# Diagonals And Offset Tables

Discussion in 'Boatbuilding' started by BASIL J WALL, Aug 8, 2012.

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### BASIL J WALLdesigner

Hi...
I have a quick question for those who use offset tables to loft a hull...
What format do you like in the table for the diagonals data?...
Do you use the height and half breadths to the point of intersection of the diagonal and the station or do you use the angle of the diagonal and its origin point and then use the length along the diagonal to the intersection of the diagonal and the station?....Or perhaps some other format...
Basil

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### gonzoSenior Member

I use both to compare. Then when you loft the lines, they most probably not intersect. That is the point when fairing comes in. You need a good eye and the ability to visualize what the lines on a flat surface mean when they get translated to 3D.

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### jehardimanSenior Member

The diagonal planes are placed in the section view and the perfered method to start the cutting plane at the intersection of a waterline and the centerline and the angle is defined by the intersection of a buttock and the baseline or another waterline. The diagonal lines themselves are drawn opposite the waterlines in the plan view. See the figure below for an example.

In the Table of Offsets, the diagonals are listed by line title and station. The distance in the table is from the Centerline to the section line whereever the diagonal falls on the section line. There is no need for any other info unless you construct the angle in some obscure method. In that case you need seperate note in the section plan to lay out the diagonal cutting plane. See the offset table for the above hull below.

http://www.heritech.com/seawitch/plans/1956/1956_offsets.jpg

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### BASIL J WALLdesigner

Thanks for this...it helps
Basil

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### PAUL XAVIERJunior Member

@JEHERDIMAN: Sir, whether the bilge diagonal a mirror image of the waterline? What i meant is if we take diagonal 1 to consideration, is it a mirror image of any of the waterline? TY

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### jehardimanSenior Member

No, not generally. Everything depends on what type of hull you need and how you intend to power it. A racing sailboat has a very different hull than a cargo sailing ship which is different that a motor cargo vessel which has little in common with a modern cruise ship whos lines are nothing like a true ocean liners (and warships hulls are altogether different, again grouped around hull types selected for the task). So the relationship between the diagonal in the run and the waterlines aft change with the needs of the vessel.

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### rwatsonSenior Member

I remember a lively discussion on the 'standard' of allowing for 'planking' thickness on boat plans.

I note with interest that these plans stipulate measurements being the outside of the planking.

That would complicate the lofting process a bit.

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### jehardimanSenior Member

Not at all, the planking thickness is easy to deduct once you have the fair lines...You have to do it anyway for the intermediate sawn frames. There is a special lofting/drafting device called a perpendicular finder just for that. I have several of different sizes in my drafting tools.

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### rwatsonSenior Member

Youre the only one in the world that has one - according to Google.

Feel free to explain its usage and concept if you are up to it.

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### SukiSoloSenior Member

Well said about whether the offsets are to the inside or outside of planking/skin. Diagonals, too. I have known 'One Design' sailboats not measure because the builder did not check this basic information....

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### jehardimanSenior Member

Nooooo.....You can't mean that! You say Google can't always give you a correct answer! LoL....

I assure you I'm not the only one in the world who has some of these things, though I will admit you generally have to make them yourself. So there are some things, like trade and fabrication skills that can only be learned and discovered by doing. Reach back to your geometry class and remember how to find the perpendicular to a curve. It will become obvious from that how to make one....teach a man to fish and all that....

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### rwatsonSenior Member

All right, prolong the mystery if you want.

The easiest way I have produced an offset to a frame, is simply a triangle with two edges that touch the outside line.

But, I foresee trickiness the planking varies in thickness from the bottom to the sides, and its not a hard chine design,

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### jehardimanSenior Member

Incorrect...While using, I assume a compass, to sweep an isosceles triangle inward from the curve will give an offset... the apex is neither perpendicular to a given point nor is it a constant distance (i.e. plank thickness) from the curve. But sweeping arcs with a compass is moving in the right direction...you just need to ensure that they pick up the perpendicular at the correct point. I'll draw up the solution tomorrow...and the illustration may not be suitable for a work environment...given the other name for the "tool".

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### SukiSoloSenior Member

Always nice to see a bit of drafting skill......

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### jehardimanSenior Member

Ok, here is the solution to constructing a perpendicular finder.

Note that I have shown the construction exaggurated and on a changeing curve to show how it works and bring out where a very very small error could creep in. In reality, you would not be able to measure it as long as Circle 4 & 5 are small compared to the radius of curvature of the hull which is why you need different size ones...tiny for drawings and large for the loft floor. You can construct the tool on a straight line or the radius of a large circle. Most ones for drawings are built flat and those for loft floors have depth with a gap for the batten so you can butt it right up against the sprung batten.

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