# Bending moment calculation

Discussion in 'Boat Design' started by Xlow3636, Jul 18, 2014.

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### Xlow3636Junior Member

Hi guys;

I 'am working on a project were we are going to transport heavy cargo with some boats.
The noble Denton Said that the bending moment for seagonin should be within the allowable seagoing limits but he doesn't specifies how the bending moment calculations should be done (Still water static , Wave dynamic and for whitch wave)
What should i do? is there any other regulation that explains more?

thank you for your help

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### TANSLSenior Member

You must study both cases and check the worst.
Consider, for example, a trochoidal wave of wavelength equal to the ship length and height equal to the shipÂ´s depth..

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### Xlow3636Junior Member

Thank you for your respond TANSL .
In fact i already done this kind of study but the results are always higher than the allowable seagoing limits even with light cargo so i was wondering if the seagoing limits are maybe only for stillwater check !!
Or maybe there is other limits for the dynamic study but aren't in the stability booklet

Thank you alot for tour help

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### TANSLSenior Member

Studies requested with Classification Societies are with the boat on a wave as I have indicated.
If the vessel does not meet the minimum, you have no choice but to increase the scantlings in the vicinity of 0.4 L in the center. It may be sufficient to add 1 or 2 strong longitudinal members (deck longitudinals and/or side stringers), if you want maintain current scantlings

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### Xlow3636Junior Member

thank so much for your help TANSL

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### PetrosSenior Member

if you are talking using existing boats, the design should specify the maximum allowable cargo (or displacement) weight, and in what kind of sea conditions (in land waters, coastal, off shore, etc). If they are of unknown design origin, than you will need a design professional with experience in exactly this kind of marine survey and analysis.

If you are building barges to move goods, the best approach would be to get the "rules" for that class and size of hull and just follow the sizing tables for hull thickness, size and spacing of frames, etc. You can build to any recognized standard such as Lloyds, German Lloyds, EU specs, or USCG rules.

Doing actual load calculations is a complex process best left to professionals, if you have the budget for that than hire an expert. It might save you money in the long run since you will likely save materials cost over using the perspective tables from the Class Rules.

Perhaps if you share more details of exactly what you need to accomplish (size and weight, and shape of the cargo), type of seas (inland, coastal, open ocean), what hulls you have available, or what building materials, or what ever, the experienced people on this forum could advise you better with specific advise.

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### Xlow3636Junior Member

Hi Petros

The Goal of my study is to check the feasibility of lifting 2 cargo of 3000 T on a existing boat (Mega Caravan) from Asia to Russia.
All the rules I founded said that the bending moment should be calculated for a wave Lpp length and 061*SQRT(Lpp) height.
the problem is the the boat is designed to lift 3 3000T cargo and all other Rules (GZ, Wind, Still water bending ...) can be checked but not the quasi-static wave bending moment.
So I was wondering maybe i don't need to check this criteria or maybe to check it for a smaller wave

Thanks for the help

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### TANSLSenior Member

It is a typical calculation of a beam subjected to pure bending.
On the one hand is the weight of the beam and all loads, punctual or continuous, that is on it.
Furthermore, to the other direction, is the force of the water. The profile of the wave (trochoidal, for example) allow you to calculate how the boat floats, the existing draft at each point of their length and therefore the upward force produced by the wave at every point.
This study will give a curve for the distribution of bending along the hull girder moment. The maximum bending moment must be within the limits indicated by the CS.
Then there are the studies of stability, but for them, I guess, do not ask that you study on a wave. To my knowledge, stability is always studied on a flat sea.

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### Xlow3636Junior Member

Thanks alot for the help it's very clear now.
Last question CS is an abbreviation for what ?

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### TANSLSenior Member

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