Sailing Yacht Stretch

Discussion in 'Boat Design' started by Polarity, Mar 14, 2002.

  1. Polarity
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    Polarity Senior Member

    Hi all

    As many of you know I am having plans worked up for a steel Ted Brewer yacht "Miami" (here ). This involves taking the boat in the link and combining it with a fin keel specified in another of Ted's designs of the same size.

    The result once put up on CAD will be stretched - and here is my question...

    The LOD of the Miami is 44'11" and I had planned on a 5% stretch to 47' - lengthways only - Ted said this would not be a problem. The NA doing the CAD work has sugested that maybe I could stretch it to 14.95m - 49' this would be a 9.2% increase.

    I have asked Ted and am waiting for his answer, but meanwhile I would be interested to see what the board thinks...Is that too much? I apreciate that if I stretch it everything moves around CLR,CB,CG etc. What about ballast, keel size and shape, rig etc should the beam be increased too?

    Any input from the forum would as always be gratefully received.

    Thanks !

    Paul
     
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  2. Gades
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    Gades Senior Member

    Definetly, if you stretch your boat the coefficents will change as well (Cb, etc).
    The relationship between the L, B, D.... of a model boat and a "stretched" one (I cannot think of the proper word in English right now), are not all the same.
    There are different ways to calculate them, but I cannot remember them right now.
    I'm sure Stephen will help you out with this.
     
  3. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Actually, if you do this by multiplying the station spacing by the scale factor (1.092) Cp should remain the same. Displacement will also increase by a factor of 1.092, but what you need to realize is that stability will not change much, if at all,
    (see http://www.boatdesign.net/forums/showthread.php?threadid=272)
    so your sail area should probably stay the same. Assuming it does this change is likely to improve your reaching performance in windy conditions, but not your light air windward performance. Nothing wrong with it in my opinion, if that's what you want to do.

    Now what if you want to scale the whole boat so that you have a comparable 49 footer? This is the problem I'm working on at
    http://boatdesign.net/forums/showthread.php?threadid=327
    Note that this is essencially what Bruce Farr apparently did comparing the Corel 45 to the CM 60.
     
  4. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Actually I need to correct myself. Stability will also increase by 9.2%, so you could either increase the dimension at the base of each sail (that would be the "J" and the "P", I think) by 9.2% or you could increase all sail dimensions by the cube root of that. Still, in the scheme of things that isn't much, so what I said before still applies. You'd see the speed improvement in windy reaching conditions, not to windward. What you could do to change this, though, is increase draft as well. If you can drop the center of gravity a bit then you can add a smidge more sail.

    You needn't feel trepedation about making the boat longer. To quote Phil Bolger "On any given midsection, a longer boat will always be faster." I'm in favor.

    Having said that, I'm also working on how much you should increase the beam if you want a larger boat with comparable characteristics to the original (just for kicks).
     
  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Another correction

    When I said "P", above, it should have been "E", referring to the dimension of the mainsail along the boom.
     
  6. Polarity
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    Polarity Senior Member

    Thanks Stephen... Keel question...

    I feel alot happier about it now I will go ahead with that, Ted is going to send me a sail plan of a similar yacht stretched to 50ft.

    Re the Keel.
    ---------------------------------
    "Stability will also increase by 9.2%"
    -----------------------

    Is that on the basis of a 1 or 2 dimensional strech of the keel? - If it is just lengthened it will change the shape of the foil - I assume this is bad, no?

    There are 2 keel options a 6' and 6'10". Wanting the best of both worlds I plan to use the 6ft keel and use lead/iron mix instead of the 12,000lbs of iron ballast specified so that I can lower the CoG to approx the same position as the 6'10 keel.
    Teds suggestion to improve the windward performance was to put an end plate on it. - " This should start at 0 at the leading edge and then gradually widen until it is sticks out 100-125mm wider than the actual keel at its widest point, and then run straight aft to the end of the keel. The end plate prevent cross flow and reduces energy lost from creating vortices. It works. "

    I have heard of the endplate effect - would it be as effective as another 10" of keel?

    Opinions?

    Paul
     
  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I think a high aspect foil recovers the energy of the tip vortex more efficiently than an endplate....that's why the latest America's Cup boats have them. I've e-mailed you the shoal draft solution that I think is best, but if you want no moving parts most of the bulb-wing keels out there, including the ones on the Hunters, are reasonably good.

    The 9.2% increase in stability is an increase in form stability. At
    http://www.boatdesign.net/forums/showthread.php?threadid=272
    I posted the following formula for initial stability:
    In salt water RM1 = [B*Awp^2 / (670*L)] + (GB*Disp / 57.3)
    As it turns out this is not dead-on accurate, but if you change the 670 to 625 it's pretty close for most common hull forms. Anyway, the first term can be rewritten as CLB^3 with C being a constant for that hullform. Then it is accurate (since the cause of the inaccuracy was my way of estimating C). If you "stretch" the hull by increasing the station spacing then C and B should remain the same, so the only increase will be in L, and therefore the increase in form stability will be linear with the scale factor.

    Now I see there's something confusing about my variables. The B in GB refers to the vertical center of bouyancy (not beam).
    GB is the distance between VCB and VCG, and should therefore have a line drawn above it to indicate that it's the length of a line segment. The change in displacement, which willl also be 9.2%, will not make much difference if the center of gravity is close to the center of bouyancy on both designs, or if the distance between them decreases by about the same percentage that the displacement increases. The 9.2% increase I'm suggesting you'd see in stability assumes no major change in the second term.
     
  8. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I should also mention that if the center of gravity (G) is above the center of bouyancy (B) then GB is a negetive number.
     
  9. Polarity
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    Polarity Senior Member

    Thanks again Stephen

    I really liked the keel design you mailed me but as you said I am shy of the moving parts in the keel. - Have you patented it yet?

    If the CG is above the CB - would that mean that the boat is relying entirely on form stability to keep her upright?

    I am afraid I am not really up to understanding your formulae yet - but I think I understand your explanation of the stability issue with regard to the stretch.

    Your help together with that of so many of the other members of this forum has been invaluable in helping me make decisions about my boat - it's a lot easier to change things around on the screen or cardboard model than it is with a gas axe!

    Thanks again

    Paul
     
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    If the CG is above the CB - would that mean that the boat is relying entirely on form stability to keep her upright?

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Sort of, but I wouldn't put it that way. There's nothing wrong with form contributing to stability - it's because of form that CB will move to leeward as the boat heels. The formula I posted is just for initial stability, however. If you want the boat to recover from a 110 deg. knockdown CG needs to be on the proper side of the new CB as the boat floats with the mast in the water at 110 deg. (and some additional safety margin is recommended).
     

  11. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Here's a spreadsheet showing how to scale up the Brewer 45 if you want an equivalent 49 footer. I am not suggesting there's anything wrong with just stretching it - I see that as also being a good and valid option. The scaled boat would be a little faster to windward, but the stretched boat would be faster on a windy reach.
     

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