Digitizing a Sail Yacht - Taking offsets from hull.

Discussion in 'Boat Design' started by Owen, Oct 27, 2008.

  1. Owen
    Joined: Feb 2002
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    Location: University of Southampton


    I would like to build a virtual model of my j100.

    I am all set on the digital side with experience in Maxsurf and ACad.

    Aware that the sail and deck plans are available online - from which a good approximation could be made - I am challenging myself to taking the offsets from the hull itself.

    The j100, 32ft LOA is up on stands for the winter, with a nice clean bottom and good access from the ground up.

    I am looking for practical advice in how to take accurate offsets from a hull shape, and would appreciate all thoughts and suggestions.
  2. Guest625101138

    Guest625101138 Previous Member

    I will be interested with what others suggest.

    If the boat is on planar ground then you can use it as a reference plane. Take offsets above a level plank vertically to the hull at 1ft spacing beamwise. The plank moved from stern to bow at 3ft stations. Take an hour or two and will be easier if you have someone recording while the other measures. Probably end up with something like 60 points.

    A less involved method is to take a few good photos from different angles. It is surprising how accurate you can get from a few photos that have good views of the hull. You then align the displacement in the model to the full size boat for the known waterline. If there is published coefficients and WL measurements you can get suitable accuracy for most modeling purposes. The software usually allows you to import an image to compare with the photos. You could even add a few measured reference points.

    So these are the low tech methods. Maybe others will have some surface scanning ideas using laser technology.

    Rick W.
  3. Knut Sand
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    Knut Sand Senior Member

    I had some sheets describing this pretty thoroughly......

    It included the use of 2 ( or was it 3?) self aligning line lasers (not too expensive these days), some wood....

    First the boat must be level.
    What they then did was making a mark on the floor, below the bow/ aft. Using a lead weight in a thin line.

    Directly to the side from this let's say 2 m, parallell to these two points on the floor they leveled up a beam, on this they marked a point for each cross section of the hull they wanted to measure, assume a distance of 30 cm.

    Diagonally from the bow, they had a laser fixed to a standing beam, with vertical marks, lets say for ea 20 cm, this laser made horisontal lines on the hull.

    Now they had a standing vertical beam, laser taking vertical lines, that they moved along the beam on the floor, going in straight angle from the beam on the floor, shooting the line towards the hull.

    Now you get a red cross on the hull..... Measure the distance (d)mm from the vertical beam (with the vertical laser); 2000 - "d", Will then be the measure form the centre plane of the hull to that point of the hull.
    Now put that measure in on a spreadsheel/ paper.

    A "grease marker"/ "grease pencil" (the stuff you can use to write something anywhere, and then is easily wiped away with the back of your hand) will be useful, to get a picture of where you are, and how far you've come in the job.

    In flatter areas with less bending of the hull shape less measures will be needed.

    Repeat a zillion times. 2 persons does this a lot quicker than one.

    You will have to have a horisontal laser from the aft area too. (there you can use the one from the front, markings on the hull will make it possible to realign properly.

    Now you have the data needed to make a model of the cross sections.

    Put these into the program, space the plane in the program according to the distance for the measures taken for the cross sections.

    press "loft" (At least that's the function in SolidWorks)

    Use the most of the next weekend to sort out problems and bendings in the hull shape....

    Sure you have that much problems with killing time?
    You already have the boat?

  4. Knut Sand
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    Knut Sand Senior Member

  5. masalai
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    masalai masalai

    Not very ethical, nor morally honest.... is theft and therefore a sin...
  6. Owen
    Joined: Feb 2002
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    Location: University of Southampton


    Good Ideas

    Gents, thanks for your input.

    A common theme is the reference plane - essential for accurate measurements over the entire hull.

    I understand the importance of a perfectly square base from which measurements are taken, and shall make this my first concern.

    The Grease pencil is a simple yet invaluable tool for marking the surface and tracking progress.

    I am a visual thinker and intend to mock-up the shared ideas in Rhino and post them later this evening.

    I have done several forum and web searches with sparse results.

    Any ideas for information sources? I was thinking of contacting Wooden boat magazine.

    Post-script - Masalai, it seems you are taking my actions and preempting my intent. If you had the decency to ask you may not be so quick to judge.

    Furthermore, the impetus of this forum is sharing a collective knowledge of boat building and design, suggesting your ethical retort is misplaced here.
  7. bntii
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    bntii Senior Member

    I watched a boat get measured for its handicap rating last summer.
    The man set up a perfectly parallel line on each side of the hull. Over this line he set up a tripod which was much like a surveyors rig. On the tripod he had a instrument head which was comprised of a protractor loaded with line with a stylus attached. Both the movement of the protractor and the line were precision instruments which recorded both the angle of movement and extension of line to a laptop computer. In use the tripod placed at point 'O' station 'O' at the stern, the line was extended and touched to the hull at a number of positions from keel to rail. As the instrument was used each position was recorded and a 'tracing' developed of the hull which represented the profile of each station. The procedure was repeated at 1' increments along the hull to develop a full record of the shape of each station and thus the total hull form.

    It was a very labor intensive way of recording the hull in the field which approximated the full blown robotic recorders used in industry. The man running the equipment told me a bit about the accuracy of the system. He was getting a measurement resolution of less that 1/16" on the hull. He felt that excessive bottom paint on the hull could effect a yachts rating. He also told me that any mistake in setup would of course cause highly skewed results. He was doing a rough analysis in the field which was confirmed on return to his office a few states away. Seems on rare occasions the software would reveal these mistakes and require a return to the field to re-shoot the whole deal.

    I got the impression that the process has been in used by the rating organizations for some time.

    He also allowed that all of the measurements he developed are sitting in the design office of the NA for each yacht. They are not released as stability numbers and such would spell out all too clearly the characteristics of each design.

    Perhaps opinion from a guy standing in the rain for two days with a tarp thrown over his laptop..

    masalai- pm sent
  8. Crag Cay
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    Crag Cay Senior Member

    If you have access to an accurate sheer line in both plan and profile then your task is a little easier (as long as there's no tumblehome).

    Weighted strings hanging down from known points along the sheer will give you lines of reference. Striking lines perpendicular to both this and the centre line will be more convenient with a simple laser level, but can be done with a builders spirit level and large square (and some helping hands).
  9. masalai
    Joined: Oct 2007
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    masalai masalai

    bntii, - - - Please don't get upset by my post, - - - - this general theme was addressed in another context/thread a while ago and my failing filing system could not remember the thread name...

    There is fair use, and that could apply to making a model because you like the look and would be a pleasant addition to your display cabinet... In that case, contact the designer direct and declare your purpose - You could be pleasantly surprised and delighted by the assistance and support available.... A brass plaque suitably inscribed with designers name and date of first build and famous names of that design in history would make a nice finishing touch...

    I don't think you could interpret the "admonishment" as a slur on your esteemed self - I meant in good grace as a gentle hint.... to prick your conscience... :D:D:D:D:D
  10. Knut Sand
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    Knut Sand Senior Member

    Now that depends of the further use of the lifted numbers, if I can use that expression. It'll be a learning process, and a good use of a brain - that's never unethical or morally dishonest or a sin.

  11. Willallison
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    Willallison Senior Member

    By way of a little explanation re masalai's comment.
    Many of the contributors here are designers by profession, and the act of "splashing" a hull design is simply regarded as theft. So whilst few would be upset at the prospect of an owner wanting to make a model of their own boat, you wouldn't be the 1st to come along and try to get us to help you pinch someone elses work.
    I'm not suggesting that you are BTW, and I wish you well in your efforts....
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    So long as it's for your own use you already have paid for rights to the design for one boat and are not going to create another, right? The simplest and fastest, most moral and safest way of course is to ask the designer. Some are rather sensitive about this as you have already seen. However, I can understand if this is just an exercise you want to try for the fun of execution.

    You can simply set up a laser line off to one side at each station to paint a line where a vertical plane intersects the hull's surface, and take digital photographs from ahead and astern, keeping the camera a fixed distance from the line and at a constant height. The laser's axis must be normal to the boat's and the line must be vertical as seen from the laser. The camera will need to be at a few hull lengths distance but so long as the camera's axis is parallel to the boat's parallax errors will not be excessive for your purpose.

    Near midships the line becomes hard to see form the camera; the camera may be offset so long as it's axis remains parallel to the boat.

    it's best done at night; if the boat's parked in a storage facility you should probably explain what you're doing to the security people ...

    Take a calibration shot of an accurately square object to check out your cameras optics however; they ain't all perfect by any means.

    A similar method was used years ago when dynamic laser measuring was in it's infancy to capture snapshot images of highspeed robots executing complex trajectories for the purpose of checking instantaneous path accuracy.

  13. Landlubber
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    Landlubber Senior Member

    To do a simple drawing of the lines, the vessel is slipped, levelled thwartships and a plumb bob used to drop to the base board, readings are recorded and them compiled later of course.
    Measurements can be taken directly from the curvature of the sections being recorded as well. The waterline is an obvious easy one to do, the buttocks are difficult but not so hard when you realise that the keel is the centreline, the plumbob hangs a certain distance out from the keel so the measurements can be very accurate. I have used this method to draw and old tug for survey requirements, and it was acceptable.
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