Offset Table Question

[Dave R]

I'm looking at the offsets for a small boat with the idea of scaling it down for another cradle. The offsets are given as inches and sixteenths. I'm a little confused by something, though. I see for example a dimension written as 19-1+ and another as 24-10-.

The first would be 19 1/16" plus something. Plus what? 1/32"? 1/64"? In the second case it would be 24 10/16" minus something. Again, minus what?

I realize that the actual dimension would be found during lofting but I'm not at that point. I'm entering the numbers into an Excel spreadsheet and letting it calculate the scaled offsets.

I've done this on several other boats. Its easy to do and quick and allows me to get an idea of the scaled down boat before I begin lofting. Besides, I can do the spreadsheet thing at work. I think the boss would frown on me lofting a boat on the floor of the shop here at work.

At what point does "19-1+" become "20-0-"?

To add to my confusion (or maybe this should clear it up somewhat, in the smae table of numers there are a few offsets written like 17-8 1/2.

[NormMessinger]

Actually, I think TOF is in feet inches and eights. Some designers go to 1/6ths with the plus and minus sign. 1-3-5+ would be one foot, three inches and 5/8th plus 1/16th or 1' 5-11/16"

In the tables I've seen your 19-1+ would have been written 0-19-1+ which is 0' 19-3/16". I've never seen one with a sixteenth indicated as a minus but then I've not seen all that many.

[ 04-15-2004, 01:44 PM: Message edited by: NormMessinger ]

[JimD]

Originally posted by NormMessinger:
Actually, I think TOF is in feet inches and eights. Some designers go to 1/6ths with the plus and minus sign. 1-3-5+ would be one foot, three inches and 5/8th plus 1/16th or 1' 5-11/16"Yes, this is the only way I've ever seen the offsets. Who's the designer and how drunk was he when he wrote up those tables?

[Keith Wilson]

Norm's got it. Feet-Inches-eights is the norm, but sometimes you see inches-eights.

Thus, 19 - 1+ would be 19 + 1/8 +1/16 or 19-3/16. It's identical to 19 - 2-, which would be 19 + 2/8 - 1/16, or 19-3/16.

My advice is to convert the goddamn things to decimal inches, particularly for scaling it up or down. There's a reason nobody else in the world but boatbuilders uses this system.

[ 04-15-2004, 02:00 PM: Message edited by: Keith Wilson ]

[mmd]

While I agree with the gist of what has been said above, I think that the replies err a little bit on the side of "over-precision". Possibly a failure induced by our collective reliance on digital clocks, computers, and laser-ranging tape measures.

Unless the table of offsets is compiled from measurements taken from faired lines drawn full size on the loft floor, all datums given in the table are open to a bit of interpretation. The designer is trying to measure distances to a real-world accuracy of 1/8" from lines that are six times that in width. (a 1/32nd inch wide pencil line - very fine indeed - at a drawing scale of 1/2" = 1 ft is the equivalent of 3/4" wide when enlarged to full size.) Add to this inaccuracy the fact that he is trying to read the dimensions on a scale ruler that will have rule marks scribed at 1/64" increments, so paralax confusion will drive him nuts trying to determine whether the centre of the drawn line (or should it be the edge of the drawn line?) is actually between rule marks or is this visual trickery?

If I may be so bold, I'd like to suggest that you read the "+" and "-" signs as "plus a little bit" and "minus a little bit" and let your batten determine how much "a little bit" is, and not arbitrarily define a fixed measurement. It is a grey area, allow it to remain so or you will be chasing your tail endlessly. When I have to put these types of dimensions into a spreadsheet for manipulation, I assume that "+" means "plus 1/3rd of datum increment" and "-" means "1/3rd less"; e.g., "1-9-3+" equals "one foot, nine and three-eigths inches plus a bit" which numerically becomes 21.4167". When plotted & faired, the digital or batten line will define the exact datum.

By the way; offset table dimensions to +/- sixteenths?!! :eek: :rolleyes: The only time I have ever seen this in the real world where it made sense was the offsets table for an America's Cup hull.

[ 04-15-2004, 03:17 PM: Message edited by: mmd ]

[Bruce Hooke]

I'm not sure that plus or minus sixteenths is any more (or less) ridiculous on, say a 14' canoe, than plus or minus eights is on a 30' sloop.

FWIW - I often use the plus/minus approach when transfering real world measurements. 5/8"+ is quicker for me to deal with then stopping to think through how many sixteenths that comes to (11/16") and it's even worse when you start dealing with 32nds!

[Keith Wilson]

mmd, you're absolutely right (as usual) - a fair curve and pieces that fit together take precedence over numnerical dimensions, certainly, and +/- .03 is too close for this kind of work.

However, there's no real relationship between how the dimension is expressed and the tolerance. You can have 23.1875 +/- .5, or +/- .0005, your choice, depending on the application. I like decimal inches because I'm used to them, and because my calculator works in decimals.

[Bruce Hooke]

Keith,

I'm starting to use decimal inches more and more too. My calculator likes them and I now have a ruler and a set of calipers that measure the same way. Actually, my calculator has a system for doing fractions but it's so much quicker to just put in decimal data. I keep this chart (PDF Format) (http://members.cox.net/bghooke/Documents/Tap%20&%20Drill%20Chart.pdf) hanging in my shop for when I need to convert back and forth between fractions and decimals.

Of course now someone is going to sing the praises of metric :D . I use metric at times too but I find that for the most part good old feet and inches still work better for me. tongue.gif

[Dave Fleming]

I'm with MMD on this.

Fugger that NA is sitting at his drawing board and measuring for the Table of Offsets from his what(?) 24 x 36 or 36 x 48 drawing.
Now he is trying to get it close but many times it just doesn't work out 'that' close.
So the + or - indicator at the end of the measurement of Feet/Inches/Eights.

To PROVE the number/s out is one of the principal reasons for LOFTING.

You folks using that decimal inch thing...it does sound interesting but unless you work with it constantly I don't see it becoming too popular for the average *AMATEUR*.

*Amateur: said in the best sense of that word.

[Keith Wilson]

Dave, I'm very glad you're posting regularly again. smile.gif Just stay out of the bilge, or bring a barf bag if you go down there.

Decimal inches are very easy once you get used to them, mostly use a calculator, and have memorized that .4375 is 9/16" ;) OTOH, most systems work fine once you get used to them: feet-inches-eighths, meters, cubits, whatever.

[ 04-15-2004, 03:59 PM: Message edited by: Keith Wilson ]

[Dave R]

Thanks for all the responses and thanks to Keith. I do mostly avoid the bilge.

FWIW, I scanned a portion of the plans to assure you that I wasn't misreading anything. I apologize for the picture size but wanted it readable.

http://www.woodshopphotos.com/albums/Dave-R1s-Album/offsets.jpg

You'll notice inside the blue boxes coroboration of what I wrote in my initial post.

Inches and sixteenths.
17 - 8 1/2
20 - 6-
19 - 1+

I will ultimately end up decimalizing the numbers while scaling but What would be a fair approximation of the - or the + ? 1/64"?

Thanks again gang for the advice.

Dave

By the way, I'd be surprised if at least one person here can't recognize the designer by the writing

[ 04-15-2004, 06:27 PM: Message edited by: Dave R ]

[Bruce Hooke]

Given that the designer also used 1/2 I think MMD has it right. I would read + as meaning maybe 1/3 of a sixteenth, or a hair over (or under in the case of - ). In the end I think the key point is that 1/3 of 1/16 is too small to really mean anything so it's not worth sweating over too much!

[htom]

I'd read it as n+ being = n plus 1/32 inch = and n- as being either exactly n or maybe = n minus 1/64 inch = (especially the latter if there happens to be an entry of n+- somewhere in the table.) The + in all of my reading has always been "half of the increment" (and I've never seen - used or refered to in this usage (a quarter increment) in boat building.)

A good batten trumps all numbers.

And yes, convert it to decimal inches or millimeters before scaling.

[htom]

bump
again

[ 06-19-2004, 12:50 AM: Message edited by: htom ]

[Charles Burgess]

Originally posted by mmd:
While I agree with the gist of what has been said above, I think that the replies err a little bit on the side of "over-precision". Possibly a failure induced by our collective reliance on digital clocks, computers, and laser-ranging tape measures.

Unless the table of offsets is compiled from measurements taken from faired lines drawn full size on the loft floor, all datums given in the table are open to a bit of interpretation. The designer is trying to measure distances to a real-world accuracy of 1/8" from lines that are six times that in width. (a 1/32nd inch wide pencil line - very fine indeed - at a drawing scale of 1/2" = 1 ft is the equivalent of 3/4" wide when enlarged to full size.) Add to this inaccuracy the fact that he is trying to read the dimensions on a scale ruler that will have rule marks scribed at 1/64" increments, so paralax confusion will drive him nuts trying to determine whether the centre of the drawn line (or should it be the edge of the drawn line?) is actually between rule marks or is this visual trickery?

If I may be so bold, I'd like to suggest that you read the "+" and "-" signs as "plus a little bit" and "minus a little bit" and let your batten determine how much "a little bit" is, and not arbitrarily define a fixed measurement. It is a grey area, allow it to remain so or you will be chasing your tail endlessly. When I have to put these types of dimensions into a spreadsheet for manipulation, I assume that "+" means "plus 1/3rd of datum increment" and "-" means "1/3rd less"; e.g., "1-9-3+" equals "one foot, nine and three-eigths inches plus a bit" which numerically becomes 21.4167". When plotted & faired, the digital or batten line will define the exact datum.

By the way; offset table dimensions to +/- sixteenths?!! :eek: :rolleyes: The only time I have ever seen this in the real world where it made sense was the offsets table for an America's Cup hull.mmd has pretty much nailed down how the table of offsets are to be measured. I would only add that the numbers are specified as they are due to the fact that in the lofting process you are using a ruled measuring device of some sort where decimals are not practical and would take forever to loft.

"1-9-3+" = one foot, nine and three-eigths inches plus a bit, meaning that the line falls between the 3/8 inch mark and the 1/2 inch mark on the ruler rather than exactly on the 3/8 mark. If it were "1-9-3-" it would be one foot, nine and three-eigths inches minus a bit, meaning that the last figure means that the line falls between the 1/4 and 3/8 inch mark on the ruler.

We also need to remember that when we saw a piece of wood, that the width of the cut, the kerf, is 1/8 of an inch.

For the traditional boatbuilder that builds in wood, 1/8 of an inch precision is more than enough given the expansion and contraction characteristics of wood...a 1"x3"x60' board can vary +/- 1" or more over time solely due to local moisture conditions that vary over time. Thus when building in wood to tolerances less than 1/8" you risk distortions of the design lines and sometimes outright damage to the wood members.

Regards,

Charles

[ 05-23-2004, 12:55 PM: Message edited by: Charles Burgess ]

[Dave R]

Bruce, I agree that it isn't worth sweating over the less than 16th" measurements.

Charles, would you agree that it is best to go by the dimensioning as indicated by the designer rather than by convention? I get the feeling you would ignore Mr Oughtred's note indicating that his table of offsets for the boat in question is inches and sixteenths. Hopefully I am wrong.

I do know that the convention in tables of offsets is to use eighths however it would appear that one get himself in trouble by always assuming the eighths are intended.

Going back to my original questions (nevermind the fact that the resolution of these dimensions is overly small for the application), given that the offsets are in inches and sixteenths, and we have the following offsets:

17- 8 1/2 Would be 17 17/32" (8/16 + 1/32)?

19- 1+ Would be 19 5/64" (1/16 + 1/64) if it was 3/32 wouldn't it be written as 19- 1 1/2?

24- 10- Would be 24 39/64"? Using the same logic as in the previous example.

Again, I wouldn't worry about these little things if I was building the boat. I'm just curious.