. gggGuest
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    gggGuest ...

    Discussed above: dent resistance. The structural demands of a dinghy hull can be served by a laminate schedule that is too light to withstand normal launching and retrieval without getting dented all the time.
     
  13. nacra5.8
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    nacra5.8 Junior Member

    I was using the layup given on the Cherub website.
    http://www.uk-cherub.org/doku.php/tech/foam_sandwich.
    Bulkheads: Transom/Mid Dance Floor/Front of CB Case/Diagonal bulkheads from shrouds to mast step/Mast step/Spinnake pole take-off point and forestay attachment.
    All bulkheads will be 3/8" foam, glassed or carboned both sides, except the diagonals which will be 6mm plywood for stiffness.
    Panels are 40" x 80" Divinycell- most of them will be normal sheets, but do you guys think I might need some perforated? Thanks
     
  14. gggGuest
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    gggGuest ...

    ewrrk. Suggest not. Unnecessary weight over carbon foam and a horrendous rot trap.
     

  15. nacra5.8
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    nacra5.8 Junior Member

    Alright, I was already a bit suspicious myself, but on 'Shu's' I14 build (http://forum.international14.org/viewtopic.php?f=2&t=865&start=315), another one of the builds I have been following, he used 6mm ply + uni + more carbon, leading me to believe that tons of carbon would be needed for a foam equivalent, but I guess I can work with it. Also, for adding plywood pads for the shroud attachment points, you just cut out the thickness of foam needed to add a plywood pad in, and then epoxy it on? I'm not sure how this works, and I don't want to rip the shell in half.
     
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