I'm trying to understand the line drawing and the table of offset provided in the current WoodenBoat Magazine. I've been able to recreate the most of the line drawing on my cad program using the table of offset but I'm don't understand how to get the transom shape from the table.
Can anybody help?
I could, I think, if I spoke CAD. Or maybe
if you spoke "ducks" and transom expansion.
Don't mean to be difficult, I just like pencils. All kidding aside, I hope ya figure this one out. I enjoyed building an ELLEN
three years ago at WOODENBOAT and enjoyed
sailing the ELLEN that John brought. A really
sweet little boat.
McMichael,
My problem was not with the cad program. It is much simpler for me to draw on paper but it does take much more space. I have taken out my drafting table and I have a large sheet of paper on it. Can you help me figure out the transom shape from the table of offsets and line drawing. I've been looking at it for some time but I don't seem to be able to understand it.
Tx for your help
[This message has been edited by yvesdavignon (edited 09-25-2000).]
Trying to be reasonable
One point, looking at the plans under magnification, is that the "T" station is 21" from #10, not 24" apart like the others. There is a mysterious dimension for the aft edge of the transom at the waterline, 3-1-5 (Good God, I wish boatbuilders would learn to use standard drafting conventions! Feet-inches-eights is such utter and complete crap! End of rant.) or 37.5", but it's not at all clear where the dimension is from; it's not Station 8, that doesn't scale out.
There is an angle for the transom shown in the view at the lower left, but it's not legible, at least to me. To draw the true shape of the transom, you need some way of finding the angle between the transom face and the waterlines, or the fore-and-aft position of some point on the transom other than the sheerline (which amounts to the same thing); after that it's straightforward. If I were doing it, I'd draw a long extension of the transom line and measure the angle, but that might not be as accurate as the number on the plans. I think you'll need to buy a large-scale version of the plans, probably not a bad thing when you consider how little money Mr. Brooks probably makes designing and building boats.
Once you have the transom angle, you can draw the transom as it would appear flat on the paper (or screen), its "true shape". Horizontal dimensions ("half-breadths", but I won't repeat my previous rant for fear of getting tedious) stay the same, all vertical dimensions including distance between waterlines are multiplied by 1/sine(angle between transom and waterline).
There's a fairly straightforward graphical method for this also. Draw the true shape of the transom projected at 90 degrees to the transom line on the side view. This will automatically stretch the vertical axis the correct amount.
I apologize for my hasty answer last night.
Having thought on this a bit I don't know if you can draw, full size and true to shape, the transom without having drawn your other
views full size. It may be somehow possible
but I don't believe it is something I could find myself to trust w/o having the other views(half-breadth,profile,and plan) drawn to
full size to check and balance each other.
There are always corrections(or at least choices to be decided on) in the lofting
process and although I find it enjoyable, lofting can be a real head scratcher. With the limited info(no construction plan, scantlings etc.) that John has provided I would not try to build this boat solely from this magazine article. HOWEVER, if I was decided on the task of building a 12' sailing
dinghy I would have to put this boat right up
there at the top of my final choices. The plans and full size patterns can be trusted
and when you're about to start wasting expensive AND precious wood, there is comfort
in knowing this.
As I understand the subject, lofting a set of plans to full size is useful for ironing out the bugs in a set of plans. You draw the stations using the offsets and then fair with battens, so that you arrive at the same hull shape as on the plans, even though the batten may not have hit all of your marks and there may be some minor differences (but the hull will be fair). The transom lofted from a full size profile will give you the degree of bevel for the transom edges. You can draw just the transom, full size from the offsets. If the transom is raked you'll have to draw the last couple of stations to establish the rake of the transom. Using a square (I like a rafter square)draw lines, at a right angle to the transom in profile at the keel, chine, sheer and any water lines to the appropriate half-breadth to the outside of the planking. Connect the dots. You will find yourself with a rough transom that will have to be faired to fit. Of course I am new to this, so I'm sure someone could give you a clearer explanation.
Jim
Keith, Keith, Keith.... I feel your pain. But feet, inches, eighths HAS been the drafting convention for many generations in boatbuilding. And 3,1,5 isn't 37.5". It's 37 5/8". 37.5" would be 3,1,4. It might be measured correctly or incorrectly but that's why one lofts. I suppose one could measure the drawing in nanometers or furlongs but you'd still need to loft. And it's only a boat, not the latest in orbit deep space x-ray telescope mirrors
[This message has been edited by TomRobb (edited 09-26-2000).]
Or decibalized 37.625 inches I like the feet inches eigths and explained it to my architectural drafting teacher who had also never heard of it have not personaly had any probles travling from house plans to boat plane and back
Best wishes
Jeffery
Trying to be reasonable
Oops! Sorry about that; of course 3-1-5 is 37.625". Where that dimension measures on Ellen's plans is still obscure. I have a little Excel spreadsheet routine that translates boatbuilders' dimensions into something my calculator and I can use, but I obviously wasn't using it when I wrote that.
(Caution, short but tedious rant follows)
My main beef with the feet-inches-eights is that it is completely different from what anyone in any other industry uses. Since wooden boatbuilding is about .00003 percent of all the fabrication work that gets done in this country, having its own separate standard is really silly, particularly on small boats where just inches (fractions if you insist, but decimals is what I'm used to) would work much better. Oh well, it's not hard to convert. There is also a lot of really sloppy and confusing dimensioning on boat plans, work I wouldn't tolerate from a green detailer fresh out of drafting school. I really enjoy lofting, it's as much fun and a lot cheaper than actually building the boat, but if I ever sent plans out to the machine shop that one had to redraft full size, either to figure out some of the missing dimensions or to correct errors, my head would be up on a pike outside the building as a warning!
Actually, I quoted a special machine a few years ago for Perkin-Elmer to grind the mirrors on the orbiting X-ray observatory - we didnt get the job, but it was fun to do. Believe me, I don't even THINK about tolerances that tight on anything I build in the garage.
My point in the original post (It's a slow morning at work, and after reading what I just wrote, the expression "more heat than light" comes to mind) was that you need the transom angle to find the true shape of the transom by any method, and you can't read that dimension on the small-scale plans of Ellen in WB this month.
[This message has been edited by Keith Wilson (edited 09-26-2000).]
Senior Member
The feet-inches-eighths convention might not make sense in your CAD program or on your drafting table, based on your experience, but it makes utterly perfect sense when you get on the lofting floor, where you'll thank yourself for whatever minor inconvenience you're experiencing now!
Well, yes Keith, wood boat fabrication is but a tiny fraction of the whole enchalada, but it's not done by contract machine shops much less Skunk Works operations. The people who do this kind of work are used to the convention and it works well with a folding ruler. Few boats are cnc machined from billet stock. Those that are are designed on cad and they understand each other.
The other reason that we use feet, inches, and eights is that when the boat is designed the offsets are taken off w/ an architects scale. These are are only marked to 1/2" or 1/4" divisions. The best you can eyeball is to about an eight of an inch. And therein lays the whole reason for lofting. You're finishing the design off for the designer.
[This message has been edited by Nathan (edited 09-26-2000).]
Senior Member
Keith,
Just to let you know that even though boat drafting may be a small part of the industry, comercial and home construction is not, and all of our drawings are in feet, inches and 1/8" of inches. With the expeption of survey plans which are in tenth of a foot measurments. It is much easier to find a tape measure in the "standard format" than in tenths. And besides most of these old carpenters don't understand that .625" is actually 5/8".
Just wanted to clarify that not everyone in detailing uses tenths. It is easier to use on a calculator but you can program your calculator to do the conversion for you.
Chad Smith
Trying to be reasonable
Wow, I didn't expect to get such a lively response! A couple of points:
I have nothing (much) against fractions, they work just fine for large work with relatively loose tolerances, such as building construction (and boats), and I realize that fractional dimensions are the standard there. I was mainly objecting to the 3-1-5 notation, which encourages errors of the type I made in the previous post. There's nothing wrong with it per se, it's just different than what the rest of the world uses, and IMHO adds an unneded complication. On the other hand, they're just numbers and easy enough to convert, so you use what you like and I'll use what I like.
This is, I think another subset of the traditionalist vs. modernist debate we see here so often. Wooden boat building has, like it or not, a large component of anachronism, of trying to preserve the technology of a previous time. There is a temptation to come up with dubious arguments for the functional superiority of techniques which are, at base, done "because we've always done it that way", or which evolved in a previous age to fit conditions that no longer exist.
I'm very much in favor of trying to preserve old skills - For example, in my field, I've seen manual drafting, a tradition that is many hundreds of years old, vanish completely almost overnight. Preserving an honorable tradition is its own justification, if that's what you want to do.
Senior Member
Keith,
I am with you on perserving the old traditions. You can learn a lot do it the old way. You get more of a feel of how a project goes together. I've been a draftsmans for 10 years now (5 by hand & 5 by CAD). I don't miss drawing by hand but wouldn't trade it for anything. One of the problems that I see with modern draftsman is they don't have a complete understanding of how it all works together. This can only be learned by doing it by hand. The modern drafting class teach us how to use the computer and not how to draft. I am afraid that drafting is a dying art.
Sorry to carry on so.
Chad
P.S. CAD is just a tool and like any other tool (i.e. power planer/hand planer) is only as good as the operator.
C.S.
[This message has been edited by Chad Smith (edited 09-26-2000).]
Senior Member
The very best reason for being able to use a table of offsets is to be able to loft and build all those grand old designs that are presented thataway. Anyone volunteering to convert all existing ship/boat plans to decimalized metrics will please step forward now...
Conversion is a fast way to drooling madness. If you want metric plans, draw them that way. If God tells you that your Ark is in cubits, so be it. If L.F.H's. plans are feet, inches, eighths, get over it and build your boat.
So, yvesdavignon, how would you compare the discussion of your question here with the responses you got in rec.boats.building?
(Forgive me WBFG's for I have sinned. But I don't go there often, honest.)
Now a question on Tables of Offsets: Where do the numbers derive from? Does the designer draw the plans then scale off the offsets, or what?
Keith, if you'd mark your rule in angstroms you wouldn't have to use decimals either.
And if I get within a 1/8" I can always fill the gap with googue and cover it with glass. Heh heh! Ya cant do that with yer old fashoned traditional building methods, Ian W.
Ooops, gotta git to work.
--Norm
Senior Member
The marine standard of using Feet-Inches-Eighths (or sixteenths) on offset tables was done to save time and space. Typically after a lines drawing was completed, the painstaking job of pulling the offsets was done. A table was prepared and the entries made. To actually write down the denominator of the measured fraction over and over again would waste time and take up space in the table. After the table was completed it would be taken to the loft and “laid down” by the loftsman who would only use the lines drawing as a visual reference. The offsets would be measured on to the floor with normal tape measures. The table of offsets is usually the only place where dimensions would be recorded in this manner. All other dimensions are shown with a denominator such as 3’ – 6 5/8” or 24 3/16”.
Trying to be reasonable
Angstroms, by God! Now there's an idea! Unfortunately neither my eyes nor my workmanship are anywhere near that good. How many angstroms of googe will I need to fill that gap big enough to stick my calculator through?
You should have seen Greg Rossel's expression when he saw me measuring boat parts with a dial caliper - worth the price of the course right there, but hey, it was easier than a using a ruler! Contrary to what y'all may think, I'm all for taking the path of least resistance, that's why I use decimal inches
Bruce has a good point about the table of offsets; unfortunately, on the plans for Ellen (which was what this thread was originally about before I started ranting) Tom Brooks put a lot of other dimensions in that form also.
Conversion is no problem if one is comfortable playing with numbers and doesn't make many mistakes in math, otherwise it's just a wealth of opportunities for confusion.
Measure and build any way you please. It's your time and money. If you can measure, mark, saw, and plane to a line defined at 3 decimal places, more power to you.
Madness
Speaking of dial calipers, mine came in very handy when I was tapering the staves for the hollow birdsmouth spars. I did the first with my tape, must have assembled and checked half dozen times. Got the vernier, cut the others to within about .003, assembled once to check then glued.
--Norm
Member #919
There's an old computer type-setting program called "TeX" (and its companion program "MetaFont", for typeface design) that does all computations in units of "sp", cleverly chosen so that 2**31 sp is approximately 18 feet, and the usual typography measures of points, inches, millimeters, etc are all -exactly- expressable as integer numbers of sp. The reason for doing this is so that all of the computations can be done using integer arithmetic, for speed. An sp is a bit less than ten Angstroms, if I remember correctly.
I'd hesitate to do numeric conversion of an offsets table, in either direction. A sixteenth is close to a millimeter, but the goal is the fairness of the lines, not the values of the table. I'd loft it and re-measure in the other system, putting stations at the "right" places for that system's measures. (This is something that a good CAD program could do.)
To start, I sent "y" a long explanation of how to develop the transom, in general, as I have not seen the Ellen plans.
And I was uncomfortable doing that,to me describing lofting with words alone is just not doing it justice.
Now as about measurements in boat building.
I can see where you younger folk with CAD experience or machine shop backgroud can be frustrated with the *** arcane *** system of Feet Inches and Eigths. I have had the revers happen to me. I was working in the loft of a maker of 200' Tuna Seiners and the plans were in F-I-E and we used them to layout the hull.
Now the kicker was that the plasma burner worked in tenths. So after we proved the lines on the floor the plans were corrected and then using a Digital Equipment digitizer table the plans were converted to paper tape for the burner. It was an experience! I had to go out and add 10ths rulers to my rather large collection of K&E and Starrett stuff, not that I minded.
But I resist those that wish to change things just because it is easier for them. I see no reason to not learn how to do it the way it has been done. NOW, that does not mean I am unopen to change, learning something new is good. But F-I-E is what is used and I see no real changes on the horizon until a new generation of NA's working purely in CAD takes over the marine design world.
What do places like Newport News or Electric Boat use for lines plans? I think I read somewhere that Phil Bolger uses Metric.
How is that shown?
dave
"read the Tales yet?" www.pipeline.com/~djf3rd
Tales of a Boat Buider Apprentice
Trying to be reasonable
Phil Bolger has drawn some metric plans and argues eloquently for the metric system, but most of his designs are still in inches, I'd guess because that's what most of his customers want.
Tom, I think you're confusing units of measurement with dimensional tolerances; they're not at all the same thing. Just because I could measure a piece of wood to +/-.001 doesn't mean I have to, or even can, cut it that close. I have a drawing in front of me right now that has a dimension of 2.901 +/-.10.
I wouldn't want to convert a inch table of offsets into metric either. Although the accuracy is independant of the units chosen, the station spacing would be pretty wierd; I don't think I'd want to set up molds on centers of 606.9 mm. Feet-inches-eights goes into decimal inches just fine.
Senior Member
I find I work in Imperial for rough work ie 'get me couple of foot of 2 by 4 please' and use mm for accuracy. My first job was a tea boy in a Quantity Surveyors office and spent hours squaring up dimensions using maths to the base 12 for feet/inches and we learnt the 16 times table at school for ounces. And then good ol' Texas Instruments came up with the pocket calculator. I drew plans in ink with a pen on linen and now use CAD which is brilliant for some jobs and too slow for other tasks where pencil and paper are better.
Come on guys, use whatever suits the job. They are all correct. And adapt. Its just as important for old farts like me to learn new tech as young bloods to get to grips with feet-inches-eighths.
Senior Member
Keith... "I really enjoy lofting..."? Is it just me, or did anybody else almost cripple themselves while lofting? Granted, it WAS an interesting exercise and really helped me to "visualize" the boat in three dimensions. Saved a lot of headaches later too. But, My God! I didn't wise up enough to find kneepads until I was half way done and the damage was already done. I could barely walk. Anybody ever heard of "drop foot"? I suddenly couldn't lift up the front of my feet. Those muscles just stopped working. Had to kind of shuffle along with my toes "slapping" the ground in front of me. Lasted, with diminishing severity, for about a month. I had a doctor as a sailing student soon after I started having problems and he explained what was happening. Seems being on your knees all day causes nerve damage to your lower legs. I'm glad to have had the experience of lofting, but I can't say I'd like to do it for fun!
[This message has been edited by Art Read (edited 09-28-2000).]
Senior Member
This all goes to show that there a three kinds of people, those good with numbers and those that are not.
Trying to be reasonable
Well, if I had ever done it on the floor, I'd probably feel the same way, not being at all flexible.
I've only done relatively small boats, often half-scale, and always up on sawhorses using Lauan underlayment plywood with a 2x4 frame underneath. Hard on the back after a while, but much easier than crawling around on the floor.
Half-scale for top and side views is fine for fairing the hull IMHO, although I'm a long way froma perfectionist. The front/back view can then be drawn at full size to make the molds - you can redraw critical parts at full size if you want. This obviously takes a bit of number crunching with a calculator, but I still think it's easier than crawling around on the floor.
Also, there's no reason you have to draw the three views all on one sheet; splitting them up may get the individual drawings for even a medium-sized boat down to a small enough size that working off the floor is reasonable. You can angle the plywood pieces like a drafting table if they're small enough, which eases the pain in the back but prevents you from working from both sides. If I ever do a larger boat, I think I'll do it on the wall with the plywood angled out about 15 degrees at the bottom.
This may prove about as popular as my earlier suggestion that decimal inches were easier than feet-inches-eights, but hey, all good ideas got a lot of resistance at first, and so did all boneheaded ones
Senior Member
I had read about lofting half or quarter size before I started my project, but didn't want to deal with the mental gymnastics of relating it all to full sized wood. I suppose that's why I always plot out set and drift corrections right on the chart rather than use all those fancy formulas. I suppose when this project is done and SWMBO lets me start another model, I'll try lofting it on the coffee table in front of the TV. (Hope I haven't scared away any would-be builders away from buying plans that require lofting... It WAS one of the most interesting and satisfying aspects of this project. Just buy good knee pads if you're doing it on the floor!)
Welll . We all agree that the English system of measurement is rational and valid . Now ; why would a dimension (9.901) with a range of error expressed to the nearest hundredth ( +/-- .10 ) be itself expressed to the nearest thousandth ?
Trying to be reasonable
Again, I think you're confusing accuracy of measurement and tolerance - they're completely different things. 2.9 +/- .01 is not the same dimension as 2.901 +/- .01. The acceptable range for the former is 2.910 to 3.890, for the latter, 2.911 to 2.891.
This is kind of a silly example, but the tolerance for a dimension has no necessary relationship to the accuracy with which the dimension is measured, nor the number of digits with which it's expressed. (There is an all-too-commonly-used drafting convention that relates the tolerance to the number of digits, but I'd argue that's not the best way to do it.) Another somewhat less silly example: Say you have a nominal 1-3/16" diameter turned-ground-polished shaft, which would be, say 1.1875 +0 -.001, and you want it to be a loose sliding fit in a bore, you'd dimension the bore 1.1885 +.002 -0, or something like that. Rounding off the dimension just muddles things, and makes it harder to get the size you really want. When you can buy a pretty good calculator for $10, there's no need to do that.
What does this have to do with boatbuilding? D*mned little, except that tolerance and accuracy are different. whatever tolerance and accuracy you're working with.
Another example, in fractions even: Say you can reasonably measure and cut to +/- 1/32, and the dimension to, say, fit a breasthook is 3-3/32". Because the breashook should be closely fitted, you'd want to make it as close as you can, say 3-3/32" +1/32 -0, and then probably shave it down a little to fit even more closely. On the other hand, say the dimension for the height of the coaming is also 3-3/32", but it doesn't have to fit against anything, so it might be 3-3/32 +/- 1/4, with the stipulation that both sides are the same within +/- 1/16, just so they don't look wierd. Same dimension, different tolerances. Most folks building boats, I think, do this in their heads without thinking about it too hard.
(Lordy, so pedantic he gets, and early in the morning, even
Keith ; This is not a rebuttal , just a continuation ( further destruction ? ) of the thread . When Tom suggested that an offset dimension expressed to the nearest thousandth of an inch was inapropriate , I don't think he was baseing his objection on a misunderstanding of Terms . We never did get the conversion from 3--1--5 quite right . 37.6" is the nearest decimal approximation . The last two decimal places of 37.625" are specious , and imply an impossible level of accuracy in measurement and execution that was never intended . That the number of decimal places in a number define its' level of accuracy isn't just another annoying old convention , but is ( I think ) one of the laws of mathematics . Similarly , in a table of offsets with 1/8ths as its' smallest unit of measurement , that's the assumed level of accuracy .----There's a typo in your first paragragh , it was +/-- .10 , or was it +/-- .100 ?
I think it was Capt. Nat that stated ,"A fair
line supercedes any given measurement".
I would like to thank everyone who help me understand the process involve in lofting the transom of the Ellen.
I have printed this thread and will try to understand it in the next few weeks or years. I have order Lofting(the book) and I hope that after reading it I will get a better grasp of lofting.
Member #919
feet, inches, eights, sixteenth.
3-1-5-0 --> 37 5/8" +- 1/32"
3-1-5-+ --> 37 11/16" +- 1/32"
3-1-4-+ --> 37 9/16" +- 1/32"
The mathematicly correct and complete decimal versions of these values are 37.625+-0.03125", 37.6875+-0.03125", and 37.5625+-0.03125".
The F-I-E-S system is much simpler to write.
37.6" is a distance between 37.5499999... and 37.650000... which includes 37.625 but it's not the best representation (mostly because the least error in the FIES is smaller than a tenth of an inch.)
The bounded 37.6 37.54999 37.6oo 37.65ooo
FIE+S true values 37.59375 37.625 37.65625
The best rounded decimal fraction is probably 37.62 but it's really MUCH better to use the entire value when it's available as an orginal datum, which is the case here. The number of digits in one system of measure is not meaningful in another system of measure, as the digits measure different sizes.
Trying to be reasonable
I really think there's a fundamental misconception here (or perhaps a disagreement, but y'all are wrong.
There are two entirely different categories of dimensions: One is measurements taken from physical objects ("this piece of wood is 5.63 inches long") where the dimensions should be expressed in units which reflect the accuracy of the method of measurement, and the concept of significant figures is very useful. You shouldn't measure something with a tape measure and say it's 5.4375 inches long, that would be spurious accuracy. If you measure it with a CMM or a micrometer, that dimension would be legitimate.
An entirely different case involves dimensions on drawings that are used to create physical objects. Here, one says "I want this piece to be 2.5 +/- .030 long", and the concept of significant figures has no relevance whatever. Dimensions are assumed to be exact, then the specified tolerance is applied. Decimals are assumed to have zeros trailing off to infinity. fractions are assumed to be exactly what they say. Tolerances are then specified, either by the convention, noted on the drawing, that 2.00 has one tolerance, 2.000 has another, or by tolerances shown by the dimension, i.e. 2.342 +.001 -.002, 4.5625 +/- .25 etc. Remember, this is not measuring the real world, this is specifying how you want the real world to be. Whether you get what you specify depends on the methods used to make the parts, and the skill of those who make them.
You may ask (if you haven't developed a glazed expression and logged off by now), what the bloody h*ll does this have to do with boatbuilding, I haven't seen any tolerances on any of the drawings from L. Francis Herreshoff or Phil Bolger. Well, I haven't either; they don't put 'em on because it would be too much work. They leave it up to the builder. The point of this is, just because something is dimensioned in 32nds, no tolerance is specified. You have to make it fit as the need of the boat and your skill and patience dictate. Carvel planking needs to have a tight tolerance or you'll get awfully damp. The cockpit seats, well, that depends on how pretty you want them to look. A fair curve or a tight fit absolutely do take precedence over any number on paper.
[This message has been edited by Keith Wilson (edited 10-02-2000).]
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