# Understanding boat plans from table of offsets

Discussion in 'Boat Design' started by Thule, Aug 13, 2023.

1. Joined: Apr 2023
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### ThuleJunior Member

Hi all

I have been reading and watching to learn about table of offsets. I tried to recreate the body plan view with this one Fragniere_offsets.pdf https://docs.google.com/file/d/0B3m28UIOtMr3N0VOaHR6QUx2eFk/view?resourcekey=0-8ikKsBz4H1SQwjhYRhkmlw and managed to recreate it and it looks like the body plan on page 3 of the PDF.

Then I tried to do it for Europe dinghy and the table I can find is this one.
https://www.europeclass.org/oldwebsite/www.europeclass.org/images/paragraph/38712.pdf

I tried to reason with it and made some perhaps incorrect assumptions that - x is distance from transom in millimeters, y is across and z is up as a point. And the offsets at each 10 degree intervals are from this point. I tried that and came up with a line looking like half cooked pasta!

How do you read this type of table or am I making a more fundamental mistake? I heard a mention of Nordic and European designs may use a different type of table of offsets but can't find a tutorial or something.

I am guessing that one can potentially build frames using the table of offsets to put on a strongback.

I am very lost on this one.
Any help appreciated.
Thanks

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### wet feetSenior Member

I might have an inkling of where your problem originates.The word originates is key to the matter;you may be trying to apply the diagonal measurement from somewhere other than the intended origin.To be of use when lofting,or drawing,the hull lines the most useful measurements are those which are perpendicular to the surface.this minimises the risk of error creeping in and in the case of sections it will mean that with the hull drawn upright the origin point will be above the base line.In the case of the offsets you posted there is no clue as to how high the origin is.It is a rather unusual format for offsets as there are no waterlines or buttocks.I know of at least one well regarded designer who uses diagonals as a primary fairing tool,but he does provide buttocks and waterlines as well.Obviously the person who recorded the Europe offsets preferred the accuracy that diagonals can deliver and you will get a result that accurately reflects the hull shape if only you can establish the height of the origin for each of those families of diagonals.They may all originate from the same longitudinal datum but my search for an image of the lines drawing for the Europe didn't achieve a result that helps.

A good while ago there was a thread on this forum about diagonals Diagonals And Offset Tables https://www.boatdesign.net/threads/diagonals-and-offset-tables.44204/page-2

This diagram shows a common way of presenting diagonals and you will see that they pass through the intersection of specific waterlines and the centreline and an intersection of another waterline and a buttock.Your challenge is to determine the relevant locations for the hull in question.I have never known a designer use angles at ten degree intervals for diagonals and in the days of pencils and kneeling on the floor to loft lines I don't suppose it was particularly appreciated.I'm in a minority that uses inverse tangents to set out angles when doing this sort of thing,using lines though intersections on a grid is a lot easier.

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### mc_rashSenior Member

Hi Thule,

the offsets for the europe dinghy might be confusing, they are polar coordinates and probabpy a little more labor is required than for a "normal" table of offsets with cartesian coordinates.

You are right, the x-value is the position of the station measured from the transom.
The z-value is the height of the deck above the baseline (the hull is flipped upside down).
The y-value is the breadth of the deck.

The polar coordinates are measured from the z-value, given an angle and a length.
To convert this coordinates to a linesplan there are different methods, one would be to draw the lines from the table with the specified angle. If every line for one section is drawn connect the ends with a fair line.

Another method would require a little, simple math.
Using the length and the angle you can calculate the y-value with:
length*cos(x)
And the z-value:
length*sin(x)
which is basically converting the polar coordinates to cartesian coordinates.

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### wet feetSenior Member

That would imply a different origin for each station and there are lots of stations.It would be more practical to use a common origin point,at least when working on a lofting floor,as otherwise there would be lots of diagonals and a high possibility of confusion when setting out the sections.Such things can happen,Atwood Manley's book about Rushton canoes presents hull lines in a rather peculiar way too.

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### mc_rashSenior Member

The all originate indeed from one origin (0,0,0). Pardon.

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### wet feetSenior Member

I agree that they are likely to originate from a common point.The 90 degree figure is the baffling aspect,is this the height of the sheer?A scan of the plans would be very helpful if the copyright holder would permit it.

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### mc_rashSenior Member

The 90° coordinates are the keel.
The z-value is the deckheight.
The y-coordinate (or 0°) the deckbreadth.

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### mc_rashSenior Member

This is station 1. I can't fair the line since I'm on my mobile but I think you'll get the idea.

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9. Joined: Aug 2020
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### mc_rashSenior Member

There are lineplans for the europe dinghy somewhere on the internet, but it might improve Thule's learning process if he tries to draw the lines from the offsets he has before he sees the lineplan and how it should look like. So I won't post a link here.

If you want us to take a look over your half cooked pasta don't hesitate to post a picture of it.

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### wet feetSenior Member

I'm extrapolating a bit but I imagine the vertical datum point is the top of the stem as I have a recollection of my Moth being limited to a length of 3355mm max and the location of Station 1 in this table is 3250 from datum.I hope I have attached a screenshot showing how Station 6 appears when drawn this way.

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### mc_rashSenior Member

Yes, the origin is at the top of the stem. The european dinghy class society has extra files for the stemdesign etc.

12. Joined: Apr 2023
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### ThuleJunior Member

Thanks Wet Feet and mc_rash.

So, reading from the replies, hull is upside down, x is the distance from transom, y and z give breadth and height of the deck at that station, above the baseline. However, the 0,0 is still from the base line. The diagonals all indicate the angle and distance of the hull intersection at that angle? I will add the station 1 here. [Hope the formatting stays!]

station 1:

x z y 10d 20d 30d 40d 50d 60d 70d 80d 90d
3250,0 5,5 81,0 81,5 84,5 90,5 100,0 116,0 141,0 184,5 266,0 399,0

Now, looking at the station 1, are all the numbers #,# as in two numbers separated by a comma like a coordinate in 2D right. Or, are they the European currency style 5,5 really means 5.5 decimal?

In either case, what will "x" be for cos(x) or sin(x)? For eg: at station 1, at 90d, what should the "x" be and the length be, to convert the polar coordinates to cartesian?

Thanks

Last edited: Aug 14, 2023
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### mc_rashSenior Member

I assume that wet feet's diagonals are a little bit off topic since you want to create a line plan based on table of offsets. The diagonals wet feet mentioned are not aplicable in your case.

Yes, the diagonals in your case are represented by a line with a specific angle.

The coordinates are SEPERATED by a comma (in 2D "x, y" and in 3D "x, y, z") a decimal would be with a . (e.g. 0.5)

For cos(x) and sin(x) you insert angle x (I should have used a different letter for better understanding), basically the angle 10°, 20°, ..., 90°.

14. Joined: Apr 2023
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### ThuleJunior Member

ok. So,
station 1, at 90 degrees will be
y coordinate = length * cos(90)
z coordinate = length * sin(90)
Now, what do I use for length? Is it (399,0 - 0,0) as a vector magnitude or the 399,0 are the y, z lengths respectively i:e;

y= 399*cos(90) and
z=0*sin(90)

Apologize if the questions are basic. Math isn't my strength

Thanks

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### mc_rashSenior Member

Since the x-coordinate is the same for the offsets of one station you only have the y- and z-components.

For station 1 it would be (I'm switching columns to rows for better reading)"

Position - z (Height, mm) - y (Offset, mm)
Deck - 5.5 - 81.0
10° - 14.15 [calculated with 81.5*sin(10)] - 80.26 [81.5*cos(10)]
20° - 28.9 - 79.4
etc..

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