Adventures in Lofting

It all starts here - basic "lines drawing" of a hull is arranged in such a way
as to represent the shape of the hull in all three axis - X, Y, and Z. 

These are quite different from what is normally called a "line drawing"!

The lines drawing depict what you would find if you were to slice the
hull up on a particular axis.

The "profile" is the side view, the "plan view" is the boat as seen from
above, and the "body plan" or "sections" show what the slices would
look like as seen from the ends.

In each view there are two sets of straight lines, and one set of curves.

The Lines

At the top of this drawing is the profile view.

The curved lines are called "buttlines" which are shown as straight
lines on the other two views.

Next down is the planform.
The curved lines here are called "waterlines". If the hull were sitting in the
water that deep, that's the shape the water would show.

The vertical lines are called "stations". They are located aft of a vertical
reference line refered to as the "DATUM" (the zero point along the length
of the hull).


Similarly, the curved buttlines in the top profile are shown as horizontal lines
on the plan view. If we sliced the hull on the butt lines, we would get the curves
of the buttlines.

Lastly are the sections, shown to the right side. These curves represent the
cross section shape at each station. I.E. as if the boat were sliced up like a
loaf of bread.

The horizontal straight lines shown in the section drawing are the waterlines.
The vertical straight lines are the buttlines.

The intersection of all these lines (plus a   few diagonals to smooth the bottom)
makes a "fair" curve.

A fair line is free of extraneous bumps or hollows.
An unfair line needs to be faired, or smoothed out.

 
It looks something like this when projected in 3D (that's the port side shaping up).

The section lines are on the right side (yellow).

The waterslines are at tthe bottom (green).

The buttlines are the purple curves at the top.

Notice how a line is represented in each of the three views.

For every line, two of the views show it as a straight line while the third shows
the curvature.

To create this 3D shape, I've simply taken the section lines and placed them at
the appropriate station. The waterlines are raised up to their correct location.
Then the buttlines are slid in place.

The Art of Fairing requires that all three lines intersect at the same point.
If they don't then there is a lump or dip somewhere on one (or more!) of the
lines. These have to be corrected.

 
In the old days this was done with long flexible sticks of wood (called "battens")
bent around nails driven into a FULL SIZED DRAWING on the lofting room floor.
An eighth of an inch was considered very good tolerence.

Today, large ships are built in interlocking sections and must fit a lot closer than
that. So our drawings are done on a computer screen with a CAD program.

Just like you see in these examples...


On these drawings I've tried to hold tolerance to 1/100 of an inch tolerence.

Here our hull has been rendered as a wire frame -
with hidden lines removed to reduce the confusion.

Smooth rendered!
The fin keel will be added afterward.
Starting to look like a Catalina 27 yet?

This one (Triton 29) is a bit tougher due to the deep keel, which must be faired
into the hull. But it came out pretty nice in the end.

Before computers came into use, this kind of drafting was done full sized on the
lofting room floor. Drive nails in to locate the points and bend flexible strips of
wood (battens) around the nails to make the curves.

Then examine for fairness and adjust as necessary.

Doing it on the computer is the same, but harder - or easier - depending on how
you feel about computer work. But that's the way it's done these days.

A couple of others "finihsed out" a bit more...

This one is a traditional Cat Bboat. Single sail set forward on a fat hull.

And a schooner for the romantics.