Giving instructions on how to measure a hull

Discussion in 'Boat Design' started by ldigas, Jun 23, 2012.

  1. ldigas
    Joined: Feb 2011
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    Location: Zagreb, Croatia

    ldigas Senior Member

    I have a chance to do some work on a motor yacht, about 12-14 meters (yet to be determined :) ). However, the boat is quite far from my place of residence, and I won't be heading that way any time soon.

    It is a steel boat, but in rough shape to which the investor wants to get the documentation straight - for a start. For some reason he can't find the original documents, apart from some which he got with the boat when he bought it.

    The crew at the place where the boat is kept is a friendly one, not afraid of work, so they've asked me, if I could give them some guidelines what would be the best way to measure the main dimensions and take measurements of the lines for a start. So we have a starting point.

    What would be the best low-tech way to measure a hull of that size? The measurements don't have to be perfect, just something in adequate accuracy, so we can use the data for stability calculations and so on afterwards?

    I have some ideas involving a drawn grid on the floor, but I'd love to hear your approach on this one.
     
  2. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Boats: a Manual for the Documentation has several chapters on measuring boats by hand and describes several methods. It's available as a free PDF download at the Museum Small Craft Association website.
     
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  3. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    http://www.amazon.ca/Bosch-GLL2-40-Self-Level-Cross-Laser/dp/B004TEIMI0

    Set up a base line of one string parallel to the keel (and full length of the boat) down close to the ground (and level fore and aft) on the side you want to measure. Only measure one side (half) the boat. Divide your base line into evenly spaced "stations" and mark the stations on your base string with tape. Using a level and sharpie marker draw the sections vertically on the hull.

    Go rent the Self-Leveling Cross Line Laser. You may also need the special glasses so you can see the lines in bright daylight.

    The boat needs to be as close to level athwartships as possible, fore and aft does not matter.

    You need a helper, one person holds the tape and the other reads and records (write down the measurements)

    Set up the laser on a post beyond the full beam of the boat at one end. You'll need another post the same distance from the baseline at the other end of the boat. Because of the curve in the side the light won't/can't get to both ends at once. You are using the laser with both horizontal and vertical beams on at once. The vertical beam is parallel to the keel (fore and aft) passing outside the boat but close enough to measure to. The horizontal beam is your waterline projected onto the hull.

    Set up the laser at each end a set distance above your baseline, working from bottom (keel) to sheer. Take measurements at each station from hull out to the vertical light line. Do each height (waterline) in turn.

    Transcribe in CAD or rhino and build surface......
     
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  4. ldigas
    Joined: Feb 2011
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    Location: Zagreb, Croatia

    ldigas Senior Member

    Thanks guys.

    DCockey - thank you for the link, it proved very useful and I think we'll be able to manage with a slight modification of one of the technics described. To my understanding they already started preparations, and I think they'll be done by the end of this week with the offsets.

    Tad - Unfortunatelly, that is rather high-tech, and 150$ is too much for them to spend "for a simple measurement". As much to my disaprovement :/
     
  5. Tim B
    Joined: Jan 2003
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    Location: Southern England

    Tim B Senior Member

    A few pieces of string, a few lead weights (at least 3 weights and strings) and a few pegs or markers and two tape measures are all that is required. Tie the lead weight onto the string (for each weight and string), to create a plum-line. Ensure that the boat is as upright as possible and on smooth, level ground.

    Now tape one of the plum-lines to the transom on the centre-line so that it is just touching the ground. Do the same at the stem. Now stretch a tape-measure from stern to stem between the plum-lines. Place a marker along this tape measure, and measure out a distance transversely from the tape. Place a marker. Now measure to the forward and aft plumb-lines, to triangulate your (XY) position. The next bit is easy. Hold the remaining plumb-line against the hull, and adjust until the lead just touches the ground. Measure the length of the plumb-line and weight, and record for your offset.

    Pretty low-tech solution, but good enough.

    Tim B.
     
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  6. viking north
    Joined: Dec 2010
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    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    Don't know if this will help in your situation but if you scroll down to heading Boatbuilding under this site. Go to page 3 and open up "The Nancy G-----" you will find a description as well as photos on taking off hull lines old style. While it covers the proceedure with the hull inverted it is a simple matter to reverse the proceedure with the hull upright. In hindsite i would have divided the hull into 10 equal stations for simplicity of scaling. I didn't because the info gathered was being fed into a computer program and it was not necessary to do so but this was a big mistake on my part as there is always manual calculations and locations that the 10 equal station spacing would have made easier both on paper and brain visualization. I got lazy and took the easy way and will forever pay the price in more complicated measuring during construction. P.S. I am not an architect nor a designer and if I can do it anybody can -- Hope this helps ---Geo.
     
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  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    George, Tim and the rest, if they can't afford to pull lines as mentioned, then this yacht is doomed to rust until scrap.
     
  8. viking north
    Joined: Dec 2010
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    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    The vision of a rusty steel hull just gives me the shivers--- God bless fiberglass and second choice aluminium :) -- Being an old wood man i feel like a traitor every time i utter those words---
     
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  9. ldigas
    Joined: Feb 2011
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    Location: Zagreb, Croatia

    ldigas Senior Member

    Teh ...

    it's a typical case of that (stupid - I've no other word for it) approach where "they just want to work", and "not waste time measuring, drawing and playing with numbers" (as
    literal quote as I could translate it).

    For some people that approach works ... others spend two weeks more balancing the hull and in the end sailing with 1,5t of lead ballast in the stern.

    Six months in the laboratory often saves an afternoon in the library.
     

  10. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Sounds like they need a knowledgeable/skilled leader.
     
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