Lofting diagonals

[Clifton Brittain]

I have read and re-read Vaitses book, consulted McIntosh and Stewart, and I am still puzzled about how to integrate the diagonal information into lofting a round bottom boat.

I understand the greater reliability of a number that has been generated "normal" to the surface in question.

I understand that one of the great benefits to lofting is the ability to bring into focus the smallest detail at full size.

I understand that one purpose of lofting is to use the table of offsets to reconcile the points within the table, especially if the designer has made the table off his own line drawing.

I understand that the purpose of the body plan (boat sliced like a loaf of bread) is to enable the builder to frame the full size boat.

I can easily construct on any scale a body plan from the heights, half-breadths, and diagonals given on a plan. I even trust myself to draw a fair (well mostly fair) line connecting the dots.

I can also draw the "long lines" between stations to get the plan-view and profile of the boat.

What I cannot do is figure out how to include the diagonals in the long lines. I am ending up doing the half-breadths, the heights on buttocks and waterlines at all stations, then constructing a body plan. When I then plot the diagonals, they are sometimes discrepant. Sometimes discrepant is an understatement.

One possible discrepancy is the measurements of my waterlines or buttocks. But I use the same ruler, pencil, and eye to lay out each point, and one or two battens to lay out each line. I rarely have to move more than one point out of 10 more than 1/4" to get a fair line.

I use a tick stick to transcribe the dimensions to the body plan, so I don't worry about misreading measurements.

But then when I go to incorporate the diagonals, it gets pretty ugly. Close, but nowhere near allowable tolerance.

I'm about to repaint my panels and start over, but what the heck do I do with the diagonals in profile/half-breadth?

[Tar Devil]

I've struggled with diagonals. I got some good responses from these guys sometime back...
http://media5.hypernet.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=5&t=005153&p=
Actually, there are a number of discussions on diagonals if you check out the search feature.

Later,

Phil

[imported_Conrad]

Diagonals are a tough issue- I haven't relied on them much for a couple of reasons. First, I don't think they have much to do with whether your eye sees a boat as fair or not. Second, they are the hardest to both pick up off a drawing and lay down on the loft floor because of the angles at which they meet the scaling grid used in their measurement- a bit of a mistake here or there just seems to get magnified as the process continues. My experience with lofting large boats is limited, but I have done design work, lofted several smaller boats, and have a pretty good understanding of geometry and the concept of cumulative error. I've never had diagonals just lie down on the loft floor as expected- my decision has been to ignore them- if the rest of the lines look fair, the data for the diagonals is probably bad. And since what really counts in the long run for most of us is an attractive, fair boat built "reasonably" close to the plans, who cares?

You would probably achieve better results putting the time into tweeking your full size molds, playing with a variety of battens to ensure a fair shape after cutting and set-up errors, than on the loft floor. I think diagonals were much more useful prior to computers, giving the designer a sense of what the heeled shape (and planking lay-out)might be. Now its easier and more useful to tell the machine to heel the darn thing 15 degrees!

[videoguy]

Clifton I'm not sure if This will help your problem but the diagonals are only used on the
body plan.I have never seen them used on the profile ............. keep building Phil

[John E Hardiman]

Clifton,the diagonal serves two main purposes. First it is a fairing check line. After you lay out the major body plan lines and a few waterlines and buttocks, strike a fair diagonal. Then as you finish filling in the rest of the lines, they should be lofted to the diagonal. This brings up the second major purpose, it is one of the major lines that determines the form resistance of the hull. Water, as all things in nature, wants to minimize its change in total energy as it flows along the hull. As you always have to pay for the forebody pushing the water out of the way, you try to minimize energy losses in the run. The rate of curvature of the aft diagonals are a major indication of how much extra energy is going to be expended by the form of the hull. Water lines are too oblique to the flow streamlines to do a good job of this, so if speed is a consideration, loft to a fair diagonal in preference to either the waterlines or the buttocks.

As an aside: NEVER ASSUME THAT THE TABLES OF OFFSETS ARE CORRECT!!!!! THERE ARE TOO MANY WAYS FOR ERRORS TO HAPPEN. ESPECIALLY IF COMPUTER DRAWN OF A ROUND BOTTOM HULL(i.e. no chines). I deal with unusual lofted shapes, some with deliberate flat spots, and I have always had the loft return corrected offsets. Even when working at large scale (1'=1 1/2"), it is easy to misplace a 16th or so or set down 4-5-6+ as 4-6-5+.

[Art Read]

I barely remember lofting my project, ('cepting for the "drop foot" and sore knees it caused...) But I do remember having the same confussion over diagonals. I wound up mostly ignoring 'em. I think I drew a few on the body view but I'm not sure anymore. At any rate, get the waterlines, halfbreadths and buttocks looking good and concentrate on translating that into molds that still look right once set up. I spent a lot of time shimming and shaving my setup even after obsessing over getting the lines right. (Maybe 'cause I didn't understand diagonals?) Anyway, it looked like the boat in the plans once it was finally all set up...

[Bill Perkins]

Clifton an epiphany I experienced after reading Viates ( p.75 ) on this subject is that the diagonals , tho included on the " half breadth " plan are not measured in the horizontal plane like everything else there . Their " halfbreadths " are measured in the plane of the individual diagonal for greatest accuracy . That's why the faired diagonals are drawn seperately from the true halfbreadths in plan view .Don't expect to integrate them with the rest of the drawing .

[ 11-20-2002, 08:24 PM: Message edited by: Bill Perkins ]