designed waterline?

Hi everyone,
I am new to yacht design and quite a few mouths ago I started reading design articles on catamarans. I have now bought the " Principles of yacht design" book. I am currently reading about the hull geometry and I am confussed with the "half breath plan". how do yacht designers work out the designed waterline length? how do they predicted how much the yacht would sink into the water to work out the draft and waterline? (How do they calculate it). This question my sound simple to all the professional yacht designers, but this is all new to me.
Thanks for the help
huw

 

The amount of hull below the water line is equal to the total weight of the boat...Displacement.
TIM

 

Elaborating on Mr. Hastie's remark, designers work out how the boat will float by comparing two calculations. The first calculation is the easy and fun one: using the lines plan, find the volume (and center of volume) of the underwater parts of the boat. The second calculation is the hard and tedious one: for every part of the boat and it's stores and equipment, add up the weight and compute combined center of gravity. The weight of the volume filled by seawater must match the weight of the boat, and the center of volume must be directly over the center of gravity. This satisfies Archemedes Law. (If this condition is not met, the boat will not float the way the linesplan is drawn.)

A wise designer incorporates some way to adjust the flotation a little after the boat is built, since "as built" is not usually "as designed" by at least some small factor. it used to be usual to reserve some ballast weight to be 'trim ballast" that could be put fore or aft as required. A modern boat factory reproduces boats pretty accurately, so adjustments can be made in the prototype (by moving a tank or making a small adjustment in the keel, for example.)

 

Take a look at

Claughton, Wellicome & Shenoi, "Sailing Yacht Design - Practice", Addison Wesley Longman Ltd, 1998. Chapter 5, pp. 43 - 72, "Case Study for a Modern Charter Catamaran" by Alexander Simonis. He discusses the design evolution of catamarans for the Moorings organization.

Shuttleworh has a number of excellent mulithull design articles at http://www.steamradio.com/JSYD/Articles.html. Especially good are "Multihull Design Considerations for Seaworthiness", and "Design develoments in high performance cruising catamarans. From the Spectrum 42 to the Dogstar 50."