Builder questioning Architech plans??

[Dale R. Hamilton]

Ray Sargent sport runabout project:
So- got the sides on and preparing to put the bottom on soon. However, due to my inexperience, I have a question. You know I was very fastidious in having these plans redrawn for computer, and I cut them out ever so careful. I spent 2 weeks hanging the frames -checking and rechecking. So how come when I put the sides on, they are almost 5" too long?. Botton up forward also several inches oversize. Moreover, slots in the frames wher engine stringers go did not line up in come cases. I'm not complaining- nothing here can't be fixed. In fact everything has been oversize and easily fixed. But I'm wondering does the architech draw these with the intention that the builder pare them to fit? Oh yes, all those changing bevels on the frames that I carefully cut- every damn one had to be planed different so the skin would bear right on the frame. If this is the case- it will make boat #2 ever so much simpler to build. In fact instead of cutting out the bottom to fit the pattern- I'll just throw a scarfed plywood sheat over the frames, and pencil in a cut line where needed. How about it mmd- whats your take?

[Keith Wilson]

In almost every case, a fair curve and a tight fit takes precedence over anything on the plans.

Like I tell folks in the shop building something I've designed," I don't care what the drawing says, the important thing is that the damn machine works!"

[ 07-07-2005, 11:33 AM: Message edited by: Keith Wilson ]

[Tom Lathrop]

Dale,

I was about to answer your email before my ISP went down but might as well do it here.

If your boat was S&G, the panels would need to be very accurate to duplicate the designer's intent. Since you say that you are building over frames, (also chine logs and and sheer clamps I assume) designers will dimension a lot of oversize on the panel layouts, if they offer such dimensions at all. Doing a computer layout of panels for a boat to built over frames is not only overkill but not very usefull as you found out.

The dimensions he gave were only to get you in the ballpark without cutting the panels too small anywhere.

Without seeing the plans, this is my best guess.

[Dale R. Hamilton]

Hey thanks Guys- thats the sort of stuff you learn as you go. Yes- its S&G, but has 3 frames and a sheer clamp. no chine.

[mmd]

My take is that you have been the victim of mistaken presumptions. A computer is merely a device that allows you to make bigger mistakes faster and with greater confidence. Don't trust them.

1.) Hand-drawn plans are presumed to be dimensionally accurate. They are often not, but the dimensions usually are. For example, if the designer drew those engine bed slots at 9" off centreline and subsequently discovered that the engine feet are at 9-1/2" centres, he/she may just change the dimension rather than re-draw that portion of the drawing.

2.) If the "redrawn to computer" plans were created by merely scanning and tracing or otherwise re-creating the original drawings by rote, many sources for errors can creep in. The original plans may be distorted due to paper stretch. The drawing inaccuracies noted in #1 above will be slavishly re-created. Errors of scale will be created - a line that is 1/32" wide on a drawing scaled at 3/4" = 1'-0" will be a full half-inch wide in reality, so the path of any cut taken from the drawing can be in error up to a half-inch. And so on...

3.) Most hand-drawn plans are created with the presumption that the builder will loft and fair the lines full-size prior to construction. This allows the builder to resolve fairness and fit issues prior to cutting any material. Also, locations and fits of structural components such as engine beds can be checked at full scale.

I have produced several builder's plans for classic yachts based on original pencil drawings, but I do not just copy them on AutoCAD, especially the hull geometry. I use the original linesplan and table of offsets as the basis for a completely new full-size 3-D model of the hull, from which I derive a new set of lines that I use for the basis for all subsequent drawings. My faith in accuracy is based on my "new" hull lines, not the original - possibly distorted - lines. From this I can go to defining structural pieces and molds because I know that the objects are derived from a digitally-faired set of lines. The only other way to get this level of accuracy is to use templates or patterns lifted from a loft floor.

I'm sorry that you have had difficulties with your project. Unfortunately, the accumulation of errors that occurs at each stage of the design interpretation (designer-to-scale master drawing, master drawing-to-blueprint, blueprint-to-computer trace, computer-to-plot, plot-to material) bit you.

[Stephen Hutchins]

I've seen (supposedly pro's) unable to draw/plot bulkheads that would fit in a hull and others (definatley pro's) give me full size curved raked transom expansions for traditional transom frames that fit to perfection. -All depends on whose doing the drawing. If it's done right, it sure does save the builder his knees and all that lofting time.

[RonW]

Don't complain.

What if the panels and other parts where only a inch or so short , here and there.

Glen-l's plans are famous for- cut to suit,- trim and fair as required.

And it all works out in the end.

[Gary E]

Could it be the software and or the operator?

As I recall the Boeing 777 was the first plane completely designed, parts machined, and all the wireing, ducts, etc etc were completely done on a computer, not one slip of paper used. I admit that they have the experience and no doubt the very best engineers and computers so it is possible, but maybe not for the little computers or the ability of those with lesser skills.

So grab your plane and take some off here and over there, they cant do that on aircraft.

[Dale R. Hamilton]

As I was waiting for your response- I had a thought. Instead of erecting my frames and then stretching the ply over the frames- what if I S&G the skin together, sans frames- like Japanese orgami- and then trim the frames to suit the skin. Wouldn't I have a better chance at creating a fair curve this way?

[Frank E. Price]

It's the builder's responsibility to get it fair. The "architect's" responsibility is only to get you in the ballpark.

Frank

[Gary E]

Originally posted by Frank E. Price:
It's the builder's responsibility to get it fair. The "architect's" responsibility is only to get you in the ballpark.

FrankDo you know any architect that would admit to that?

If I ever met one that did, he would not work for me.

[Tom Lathrop]

So it is S&G. If the design(er) is any good, the shape of the boat will be primarily determined by the dimensions of the hull panels. Intermediate frames are usually only to make the shape conform to specific dimensions at a few key points. It is not practical to try to fair the hull over a few frames if the panels are not correct. This where chine logs and sheer clamps come in in ply on frame construction.

Something is badly wrong somewhere. On S&G boats I have built or designed, the hull panels have been accurately designed, accurately laid out, accurately cut and then wired together. On the first boat built to a new design, there is some excuse for slight length differences of hull panels but not 5". Plywood does not often bend and fit just like the "ideal" panels the computer assumes. Operator experience comes in here.

That is, if I understand your situation correctly. It is a lot easier to give a good answer if the question explains the situation completely.

[RodB]

The plans for my stitch and glue skiff by Tracy Obrien came with "exact" dimensions for the bottom panels and side panels plus the transom (with proper bevel angles for the sides and bottom edges. Three stations called for knees with exact temporary spreaders to keep the sheer positioned along with the transom for exact alignment. All went together perfectly and stitching was as smooth as pie. Once I began checking for squareness and all being level fore and aft and side to side, it was clear how accurate all parts were dimensioned.

From that point on all bulkheads, cockpit sole panels, etc were cut to fit...and I was learning that as I went along. I had the misconception at the beginning that most all parts would be drawn out and go together like a puzzle, but soon learned the way it worked and I would have to fabricate all the rest "to fit". It was a good learning experience which allows for building other boats in the furture with less and less details. [I must confess that much of the joinery illustrated in the traditional boatbuilding books is daunting]

Once I had bonded the hull panels and was checking against the table of offsets for the heights of both bow and stern corners, I found that I was only 1/8 inch off from the table of offsets with the hull sitting on level cradle. I would say that the plans were accurate for an 18 foot boat.

RB

[ 07-07-2005, 09:38 PM: Message edited by: RodB ]

[Tom Lathrop]

Originally posted by Dale R. Hamilton:
As I was waiting for your response- I had a thought. Instead of erecting my frames and then stretching the ply over the frames- what if I S&G the skin together, sans frames- like Japanese orgami- and then trim the frames to suit the skin. Wouldn't I have a better chance at creating a fair curve this way?Just saw your last post Dale. That is the way it is done. There is still an error somewhere. Be interesting to know where it is.

[Dale R. Hamilton]

Tom- I was following Ray's instructions for assembly- provided when you buy his plans- which call for setting up the frames, adding the engine stringers, transom, sheer and bow- then bending the flat developed ply panels over that assemble. Although I can't find an error (yet)- it seems to me that it would be more likely in the station assembly that in the computer generated hull panels. I'll go ahead and solve this problem- its too far along now- and it does look pretty darn good. But next time, I'll stitch the panels together- so I know thats right, then add the frames.

[Paulyboy]

A computer is merely a device that allows you to make bigger mistakes faster and with greater confidence. Don't trust them.

[/QUOTE]
I've worked in the car parts biz for 22 years as a second career, most of my time with computers. This is perhaps the most profound statement about them I've ever seen. If you gentlemen don't ind, I'm going to copy this and put it at every terminal in the dealership where I work.
P.S. I agree with most responses in this thread regarding the intentions of the designer. There is no such thing as an overage until you've found the god and true fair lines and trimmed away to those lines.

[ 07-08-2005, 04:04 PM: Message edited by: Paulyboy ]

[Frank E. Price]

Gary,

The S&G plans I've dealt with from Bolger and from White state on them somewhere, if I recall correctly, that the panels are drawn a bit oversize and the builder is expected to cut them so and then pare them down to fit accurately and fairly (though not in those precise words).

The Chapelle tables of offsets I've built from or just studied each contained at least one egregious error; obviously from a simple transposition of numbers or misreading his scale by an inch or so. The sail plan for the 18' sharpie skiff contains an error of an even foot. The plans I've studied in the back of Buehler's book also contain an obvious error or two. Buehler makes a statement in the book similar to mine.

The one or two smalltime professional builders I've worked with faired by eye, not by numbers from a page.

I've only built seven small boats on my own and my earlier post reflects my experiences on all of them, and my experience with each of the handful of professionals I've worked with. I have never worked with anyone who would claim an architect's drawings and tables could be built to without further regard for fairing. Who is your architect?

Frank

[ 07-09-2005, 02:51 PM: Message edited by: Frank E. Price ]

[Gary E]

Frank,
The last thing I want to do here is get in a pissing contest about who's design software or plans or who's building methods are better. All I am saying is that if what you are paying for is labeled "Computer plans" or some variation of that and what you get is a component or pattern that requires trimming with so much as a file to make it fit, you have been snookered into paying for something you did not get. This can be attributed to several things, a less than adequate design software program, a less than proficient designer/operator, or a combination of both, or maybe just a marketing ploy to get you to buy from them.

So many products are built today from the keyboard you use to type to a auto body to a Boeing 777 and all from a computer generated set of files defining the shape down to a tolerance of less than the thickness of your hair.

Look here for one companies software..
http://www.manufacturingtalk.com/news/dea/dea250.html

or..
http://www.nccs.com/pages/Products_main.html

Look at some of the 5 axis shapes cut by the CAM systems that used CAD as the input. The CAD was done by the designers.

Now before you say, but that's a pro outfit and I cant aford that, well maybe your right, but the designer who passes off what you have to trim off or fit to, or any other method of make it fit as a computer design is just not delivering what you paid for.

[Tom Lathrop]

Gary is on the mark. A panel layout for a S&G boat is either accurate or it isn't. There is no good way to trim to fit the hull panels in this kind of building method. I am talking about the keel, chine & stem joints here. There may well be some slight difference in length to the side and bottom panels at the transom on most S&G boats. This is usually due more to builder assembly technique than an error on the plans.

That is not to say that the measurement points on the layouts are not to be faired with a batten before cutting. Such dimensional changes brought on by such fairing should be very minimal. This step is the nearest thing to "lofting" that happens with a S&G plan.

[Frank E. Price]

Okay, my apologies. Guess I'm way behind the times. I don't care for S&G construction in general and the older I get the more comfortable I feel with traditional construction. I know nothing about CAD-generated plans. Maybe newer S&G designs don't require trimming panels to fit, but that hasn't been my experience or expectation. Given my ignorance, it still seems strange to me that one could expect to build a boat without doing some fairing.

Now I'll try to be quiet on this subject and maybe I'll learn something.

Frank

P.S. As the S&G plans I've used (they were not CAD-generated) were drawn for amateur construction, perhaps trimming to fit was called for in recognition of most amateurs and their tools not being likely to cut panels accurately enough to fit without trimming. Or not.

[ 07-10-2005, 02:18 PM: Message edited by: Frank E. Price ]

[G. Schollmeier]

Here I stand, hat in hand, admitting to being the one who lofted the panels for Dale. The fact that there are discrepancies troubles me. The stations and panels were lofted from an offset table and dimensions from the drawings. I used Rhino 2 for the lofting, a program I have used daily to draw aircraft parts to tolerances of +/- .0005. The software can’t be faulted here; the error must be in the numbers on the drawings or in a scaling error when I had the full size plans printed. I checked the printed panels with a steel tape against my file and they looked good. An easy check would be to measure the patterns and see if they match the drawing, if not then the fault is mine. I don’t want people to get the wrong idea about CAD software, this error was either mine or the designers, or it was the designer’s intention to allow for trimming. I took this on as an instructional project and have learned much along the way. Dale I’ll help in any way I can to figure this out.

Gary

[Gary E]

Gary,
It sounds like you have a lot of CAD knowledge, I guess I used to have some.
From what you say here,

The stations and panels were lofted from an offset table and dimensions from the drawings. I used Rhino 2 for the lofting, a program I have used daily to draw aircraft parts to tolerances of +/- .0005. The software can’t be faulted here; the error must be in the numbers on the drawings or in a scaling error when I had the full size plans printed. I take this to mean you either design or you use a design from another. I used the design of others. Let me clarify what I used to do. I was furnished drawings by the OEM, those may have come from the OEM or most likely from the Government, as most of what I did was on Navy contracts, some directly for the OEM. Those drawings contained complete information that I used to generate a toolpath for CNC Milling and Turning equipment, so if the drawing was right the part was right. Never had a reject so, I spose they were ok.

This brings me to the current problem. Could the data that you used be wrong? You mentioned "scaling full size plans", I was tought long ago, NEVER SCALE DRAWINGS, but that is sometimes or was sometimes done on vellum drawings made by the OEM such as Boeing, again, it's THEIR DRAWING.

I am not familiar with Rhino, is it in wide use? Do you import DWG, or IGES, or other files into Rhino?

Hope that this can be worked out, becasue CAD/CAM has been and will contimue to be used more every day.

G

[G. Schollmeier]

Hi Gary,

At my day job I get paid to draw parts from sketches and written instruction, import that file to a CAM software, produces the NC code, and cut the part. I have been doing this for 15 years and would consider myself a skilled professional.

In this case I’m dealing with a set of plans, instead of lofting them on the floor I lofted them in Rhino http://www.rhino3d.com/ , simple 2D drawings, curves connecting points from an offset table. The original intension was to cut the panels on a CNC router. Due to lack of a shop to do the cutting, I agreed to change the drawings to full size plots. I imported the files to AC as .dwg I then plotted them to file using the driver for the plotter at a near by print shop. The longest one was just over 15 feet. When I got the plots I no longer had the drawings but I compared them to my file and they looked right. AC is a great tool but there are always issues when plotting so I am very careful with this step.

Because I no longer have the paper drawings and the file is on a disk that is not handy right now I have no way of checking my work at this point. I did this for the experience and to help another forum member out, there was no fee for service. But it troubles me that my work may have been in error.

Gary

[Gary E]

Sounds like you did the best you could and that the table of offsets is not up to par.

I started this stuff back in 1959 with the Cintimatic, then on to all the newer Acramatic controls, so you know where all this came from, even had my own small shop for a while with several CNC machines.

Thanks for the Rhino link, I'll take a look

[ 07-10-2005, 08:11 PM: Message edited by: Gary E ]

[mmd]

The method that I have had most success with is to use the paper drawings and offsets to create a digital model of the 3-D hull. I use ProSurf, but Rhino or other 3-D modelling software would be fine, as long as it has the feature that expands plates into 2-D geometry. When the model is complete, fair it and check to make sure the panels are developable. Then deduct the panel thickness. Now develop the panels of the inside surface of the shell into 2-D layouts. A good check is to plot the panel layouts and glue the plot to stiff cardboard. Cut out the panels and stitch them together. If the panels are correct, they should go together without gaps, overlaps, or buckles.

[Dale R. Hamilton]

Gary S and others- Don't forget- I built it. Because I have not found an error yet does not mean I didn't make one. Maybe I errected the frames 5" too short. Still looking.

[djtil]

You may not be dealing with any kind of measurement or lofting problem.

FWIW I also used Rhino to develop CNC cut file for the lapstrake planks for my Fulmar. In my case the plans included a full size pattern showing the plank laps at each station. I used these to create a 3D model of the planks, which Rhino could then unroll. I then used this to produce the cut files. This worked very well and was very accurate when cut with the CNC, because it would cut exactly to the drawing file.

Since you were working to paper plots from the drawing files, you may have been bitten by the issue of plotter scaling. We have a very good plotter where I work, but it has never been possible to get it adjusted to a tolerance closer than about 1% in the feed direction. For a 15 foot long plot this would be nearly 2". 5" only represents 2.7%. This is not unreasonable if the plotter has not been carefully calibrated.

Just a thought.

David

[Gary E]

Since you were working to paper plots from the drawing files, you may have been bitten by the issue of plotter scaling. Very good point, it has been a long time since I played with plotters so I am not familiar with how they work today. Could you tell us more about it?

For example, if the paper runs over a drum or feed roll and that drum rotates to provide the longest direction, how is that drum controlled? a stepper motor? If a stepper motor then it is possible that one step is not really what ever it's distance is suposed to be. If that is the case, lets check it by drawing a simple box. Make the box an inch inside the papers width, and the length of the box 10 or 20 ft or whatever long. If the length measureres wrong, maybe a sheet of paper could be added to the diameter of the feed roll to increase the length of the 10 ft mark from the zero line. If the actual line is over 10 ft from the zero, well, can you take some off of of the feed roll's diameter?

[John Meachen]

I have known the problem with plotter slippage in other contexts.With differing plotting media there can be a lack of accuracy along the plot but not across its width.This means that using a scale factor is difficult unless the slippage is known and it can be applied in one direction only.With the older generation of plotters that used punched holes at the edge of the film or paper,the accuracy tended to be better.I have found it helpful to have a grid drawn with dashed lines at 100mm intervals in both directions on the plot and to check the result with the longest accurate rule available.I you have the means of sending the information direct to a CNC router this becomes irrelevant as the production process is streamlined.It has been several years since I saw a demo version of Rhino and I cannot recall whether there is a curvature analysis function.If there is such a facility and no anomalies appear,the panels should fit together if cut to the CAD model.The model should take precedence over the plot as there is less chance of it being corrupted by an extra stage.Isn't it wonderful to use technology to revert to a process of design by modelling rather than just using lines on paper?

[djtil]

I don't know that much about the mechanics of the plotter, or what might be done to improve accuracy. I just know about the problem, and that as a result we do not scale to paper plots.

You are right in thinking that plotting an object of known length and then measuring it, would at least let you determine what your accuracy is.

David