Lines Plans

Why is it that lines plans don’t show all of the buttock sections? I am new to seeing actual design drawings (lines plans) for a real boat and this seemed odd to me.

Probably because showing all the buttocks on a fairly flat bottomed hull can be confusing. The full lines for the builder should show the lot.

What frustrates me even more is that these days nobody shows the diagonals.

I’ve seen but never used them - Education Time is here - will the class please come to order?

The floor is all yours - see if you can keep it to 500 words or less for some of us who have long since left the classroom. :scared: :sly:

There is a belief, which I share, that the buttocks and diagonals show the effective form of the hull better than the waterlines and sections. If you read Uffa Fox, his commentaries on various designs quite frequently start with his assessment of the buttock lines. Diagonals are, of course, essential to the process of “paper fairing” a design before it is lofted for real. The modern assumption seems to be that the CAD programs will fair the design for you, so the diagonals don’t need to be shown.

A common phenomenon, often seen in the designs of L Francis Herreshoff, is that the fore sections are somewhat U-shaped, leading you to thing the hull is “tubby,” but when you look at the buttock lines they are fair and easy.

Hope this helps,


Similarly I have had in my hands the lines plan of a Tartan 37 centreboarder (it may be 38, I’m not that familiar with American types). This was allegedly the last hull to be drawn by Olin J. Stephens II personally. Once again, on the profile and buttocks the ‘bustloid’ stern looks dumpy and contorted. The diagonals are as clean as a whistle.

Dick, one of the other reasons for using diagonals is that with many hull forms they give greater precision than waterlines and/or buttocks in that they can be arranged to cut the skin at something approximating more closely to 90 degrees. This also reveals interesting things. The 12M Constellation had a distinct trace of the ‘double chin’ that first saw the light of day in its full-blown form in Miller/Lexcen’s Southern Cross and is now commin in A class, 6M, etc. I first dscovered this from a kit model hull where you can feel it and just about see it if you hold the thing upside down. I have since seen the original lines plan at Mystic and it sticks out like a sore thumb - on the diagonals.

The plans I have are def. missing maybe the outer 2-3 buttock sections. I could understand some of the inner ones where the hull is relatively flat. I’ll have to scrutinize them a little more, but I don’t think the info can be gained elsewhere. I will keep you guys updated.

Forgot to mention: a complete treatise on “traditional” design methods, from Thomas Moore’s 1928 book, can be found at:

The only other rule you need to know is from Alan H. Vaitses’ classic “Lofting”: “A fair line trumps any given measurement.”