Lofting the Coot

Okay, so as I've continued to study lofting, I realized trying to tackle a full description of lofting might be a bit too much to bite off because a) it CAN get really complicated and b) I don't understand all of it.  So I've decided to just go into detail about the portion of lofting that will actually apply to my first project.

The Coot is a simple, 8-foot boat with few bells and whistles.  As discussed in my last post, the offset tables for the Coot describe how to build the frame of the hull (see below).  In strip-planking, the hull is not actually a part of the finished boat; it is simply the frame that the finished hull gets bent around.

 The offset charts give you a list of coordinates for each cross section of the hull (often referred to as forms).  Each form is graphed on large sheets of graph paper, cut out, glued to plywood, and then the plywood is cut to match the contour of the graph paper.  Then, each form is placed at set intervals from its neighboring forms along a straight backbone, and, once complete, you should have an accurate frame to wrap your wooden strips around.

 But first, you have to learn how to read the tables.  Reading an offset table is a lot like reading the x and y coordinates of a graph (if it's been a while since you took geometry, no worries!).  The only catch is that there are a few keywords that have to be explained first.

This is an example of a column from a typical offset table.  The 4 main keywords to focus on are half-width, elevation, keel line, and DWL.  Basically, the keel line is a vertical line that goes through the center of a form, and the DWL (design water line) is the theoretical water line of the boat, but most importantly, it is out main point of reference for measuring vertical displacement.  To really grasp how to read this thing, let's set up our graph.  Your horizontal axis is the DWL and the vertical axis is equal to the keel line.  The elevation is the distance of a point on the outline of a form from the DWL, and the half-width is the distance of a point from the keel line.  I have a rough illustration of this below.

  There are a couple important things to note on this graph.  1) Replace "Height" with "Elevation" (Oops!), and 2) the negative values for elevation will actually be charted ABOVE the DWL on the chart (the hull is built upside down, so what will actually be below the water line (negative elevation) is above the DWL while you are building it).  3) The offset will only make one-half of the cross-section.  The other half is simply a mirror image across the y-axis.

That is really all you need to know to get started with drafting, however let's take a look at the offset table again.  We have briefly discussed waterlines and buttocks lines in my last post, but I have not mentioned gunwales or feature lines yet.  A gunwale (pronounced "gun-null") is simply an area along the top of a boat hull that an extra strip of wood is added to prevent wear and tear on the hull (in the case of the Coot, most likely from oars).  The offset table simply tells you where to place the extra strip of wood.  The feature line is point on a form where the outline makes a hard angle, and from that point to the tip of the gunwale, the outline of the form continues as a straight line instead of curved (my graph above does not have a feature line on it).


And thaaaat's all folks!  Thanks for tuning in!



Term of the Day: To Freeze the Balls Off a Brass Monkey

Nowadays, we think of this term as a crass way of saying, "It's REALLY cold!" However, it's roots might surprise you.

Oddly enough, this term is a throwback to the times of cannons and sailing ship warfare.  The true saying actually went "to freeze the balls OF a brass monkey", and did not refer to any crass references our imaginations might conjure up.  Instead, it dealt with the science and freezing points of various metals!  A "brass monkey" was a small brass cannon on the deck of many ships.  When the temperature would get extremely cold, the brass of the cannon would shrink faster than the iron cannon balls inside it, and when this happened, the cannon was rendered inoperable because the cannon ball was stuck in the cannon.  On cold nights, sailors would exclaim to one another, "Tonight could freeze the balls of a brass monkey!"

...And sailors are always the ones accused of having dirty minds...