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Boat Forum / Building / November 2007Adventures in Lofting
I thought I was a pretty good draftsman. At least until I tried lofting hull shapes... Buildings, machine parts, entire aircraft - no problems. But fairing a hull can be a humbling experience. Well, for grins and glggles, here are a few of my efforts so far. http://www.home.earthlink.net/~cavelamb/draft.htm Richard > But fairing a hull can be a humbling experience. By defination, you're hull is "fair" when your arms are so tired you can no longer pick up a fairing board<G>. Lew >> But fairing a hull can be a humbling experience. > > By defination, you're hull is "fair" when your arms are so tired you > can no longer pick up a fairing board<G>. > > Lew Amen to that Lew, but I have a great solution to the sore arms problem. When I started fairing our mahogany runabout I called in some favors from my youngest son. He is 6'3" of solid muscle. I just showed him what to do and he faired pretty much the whole hull. Tom Check out our boat www.edison-marine.com >>>But fairing a hull can be a humbling experience. >> [quoted text clipped - 10 lines] > Tom > Check out our boat www.edison-marine.com Having done quite a bit of aircraft fiberglass work I can relate to what you guys are saying. But MY issue was with _drafting_ the damned things. My mouse button finger is what's sore! Richard Trying again... http://www.home.earthlink.net/~cavelamb/draft.htm > I thought I was a pretty good draftsman. > At least until I tried lofting hull shapes... [quoted text clipped - 7 lines] > > Richard Well Richard, This is no surprise to a third generation boat builder. The pictures look pretty good. It that first one your design? I don't like working with a baseline other than zero draft because I always get screwed up doing the TPI, trim and stability calculations. It does get a little easier when working on the lofting floor at full scale. The you can go back and correct the offset table. Matt Colie >> I thought I was a pretty good draftsman. >> At least until I tried lofting hull shapes... [quoted text clipped - 23 lines] > > Matt Colie It's (supposed to be) a Catalina 27. >>> I thought I was a pretty good draftsman. >>> At least until I tried lofting hull shapes... [quoted text clipped - 25 lines] > > It's (supposed to be) a Catalina 27. Gee, I though it looked real familiar. Matt >>>> I thought I was a pretty good draftsman. >>>> At least until I tried lofting hull shapes... [quoted text clipped - 28 lines] > Gee, I though it looked real familiar. > Matt I had this lame idea I was going to try when I pull my boat out for the winter. But I can't bring it home because it's too tall for the city standards, so I dunno now. Anyway, the idea was to level the boat and use a few laser levels to pick off the frames shapes. Set up the lasers so as to draw a vertical line every 2 or 3 feet and one horizontally at the water line. Add a couple of vertical lines to pick off the buttlines too (side shot). Take a digital photo - one front - one aft with the "waterline" reference at the same height on my tripod... Load the photos into CAD and trace off the frames. I can't capture the waterlines that way, but the buttlines and frames should be enough to get a fairly accurate start.... Should work ok, don't ya think? Richard >>>>> I thought I was a pretty good draftsman. >>>>> At least until I tried lofting hull shapes... [quoted text clipped - 7 lines] >>>>> >>>>> Richard >>>> Well Richard, >>>> [quoted text clipped - 36 lines] > > Richard Richard, Did you thing to try to get a trailer made that would allow the foil (keel) to drop between the axles and only clear the pavement by a few inches? Next.... Did you just guess at the offsets you used to generate that set of lines? If you did, why didn't you just get a hold of either the class association or Frank Butler (I think he still runs that show) and get the offsets? (If Catalina says that this is proprietary and they don't want to release them, then go ahead and develop an offset table and sell it to owners yourself. This is not illegal because you are making available the observations of a lawfully purchased physical object.) Real world efforts to capture 3 dimensional data have never been simple. We (the family) once did this to reconstruct a much loved dinghy and it was a major undertaking with strings and levels and plumbbobs (lasers would not be available for another few decades). How would you get the image of the frame stations or buttlines with the cradle structures in the way? One of the little "gotchas" out there is the actual accuracy required is quite high. I do the odd keel refairing job from time to time and that is simple compared to this because the measurement survey is largely done against and almost vertical surface. It is still really hard to get better than +/-1/8" and that is just not very usable data. I was struggling with this issue myself for a number of years. I owned and S2-7.9. No lines or offsets are available. I got real close to being able to barrow a large scale coordinate measuring system. I was going to either hang the boat by the chainplates or roll it over, but the entire plan collapsed before that became the big issue. Keep working on the plan and let me know how you make out. I'm here a lot. Matt Colie >> I had this lame idea I was going to try when I pull my boat out >> for the winter. But I can't bring it home because it's too [quoted text clipped - 40 lines] > was a major undertaking with strings and levels and plumbbobs (lasers > would not be available for another few decades). The top pic, the basic lines layout was found on the web. The others I've either taken from books and traced, or used the published offsets. But I'm not intending to sell anything. At least not at this stage. Just trying to learn how to do the work. Tracing sucks. Terribly inaccurate since he lines look like they were drawn with a 3" paint brush. Makes fairing a lot harder than necessary. And once faired, there is no telling how accurate the generated shape is compared to the original. > How would you get the image of the frame stations or buttlines with the > cradle structures in the way? My trailer has very little to interfere with the laser. This CAD package used a splined curve. Outta fill in the small gaps ok. At least it would be cheap and easy enough to try. > One of the little "gotchas" out there is the actual accuracy required is > quite high. I do the odd keel refairing job from time to time and that [quoted text clipped - 10 lines] > Keep working on the plan and let me know how you make out. > I'm here a lot. > Matt Colie > The top pic, the basic lines layout was found on the web. > The others I've either taken from books and traced, or used the > published offsets. Sounds Like you have found effective sources. > But I'm not intending to sell anything. > At least not at this stage. > Just trying to learn how to do the work. Learning is always good, > Tracing sucks. > Terribly inaccurate since he lines look like they were drawn with a 3" [quoted text clipped - 3 lines] > And once faired, there is no telling how accurate the generated shape is > compared to the original. Just so you know.... The final hull and foil shapes of a number of boats out there don't always have a lot in common with the designed lines. (That's what I often have to fix.) >> How would you get the image of the frame stations or buttlines with >> the cradle structures in the way? > > My trailer has very little to interfere with the laser. > This CAD package used a splined curve. > Outta fill in the small gaps ok. Prolly work, just watch out, computers can be real stupid sometimes. > At least it would be cheap and easy enough to try. Actually, it sounds like it could just be a lot of fun. If you could get about a box full of those line creating lasers and set some up as waterplane, some as station and some as butts - you might be able to reverse engineer a hull shape pretty effectively. I'm interested to see how it works out. Matt Colie Richard, You have done a very good job of recreating a lines drawing on a computer, but your effort has nothing to do with lofting. Lofting is laying out on a wooden floor the full lines drawings full scale for the purpose of template generation, one on top of the other using the table of offsets as an initial starting point. Fairing the lines occurs during this process by driving nails in the floor at line intersections and subsequently bending an appropriate batten around the nails to scribe the line. When the battens fails to bear on a nail, the nail is then removed and redriven at the correct point. The offset from the table is then remeasured and entered into a new table that is called the "corrected table of offsets". Only then can templates be taken off the lofting for the manufacture of the individual components to stand up the actual hull. This is not done with a mouse while sitting comfortably in a chair. It is extremely hard work done on your hands and knees for many days on end. No computer will ever replace this task, even big ship yards still must bulletproof the table of offsets for CNC cutting machines to be accurate. Steve >I thought I was a pretty good draftsman. > At least until I tried lofting hull shapes... [quoted text clipped - 7 lines] > > Richard > Richard, > You have done a very good job of recreating a lines drawing on a computer, [quoted text clipped - 28 lines] > > - Show quoted text - :O > Richard, > You have done a very good job of recreating a lines drawing on a computer, [quoted text clipped - 26 lines] >> >>Richard Hi Steve, Actually, I erased all the nail holes - trying to be neat. Richard > No computer will ever replace this task, > even big ship yards still must bulletproof the table of offsets for CNC > cutting machines to be accurate. > Steve Steve, Maybe a decade ago this was true, but in the real world of today's shipyard, the cut files are all done on computer, the plate is burned, and erection starts the next week. The loftsman is a dying breed. We (commercial naval architects) seldom provide tables of offsets any more; just send 'em an IGES file of the 3D hull surface, or we produce all the steel parts in house. No lofting required. Evan Gatehouse > > No computer will ever replace this task, >> even big ship yards still must bulletproof the table of offsets for CNC [quoted text clipped - 12 lines] > >Evan Gatehouse Aw shucks! You didn't even mention being able to run a ball bearing along the panel seams before weldup! :-) Brian Whatcott Altus OK > > No computer will ever replace this task, > [quoted text clipped - 13 lines] > > Evan Gatehouse While the drawings look small, they are drawn in full scale. Actually, sometimes much larger than full scale. When zooming in to locate a point between a waterline and section I may be as much as 100 x magnification. Trying for .01 inch resolution. Can't do that on the floor. But that's the way it's done these days - and why the parts fit so well (hopefully!). Richard Evan, I did a bit of research and you are correct, it is now done that way. I guess I am a dinosaur. God bless computers, lofting is a horrible task. I know, I have done a lot of it in my life. However, I hesitate calling mouse work lofting in deference at least to the way it was. Steve > > No computer will ever replace this task, >> even big ship yards still must bulletproof the table of offsets for CNC [quoted text clipped - 12 lines] > > Evan Gatehouse |
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