Reverse Lofting a boat

Discussion in 'Boat Design' started by WHumphreys, Aug 2, 2009.

  1. WHumphreys
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    WHumphreys Junior Member

    Are there any good write ups / books or anything anywhere on reverse lofting a boat. I would like to draw some plans of my boat in autocad but would like to know the best / correct way to get the measurments.
     
  2. wardd
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    wardd Senior Member

    a program that uses photogrametrics might be helpful
     
  3. apex1

    apex1 Guest

  4. WHumphreys
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    WHumphreys Junior Member

    I have debated it but to be honest Id just like some line drawn plans with accurate measurments of the boat Im restoring. At this moment in time Im not really that interested in being able to do calculations on the shape.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Chapelle has a little bit on in in one of his books, "Boat Building" most likely. It's not an easy task, if you expect a fairly high level of accuracy, without employing lasers and rather costly equipment.

    The short of it is establishing a base line, under or over the plumb and leveled boat, then make your measurements from it.
     
  6. Guest62110524

    Guest62110524 Previous Member

    get some strip ply, and a hot glue gun, , invert your boat, then mark off some stations, then make patterns of the body plan with the ply and hot glue gun

    shoot off the sheer height with a dumpy or water level, lay your sections out with the heights as taken etc etc etc
     
  7. wardd
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    wardd Senior Member

    a digital camera with good resolution some paste on dots and pics at different angles and photogrametric program, and ive seen some rather cheap programs

    this will get you reasonably close

    o and a tapemeasure will be needed

    the process is used in aviation to inspect jigs and fixtures so if used right it can be highly accurate
     
  8. peter radclyffe
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    peter radclyffe Senior Member

    i made the lines plan for Lulworth by reverse lofting her, on my own , she had lost her shape, so i battened her, faired her in & took new patterns for every, 75 frame sections, then bevel boards every 3rd or 5th plank bevels on steel frames are plus 90 degrees, on wood there minus , 36 planks, then butt plan which i redrew having 14 metre planks, i patterned all the deadwoods , deck, stringers, sheer, etc , then lofted the body plan & drew offset tables, waterline, buttock, diagonals, keel & sheer heights ,etc another reason for doing this is because fairlies didn't know how to take the shape off for the new 70 ton keel, so i did it for them, some 4'000 measurements later, i had a new lines plan, but for your hull, level the hull, set a spinning laser at your frame stations, or less stations outside the hull, then measure in level at set waterline heights, & build an offset table, for a new hull, take off the planking thickness sq to plank, pattern the rabbetts, & profile,
     
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  9. kroberts
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    kroberts Senior Member

    Not exactly sure how big your boat is, but you can fairly easily get an outside cross section the "old world" way if you are not comfortable with the computers and photo software.

    Set your boat upside down and level. Set up a rail above it, at the station you want, above the boat. You only need to access half the boat, from centerline to one side. Set up a piece of light plywood so it hangs from the rail and can slide closer to the boat. You can start with a general guess of the shape, as long as it's too tight. Cut out your "guess" shape, and then put the plywood on the rail and move it toward the boat till it touches. Make sure the plywood is hanging straight down. You might want to have a vertical board be part of your jig, so the plywood can be pressed against it.

    Now, take a pencil or marker and rest it horizontally on the boat, so the point touches the plywood about 1/2 the diameter of the pencil from the boat. Draw a line the entire contour, anywhere the plywood is close enough.

    Sand or cut the plywood along the line, and repeat it until you have a nice tight contour for the entire section. You now have the outside cross section view of one "rib" section. Move your rail to the next station, start with a new piece of plywood and keep going until you have enough examples.

    This is slow going, but it's reliable. If the boat has a flat bottom, you might want to tilt the pencil closer to the boat for the bottom, the idea is to just get the tight spots sanded off, but a little practice will get you there.
     
  10. keith66
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    keith66 Senior Member

    A few years ago i lifted the lines from my 28ft Saunders launch, this i did with a fair degree of accuracy. My father in law wanted to build a replica in cedar strip, He insisted on putting the offsets into Autocad, Now it is a fine program for engineering and designing interiors & components but it cannot fair a hull properly. It fairs through all points regardless of how lumpy the surface turns out.
    On the small screen of the monitor you simply cannot see how unfair it is, He tried & two other engineers tried all using Autocad and it produced a table of offsets that was so innacurate that when the molds were set up the whole lot was so bad i had to fair it by eye to put it right, this took me a week.
    It would have been easier to lift the molds straight from the master body plan i took.
    Now if it had been put into delftship or lofted full size a fair hull would have resulted.
    As it happens the boat has turned out ok but at the cost of a lot of totally unnecessary work.
    Moral is dont try to force a program to do what it wasn't designed to do.
    And yes i do rub it in on occasion!
     
  11. peter radclyffe
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    peter radclyffe Senior Member

    thank you, this is interesting, are you saying delftship is more accurate than autocad
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    To keep it simple :rolleyes: without trigonometric functions or sophisticated software basicly what you need is two planes, the other one horizontally and the other one vertically along the longitudinal axis. If the boat is open, deckless, it's easy to arrange the planes "inside" the boat. It's indifferent how you "make" these planes, with lasers if you have them, cartboard, paper with strips, imaginary.. anything you find convinient to use.. With bigger boats with closed interiors it's easier to have these planes outside, also smth like level ground or floor under and a wall on the side are possible. Make a grid on the planes and measure distances btw grid and hull points.. and make a drawing according the measurements..
     
  13. keith66
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    keith66 Senior Member

    In reply to Peter, its not that Delftship is more accurate but that the way the programs generate curves & modify them, Autocad is used for enginering components, structures etc, it is not designed or intended for fairing hull surfaces, it will take the fairest line through a series of points but the points themselves stay in the same position, of course if you cant see the dents or bulges at small scale at full size they might be measured in inches.
     
  14. WHumphreys
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    WHumphreys Junior Member

    The boat Is 20 Foot GRP and I have removed all the interior bar a few bulkheads. I debated using the tick stick method and making cardboard bulkheads at intervals along the inside of the hull and then measuring them.

    keith66 - 'It fairs through all points regardless of how lumpy the surface turns out' what does this mean.

    To be honest I may not even use a cad program and do it the old way on paper.
     

  15. Guest62110524

    Guest62110524 Previous Member

    when your boat, or any boat was drawn, , the lines were laid down , faired and from there the sections (body plan ) was arrived at From the body plan frames were made, either permanent of to build a mould from
    So you have to do the reverse
    If you do as I recommend, and take patterns from the out side, using a hot glue gun and strip, you will arrive at the body plan .
    when you have your sheer heights, you can lay the sections on the loft at that height
    From there you now need to lift the chine height and half breadths, and sheer h and 1/2 and check the fainess with a long batten,
    You set out your long loft with stations , same distance apart as you took from your boat
    Buy a book on lofting, it,ll help, Chapelles was the best, but they are light years outta print, and can find from time to time ebay
     
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