Yacht design the hard way. learning from Kav800 (most beautiful boat EVER...etc LOL!)

Discussion in 'Sailboats' started by Schoonner, Jan 12, 2012.

  1. Schoonner
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    Schoonner Senior Member

    I'm focusing on the keel right now as it will be the next to tackle after the hull is shaped, but I have a question about a swing rig that I will eventually want to try.

    With conventional swing rigs the foot of the jib uses two booms. One that pulls the jib around side to side as an extension of the main boom, and another that is attached to that extension and controls the tack and clew. This seems to me to cause the jib to come windward of the centerplane of the hull so that the boat would see a force pointing it a little windward. Is that correct?

    EDIT:: Wouldn't that be a bad thing I mean?
     
  2. Schoonner
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    Schoonner Senior Member

  3. Perm Stress
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    Perm Stress Senior Member

    This is very good book for general understanding of what the physics of yachts are all about.
    For actual design work it a little scarce on data.
    For scale modelling next to nothing -it is all about full size boats, they even do not describe model tank testing procedures in much detail.
     
  4. Alik
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    Alik Senior Member

    There is new book by Molland 'Ship Resistance and Propulsion' (2011). I got a copy; it deals with model testing and also includes best collection of resistance prediction methods up to date.
     
  5. Schoonner
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    Schoonner Senior Member


    Maybe I can get someone from department of vocational rehab to help me with the cost of this book. The best price I could find was $91.24 US dollars.:mad:

    http://www.amazon.com/Ship-Resistance-Propulsion-Estimation-Propulsive/dp/0521760526

    I had a dream last night though. I dreamed that I was hanging weights in the spreaders and trying to capsize my foam model and couldn't get it to capsize with my fan blowing full blast and the equivalent of 300lbs or about 150kgrams slung up high on the mast.

    Then I realized that it was actually more than that because the leverage changed as the weight swung away from the mast. :D

    weird dream. Do people actually do things like that on sea trials or anything?
     
  6. Perm Stress
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    Perm Stress Senior Member

    Yes they do.
    Normally not at sea.
    Normally we do in mine home waters when it is necessary to reduce draught of fixed-keel boat to float off the mud shoal.
    I did it late autumn last year for Skippi 650 I did skipper most of the season, in the marina just before hoisting for winter storage.
    (with bulb keel down of course ;) )
    Just for fun, we laid her down to 90 deg heel. It took 2 of us pulling hard on spinnaker halyard.
    When down, she was able to hold about 30kg on her spinnaker halyard and remain in position.
    WHAT A FUN IT WAS!!!

    For scale model, this would be x*x*x the this weight, x being the scale of model.
     
  7. Schoonner
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    Schoonner Senior Member

    Another question. Would it be plausible to set the CB and CF for the model KAV 800 in such a way that when sailing and excessive heel is reached it so that it would force itself upright by turning into the wind? maybe not when pointing downwind, but at least when it is close hauled or 90 degrees off the wind?
     
  8. Perm Stress
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    Perm Stress Senior Member

    Positions of CB and CF are determined by hull shape already.
    For model with closed deck and bulb keel it is no danger to be laid down. As the gust is over, she will came upright and carry on.
    I would not care about modifying anything, except for increase in depth and width of keel and rudder and placing "leftover" weight (i.e. weight you will manage save in hull, rig, RC equipment and so on) in the bulb.
     
  9. Schoonner
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    Schoonner Senior Member

    But, if I place the bulb keel too far forward or something won't that throw the CB off?

    I want to make a fairly long keel bulb, but I have a problem with it. There is stuff in the pond outside my house (http://en.wikipedia.org/wiki/Myriophyllum) that will get caught in props and on keels making disabling the boat.

    I am thinking that if I angle the keel like the centerboard is angled then maybe it won't hang up. That makes for a strange keelbulb though right? So, maybe an extra long full lead centerboard that can be raised up would actually be the better idea? I want to make it from lead by cutting out a hardwood mold and pouring it into the mold, then cutting away the excess wood so that I have a wood keel with a lead core and brass eyelet. That way I'm being at least a little environmentally 'green'. LOL!

    What if I made the tip and leading edge of the rudder out of lead as well? Would that cause problems? I plan on using lead for the centerboard box thing too. That way she is plenty heavy on bottom.

    Alik, could you please send the alternative plans to my email? ( tegmccall.iii@gmail.com ) That way I can study both designs and maybe make a good decision.

    THanks,

    Schoonner
     
  10. Perm Stress
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    Perm Stress Senior Member

    Just make the keel/bulb L-shaped with bulb pointing aft.
    Make sure there are no "hooks" on the front edge of keel.
    A few degrees of sweep will not hurt either -it is not that necessary to have 30 degrees of it.
    Than, with some luck and skill, she will not catch much grass.

    Lead is far more effective in the bulb than anywhere else and have (almost) no structural value.
     
  11. Schoonner
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    Schoonner Senior Member

    I seem to have a problem. my model is 1/131 scale. The way you said to calculate the ballast would be 131 * 131 * 131 which is 2,248,091. If I divide 350kgs by 2,248,091 I get 0.000155688 kgs which I believe is about 0.01 gram. That's not very much ballast! LOL!
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ====
    Schoonner, here is an example from my own test model-for this method you don't need to use the "scale"-just the actual length of model and fullsize in feet:

    OK: weight(displacement) varies as the cube of length:
    1) model length=60.75"=5.0625' ;5.0625' cubed=129.75
    2) full size= 18' ; 18 cubed = 5832
    3) model to full size= 5832/129.75=44.95, so multiply model weights by 44.95.
    4) full size to model=129.75/5832=.02225; (example: .02225X 750 =16.688lb)
    ------
     
  13. Paul B

    Paul B Previous Member

    The models you have been showing are not 1/131. Not even close.
     
  14. Perm Stress
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    Perm Stress Senior Member

    length of your model by eye measure from photos is ~1.2m
    for easy numbers I'll assume 1.0m;
    if hull length of the original is ~8.0m ,
    the scale will be :
    8/1.0=8
    Approximately, of course, as do not know the actual length of your model.
    than scale for all weights would be 8^3=8*8*8=512.
    i.e. all the weights and forces will be 512 times less than the original.
    It is not absolutely correct, but as close to the target as possible without going deep in to detailed analysis of EVERYTHING in actual size of model at hand.
     

  15. Schoonner
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    Schoonner Senior Member

    Okay, this is how I make my dimensions.

    The full size model is 7930mm from transom to frame "A"
    I took 7930 and divided it by 131 and came up with 60.53mm. I rounded to the nearest mm. The length of my model is 60.5mm

    It was supposed to be 7930mm/120 = 66.0mm, but I didn't cut the foam straight and only had 14mm maximum depth to work with instead of 15mm so I had to trim the height to fit. :(

    It sounds like the way you describe the scale would mean that it is 1/12th scale because 7.9/.65 =12.

    So, if that is correct I cube 12 and get 1728. Therefore my ballast might be 0.2kgs or 200 grams. :( That's less than 10 oz.

    I'm doing something wrong!!



    I like the second way better which would be like this:

    because the model is 60.5cm that would be 6.05 decimeters.

    1) model length is 6.05dm so when cubed is 221.445125.

    2) full size 79.3 dm cubed is 498677.257

    3) model to full size is 498677.257/221.445125 = 2251.922488698

    4)full size to model is 221.445125/ 498677.257 = 0.000444065

    So, 350kgs *0.000444065 = 0.15542275kgs which is even less!!

    I bet I'm doing it wrong again!! :confused:

    EDIT::
    I want to put something like 3.5 lbs, or 1.5875733 kilograms, on her but that is just because in my head that's how much seems reasonable.
     
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