Diagonals

[johnh94927]

Boat plans, with their typical three views, are orthogonal drawings.
Yet, they sometimes also incorporate the mystery specs for "Diagonals".
Diagonals do not seem to have a canonical explanation - hence this posting.
My (mis)understanding of diagonals is as follows:
Diagonals originated in ship-building, not boat-building, because of the massive scale of wooden
shipbuilding. Diagonals informed the shipbuilders of how to arrange the huge wooden components
of the ship's ribs on the assembly platforms prior to the connection of the components and
subsequent raising of the ribs into place.
Because of the much smaller scale, diagonals do not play the same role in building smaller vessels.
Yet - they are sometimes included, and seem to have a nearly-mystical value in the minds of some
builders. I suspect that this is a carry-over from the traditions of ship-building, and that with
some "backing and filling", values were attributed to diagonals that were not originally intended;
such as: smooth diagonals in the stern mean smooth-flowing water past the vessel.

Anyone have info on diagonals?

[Beowolf]

Hey there.

As I understand it, which is a massive qualifier to everything written below....

Diagonals serve no informational purposes that aren't already served by the traditional three sets of lines. However, since diagonals typically run perpendicular, or nearly so, to the sections, they are very useful in fairing out the lines. Thus, they become a very useful piece in the lofting process.

The set of plans for my boat, a small open daysailer, had one set of diagonals, this created a flatter bottom to the craft. Suppose that more waterlines, could have provided the same information, but would have been more work for the designer.

Hope I've helped.

Jeff.

[ken mcclure]

For boats that have a LOT of shape in the bow or particularly the stern, a lines drawing with waterlines and station lines can get a little hard to decipher - lines bunching up and at odd acute angles.

The diagonals can clear up that confusion and make the shape clearer to see.

[Thad]

Diagonals are included for the a few practical reasons. Because diagonals can be drawn nearly perpendicular (normal to the planking through the turn of the bilge, their measurement is much more consistantly accurate than waterline or buttock measurements which are crossing those surfaces obliquely. In the lofting process the lofter should favor the measurement most perpendicular to the line, for the simple reason that there is less margin for error in that measurement. The eye and batten will of course make the final decision. Even for boats where sawn frame bevels are not being taken, having diagonals drawn can be used to get a sense of the shape of plank runs.

[Todd Bradshaw]

On a sailboat hull plan the diagonals can also sometimes give you a much better idea of what the "in use" - heeled over shape of the submerged portion of the hull is going to be.

[B. Burnside]

Diagonals are put in to provide a reference point that is approximately at right angles to the plank in question. They can be placed anywhere, and are especially useful where the buttock lines, waterlines or station lines are cutting through the surface of the boat at an oblique angle. They give a fourth point of reference to be sure your hull is sweet and fair. Builders or lofters can also add a diagonal to the plan if they want to check something more closely.

They have nothing to do with "rib" or frame placement.

Barbara

[Ken Liden]

It's like piloting John. Everybody knows that a three way fix is much more accurate than two.

[pjwalsh]

what Thad and some others said - plus if you are a designer who is drawing a lines plan by hand it is much quicker to develop the sections using diagonals first then do the waterlines and buttocks as needed.

[johnh94927]

Very informative. Most of the reading I've done on the subject has been unclear.
The use of a radial line relates to the way they were employed in big shipyards - for laying out the components of a rib, or frame, and giving a check to its size and shape, relative to one or more easily fixed points.. Locating the assembled frame would be from a different datum, of course.
Thanks for the education. - John

[David Payne]

A late entry to the diagonal question. As a designer, its exactly as previously stated, they help fair out difficult parts of the hull shape more accurately, and are very helpful in being able to draw a shape up quickly by hand, the way I mostly work. After a while you learn which of the three grid lines is the one to pay most attention to in different areas on the drawing, as it develops. I have even used a diagonal as a sheer line or a chine line on a couple of simple plywood boats, and it has worked out well.

[Colin Stone]

I am using Hull Form to design a 80 ft boat with relatively simple lines and have also wondered what the purpose of diagonals are, hull design books have not really explained them that well!!. Hull Form software allows me to select the diagonal interval, and vary the angle if so desired away from the default 45 degrees. I find the diagonals useful in checking the elevation view in particular. If the diagonals are nice and smooth, then it confirms that the hull is fair. It has also been very satisfying to progress from the original drawings in CorelDraw, through an offsets table in Excel which was imported into Hullform and faired/adjusted. From Hullform the hull can be exported to a "vrml" file and viewed in a 3D viewer, or in a CAD dxf format. The hull can then be directly matched with the original CorelDraw. In addition, the hull plates can be developed in Hullform and exported to the CAD programme. The final plate measurements can be compared with some of the original dimensions, and they are accurate to 0.2mm or better.

[TR]

One use for diagonals that has not been mentioned is in fairing a reverse or keel filet. The buttocks are too far outboard, and waterlines do not follow the fairbody curve. Diagonals can run out from the centerline at any handy angle. So I can place a diagonal running right through the filet from stem to stern, it's the only way to fair a reverse in hull sections up close to the keel. All the best. Tad