All boat plans
required lofting in the days
before CAD (Computer Aided Design), when the plans were hand drawn.
The job of the 'Loftman' was to scale up the original drawings to full size.
Drawing the plans full size had two main objectives.
Fortunately, for today's builders, CAD drafting should remove any of the mistakes inherent in hand draw plans.
Many plans are supplied with full sized patterns, particularly for small boats.
And kit boats are supplied with the parts already cut out or at least marked on the wood.
However for anyone who does need to do their own lofting, be assured there is no magic art involved.
But it does take a couple of attributes.
You will also need a suitable space to work, a few tools and an understanding of how plans are drawn.
The name lofting comes from the space where traditionally the 'mold loftsman' worked.
Like a sail loft this had to be a large open area where the work could be spread out flat without any obstructions.
Most of the tools you will need are for marking and measuring.
The Splines are long strips of springy material used for drawing curves.
It is preferable to have three long splines and several shorter lengths.
The longest should be longer than the length of the full sized boat.
Traditionally splines were made from fine straight grained wood such as fir, which is available in long lengths.
Sprung steel and fiberglass strip can also be used.
The advantage of wood is that it can be tacked to the loft floor.
The plans are a set of line drawings.
The lines of the boat are drawn over a grid.
The plan will show three views, a plan, a profile and a body plan which, is the view from bows on.
Heights are measured vertically off the base line, though sometimes from a datum waterline.
The hull length will be divided vertically into 'stations', positioned in relation to the centreline.
These station lines are datum lines and are not necessarily the positions of the frames.
The next job is to transfer the grid full sized in to the floor.
Check the base line is straight by stretching the string along it.
Then erect the center line at 90 degrees to the base. Check this by using Pythagoras' 3-4-5 triangle.
Once the first set of grid measurements have been made and checked they can be transferred to a baton which can then be slid along to mark the rest.
Label all the grid lines as you go.
Constantly check and re-check your measurements.
All diagonals must be marked accurately as these cannot be checked for parallel against anything.
Once the grid is set up you can then interpolate and draw in the frame locations and the curved shapes by joining all the relevant points using the flexible splines.
Most sets of plans will also have a table of offsets.
With a full set of plans these are more of a convenience than a necessity.
They show the dimensions of the boat taken from fixed datum points
You need to check if the dimensions have been taken to the outside or the inside of the planking.
However if these have been taken from the line drawing they may contain inaccuracies.
But, this is why you are drawing them full size anyway, to rule out any drawn errors.
The traditional way to show dimensions in the offsets for a ‘station’ is in "feet" - "inches" - "eighths of an inch.
For instance; O-6-2 = 6 and 1/4 inches, 10-1-6 = 10ft,1and 6/8 inches, 13-0-2. = 13ft and 1/4inch.
Just to complicate things, if there is a + or a – after the last number that means you should add or subtract 1/16 of an inch from the last number.
So, 0-6-2+ would work out as 6 and 5/16 inches.
Once you have your plan lofted out full size you can now use it to make templates for many of the parts.
Patterns for the keel, the stem, beams, and combings can be accurately made from stiff material such as plywood.
This will make marking the timber prior to cutting much easier.
The template is also a handy way to check the timber for any knots or splits which might fall in an area where a rabbet might need to go.
The most accurate way to assemble molds and frames is to do it on the loft floor directly over the lines.
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