Lofting

What exactly is the process of lofting? The only thing I really know about it is that its laying the lines of the hull down. Whats the formula for lofting if there is one?

 

Elcapitan,

Each boat design has a set of offsets & lines given in numerical form. These numbers correspond to measured distances from a reference point(s).
Vertical distances are usually given (+ or -) from the waterline or bottom of keel (BOK). Width distances are given from the centerline. Length is usually given from approx. the boats center.

Any point on the boat can be located by giving it's position using what are basically it's coordinates.

When lofting you basically lay out the transverse frames and the fore & aft lines full size. The transom and other parts may be laid out also. This is usually done on a plywood or simularly smooth surface. Nails are driven into the critical and called for points. By laying wooden battens (straight, long pieces of flexible wood) across these, nails one can check for fairness. Critical errors will stand out clearly.

Frames and transverse bulkheads shape and size will be "lifted" from the lofted lines.

I'm no expert, so I am sure others will have some good input for you. Any proper boatbuilding book should also explain the process.

BYW, in 2003 I made a very enjoyable 3 day skiff trip up El Capitan Passage on the west side of Prince of Wales Island Alaska.

Take care.

TGoz

 

Get this book. Great help.
http://www.woodenboatstore.com/Prodinfo.asp?number=325-114&item=1

 

I loft all my boats even ones with the most accurate CAD offsets

the reason is that there are so many componants that need to be drawn full size, like the stem keel backbone, , fabricating these long pieces on the loft and marking the waterlines frame stations etc All the tank detail mast steps, fllors the whole thing is drawn full size, from these you take Mylar and lay down and mark, then you lay the mylar on your plates and centrepop, ready to cut
Even with cad I prefer this method
From the lines in days gone past you woukd have to PROVE the offsets, that is change them to make sure the lines are fair, because you see the architect could only make a calculation from a scale rule, a pencil width out could mean 15mm, and so on
We would start by drawing in the sheerline and profile, then the water lines and buttocks,After these lines were corrected in the longtitudanal then we would then draw the body plan, (sections looking at the boat end on) in theory then all the points on the waterlines would be fair( that is without humps) if a point missed the the long lines would have to be adjusted
over and over until all was ok, then after all thsi we would do diagonals to recheck all was ok
A bad design could take days to get right on your knees,
Nowadays you still need to draw the full body plan because with pressed frames like flats and t sections, you need the full size thing on the ground
Aloftsmans job was a very skilled one, he had to develope flat raked transoms and raked radii ones, i used to enjoy it very much, it required lots of space, but when mastered gave a great understanding of the design process
I have never been able to get to grips with Cad even after years on my knees!!makes me frustrated at times
Old book cant find mine, with chapter on lofting, Howard chappelle, best I,ve seen

 

I always loft full size using a vertical plywood faced loft on the wall nearest the building area.

This enables me to get everything to a pencil line in accuracy quickly and if any changes are needed the simplicity of another color pencil is all that is required.

 

how very facinating, how do you weight the battens on a vertical face, given that doing the body plan on a round bilged boat you sometimes need 20 kg weights to hold the batten around the forefoot?
How do you lay a stem on a vertical face, I,m not quiet sure what you are saying here

 

Huh?? I use about 10 different width battens plus some tapered specials for the parabola shapes in some of my stuff

BTW I also use Ice Picks much sharper than any nail.

My Keels (I am a Cat builder) are picked up with rocker sticks with a batten attached simple and quick.

 

Drawing half of each view of the boat full size is a correcting and then a transferring (to the actual boat parts) procedure.
The table of offsets are taken from a drawing, during which the drawing itself is measured and then the measurements are multiplied to represent the full size measurements.
The problem is that pencil lines, no matter how thin and nice, are never measurable to the degree that they would scale up accurately.
The designer (traditionally, before CAD programs existed) would always measure the location of crossing lines or points to some arbitrary fraction of an inch relative to the view's baseline or reference line, and write the measurement on a handy set of tables called offsets. Often getting as close as an eighth of an inch was sufficient.
Now the scaled up offsets could transfer the drawing (magnifying it) to a large open floor area, and the minor inaccuracies would show up when the points measured got averaged using long and consistently-grained battens of wood of various thicknesses.
The establishment of one very accurate and very fair-curved line, for instance, the midsection, created a whole set of corrections in all three or more other views, each of which would allow even more corrections. A back and forth correcting process would eventually correct each offset, and a batten sprung fair along any curved line would prove this.
Traditionally (and it's still done a lot today), frames, stems, keels, sternpost, virtually every part of the structure would be traced onto the wood going into the boat in a process called picking up. Commonly, tacks laying on their side would be tapped down on the lofted line so that the heads would make little dashes when the wood was pressed down on them, transferring the shape pretty accurately.
CAD can produce extremely accurate measurements to begin with, so lofting time is surely a lot shorter, but I'm not as familiar as these other guys in that area.
There is a difference between building one or two boats and using a hand-drawn design, where it doesn't pay to purchase and become profficient with a CAD program unless you want to learn more than just what's needed to build your own boat.

Alan