Table of Offsets: How?

[NormMessinger]

How does a designer get his (or her) table of offsets? I've made the wild guess that the design is worked out to a given scale on paper then measurements made therefrom to develop the table. The discussion in another thread here suggests otherwise.

--Norm

[mmd]

Traditionally, the design is produced on paper in three views (plan, profile, & sections) and faired as accurately as the scale allows. The preliminary (termed unfaired) lines are reproduced full size on the loft floor and faired. The loftsman then makes a table of the measurements from centreline or baseline to the appropriate line (waterline, buttock line, diagonal) at each station and also records the profile shape of the stem, stern, and rudder. In rare cases the lofted lines would be revisited and changes made if, during construction, the vessel could not be built as lofted. The recorded offsets would be revised to suit. When the hull was complete and critical datum points checked for accuracy, the table of offsets would be finalized and either incorporated with the lines drawing or recorded on a seperate drawing sheet.

It was common for a designer to deliberately include erroneous figures on the published table of offsets and record the proper figures on a document kept in his personal files for the design, thereby making it difficult for others to copy the lines without permission.

In earlier times, a half-hull was produced to a certain scale and a table of offsets would be developed by measuring directly from this model. N.G.Herreshoff used this method, as well as employing the erroneous-figure subterfuge to protect his designs.

Modern computer software can eliminate the necessity of tables of offsets because, if used properly, the 3-D model of the hullform can be manipulated digitally to produce the CNC code for CAM machinery to produce the properly shaped pieces without ever requiring human contact prior to setting up moulds. There are a number of stitch-and-tape ply boat kits that are produced in this manner. I am currently working on a project with a couple of people who wish to establish a facility to produce building moulds for custom, one-off cored FRP yachts using this method. The idea is to supply numbered, sequential ply panels, with appropriate stiffening attached, to the builder's yard so that he can dispense with lofting and mould-making and get right to creating the hull. This would work well with strip built boats, too.

[Cosmo Lengro]

Mr.mmd, can a design as artistic, my personal view, as L Francis Herresoff's 'Ticonderoga' be created on such a computer program?

Thank You

[NormMessinger]

Thanks, mmd. I wonder if, say, Iain Oughtred lofts his designs full size before he finalizes the table of offsets. I lofted Prairie Islander 1/4 size to learn how and found a couple of minor error in his table (more like transposed numbers) but I could measure from his 1-1/2" to the foot lines drawing accuratly enought to confirm the error. The same error showed up when I lofted full size. That lead me to the conclusion that one could measure the lines drawings at least for a boat that size.

[Bruce Taylor]

Where's the thread discussing offsets, Norm? I must've missed it.

When designing Blackfly, I used a scale rule to lift offsets from my drawings (1 1/2" to 1'). I then used these preliminary numbers to generate a second set of drawings. I faired the curves on these new drawings, and corrected the offsets accordingly. I repeated the process a couple of times, until all buttocks, WLs and sections were completely fair and in agreement with one another.

I lofted the boat from these offsets, expecting to make further changes on the lofting floor. The lines came surprisingly fair at the larger scale, but there were a few places where further corrections had to be made.

If, after it's been built and tried, the design seems worth keeping, I'll use my lofting to take a final table of offsets.

[On Vacation]

I can add one more item to the mix. In the old days, lofting was the only way to produce the proper frames for the boat hulls. It required a lot of room for a big boat. Many old timey boat builders had an upper area or loft built upstairs overhead to do this. When a lot of the plan sellers began selling plans, some came with paper patterns. This was an improvement, but in the transfer of measurements in the old days by the old Ammonia bluleprint copiers, we found that many were close but no cigars.

I am not sure how accurate the new ones are, but I have used this method that seems to be the happy medium betweeen lofting and dealing with unfair hulls with finish woods in them.

The picture below shows my elementary way of compensating for this problem. I cut the jigs as close as I can, set the boat up, cut scabs the the shape of the side patterns and screw them to the jig, using a battern to find the fair point of the building hull.

This works even with a curved side surface. I build the sides first. Then I will cut the good woods and dry screw them to the other side of the jig into the hull. In the case of longitual battens, I use pieces of luan and make a pattern, screwing the pieces together to match the sides with appropiate notches and then tranfer the shape to the good wood. Then I will check for the keel alignment and block up from the jig, and then cut the good bottom frames. After getting them in place, shape the angle if a vee hull and then plank the bottom.
http://www.imagestation.com/picture/sraid45/pee57665cfca181bf1d62272911005b98/fcd4e275.jpg

http://www.imagestation.com/picture/sraid45/p4348a0b4bd3d7455e870a94f70fba154/fcd4e2c9.jpg

The new CAD program, now, can produce a jig cut on a CNC router in approx. six to eight hours for a 22 foot boat. You can notch the paterns, and put anything you wish in them on the table and just sit them up in place.

http://www.imagestation.com/picture/sraid45/p0123d38ecda6e2859d2566445ce436ef/fcd4d788.jpg

[ 01-04-2003, 04:21 PM: Message edited by: Oyster ]

[mmd]

Cosmo, last year I created a 3-D model of both Mobjack and Bounty from prints of the original lines by LFH to assess stability and to provide an accurate "envelope" to design an interior to the client's specs. I self-imposed a fairness error of 1 millimetre. So, yes, such a sculpted design is certainly able to be done. It is, however, not as easy as software manufacturers would have you to believe.

As with most things, computers don't necessarily make things easier. One has to be concious of the restrictions imposed by the software and allow for it. It is very easy to do things the way the program likes it because it is easier. One of the fussy things that I don't like about a "software boat" is the inability to lay your face down on the lines and sight along it for "eyesweet" fairness. This is particularly needed when defining the sheerline. Personally, I prefer to rough out the hull form on paper "the old way" with ducks & splines to define the basic hull form, then digitize the bodyplan and tweak & fair it from that point with the computer.

I draw all my work on the computer full size and merely print it to scale, so I can give dimensions to thousandths on an inch. Repeatability is a huge asset, as is the ability to extract data quickly from the completed model (need a section thru the hull to see how the engine will fit? this is a snap on the computer, a substantial drawing exercise by hand), and accuracy is unbelievable.The really powerful feature that I use a lot is to stretch & pull an existing design like taffy to produce a new hull form. For example, a client recently asked me to create a design proposal for a 42-ft commercial fishing boat hull based on his successful 38-footer. I already had created an accurate model of the 38, so I just scaled it up by 20% longitudinally, 12% athwartships, and 15% vertically, all in one operation. In the same program I was able to calculate volumes, displacements, static stability, and form coefficients. Using the latter I was able to reassure the client that the vessel would have virtually the same performance and seakeeping characteristics as the base boat. I then transferred the 2-D lines generated by the modelling program into AutoCAD and created an accurate preliminary plan & profile for presentation to the builder's prospective client. Total time from phone call to delivery of preliminary design package complete with lines, GA, initial stability calcs, and prop & power calcs was ten hours. You just cannot do that with pencil & paper.

Norm, small boats at large scales are wonderful for just the reason you described - the scale is large enough to be easily measured accurately. A half-inch error at 1-1/2" = 1'-0" is glaringly apparent; the same error on a 70-footer drawn at 1/4" = 1'-0" is the difference between a sharp pencil and a dull one.

[Meerkat]

Originally posted by Cosmo Lengro:
Mr.mmd, can a design as artistic, my personal view, as L Francis Herresoff's 'Ticonderoga' be created on such a computer program?

Thank YouOf course a boat as nice as "Ticonderoga" could be designed using a computer program - and perhaps even better. Think of the computer program as an advanced drafting table that can help with some technical matters. It's not a substitute for the designer's creativity, nor can it actually create a design without a human's input.

Where the "perhaps even better" comes in is in measuring the technical aspects of the design much more accurately then old methods are capable of. Some of that may be a matter of style, but much of it is measuring and plain old number crunching - which is what computers do.

[On Vacation]

MMD, what is your thoughts on the plan accuracy with the method used on this thread about the Biscayne 22? I questioned of depending on the drawn lines for finish wood. I think Shallowater is in the premlimiary stage of the same boat and same building methods.

http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=006237

[mmd]

Oyster, I am going out tonight for the last blast of the season, so don't have time to post a proper reply right now. I'll review & reply tomorrow, OK?

[On Vacation]

Thanks. The pencil width was discussed with dealing with table of offsets and pattern transfering.

[Tomcat]

You can do it with CAD, and the conputing and redrawing power is awesome. You can also use that design as a template later.

It does seem though, since computers, we see a lot more things that look like tubes of toothpaste, than we do the boats of the past. There are technical reasons why people have turned against certain shapes, but it is also easier to generate certain shapes with a computer.

When working with ships curves, it is interesting how often certain designs will pop off the page in close relationship to certain curves. I think it is generaly easier to draw initial views on paper. Seing subtle changes in curvature even on the best screens is unlikely. The resolution of graphite is like 1028(to the 15thpower)X 840(to the 15th power).

Could you draw the Mono Lisa in Paintbox? I doubt it, even if when you did it gave all kinds of fascinating information on the amount of paint to order or what the print would look like with a different aspect ratio to the face.

Throughout time new tools, perceptions, and designs have been swept along an evolving spiral. We don't use CAD CAM so much to make better horseshoes as we do to make cars that look like jellybeans.

[Cosmo Lengro]

Oh Good, the pause while Mr.mmd enjoys himself allows me to pose some further questions for review.
Norm asked, if I understand correctly, how does a designer make the Table of Offsets from the drawing. The drawings are in a scale, correct?
How does one translate that scale into yet another to come up with the feet-inches-eigths of a Table of Offsets?

The next question has to do with the CAD software.
Am I reading accurately that a designer uses one program ( which to you prefer, mmd?) to design the boat and yet another to make it universally understandable to anyone?

If mmd, you prefer to draw the sheer line on paper how to you get that very same line into the computer design program?

Gentle Folk, this is the kind of discussion that I visit the forums for! I tried to elicite some information in one of the misc forums regarding a members comments on bullwarks, a mrmilktoast. Unfortunately he seems to have decided not to answer me on some points or it slipped his mind. I do wish that this does not happen here.

Gratefully and with Thanks
Cosmo

[NormMessinger]

Mr. Milk got confused. Non-boating related vs boat building. You are more likely to get an answer here anyway, Cos.

[ken mcclure]

CAD programs work off of a set of coordinates, if you'll remember back to geometry class, that are represented in an X-axis (horizontal), a Y-axis (vertical) and a Z-axis for the third dimension in those programs which work in 3-D.

To put any line into the program with accuracy, you need to at least put a representation of the X and Y axes on your paper, and measure the points on your line in reference to those axes.

Then you simply input your measured points into the CAD program and it draws the line.

Getting stuff into the CAD program is not much different than drawing on a piece of paper. The real benefit comes when you go to modify that line or do calculations based on what you've put into the CAD program.

CAD is to drawing what Word Processing is to writing.

[Bruce Taylor]

How does one translate that scale into yet another to come up with the feet-inches-eigths of a Table of Offsets?I use an architect's scale rule. I assume that's the usual way.

The 1 1/2" scale is divided into 48ths. At 1 1/2" to a foot, each of these corresponds to 1/4". It's easy enough to eyeball 96ths in order to get something close to an 1/8" (if your eyes are bad, there's always the 3" scale).

[ 01-04-2003, 06:38 PM: Message edited by: Bruce Taylor ]

[On Vacation]

The end result cannot be formed in the computer. It still requires the hand and eye, unless people only look for shine. A novice will not know what he or she looks at anyway.

[G. Schollmeier]

:D

Can I add a few tidbits here? Recently I took a file I drew in CAD software, to a printer to have full size plans produced. It was a canoe so all the lines fit on a 24” X 36” sheet. It was a test so I had them print it on paper. The drawing included a 12” scale so I could be sure they were full size. When I picked them up, I put a 12” ruler on the scale and it matched. After 2 weeks in my study that 12” scale now measures 11 ¾”. That is a very BIG difference. So if you’re taking measurements off of full size plans I hope they’re printed on velum or vinyl or some other stable medium.

The 3D software I have is able to produce an offset table from a 3D model with the push of a few buttons.

Yes it is very common to draw the model in one program and use another to produce a file for documentation. And yet another to write CNC code.

Gary :D

[On Vacation]

Thank You, Gary. Time has proven printed plans are not true. Beware.

Sometimes plans that have been folded up for storing and then taken out, will really throw you off, also.

[ 01-05-2003, 04:39 AM: Message edited by: Oyster ]

[On Vacation]

I will post this link off of a web page on both threads, the Biscayne 22 for those that have not read it, and here. This that is one of the finest online about fixing an untrue hull. A lot
of work is involved long before the hull is built to make the outside "look right" and true.
http://www.bronkalla.com/Fairing.html

[mmd]

Whew! You guys have been busy while I was out with the Missus at the Uproar - and recovering from it. (No, not a hangover, but wuz up pretty late and my voice is shot from too much hollerin' and bad singing.)

Oyster
I re-read your post on using paper patterns and using ply moulds to set up. There are many ways to do this job right, and yours is one of them. So much depends on the level of experience of the builder and his/her preferred method of construction, as well as the type of construction of the hull. In fact, one of the biggest headaches in my job is when I do a design job for a builder I have not worked with before, I have to try to find out what is his preferred method of work so I can tailor the design to his shop processes and tools. To all the uninitiated, yes, paper patterns shrink and swell and twist as the relative humidity fluctuates, and can never be viewed as accurate. How inaccurate is the wild card, and guessing that, of course, comes with experience. Some are happy with adding 1/8" or 1/4" to the faying surface of the timber, knowing that they will dubb off the excess in the process of fairing up to a batten after setup. You gain time by not having moulds, but you lose it in fairing and you'll be FUBAR'd if you didn't allow enough extra material. Personally, I like a variation of your method, setting up accurate temporary moulds in place of,say, every third or fourth frame, springing fair battens, and fitting the framing to the battens. But to be fair about it, I haven't personally done too much boatbuilding in the past decade; I just provide the techie stuff to suit the building style of professionals who I am in awe of and enjoy working with.

Tomcat
Referring to your comment on seeing relationships between certain designs and ship's curves, when I worked in a much larger design office than my current little cubbyhole, my colleagues and I had many discussions on which ship's curve was an individual designer's favourite as indicated by the lines in front of us. To this day I maintain that LFH's favourite was a K&E #57.

Cosmo
To get the dimensions from the line for the table of offsets one measures the completed linesplan with a scale rule, as Bruce Taylor described (I like the triagular architect's scales more than the flat cartographer's scale rules). Using computer programs to design a boat is analogous to baking a cake - you use various ingredients and tools to create the final product. Bearing in mind that everything changes constantly in the software world and that what I am about to say might change next week, my current suite of software is as follows:

ProSurf (3-D modelling, initial stability)
ShipHull (intact & damaged stability)
The Nautilus System (floodable length calcs, station properties, intact & damaged stability, planing boat resistance calcs, prop selection, and sailing velocity prediction)
AutoCAD (2-D drafting)
MS Excel (I've developed several proprietory programs for strength analysis, propeller selection, scantlings calculations, rudder calculations, mast design, etc., as well as for tracking hours, doing invoices, and other admin tasks)
MS Word (correspondence, reports, presentations)
MS Project (project management)
Intuit Quickbooks (financial management)
MS Works Database (database storage & analysis)
Visual C++ (various proprietary applets, such as regressional analysis of data from my Works database)

Once I layout the rough design on paper, I digitize it into my Prosurf program using a CalComp desktop flatbed digitizing tablet, which allows me to "trace" the lines - selecting points along the curve as coordinate points - and convert them to X,Y, & Z datum points in cyberspace, as Ken McClure has described.

As Oyster so perceptively commented, a computer is just a tool and it is the style, eye, training, & experience that make a designer. If one is missing any or all of the above requisites, a computer will only help you make ugly boats at a faster rate.

OK, that's all. I hope I found all the questions I was supposed to answer. Caio!

[Cosmo Lengro]

Thank You Mr.mmd, so much in this thread I am going to print it out and digest it slowly!

And to all others here who provided alternate view points my Thanks too.

[Mark Van]

Most designers start with a hull design program, and transfer it to a 2-D CAD program like AutoCad to do the finished drawings. I designed 3 boats using Design Cad, the latest version that I have is Design Cad 97, which is an inexpensive ($200 to $300) 3-D and a 2-D Cad program. It is a much easier program to learn than Auto-CAD. I draw the hull in 3-D first, and then use the 2-D program for the finished drawings.

All the displacement and stability calculations can be done with a genaral purpose 3-D cad program, it just takes a bit longer than the special hull design programs. You can sight along a shear line to see if it is fair, just change the viewing angle. You can also find the true expansions of the pannels for plywood boats, it is tedious and takes a bit of time, but it can be done, much easier than without a computer. Round bottom hulls can also be disigned that way, sometimes easier than hard chine hulls.

As to wheather buitiful boats can be designed with a computer, I was talking to a guy that was admiring my last boat ( a round bottom strip-planked river cruiser), and he said that it was one of the nicest shearlines he had ever seen. Later on, he mentioned that you can't design buitiful boats with a computer. I told him that I designed my boat with a cad program, I drew the midship section, the stem, and the transom, and the computer drew the shear line. I liked what I saw, so I didn't change it.

Mark

[On Vacation]

Thanks a bunch, MMD. Yes your variation is a nice compromise. But one thing I have found in many of the more extreme flared boats is a batten across a bare amount of jig style frames will not give you a full hull in the foward section. This may sound a little funky, but what it does in the middle of the side without staging or a rough jig will give you a huge flat spot in one side or the other sometimes. When doing this method, you may need a thicker batten than one that you would normally use to keep it true. That gets a little tricky in the compound turning in the most foward areas.

I read some of the plan boards of some people building their first boat and wonder how some of the designers and staff stay sane and without a big drink in the middle of the day. Thanks again for your time.

[mmd]

Oyster, when we built the plug for the Huckins 44 we ran into the problem you describe with the flat spots just below and aft of the most extreme point of the generous flare of this hull. We halved the station mould spacing and stiffened up the battens in the area by laminating on extra 1/4" of material on the inside of the batten before beginning to apply the double diagonal plywood skin.

[On Vacation]

Huckins, your a long ways from Jacksonville. We actually have a new 44 right here next to me. If the weather insn't too bad, I will go over and shoot some shots of it. We have another fellow here that has just gone down to look a a new one, I understand that they have started. I have done a few of the old ones and reworked and taken care of a few of the AIrex Cored one-offs.

Well I will wind it up , here. I must say that I really didn't meant to hijack Norms thread. My extreme apology Mr. Norm, and thanks to you Mike for your time and information.

[mmd]

I agree, we should stop the active highjack of Norm's thread. I'll finish with a pic of the first Huckins 44 hull & deck from the moulds we built. Maybe it is the one that is next to you. The guy being interviewed by the pretty lady from the local teevee station is the yard owner, and yes, we are a lo-o-ng way from Jacksonville. Location is just outside Charlottetown, Prince Edward Island, Canada; photo was taken late July last year.

http://www.imagestation.com/picture/sraid45/p271aa4486abb4de5223d0555fdc8092c/fcd2fa18.jpg

[NormMessinger]

Hijaking? This has turned into a very interisting thread. Jack away.

Please.

[G. Schollmeier]

Thanks to all, this sponge isn't full yet.Oyster the site you posted is a great one.

Gary

[Eric Sea Frog]

Originally posted by Bruce Taylor:
When designing Blackfly, {snip}What does this Blackfly look like, Bruce? Are you an amateur designer or going pro?

[John E Hardiman]

I'm off on vactaion and a nice techie topic pops up :( .

Just a few notes and points about things that have already been well covered.

We draw on H-size mylar ~ 30"x60" in scales up 1"=3'-0" and expect accuracy up to +/- 1/8" with 0.3mm vinly lead. Usually, we loft to +/- 1/16" as that is what is all that needed for steel. We'll go to +/- 1/32" for Al to decrease warpage and for glass to improve strength. On paper plans expect a dimensional change up to 5% with changes in humidity, normally in the direction of the roll.

Due to the limitations of AutoCAD b-splines, either expect large errors to true fairness (we've seen up to 1" on large structures with few control points) or you need to put in a lot of control points, which means that you have to draw a fair set of lines first. Additionally, AutoCAD cannot do a true surface/surface intersection. We use Rhino 3D to generate surfaces then fold them into AutoCAD for flat plans.

Finally, everytime I was asked about SUBHUMAN II and "How the computer designed the sub" I had to explain that computers don't design anything. The man designs it, the computer just allows him to create many designs quickly so he can discard his mistakes more efficiently. :D

[mmd]

Hi, John; welcome back to the real :rolleyes: world. I trust your vacation was a good one. I had been wondering why you had not jumped in here as I knew it would be a subject of interest to you.

A question, if I may, on your comments on accuracies in the drawings from your office. Forgive me if I'm too stupid, but maybe it has been too long since I drew big boats with a pencil. If you are drawing to 1:36 (1"=3'-0") scale with a 0.3mm lead, how can you be confident in an accuracy of +/- 1/8"? Presuming that the pencil line is centred on the datum and therefore the error due to the thickness of the drawn line is no more than half of the lead width - 0.15mm - would not the margin of error be +/-0.15mm divided by 25.4 (to convert to inches) multiplied by 36 (scale factor), equalling 0.2126" or approximately +/-7/32? This problem reared up and bit me in the ass about ten years ago when I was working on a refit for a freighter in that the client insisted that the drawings done at 1:48 (1/4" = 1'-0") and was not pleased when compounded tolerance errors in the cut plates resulted in over a half-inch of air between parts that were supposed to touch.

[Bruce Taylor]

What does this Blackfly look like, Bruce?An open sailing dinghy of lap. ply construction, suitable for cartopping. If it works as it's meant to, I'll use it for camping on some of the bigger lakes around here (hence the name "Blackfly").

Here's a sketch (I'd post the full plan, but it's too large to scan):

http://www.imagestation.com/picture/sraid37/p81d1f814346341743d9e6d53c1289cb5/fd1e3504.jpg


Are you an amateur designer or going pro?An amateur, and barely that. I've had a lot of help from the impressive people who frequent this forum.

[Eric Sea Frog]

Thx. Lovely. 14 ft. Add water. The kind of boat you keep all your life whatever else you'll build. Chances are that some of the blokes who'll ask you about the design at the put-in actually own a big plastic boat that's getting rotten at the marina cuz sonny learns to drive and the wife is shopping! :D

[John E Hardiman]

Originally posted by mmd:
A question, if I may, on your comments on accuracies in the drawings from your office. Forgive me if I'm too stupid, but maybe it has been too long since I drew big boats with a pencil. If you are drawing to 1:36 (1"=3'-0") scale with a 0.3mm lead, how can you be confident in an accuracy of +/- 1/8"? Presuming that the pencil line is centred on the datum and therefore the error due to the thickness of the drawn line is no more than half of the lead width - 0.15mm - would not the margin of error be +/-0.15mm divided by 25.4 (to convert to inches) multiplied by 36 (scale factor), equalling 0.2126" or approximately +/-7/32?Well, first of all I may have been a little too general in my statement. redface.gif If I was doing a 1"=3'-0" dwg I'd use a pointed "lead", 3 or 4E, to draw the lines, then darken them with either a mechanical ink pen or a 0.3mm. Second, I never draw or measure to the center of the line, but rather to the 'inside' of the line. When I lift the lines from the waterlines or sheer to the body plan I use tic strips, I never measure in the set up except for key dimensions. When the lines are complete, I work up a table of offsets to send to the loft floor for correction and they send back the corrected offsets which also go into a new rev of the plan (while keeping the original offsets legible). Of course, once the stucture is built, we never consult the offsets again; but rather template off the structure.

PS: Failed to mention it but the most I've been off ( excluding an obvious transposition or two) has been 5/16", and I still question wether they lofted the flat spot I wanted there correctly. :confused:

[ 01-06-2003, 11:36 PM: Message edited by: John E Hardiman ]

[mmd]

Yep, that all sounds pretty familiar. I thought that I might have missed something in drafting class and was hoping to learn a new trick. Thanks for the reply.