Boat plan lofting - What does it mean?

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If there was ever a dark side to boatbuilding this would be it!


Lofting: "the full size transfer of a plan measurement to a piece of timber!"


That's the simplest way I can put it. Sounds quite basic but there are many problems to be aware of. Let’s exclude the obvious for now, spacing of frames or stations are uncomplicated as long as you can use a tape measure. Getting a bunch of points though to form a shape accurately is the real dilemma we all face. For those that are not aware of standard boat plans, they are only points given to the outside of the hull...Yep that's it! So, every foot or so you have a series of points that specify the shape of the frame as heights from a base line or widths from a centreline.

These heights and widths are referred to as waterlines, buttocks and diagonals. There is no set spacing for these as they vary from boat to boat. An easy way to understand these is to think of them as a crosshatch or grid pattern. The offsets given in heights and widths intersect along the grid and a line can (should) be drawn through them forming a frame outline.
A major point of interest here is that offsets are only provided after the first boat is built!
Offsets are always given to the outside of the planking or hull, a bit of a giveaway!
This is somewhat annoying as you can no longer use the chine or sheer height as a set-up point since it’s generally accepted that these two points are removed for the logs prior to assembly on the boat.  I specify my heights as “height to keel point”, this is the flat part lowermost on the frame that meets the keel.

The Dark Side
Inevitably errors have ended up within the offsets, understandable as it’s an arduous task of measuring and logging on number in feet, inches and eighths etc.
The minor errors are the worst to spot on a finished frame. These cause bumps or divots on the hull and are only visible when planking. The major errors when spotted can cause the complete standstill effect where you scratch you’re head checking both fore and aft frames for clues!  This can be compounded when the errors are on the chine, sheer or keel points. There is no shortcut here as each error has it’s own set of problems, and I am still just talking about 1 error across say 200 offset points. When there is more and this is almost always the case, the plot really thickens.
This was my introduction to boat plans! Erroneous boat plans with no reference back to double check against anything. To this day I am still confounded by the percentage of offsets that contain errors. Coming from a manufacturing background I guess I expected 1 plan per part, not 1 plan for 70 parts…

 

There are ways around all of these problem that I will cover in my next guide...

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