Lofting

Cutting wood is like cutting paper.  Draw the shape of the cutout, then cut.  Unlike paper airplanes or dolls, life-size wooden boat cutouts are big; six feet by four feet big, in this design.  So how do you move the lines from your paper design onto the actual boat?  Lofting.

Lofting is an old form of copying.  Take lines from a plan, scale them up, and put them on wood.  It sounds complicated.  It’s not.  The shipwrights of the 17 and 1800’s had to loft hundreds of pieces.  For our boat, only the bottom part needs lofting.

Recall the simple plan:

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In this chapter, that oval drawing will be scaled up and put on a 4’x8′ sheet plywood.  When cut, that oval will look like this:

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And here are the lines.  Only the curves will be cut.  The other lines either help determine the shape of the curves, or direct us to place beams and screws in later stages of assembly.

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What you need

  • A flat surface.  A driveway works, but I prefer a razed platform.  I’ll use the boat platform that we built earlier.
  • A carpenter’s square.
  • A tape measure. This is optional.
  • A pencil.
  • A long, strait edge.  This can be a metal strait edge or be a 4′-8′ length of high-quality wood.
  • A 4′ bendable piece of wood for drawing curves.  A strip of trim or scrap plywood works.
  • A 4′ by 8′ 1/4″ plywood sheet.
  • 3-4 clamps.
  • Random scrap wood.

Setup

The boat platform is complete, so a handy, movable table is already available.  The platform isn’t wide enough to securely place a 4’x8′ sheet of plywood.  To fix that, take a few 1’x4’s and line them up along the top of the boat platform.  They don’t have to be even. About six is enough.

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Only the far ends of the plywood hang out.  That is no problem.

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The big oval is symmetrical from front-to-back and side-to-side.  The first step is to divide the plywood sheet into four equal rectangles.  Slide the sheet so one short side is no longer drooping.  Let this short side rest on one of the 1’x4’s.

Line the carpenter’s square up against a corner.  Mark the 2′ point along the short side.  If you buy expensive plywood – or if you are lucky and the sheet is part of a good batch – this 2′ point is at the exact center of the narrow end.  Chances are, it isn’t.  To check your mark, place the square on the opposite corner and mark the 2′ point again.  Here, I was lucky.  Both of my 2′ points are on the same spot.  If the side isn’t exactly four feet wide, you’ll have two marks. These marks should be close enough for you to eyeball a center point to mark off.

To find a midpoint, always measure from two opposite sides.

How accurate should these measurements be?  An eighth of an inch is passable.  Nobody will notice a 1/8″ error when the boat is on the water.  Keeping measures within 1/8″ is easy enough, too.  With practice, measuring to within a 1/16″ easy.  Practice away.  You can always erase pencil marks.

We double-check measurements for big errors, not the 1/16″ goofs.  It’s those 1 or 5-inch error that we need to watch out for.  I built a chair and was so proud of a super accurate 5 15/16″ rabbit … until I realized the plan called for a 3 15/16″ rabbit.  In the saying “cut once, measure twice”, “measure twice” is for brain fart insurance.

On to the long sides.  If you have a tape measure, start at a corner and mark 4′ point. Approach from the opposite corner and repeat.  Again, we want two marks from opposite ends to make sure the point is the exact center.  If the two marks don’t overlap, eyeball and mark the mid-point between them.  The goal isn’t to find the 4′ mark: the goal here is to find the center point.  In my sheet of plywood, the center is at 4’1/8″.  The sheet is 8’1/4″ long – I got an extra 1/4″ for free!

A carpenter’s square also works on the long side.  Mark two feet, place the square on that mark, mark the next two.  Two plus two equals four.  Get the other 4′ point from the other side.  Eyeball the midpoint of those marks.

The short sides have mid-point marks.  The long sides have midpoint marks.  That’s four marks, one in the middle of each side.  Time to connect the dots.  The goal is to form a cross that divides the plywood into four equal rectangles.  Enter the strait edge.  Four feet is a standard length.  I own one and find it useful for many projects. The cheaper alternative is a strait length of wood.

Lay the wood (or strait edge) across the narrow part of the sheet at the 4′ marks.  Hold it flat, and draw your line.  That mark divides the plywood into two 4′ by 4′ squares.

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Mark the center of that line.  Again, measure it from one side of the panel, then the another.  From one side of the sheet, measure 2′ in.  Then do the same from the other side.  Find the midpoint between the two marks.  That is the center.

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Place one end of the wood (or strait edge) on that center point.  Place the other end at the midpoint of one of the short sides of the sheet.  Draw the line.  Repeat for the other side. The plywood is now divided into four equal 2′ x 3′ rectangles.  Easy!

Things get interesting soon.  Before then, we need four more lines.  The plan calls for a 3′ 6″ wide by 6′ long oval.  So far, we have a drawn a cross.  One line is short (4 feet), the other long (8 feet).  On the short line, mark the point that is 3″ from the each edge of the plywood.  For the long line, mark 1′ from each edge.  Then mark the edges.  On each short edges, mark a point 3″ from each corner.  On each long edges, mark a point 1′ point from each corner. Connect the dots.

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You should have a 6′ x 3’6″ rectangle inside the 8′ x 4′ plywood panel.  In the picture above, each red dot is a mark, the thick lines are the lines that we drew.

Now for the curves.  Start by drawing a diamond. Draw it like so:

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No measurement is required here. Just connect the points.  With the diagonals drawn, find and mark the midpoint of each.  By now we’re experienced mid-point finders. It’s probably second nature.

Then, from each midpoint, use the carpenter’s square to draw a 90 degree line that points outward from diagonal:

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Time to get out the clamps, a long 1’x4′, bits of scrap wood, and a flimsy long piece of scrap wood. I find scrap plywood works well as the flimsy piece.

Place two clamps on the edges of the plywood: one near the center of a short edge, and one near the center of a long edge.  Put pieces of scrap under the clamps.  The idea is to brace the flimsy piece to the points of the diamond we just drew.  The clamp at the long end of the diamond probably won’t reach, so use a large piece of scrap.

Now line up a 1′ x 4′ over that 90 degree diagonal line that juts out from one of the diamond lines.  This 1′ x 4′ will push the center of the flimsy piece of scrap outward.

Place the flimsy piece between.  Make sure the INNER part lines up with the diamond points at the ends.  To hold down the curve, you may want to brace it with a fourth clamp and some scrap wood.  Does it look like a crossbow?  Good.  That’s what we want.

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Adjust the 1′ x 4′, pushing the flimsy piece out as far as needs to be in order to look like an oval curve.

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And trace along the inside of the flimsy part.

DSC01180.pngThis is Leonardo-esque curve is the result:
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The 1′ x 4′ blocked us from tracing the junction of the curve and the diagonal line.  Fill the gap using a strait edge or ruler.  Then, starting at the edge of the diamond, measure exactly where the curve meets the diagonal. This step is important.  It tells us where to place the middle of the flexible scrap on the remaining curves.  My mark is at 6 3/8″.  Therefore, I marked the 6 3/8″ point along my other three diagonals.  Double check this measurement: accuracy will make the boat symmetrical.

Make three more crossbows.  I made sure each flexible part lined up at the 6 3/8″ mark along the diagonals.  Do the same for yours.  Draw three more curves.

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Your reward will be an attractive and symmetrical oval.  Before we wrap up, there are two more lines to draw.  These, the thick red lines below, tell us where to place some ribs later on.

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From each tip of the oval, measure inward and mark 1 foot. Using the carpenter’s square, draw a perpendicular line from this point to the curve.

Does your plywood look like this?

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Great! Get out your jigsaw.

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