"Diagonals"

[bromleigh]

Whilst on the subject of Gartsides' "Jessie". Does anybody know why Lyle Hess drew in the diagonals for Larry Pardeys' Taleisen the way he did.
Am i missing something???All other BC Cutter type deigners draw in the diagonals at evenly spaced increments of approx 45deg, which seems to work well
for for this heavy chested type.I refer to Gartside,Irens,Paine and their ilk. I know I'm rambling, bear with me. Why would Hess draw his diag's (and only three sets
which is even more astonishing) at different angles...Inquisitive!!

[John E Hardiman]

The number and angle of the diagonals is usually more a function of the number of waterlines and buttocks than anything else. While I've not seen the plans in question, usually (big assumption :rolleyes: ) diagnonals are defined in the body plan by starting where a WL intersects the CL and ending where a buttock intersects a lower WL trying to give a line that is close to perpendicular to the skin where the designer wishes to control with the diagonal, usually the turn of the bilge and the run. This greatly simplifies set-up.
If the buttock and waterlines are evenly spaced, this gives a 45 degree diagonal. Now, buttocks, waterlines, and even stations don't need to be evenly spaced, but it makes things easier. Additionally, diagonals are most effective when they are perpendicular to the skin. This leads to a trade-off between how you define a diagonal and where you place the waterlines and buttocks. each design, and designer, is different so expect differences. :cool:

[MAGIC's Craig]

Bromleigh:
Lyle drew from the perspective of his experience as a builder and he indicated to me that he used the diagonals primarily as a check for the planking fairness. He was perfectly comfortable having the individual diagonals run at different angles from the centerline, as long as they "checked" a portion of the hull shape which might not be well served by the angles of the intersections of the section lines with the waterlines or buttocks.

As John pointed out, diagonals do not need to be at a given constant angle and I have even seen lines where the diagonals are twisted to follow the planking strakes, for what that might tell us.

HTH,

[bromleigh]

Thankyou Craig and John, I obviously was missing something. The only reason the question arose in my mind at all is that I am using Larry's book on traditional boat building as a rather loose pictoral guide for my 29' cutter.
Thanks again.
Roger McNeil

[John E Hardiman]

O.K.... I've found the definitive example of the how and why of diagonals.

In Chapelle, Boatbuilding, take a look at Plates 3 and 4. They are the lines of a 24' Hampden boat. In Plate 3, notice the waterlines and buttocks in the after body. They are not smooth and even, but have humps, hollows, reverses, breaks, etc. But notice the diagonals...all smooth and even (except the space between #'s 3 & 4) indicating an acceptable fairness to the hull for flow condiserations. In Plate 4, notice how the diagonals are laid out. Some start and end on WL/buttock intersections and others are defined by where they intersect the CL and BL, but they all with the exception of Diag #4 pretty much cross the midships section fairly perpendicular.

All in all, a good example of how to lay out diagonals and the reason to draw them. The rest of the lines layout is not that bad either ;)