How to construct deck line at Centerline

Discussion in 'Sailboats' started by DIY sailor, Jun 28, 2010.

  1. DIY sailor
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    DIY sailor Junior Member

    I am currently working on the design of a sailing yacht that I dream to build in the near future. The hull now seems to bee quite in order, all parameters and displacement are spot on where I want them and also the transom and sheerline at the side now look nice. For the sheerline aththe side I have constructed an intersection line of a plane with the hull surface. This seems to look just fine at every angle.

    The next step would be the Deck line at the CL, a straight line will look very silly and not right. Therefore is there anybody at this forum having experience ow to construct a nice looking Deck Line at CL. Your help, experience is much appreciated.

    The boat I am designing is a slender but modern hull.
     
  2. Gilbert
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    Gilbert Senior Member

    I think you are asking how to draw the deck centerline.
    There is nothing particularly wrong with a straight line for the deck centerline in the profile view; but if it is straight there must be enough curvature in the profile sheer line to allow enough crown in the deck beams to ensure water will drain off the deck. It is quite common to have somewhere between one quarter to one half inch of crown per foot of beam. It is quite common to not be much concerned with drawing the deck centerline. Just choose the amount of crown you want in the deck, make a deck beam pattern and the shape of the deck centerline profile takes care of itself.
     
  3. DIY sailor
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    DIY sailor Junior Member

    Gilbert thanks for your reply. You have understood my question correctly. I am trying to define a deck line at CL as this is in my oppinion the easiest way to draw a deck and a deck with constant curvature over the full length is no option as I think this will look very ugly from some angles.

    My fist plan was to go with a straight deck line at the CL. But will a straight deck at CL not result in a very strange change of deck curvature. Less in the middle and then quite a lot at the transom?
    Or will the cockpit and coachroof maybe kind of "hide" this effect?

    I would not like to have the CL deck line look like it has a negative sheer. and i am afraid thsi is wat i end up with if I make the crown ratio like a constant factor to the width. I have to check this out.

    In my opinion the deck is quite critical for the looks of the boat and it is very hard to judge from the computerscreen. In the real world something that looks nice on the computer may look completerly different when jou seen the real thing. The eye is very sensitive to subtille adjustements to deck and sheer lines. Therfor it would be really appreciete some advice from more experienced people.

    If there are more oppinions on this subject I am very eager to learn...
     
  4. Gilbert
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    Gilbert Senior Member

    Yes, you are correct about the coachroof, cockpit, mast, deck fittings and bulwarks or toerails all distract the eye from the deck centerline. And even if they didn't, it is in effect a 'hidden' line just like all the other lines that are drawn for fairing purposes. The only hull lines that really stand out to the eye are the sheer and waterline. That is not to say the other lines don't matter, it is just that they sort of disappear and you see curvature but not the line itself.
    There may be a discussion of different ways to approach deck design on the forum. You may have already done a search.
     
  5. jehardiman
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    jehardiman Senior Member

    I concur with Gilbert. The deck needs some camber to drain, but as long as the camber is low, the deck CL will be hidden in the 3d shape. Only very rarely is the observer dead abeam at deck level like a lines drawing, mostly you look down on a vessel. Additionaly, the shear is usually marked to bring it to the eye, cove striping, toe rail, lifeline, sail tracks, etc all provide definition to the shear line.

    FWIW, camber is historically constructed by bisecting a circle with a radius of the maximum camber at centerline. The beam is then divided into eights (i.e. quarters on the radius) and likewise the arc is divided into eigths. The height of the camber at the first quarter is equal the height of the first quarter of the arc, etc.. This tends to make the deck camber have less curvature near the deck edge which will lessen the apperance of a curved deck. See this site: http://joliebrisemodels.co.uk/tenth/calculator.html
     
  6. Repelsteeltje
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    Repelsteeltje Junior Member

    Guy's this is quite an interesting thread. I was wondering if for a modern sailing yacht the old fassioned deck curvature is still used.
    From the looks of many modern yacht I have the impression a circle arc has been used. Can anybody comment on this please?
     
  7. daiquiri
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    daiquiri Engineering and Design

    I don't think there are any formal rules about it, apart the considerations about seakeeping and deck dryness.

    Beauty is in the eye of (an experienced) beholder. ;)
     
  8. jehardiman
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    jehardiman Senior Member

    Too true. FWIW, I have seen decks from circular arcs to flat. Often in metal they are knuckled. IMHO, I perfer elliptical arcs to circular ones. Curves that are too regular don't look good to me.
     
  9. Gilbert
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    Gilbert Senior Member

    I still use the old fashioned deck curvature. The way it became the old fashioned way was that it worked. If it ain't broke you don't really need to fix it.
    I suppose folks use what works for them if they use something different.
     

  10. tspeer
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    tspeer Senior Member

    If your sheer line is the intersection of a plane, then it must be a straight line when viewed in elevation. In order to have some crown to the deck, the deck centerline will have to be convex. I assume there will be a house over some portion of the deck - unless you are going for a flush deck - so the forward portion of the curve is probably the most important.

    A cubic curve, like a Bezier curve, would allow you to control the slope of the curve at each end and the degree of crown in the middle. You establish the longitudinal slope of the curve at the bow and stern by the angle of the control points. Then slide the control points along those slopes until you get the desired height of the crown amidships.

    Personally, I'd be inclined to define the sheer as the intersection of a concave surface and the hull, giving it a bit more sheer than the planar intersection you described, then use either a straight line for the centerline or have it coming horizontally from the bow before curving down in the middle to meet the stern.
     
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