cross section from hull - techniques?

I'm sure there is a clever way to do this, known to craftsmen of past ages.
Say you have a complex curved object such as a full sized boat hull. You want to extract an accurate drawing of a cross section for, say, making a one-piece ply bulkhead that will fit in with minimal error - how do you get that inside curve profile?
thx!

 

You know what a body lines drawing is?

 

Rip some thin strips from a thin sheet of plywood or a piece of spruce or something. lay the strips vertically along the inside of the hull. Cut some more strips and wedge them across the hull, pressing against the strips that are vertical so as to hold them tightly up against the hull.

Take a hot glue gun and melt some gobs where the pieces contact so as to hold it all together. You can go around complex shapes the same way by cutting and gluing pieces to the template. put a few extra pieces diagonally for strength so the template holds its shape.

Remove template and scribe your piece. Good luck!

 

As above can be good- as well the use of a spiling block or a dagger shaped spilletto- strait one side with some unique cut outs V, half V, square, round can be used to reach the inside shape, just trace around at as many locations as required then once the spiling is taken off reposition the block or spilletto to find the points & or some minor strait sections can be referenced say around stringers.... join the dots & cut to shape, have also seen referenced as a "joggle stick" in the modern world..... Hot melt glue, thin ply & some short self tapper screws. Charlies explanation proves the piece can actually get in & out of the space.... where as the spiling is often a bit smaller & sometimes around stringers there is no way to get in or out without some angled relief.

Jeff.

 

I use a piece of plywood clamped or screwed in the plane the bulkhead will be installed. Then make marks using a jogging stick and transfer that to the bulkhead material. Once you get the hang of it, the bulkhead will fit on the first try.

 

Any reason to use a lines drawing as the primary reference for the shape of a bulkhead which will be installed an existing boat rather than taking a template directly off the boat?

 

jogging stick? I googled it - not a stick for hitting dogs when you go jogging, I assume?

 

yeh, spiling block ?!? Had to look that one up. Here is an article:
http://www.sandypointboatworks.com/...oat-building/114-spiling-and-it-s-application

spilletto - no such luck. If you want a laugh, google it and look at 'images'

joggle stick - got it
thanks!

 

thanks all
I'd already imagined a cobbled- together odd pieces solution but it didn't seem elegant. Hot glue makes it a bit more elegant. or at least quicker.
The joggle stick approach will take a lot of measurements. I'm thinking foamcore and hotglue now.

 

Any reason to use your method, if there is a lines drawing?. I'm sure not.

 

There is no drawing.

 

Even if there is a lines drawing, there is no guarantee that the boat was built accurately, or that the shape didn't change over time.

 

So no choice but to take measurements on board with any of the procedures described. If you can have a laser meter, you will save a lot of work. If you take measures from the outside do not forget to subtract the hull thickness. Good luck.
I would not say that "there is no guaranty", I would say you can be sure that the body lines plan does not have great accuracy. Although the lines plan may not be very accurate, it sure is more accurate than to take measures by hand with a tape, with totally inaccurate reference lines and all the difficulties, work in situ, carry.

 

I remember seeing a simple tool for such a task made from wood strips, I could not find an image of it. It is made of a number of vertical strips of wood that are each clamped to a horizontal board. You set the board across the gunnels at the location of the bulk head, adjust each strip downward until it makes contact with the hull. The end of each strip comes to a point so it contacts the hull at the tip. One they are all adjusted, carefully lift up the assembly and lay it on a piece of cardboard or stiff paper and mark each point, cut it out and check the fit. That becomes your template.

 

These are some tools to copy curves

http://www.axminster.co.uk/axminster-large-flexible-curve
http://www.amazon.com/General-Tools-Instruments-Contour-Duplicator/dp/B00004T7RA