# Weight Distribution Diagram

Discussion in 'Boat Design' started by LePrince, Sep 26, 2017.

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### LePrinceJunior Member

Dear all

i want to calculate and draw weight distribution diagram of 85 meter length pontoon

it's longitudinally framed so the weight distribution of most members will be (member weight/member length)

my question is how to deal with transverse bulkheads and fore and aft bulkheads ?

should i calculate them as concentrated load ?

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### gonzoSenior Member

Fore and aft bulkheads would not be a concentrated, but a distributed load. However, it is common to divide the boat into sections and calculate the weight as a concentrated load at the stations. It simplifies calculations.

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### TANSLSenior Member

The weight of the transverse bulkheads should be considered as a concentrated weight in the corresponding frame. There are no simplifications here. For what? It is a weight placed in a single frame, simple. Another thing would be different if you had a corrugated bulkhead in which case you would have to distribute its weight between the two frames that support it.

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### JSLSenior Member

I assume this weight estimate (diagram?) is for hydrostatic calculations input for vessel conditions (draft, trim, stability, etc)
If for bending moment diagram you will still need the same input.
What is the material..steel?

Last edited: Sep 26, 2017
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### TANSLSenior Member

If I am not mistaken, calculations of the hydrostatic values need not know the weight of the ship and its distribution.

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### JSLSenior Member

Thanks Tansl- you are correct. Mine was a quick response which I have edited (above).

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### LePrinceJunior Member

i need the load distribution diagram for bending moment calculations

so i'll go with TANSL opinion by considering transverse bulkheads as concentrated load

anyway is there any software i could use to get this diagram accurately ??

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### TANSLSenior Member

We agree, @LePrince, the law of distribution of bending moments generated by a point load on a beam is totally different from that of a uniformly distributed load. Therefore the results are totally different. In a case as clear as that of transverse bulkheads, doing something else is more laborious and, to top it all off, totally wrong. Judge whether other methods are worth using.
As far as I know there is no software that generates the distribution of the lightship weight because for this it should take data of the structure, its elements and its weights, in each zone. I think there is no choice but to do it by hand, with the help of a spreadsheet or similar program. There are programs, or can be created with little effort, that distribute the load, or the weight of the liquids in tanks, between several frames. There are also programs that calculate the curve of distribution of buoyancy in function of the displacement and the trim of the boat.

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### LePrinceJunior Member

Thanks alot @TANSL

does the same thing go with web frames ?

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### TANSLSenior Member

No, I think with web frames it is not necessary because its weight is much smaller than that of a transverse bulkhead. But if you have enough time and patience, the more detail the weight distribution curve has the better.

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### gonzoSenior Member

I disagree, since the OP is not calculating bending moments. He is asking for weight distribution.

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### TANSLSenior Member

I know that the OP is trying to calculate and draw weights distribution, not bending moments.
It seems to me very well that you disagree, rather, I do not care. I know what I am talking about and I see that you have not understood my reasoning, which does not surprise me, and I also know that what you propose, to distribute the weight of the transverse bulkheads in several frames, is incorrect. There is no justification for doing that "simplification" which merely distorts the weight distribution curve. Why that distortion occurs is what I explained in my previous post but I see that you did not understand it. Sorry, I do not know how to explain it in a simpler way, although I do not think there is another.
I think the OP has fully understood my point. Do not let your comments confuse him, try to make the task easier, do not complicate it more.

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If you are doing this correctly, there is no difference, per se. Since the weight rate curve takes all structure into account. A WTB has a weight over an infinitely small length, longitudinally - thus the rate is disproportionately high and yields a very high peak. But over 1 frame bay, is low, and blends into the main structure, in terms of weight.

This is of course very different from say a main engine, these are treated separately as lump weights. This is a lump mass over a finite length. But the length is not finite in the same sense as a WTB, which is ostensibly the thickness of the web frame. So the weight/length for a WTB is totally different for an engine, despite being heavier!

So if you are calculating BM for the pontoon, the structure curve should be very straight fwd and simple and follow the typical trapezoidal shapes that result.

As for software, there is always softwre for "something"..but 1) it costs money and 2) how accurate is it and 3) why not do it by hand, it is not difficult. Just takes a bit of time.

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### TANSLSenior Member

I think it is convenient to clarify so that the OP does not feel more confused, that is to draw a curve that in abscissa has the linear meters of the length of the ship and in ordinates has the weights per linear meter of the structure. Therefore, it is not a matter of representing the weight at one point but the weight per linear meter in each frame. Just as the weight of the engine is distributed in all the meters it occupies (and what is represented is therefore the weight/meter due to the engine), the weight of the bulkhead is distributed in a linear meter and that figure is assigned to the specific frame in which it is located.

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### gonzoSenior Member

Engine weights are calculated at the CG or at the engine mounts. Unless you have a curve for the distributed weight of every component, your method in nonsense.

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