Taking Boat Lines

Taking the lines off of an existing boat is done to replicate or document a boat for which no plans exist.  When the lines are taken off of  a boat, a 'lines drawing' and /or a 'table of offsets'  can be produced which can be used to:

  • build a copy of the boat
  • make a model
  • feed into a computer boat design program to make a 3-D virtual model
  • document information about the hull form for posterity or study.

There are many ways to take the lines off of a boat; here's a sample of  one of them:

The owner of this late 1800's steam launch wanted a larger version of the same boat built for taking on more ambitious expeditions.  In order to capture the hull form and plug it into a design software program to develop the design for a larger version,  the lines needed to be taken off the existing boat. 

With the boat levelled on its trailer, plywood panels were set up on the planes of the forward and aft perpendiculars of the hull and securely braced.

The grid of waterlines and buttock lines was drawn on the plywood panels and extended beyond the dimensions of the hull to give reference points that were outside of the hull shape.  Small holes were drilled at seven of these reference points and mason line was threaded through the holes and stretched tight between the forward and aft perpendiculars.  A tape measure on the floor gave the locations for station lines, which were shot onto the hull with a laser level and marked with strips of masking tape.

A large compass was then used to take the dimension from each reference string to each significant point on the hull at each station line and plot those dimensions directly onto a full-sized 'scrieve board' or body plan.  Significant points include the bottom of the keel, the rabbet line, each plank seam, the chine, the top and bottom of the rubbing strake, etc.  

The positions of all internal framing, joinery and machinery were documented, as well as the details of the coaming and superstructure.

All of this information was turned into a lines drawing and table of offsets.  Here's a waterline being faired on the half-breadth plan:

Here is the completed lines drawing: