Lofting Diagonals

[GRussell]

My plans (Ken Bassett's LIZ) have diagonals drawn on the body plan and the half-breadth views but they do not appear in the Table of Offsets. The drawings are 1/8 scale so taking them off the drawings could be risky. Do I really need to worry this? What is the smart thing to do? Thanks in advance.
Guy

[Peerie Maa]

The diagonals are drawn to check the fairness of the other curves. Some designers also preferred to use diagonals rather than waterlines and buttocks as the primary lines in fairing, a lines plan with lots of diagonals may have been designed this way.
When the lines are all fair and in agreement, the designer prepares the table of offsetts from the waterlines and buttocks.
The loftsman will then fair full size from the offsetts. He may chose to reproduce diagonals to check the fairing of the full size lofting, although some do not need to.

[Todd D]

You don't need to worry about the diagonals. They are there for the purpose of fairing the original drawing AND giving a better idea of the hull shape to the those looking at the drawings. You do not need them.

[Roger Long]

You don't need to worry about the diagonals.

I totally disagree. These lines most closely correspond to the way the eye runs over the hull and how the planking lies. Chapelle and other designers didn't appreciate them and did use them only as a final fairing check. Some don't give offsets and on some plans, where there are offsets, those for the diagonals will be for a slightly different hull.

Here is the sequence I would follow.

1) Lay out the sheer in plan and profile. Don't forget to expand it. (Search for thread).

2) Lay out the rabbet or fairbody in plan and profile checking actual siding of keel stock to be used.

3) Lay out the body plan lightly using offsets for waterlines and diagonals and lofted points picked off for sheer and fairbody. Fair up sections lines as close to offsets as possible keeping sheer and fairbody points fixed.

4) Draw the diagonal locations on the body plan. You may want to add some and don't even need to use the ones the designer did. Use the offsets table to wrap up that sandwich you didn't finish for lunch.

5) Pick off the diagonal offsets from the body plan as drawn on a marking staff(s) and layout on station lines. Run full length batten through each set of diagonal points and fair carefully keeping to the average of the offsets.

6) Transfer faired offsets for diagonals back to body plan with marking staff. Resolve any fairing conflicts between body plan and diagonals.

7) Now pick up waterline and buttock offsets from body plan with marking staff and lay out on plan and profile views. Every time I have followed this sequence, the waterlines and buttocks have fallen right into place and needed no additional fairing. It is the waterlines and buttocks that should be the final check in lofting.

Chapelle describes lofting according to the sequence for developing a set of lines from a blank sheet of paper. Since the lines are already fairly close by the time you get a set of plans, the sequence above works better.

If your lines plan doesn't show a ghost station for the transom at the extreme aft end, you should add one as developed by projecting the diagonals. You'll get a better result projecting a transom into a set of lines faired out beyond it than trying to get the lines to fair into the complex projections of a transom.

[mmd]

What Roger said, 'specially in the first paragraph...

[Songololo]

While on the topic of diagonals...

Can they be used to read off the bevel angles for floors and frames?

[Peacefuljourney]

No need to for knowing bevel.

You can do it on the body plan, at 90 degres of every stations.
Worked fine forme.

[Peerie Maa]

What Roger said, 'specially in the first paragraph...

Rogers last paragraph is vital as well.

[Bob Cleek]

While on the topic of diagonals...

Can they be used to read off the bevel angles for floors and frames?

You betcha! And that is another really valuable use for them. I suppose if you are bending half inch plywood over half inch plywood bulkheads and slapping googe all over the thing, it might not matter, but if you are sawing floors and frames, they are pretty essential to take off the bevels.

[JimConlin]

On my Liz, I cheated and used the full-size body plan blowup from our hosts. I didn't have to tinker the setup very much.

[Songololo]

Peacefuljourney - the bevel I refer to is the one that is present fore-and-aft at a particular station. You seem to be referring to the athwartship profile?

Bob - My understanding, is that in order for a diagonal to accurately reflect the bevel angle at a specific point, it's plane needs to be perpendicular to the hull. In most instances the diagonals don't seem to be constructed like that; as an example, here is a plan set (Shearwater 39?) from the Dudley Dix site:

http://www.dixdesign.com/loft1-1.gif

[Tom Robb]

I expect to be embarassed by the extent of my ignorance, but what's a "fairbody?"
Never heard the term before.

[Roger Long]

what's a "fairbody?"

http://www.brazilian-bikinis.org/ProductImages/sheer-bikini-2.jpg

[Eric Hvalsoe]

Station bevels? Diagonals tend to pass in a somewhat more perpendicular fashion through some station lines in body plan, so they can be somewhat more useful for determining station bevels, but to the extent they are not square to a particular point on a station, they are not an accurate read of the bevel at that point. Look at the Gougeon Brothers boatbuilding manuel if you want to understand an accurate method (courtesy Joe Trumbly). It does not require a computer. Greg Rossel also has a technique which seems a little cumbersome, but should be accurate. People will have different interpretations of what 'close enough' is. The above methods are more than 'close enough'.

Roger is correct about the function and importance of diagonals,
but the fact of the matter is, many small craft builders will get away with without them, with satisfactory results. They can be added to your body plan grid, with points scaled off the lines sheet. They might fill some important gaps. If you fair them out in half breadth view the trickiest part is finding the endings. With some traditional designs you will get odd looking looking diagonals, especially in the bows, you just will. I would not stress out about them too much. If I feel the product is from a reputable source, I would largely follow the curved butts and waterlines.
My attitude about the diagonals may also be inflluenced by the chosen method of construction, strip vs cedar lap for example.

[Peacefuljourney]

I was talking about the bevel for frames.
As I do sawn frames ... so I was speaking about getting the bevel of frames, which is good to take on the body plan with a 3d triangles at 90 degrees of every stations.

Worked good for me, as they all look fair and I can take several reading where I want or where degree tend so change faster.

[Tom Robb]

Roger, Roger, Roger...she's obviously an exceptional body. Fair body indeed!

Anyway, on a boat, what's a "fairbody?"

[Roger Long]

Anyway, on a boat, what's a "fairbody?"

It's one of those terms which have different meanings or are sometimes miss-used, depending on your point of view.

The general meaning is the lowest line that station lines run to, i.e., the rabbet line in a conventionally planked hull with lines to the outside of the plank.

I prefer the more precise meaning of "Fairbody" being an imaginary line that shows the shape of the hull as if there were no keel and the surface projected to the centerline as a knife edge. This is a very useful lofting method even if the designer has not defined a fairbody. Since the rabbet line already has a name "Fairbody" leaves a term available for additional definition of the hull shape. If you think about it, you'll see that the hull can't be truly fair if this line is not fair. Many designs; especially those with a tight curve or knuckle just at the forward end of the straight keel, have an area that isn't strictly fair but the planks are just made to fit.

Projecting a fairbody and using it for the ending points of your diagonals makes that process much easier. There is a lot that can go wrong doing all the projections down the face of the stem and them back to the rabbet. The ends of the diagonals don't provide much useful construction information so going right to the more easily projected ghost line of the centerline fairbody is easier.

The fairbody can get a bit strange in boats with a lot of planking tuck at the stern post. If the planking runs down so that it is nearly vertical right at the stern post, as is the case in many boats, the fairbody line will suddenly dive down to a few hundred feet or even infinity below the baseline. In such a case, the straight sections of each station line can be projected to the centerline to create a fairbody that doesn't include any of the reverse curve or tuck in the sections. This is a very useful fairing tool; especially when developing lines. Again, if that line is not fair, the hull is not going to be really fair.

Honestly though, didn't you like my first post on the subject better?

[Tom Robb]

You can post lovely lines anytime, Roger. Yes the first answer was rather more interesting, but thanks for the information anyway.

[Songololo]

Eric - thanks for that. For others not familiar with this method, it is covered in The Gudgeon Brothers on Boat Construction, pp182-183 , hidden under the heading Subtracting Planking Thickness. It makes use of a master bevel board and derived tick stick(s) for determining the bevel angle (and if needed, plank thickness reduction) at any point on a station line in the body plan. I'll now go and have a look at Rössel...