Offset Tables And Lines Drawings

It is all relative to era and yards production/experience.

If we were having this debate 50years ago….the only way to transfer the shape of the hull, from a lines plan, was to lift off X and Y co-ords, at defined long.t intervals, thus creating an off set table. Then as JEH notes once taken into the mould loft, re-faired at the 1:1 Scale for template for frames.

Some yards today, still use this method.

However the “younger” generation would have been educated, at shipyards, in the CNC machine era or just sitting at a PC all day.

The method of generating lines and template for frames has been moved from the mould loft to the desk top PC, yet the methodology remains the same, just different process involved. The surfaces used to create lines, is no different from “old days” save for the technology being used. The end result is still the same….one uses a set of offsets to create a faired 1:1 line to produce the frames (via the scrieve board) or the software does this by creating transverse lines in a faired 3D model of a hull (just like a hand drawing), then slices at defined locations longitudinally. These lines are then imported directly for frame shapes. The offsets are “hidden” as such, since it is dealt with via the software.

So, if a yard solely uses CNC plasma/water cutting machines, the frames can be simple cutting files taken from the 3D lines. Not a single table of offsets in sight. This is “easier” if the frames are cut from one sheet. However, as JEH notes not every yard is comfortable with the results via 3D software and still wish to fair or check on a scrieve board.

If a frame cannot be cut from one sheet of metal, then you require a scrieve board. Since once fabricated, how will the fabricator know if the shape is still 100% correct?...they wont. So after a frame has been fabricated, it is offered up against the hull frame line, on the scrieve board to ensure it is the correct shape and not distorted during fabrication.

Also the 3D software driver has replaced the expert mould loftsman. These guys were highly skilled. They could spot a wonky line from a thousand yards in the dark. Many 3D software users are just play station users with no appreciation for 3D lines nor lofting. Thus to them, it looks fair, computer says so!!!, but, again as JEH notes, the final result cut when offered up on the jig, can often be somewhat lacking in accuracy!

 

Agree.

When a naval architect says "offsets" do they mean any coordinates, or is the accepted marine industry standard use of "offsets" to mean the numbers in a standard "table of offsets"?

A cutting file produced by a computer contains coordinates, but those coordinates will not neccessarially be at locations used in a table of offsets. It would be less confusion to simply use "coordinates" rather than try to stretch the meaning of "offsets".

In another industry where the requirements for fairness of surfaces and curves exceed those typically used in the marine industry, there are highly skilled individuals who's job is to take a design, ensure that it can be built, and to adjust the math data of the surfaces and curves so that the product will have surfaces which meet the stringent requirements. Tooling is then built directly from the adjusted math data. I understand this also occurs in the marine industry but I don't know how common it is.

 

Hmm..tricky. It has become bit of an anachronism these days. But still used simply by some, like me, who were initially brought up in yards before the age of computers. This “offsets” is related to the lines of the hull or superstructure. Since this is how a lines plan was “communicated” between departs where small AO dwgs were not sufficient or a reduction was required. So, in a sense it is “any coordinate” but generally related to a table that was created by the person who did the lines plan of the hull/superstructure. Even though this process no longer takes place (but i was at a yard last year that still does this...)

Quite possibly. But many terms in naval architecture/ship building have been around for centuries, thus almost “cast in stone”, even if misnomers.

It is related to the each yards preferences.

At my old yard in the early 90s, the first CNC cut parts we did all had “green” on. This was an allowance to make sure when things didn’t fit, the fabricator could cut off and reshape on the job. As the whole process evolved and more confidence came from each member in the chain of events, no green became the norm. But the process of offering up a finished frame onto a scrieve board before setting up on the jig for ******** remains, otherwise the plater will be struggling with a “donkey’s hind leg” shaped hull.

 

Quite true, even today, ships' steel plating is laid out in strakes, we talk about garboards and gunwhales, deadwood and keels.

One important point I would like to make is a CNC cut vessels is no better, fairer, or straighter than a lofted one, and often less, for all sorts of different reasons. Welding skill, fitting skill, time of the year and day, cutting/fabrication methods and temperatures, way orientation, etc all contribute to culmulitive errors that get built in. Even molded boats all have little variations in dimensional tollerance and surface flatness well in excess of the difference between a computer faired line and hand faired one.

IMHO, there are too many CAD engineers/drafters who have never ventured out on to the ways to see how the pieces actually get put together. And even fewer who have ever corrected thier CAD drawings to "as built" drawings. This is why most shipyards have an apprientice program to move shop workers into the design office, they bring a wealth of knowledge. To this day, Webb requires the first winter work period to be in a yard, just so you can learn that it is not computers, but people, who build ships.

If any of you people who believe that a vessel is built exactly to plan ever get a chance to do a docking survey on one of those vessels, TAKE IT! It will be eye-opening just how far you can be off the lines just through normal fabrication.

I still tell the junior engineers that don't have shop experience that they need at least to learn how to fit and weld, even if they never become a planner (i.e. someone who plans the layout and assemply sequence). You might CAD the part to 0.0001 step file accuracy, but you are going to cut it on a plasma table that has an accuracy of 0.01+/-0.01 with a worn tip and skewed jet and it will be cut at 26C waterbath, runover by a forklift, stored sideways in the plate yard, presented at 0C and bent and beat into place and welded up cold to a hot plate....How is that different than an exact template lifted from the loft, punched on the plate by hand at 15C on the platten, marked with a soapstone, cut with a torch by an old chain smoker with just a little in him from lunch to "steady the hand", touched with a grinder to remove the line, bowed while being craned outside, and presented at 0C then bent and beat into place and welded up cold to a hot plate....

 

See attached a screenshot from Delftship

where you can export Offsets and Co-ordinates

here is some of the values in the exported table

0.217271 0.531393 0.025632
0.440179 0.805905 0.031381
0.646134 1.104840 0.301646
0.177471 1.079530 0.314138
0.101425 1.113018 0.849254
0.606364 1.142066 0.835727
0.412498 1.092248 0.307583
0.626249 1.123453 0.568686

 

See attached a screenshot from Delftship

where you can export Offsets and Co-ordinates

here is some of the values in the exported table

0.217271 0.531393 0.025632
0.440179 0.805905 0.031381
0.646134 1.104840 0.301646
0.177471 1.079530 0.314138
0.101425 1.113018 0.849254
0.606364 1.142066 0.835727
0.412498 1.092248 0.307583
0.626249 1.123453 0.568686

Offset tables are invaluable for one thing at least -

for checking the shapes cut by a CNC router, as the computer doesn't always get it right !

 

Orca3D is an "add-in" for Rhino which generates a table of offsets, and MultiSurf also has a command to generate offset coordinates. However for both as well as FreeShip and DelftShip the offsets are not present in the internal workings of the software as they are in manual lofting. Rather the offsets which are exported are generated when the command to export them is executed.

 

That is the same process as manually drafting. I draw a line with a stick, then measure points at convenient locations.

 

The best definition of offsets I have heard. Arbitrary points on the hull surface, usually aligned across the same station for convenience,

 

However, I also use the lofting board to take lines for a fuel tank or an inclined bulkhead like in a chain locker. The computer program does the same.

 

Sounds like naval architect talk! I've worked in various yards mostly small, and lofting full size or half scale, I have never found any computer drawings that couldn't use a hair bit of tweaking. though I might be biased because I usually do my work in "cheapo yards".

 

Sorry Briggs. I sense some sarcasm there. I didn't mean to be insulting, but when I read what wrote I can see how it would seem that way. Neither was I trying to imply that the computer was perfect or the person using the computer. Errors are made using modern technology just as they were without it. In both cases it is expected that the builder will keep an eye on the fairness as they go.

What I was referring to when I talked of cheapo builders wasn't really what you thought I meant. I am a naval architect, but I am not an independent naval architect. I work for firms with lots of employee's. When billing on a time and materials basis the cost for services can range up to 200 dollars an hour. I have worked with clients and yards all over the world. Most of them are quite large. These aren't small boat yards with shipwrights with a life time of experience. These are start ups in China with money and drive, but no experience. Or big builders with several vessels being built at the same time. Many of the actual workers don't know the stem from the stern. Very detailed drawings are made that show every nut and bolt. Every connection is detailed. These type of builders often cannot think for themselves. If you draw something wrong they will build it wrong, no questions asked. It often isn't possible for these type of yards to loft full size. Both because of the space required and because they often don't have the experience to do so. Not to mention the time spent with 200 guys sitting around waiting for steel to be cut. When I said cheapo yard I was specifically thinking of a yard that preferred to throw man hours at a project instead of paying for design work. The naval architect could produce CNC cutfiles at an hourly rate that would normally be much cheaper than doing it by hand. But if labour in the yard is dirt cheap they would rather just have full size mylar printouts that guys can spend days tracing.

One other point that I wasn't clear about. As I said I didn't mean to imply the computer was perfect. Only that a linesplan and an offsets table aren't usually needed. The reason they often aren't needed is because the builder will have a CAD copy of the linesplan and possibly even the 3D model. The builder can feel free to loft it fullsize right from the CAD. If he wants to check a different section he can measure it to the infinite decimal place right on his screen.

 

I apologize David, I understand where you are coming from. I was also in a bit of a mood yesterday, nothing to do with boats, boat design, or boat building. Actually did a job about 5 years ago where the whole boat lofting was plotted full size, pretty cool! I have to say we are getting pretty close to the point where computers are going to have to do it all because an accurate loftsman is about as rare as a do-do bird. 1/2" of fairing compound is pretty common place these days and it is a lot more expensive than colored pencils if you know what I mean. Again, I apologize for the sarcasm...also I think I'm a little intimidated by this computer stuff, a manual milling machine is the most advanced piece of equipment I own, LOL.

 

I've used offset tables for the kayaks I've designed, but I haven't been able to figure out what the waterlines and buttocks tables are good for. Sure, you could theoretically use them to make a strongback or form, but does anyone actually do that?

 

What do you put in the offset tables for the kayaks you've designed? Do you use software which automatically generates the offset table?