# Positioning x,y,z cordinates for stability calcs

Discussion in 'Stability' started by Jeandupont, Nov 8, 2011.

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### JeandupontDesigner/Boatbuilder

Hi,

How i positioning in boat top view the x,y,z axes for i find the centre of equipaments for stability distribution calcs.

example: x axes for BE and y axes for centre line......

Thanks,
Jean Dupont
Portofino Yachts
www.portofinoyachts.com.br

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### jehardimanSenior Member

For the weight report, there are a couple of common conventions. Generaly the axis are selected so that a positive longitudinal moment generates a pitch up, a positive transverse moment generates a starboard down moment. Two common systems in the US are:

0,0,0 is located at midships, centerline, baseline. X is positive forward, y is positive port, z is positive up ( i.e. z up right hand rule).

0,0,0 is located at the forward perpendicular, centerline, baseline. X is positive aft, y is positive port, z positive up (a z up left hand rule)

Some European systems start from the aft perpendicular using a z up right hand rule.

For dynamics, the common convention in the US and Europe is:

0,0,0 is located at the CG, X is positive fwd, y is positive starboard, z is postive down (i.e. z down right hand rule).

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### JeandupontDesigner/Boatbuilder

Thank you for answering Jehardiman! it really helped me..

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### yuyewuyuconanJunior Member

i have saw y is positive forward yet.
So unnormal cordinates can be used,but is not a good idea.

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### Pascal WarinJunior Member

Usually Y is for transverse.

British decided to put it positive on starboard which usually create an indirect system.

In modern calculation tools it is usually positive on port.

But it does not really matters as soon as you are cleraly stating which convention you will use.

I also saw vertical origin at summer draught level and positive downward which is one of the most strange and unpleasant idea.

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### HakimKlunkerAndreas der Juengere

As long as metric and imperial system is both used, I see no sense in standardizing axis'.
Make sure that the reference plane (or point) is specified and the rest is common sense

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### DCockeySenior Member

Why not have a common axis system? Then all that's required to go between metric and "imperial" is simple multiplication/division of the coordinates.

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### HakimKlunkerAndreas der Juengere

I agree. But back in Germany we drew boats with the bow to the right.
Meanwhile I see (saw) many drawings the other way round. It seems that everyone does what he wants to and that there is not really a will to submit to a standard unless it is the own standard.
(To quote Miterrand: ' how can you run a country that knows more than 400 ways of making cheese! ' / In boat building it looks similar I guess )

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### DCockeySenior Member

Direction the drawing faces has nothing to do with metric vs imperial.

The direction the bow faces on a drawing isn't important for calculations. What is important is the location of the coordinate origin, and the positive direction of the axis. Another recent thread had of discussion of the some of the various standards used for different types of boats in different parts of the world.

I worked in the auto industry designing cars. One standard we adopted was to position the coordinate origin under the ground, ahead of the car on the centerline of the car. That made the longitudinal coordinate value and the vertical coordinate value always positive. The transverse coordinate value sign depended on whether it was on the right or left side of the car.

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### HakimKlunkerAndreas der Juengere

Drop the metric question - it was just an Hors d'Oeuvre...
Direction: Indeed not important for the calculations. But when we talk about standards we should not patch one here and one there only.
I find it very handy to see that my colleagues 200 years ago drew it the same way I do it today - it makes things easy to find.

If there is one day a global agreement of where to place a reference point - we will see. That will be decided by some committee. And I am not important enough to be one of them, so all I can do is watch and adjust myself. But I would certainly like to see an agreement that matches my own procedure.

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### AlikSenior Member

We use origin of x,y,z at fwd perpendicular (FP) and consider it more comfortable. Why? For drawing reasons: bow is always fixed but stern getting trimmed for transom during process, and also some designs being extended having transom extension in mold, etc. Then, there might be different transom versions for different types of drives. Thus we place origin at FP and there is no problem later re-calculate table of weights and hydrostatics during design and modifications. Using middle of AP would require re-calculation every time.

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### DCockeySenior Member

What do you use for the vertical position of the origin? My preference is below the bottom of the boat so the vertical coordinate values are always positive.

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### AlikSenior Member

Depends on design. Sometimes at keel (from planing boats), sometimes at DWL.

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### HakimKlunkerAndreas der Juengere

Makes sense. But still is variable in procedure and not really a standard used by all...

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### AlikSenior Member

It is standard in our office, so the guys doing weight calcs know exactly where they start.

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