Slocum's "Spray"

[wjt99]

The hull sections of the lines plan of this vessel show diagonal lines. Can anyone advise what these lines are for?, how are they used in the design? Thanks, Bill Thompson

[Stephen Ditmore]

"diagonals" more closely approximate flow lines and the orientation of planking than do sections, waterlines, or bottocks. Many designers prefer to be guided by them in the final stages of fairing the lines. The hollows you sometimes see in bows are often the product of fairing on the diagonals.

thank you Stephen for your response. I wonder if you could elaborate further. I would like to know in very simple terms how diagonals are established, what rules define their origins and destinations? I am not into boat design, just an interested model builder. Thanks, Bill Thompson

[Stephen Ditmore]

It's less about rules than about the practical role diagonals play in going from the drafting table to the loft floor. Let's say you have begun a lines drawing. You've drawn a few waterlines and have sketched in sections at stations 2, 5, and 8. It's time now to define a diagonal on the "body plan" (the bow/stern view looking at the sections). You might start at the intersection of DWL and the centerline, and you'll probably look at the bow section and the midsection that you've drawn and pick the easily defined and drawn angle (30 deg, 45 deg, etc) that is most nearly perpendicular to both sections.

That's as detailed as I'm going to get here, but you might check YACHT DESIGNING AND PLANNING by Howard Chapelle or other classic design/lofting texts for more information.