what next-lofting

Ok, so I have two 1/2 hulls that I've carved. One is a 16' Chesepeake Bay crab skiff and the other is a 20' round bilge rowing boat. How do I get a referance line to work from in order to have the molds all set up at the proper height, and how do I get things scaled up to get to a 1 to1 scale to loft from? A second question would be a recommendation for a good beginners book on lofting. The crab skiff is to be traditionally built and the rowing boat will be bead and cove redwood strips

 

Working from half hull models requires the same amount of hydrodynamic expertise, as working the hull shapes on paper or computer. Target displacement, CG and other "centers" locations, scantlings, volume distribution, a hull form suited to the speed targets established, etc. In fact, it's harder to work from a model as you really have to know what shapes to use and a good understanding of volume distribution, before a half model can become a successful full size.

Assuming the half models are accurate representations of known hull forms, the next step is to take off the lines. This can be done one of several ways. You can physically divided the hull into stations vertically or waterlines horizontally. You could even do this diagonally if you want to. Of course you'd have to cut the hulls and account for the saw kerf. You could divide the hull with a pen line, which saves the hull, just wipe the pen marks off when you're done. Doing it this way requires some way to accurately make straight lines on curved surfaces, but a simple angle iron jig could be arranged to accomplish this fairly easily. If the model is of substantial scale, a laser level could be helpful. You also can scan the hull and import the data into a program where you can render a 3D model, from which you can plot and print accurate, scaled lines to loft. With the lines in hand, you can generate a set of offsets, from which you can loft full size. Of course, this requires a new set of skills and you'll be fixing the errors inherent in pulling lines from a model, as the seemingly slight deviations in a pencil line width can amount to a huge bump or hollow area on the hull in full size. These are fairly easy to identify and fix on the loft floor.

The book store on this site will have several references that will help.

 

I was hesitant to post on this thread because I don't know anything and I'm quite impressed by someone doing all of that lofting.

However, I'm a lazy guy.

I took a full scale 1:1 CAD file and just printed out all my stations and bulkheads, then cutting them from the appropriate materials using the print outs as a template.

I know this isn't answering the question, but I'm not sure if the OP is in it for the traditional art of lofting, or if he just wants to build a boat. Can you get the models int any CAD program for full size station and bulkhead prints?

 

The original poster isn't at the lofting stage yet. He needs to develop the lines first, then offsets, then fix everything on the loft floor or in software.

 

Half models are traditionally made with thin boards that correspond to the waterlines. Is that how you built yours?

 

nordvindcrew, you might want to consider taking the lines and offsets for several traditional small boats and either redrawing and fairing them, perhaps at 1/4 scale or so, or even lofting full size. Once you become confident with that proceedure you could go back to your half models, lift the shapes from the models, and then draw a set of lines or proceed directly to lofting.

 

Howard Chapelle's Yacht Designing and Planning (revised edition at least) has an appendix on Instructions for Taking Off Lines of Half Models and Drawing Lines. Included are procedures for both models which cannot be disassembled and models which can be disassembled into waterline "lifts".

 

If I remember my reading correctly, one of the methods used to take lines off a half-hull in the old days was to press pieces of lead wire against marked station molds and/or stations, then lay them on a piece of paper to be traced.....

 

Same here, I've read that solder wire pressed against the model was a popular way to take shapes off of models.

Contour/profile gauges are another possible method. Two examples:
http://www.leevalley.com/US/wood/page.aspx?p=32522&cat=1,42936,42958
http://www.generaltools.com/837--6-CONTOUR-GAGE_p_246.html

 

Keeping the moldable, stiff wire straight is the biggest issue with this method. A simple jig, where a perpendicular can erected over the model and a sharp pointed pencil can scribe a straight line on a curved surface fairly easily, from which you can take measurements or a template tracing.

If I was doing this, I'd mark the hull with station mold and waterline spacing, then draw these straight lines with a simple jig. At each cross vector, I'd carefully take a measurement and create my offsets, probably with a pin point set of dividers, insuring I measured straight down from the jig during the process.

 

half hulls

both half hulls are solid wood; 1"/ft' scale. I carved them just to have something to do last winter. I like them so much that it seems one of them would be a perfect candidate to learn more about lofting. I've lofted one boat from a drawing but need to learn a whole lot more. Both are conventional hull shapes. The crab skiff is V bottom whilethe rowing boat is round bilge and similar to but more burdensome than a whitehall. I have 2600' of 3/16" bead and cove Western red cedar as well as a pile of 3/4" white cedar that are just begging me to become boats. As a referance, I've built 9 boats, all were built from plans I, or my brother drew and they've all worked out pretty well, I find that in a small boat the rower and passenger may weigh twice what the boat weighs.To me, that means that trim and stability are easily corrected with thwart placement. Thanks for the input so far

 

Use a laser line pointer to illuminate a line on the half-model and a digital camera to view it from either bow-on or stern-on. Mount the camera and line pointer on a wood base to keep them a constant distance apart so the scale of the photos is constant. Move the half-model between shots to capture the stations. You can also do buttocks, diagonals or waterlines. Then enlarge to a convenient scale and print. Doing the final lofting from a large flat picture will reduce errors.

If you wish to get fancy, take the pictures in the dark for a black background then with the right software you can make the background color transparent, superimpose the pictures (bow shots on the left, stern shots on the right etc) and add a dimensioning grid . . .

 

How does a laser line pointer illuminate a line on the surface of a half model? Wouldn't it be just a point.

A laser level puts out a sheet of light would illuminate a line on the surface.

 

Terminology confusion? For me a laser pointer puts out a point, a laser line pointer puts out a line (like the laser guides built into many modern tools) and a laser level is an expensive beast that puts out a horizontal line all around the room, but whatever you call it you want the one that generates a line.

BTW the cheap ones generate a line that tends to curve a little at the ends so use the middle 30% of the line for accuracy . . .

 

So when you say "a laser line pointer puts out a line (like the laser guides built into many modern tools)" you mean a device which puts out a sheet of light which forms a line when it intersects a surface.