First Boat

Discussion in 'Sailboats' started by TimClark, Nov 20, 2005.

  1. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    Well, I designed my first boat. It looks like a Pixel with some minor modifications. The color is the same color as my old, fast blue jay so I thought I'd carry it over. I'm open for critique.
    LOA-14 ft
    Beam-3.7 ft
    Draft-.375 ft

    Tim
     

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  2. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

  3. Doug Lord

    Doug Lord Guest

    Technical details

    Tim, can you show any technical details-displ., position of CB,Cp etc. ,crew and SA.
    My apologies if they're in the dxf file but I can't open it with web tv. First impression: beautiful! Second impression : perhaps a little too much rocker aft?
    Good work!
     
  4. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    Well I made a new boat because I didn't like the general design and numbers that boat had. Here is the new boat and numbers. I don't know the equation for how much SA I could get for the LWL so I haven't designed the sails yet. The person is a 2 man boat, it will have a main, jib, and spinnaker. It's like a modified trainer boat and is very stable (hence the wide beam).
    LOA-13.962 ft
    Beam-4.963 ft
    Draft-4.4 inches
    LWL-12.9 ft.
    Displacement-The program says 470 pounds?? But I would like it to be around 190-200 lbs
    Displaced Volume-8.229 ft.
    VCB-.236 feet
    CP-.5602

    [​IMG]
     
  5. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    If anyone knows the equation for the ratio of SA to LWL that would be great if you would let me know. Also, is there any software programs for designing the sail? Thanks.

    Tim
     
  6. Tim B
    Joined: Jan 2003
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    Location: Southern England

    Tim B Senior Member

    Sail design is a complex job. Not impossible, of course. basically, one balances the righting moment of the boat against the heeling moment of the sails, and calculate your areas from that, based on probable side force, crew weight/location, hull shape and, obviously, speed.

    Despite the fact that initial stability is generally taken to be k*length*beam^3 , and thus it has immediate impact of sail design, I think (considering the weight of the boat) that you could probably reduce the beam quite a bit, say by 20% to 30%, which would give you a much faster boat, at the price of needing to sit out a little further.

    Tim B.
     
  7. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    Thanks I'll work on that. I'm also wondering if there is a certain size centerboard I should make for the boat. I know there are different variables, weight, size, position, etc. but is there a general ratio or equation as to where and what size the centerboard should be in relation to the boat? Thanks.

    Tim
     
  8. Doug Lord

    Doug Lord Guest

    Statistical Comparisons

    Tim, this site
    http://www.contractsolutions.net/J18/jn-18_comparison.html
    has a wealth of data for small boats including sail area, hull weight, number of crew and "standard" crew weight which is an absolutely invaluable collection of information in one place! But they use a weird (to me ) form of SA/D ratio. So I set up my own table and used the "normal" formula for SA/D(which is: SA in sq.ft. divided by[displacement in cu.ft. raised to the.666 power]. It can be found on page 281 of Larrson and Eliasson and is the SA/D formula I've used for many years.I guess the biggest difference is that the numbers in the standard formula are higher than in the one used on the Johnson site. For boats under twenty feet I also included in the table "Wing Loading"(sail loading which is the total weight divided by the sailarea-- done for both Upwind and Downwind as is SA/D).
    Once you put this table together for the boats on the site(and your boats as time goes on) you'll have a tremendous statistical reference for small boats and you can add others as you get information on them. This can be used to give you a ballpark idea of what you want to shoot for in any design under 20'. Of course later you could expand your table to include lead bellies ,cats ect. Really worth the effort!
    Also, for small boats is Bethwaites ratio of Sail Carrying Power(SCP) to total weight which is illustrated on page 181 of "High Perfomance Sailing". It's probably the only ratio that takes into account righting moment for small boats and can predict whether or not the boat will plane upwind. SCP is the maximum force on the sail at a given height of CE(center of Effort) for the available righting moment. The ratio [SCP divided by total weight] gives a percentage which, for instance, must be above 30% to plane upwind according to Bethwaite. This deserves it's own spreadsheet since you can play with the height of the CE, weight, crew weight and Righting arm a lot. Get this book and you'll see what I mean.
    In my opinion you don't actually have to design the sails; you design the planform of the sails so that the area, CE and rig position fit in with your concept. You can design the rig to behave in a specific way in terms of gust response(see Bethwaites book) or just do a simple rig. Square top mains have excellent gust response on rather simple rigs-don't ignore unstayed rigs! If you want to do something really different consult with a sailmaker preferably someone local but if there isn't a local sailmaker check out Sandy Goodall's site- he makes a living consulting and designing sails but there is a lot of free info available there. Above all research what is already available at the length boat you want to design and try to get as much info as possible then use all your statistical information and your own brilliant insight to do a (hopefully) better boat. Believe in yourself and do the research as best you can!
     
  9. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    Thanks for that info. My skipper's Dad is a sail designer for North Sails so I bet he could help me out with that. But relating to the whole centerboard size, should it equal the force the sail will cause? Thanks.

    Tim
     
  10. Doug Lord

    Doug Lord Guest

  11. Robjl
    Joined: Nov 2005
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    Location: Adelaide

    Robjl Senior Member

    Tim, in relation to your query about the displacement??
    "Displacement-The program says 470 pounds?? But I would like it to be around 190-200 lbs "
    This is the displacement on the waterline that you designed..I think. If the boat only weighs in at 200 lbs it will just float higher, if you check out the hydrostatics part of your program, (if it has one), you can probably work out what you need.
    Good luck with the project.
    Rob
     
  12. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    Rob, thanks I ran the hydrostatics after I altered how the boat floats and it lost a lot of wait just my reducing it by .7 feet. Now the weight is 176 lbs. Thanks a lot.

    Tim
     
  13. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    I got the displacement up to 188 pounds and 3.298 cubic feet. Thanks. Now I just need to work on how to determine RM and what size centerboad or daggerboard I should make. Thanks.

    Tim
     
  14. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Isn't 188lbs a little light? The boat's got to weigh something (150-250lbs) and I'm sure you weigh something too. My best guestimate is that 300lbs displacement is the minimum you should shoot for. The old 470lb displacement sounds good for a boat that size. Particularily if you plan on having more than one person on board.

    As for your dagger board or center board, I'd make it at least 1/40th of your sail area in area if its narrow and deep. More if its shallow and long. And remember, only the part that's in the water counts.

    90sf would be a good place to start for sail area, if you want a high performance boat (like your lines imply) that is reasonably easy to handle.
    If you went with 90sft your minimum board area should be: 90/40 or 2.25sf.

    Your rudder depth should be at least 1/2 the sailing depth of your board (3/4ths is more the norm) and have at least 1/2 its area.

    If you went with a 3/4 fractional sloop your mast would be around 19ft tall.
    Hiking straps, at the very least, would definitely be in the program.

    (the beauty of a 3/4 sloop is that you can drop the jib and sail reasonably well under the non-reefed main alone, which could be convieniet when sailing single handed on a windy day)

    Best of luck,

    Bob
     

  15. TimClark
    Joined: Nov 2005
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    Location: Fairfield County, CT

    TimClark Senior Member

    I changed the boat around so its narrower and a lower draft. 190 pounds is generally a good weight. Look at the Pixel, its 190 pounds and generally the same lines. The only boat of that size that I know that is even close to 470 pounds is the Blue Jay and that weighs 270 pounds!! Thanks for the help with the centerboard area. Thanks.

    Tim
     
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