Deriving waterlines from body plan

[Harper57]

Can someone explain how waterlines can be derived from section lines on a body plan? Assuming a typical body plan shows sections parallel to the midship section (longitudinally?), and one wished to obtain waterline section for a bread and butter hull (sections parallel to the water line), how can this be done? I assume one could use the body plan to create a 3D model, then slice that up into pieces and take the lines off those sections. Is there a better way? Even if it's using paper and pencil (perish the thought, I know!).

Forgive me if this had been asked already. Especially if it was by me!

[biggles2]

I know I'm in very deep water here , but isn't the waterline determined by the displacement of the ship (or boat) rather than through any kind of geometry?

[wefalck]

Dead easy, actually. On the longitudinal section the 'stations' of the cross-section are marked. The cross-sections are marked with either numbers or letters, so that you can identify them in both plans. When you use paper and a pencil and want to draw the waterline(s) at the same scale as your plans, you can do the following:

a) take a piece of paper of the length of the longitudinal plan and draw a line along the middle of it

b) take a compass (or ruler, but a compass is better) and mark all the 'stations' of the cross-sections along this line

c) choose the waterline you want to draw, it can one already marked in the cross-section (e.g. the CWL) or any other; in the latter case you have to draw a horizontal line at the desired height across the cross-section for that purpose.

d) with a compass take for each cross-section the distance to the centre-line and mark-off this distance on each station line drawn under (b), port and starboard.

e) connect the points so marked with a 'spline' (a flexible thin lath), shipbuilders' (French) curves, or a flexible ruler (less desirable).

Voil�, your waterline is drawn.

You can do the same thing on the computer with a graphics package by taking off the distances from a scan of your plans.

Hope this explanation was suffiently clear.

wefalck

[Guest]

Literally as I pushed the "submit" button I realized that your explanation was exactly how to do this. I think I went through the mental process to figure this out once before. That's what I get for posting while still groggy, after the cat got me out of bed, demanding her early morning feeding!

To the first poster to respond, I was not referring to the one "waterline" where the hull meets the surface, but rather I was using the term as it applies to all such lines that are parallel to that one section. Water lines in this case are all sections taken parallel to the top view, section lines are all sections parallel to the front view, and buttocks lines are sections parallel to the front view of as hull.

Explained here;

http://www.themodelshipwright.com/proto ... ship-plan/

[biggles2]

DOH!...

[NelstoneB]

This is best done in CAD.
Let X axis be horizontal, Y axis vertical, and Z axis along the keel.
Draw the offsets, or frames, in the XY plane. Move each frame along the Z axis until is it positioned according to the YZ plane view. Now, set a constant elevation W (usually 2', 4', 8', 12', 16', etc.) relative to the XZ plane (Top View). Place a Node at the intersection of each frame at that elevation. (If you don't node what I'm talking about, I can't help you. Heh.) Connect the Nodes using a Spline, and there you are. Repeat for each waistline desired.
Easy.