# Which method is better for calculating shear force and bending moment of ship by usi

Discussion in 'Boat Design' started by xichyu, May 18, 2016.

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### latestarterSenior Member

The SF not being zero is due to the difference between the stated deadweight of 11777 not matching the sum of the weights in column 4 which I make to be 11569 so an arithmetic error has occurred or a weight missed out.

If this is a question to answer, have you been told to assume a uniform cross section and even distribution of lightship weight for simplicity?

I note that the forward draft and aft draft are different so the buoyancy of the sections you are considering will need to be adjusted to take account of the increase/decrease in depth of water.

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### gonzoSenior Member

Zero is the theoretical goal. However, you usually end up with a low value, which is OK. There are always errors introduced by approximations and rounding up decimals.

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### TANSLSenior Member

Sorry to disagree with this. In the case under study, the BM curve always has a point with a maximum value in which the curve changes the sign of its tangent (the curve goes from increasing to decreasing, or vice versa), and at that point , the SF MUST be zero, not approximately zero, but zero. It's not a matter of rounding decimals. Finding the exact point for SF = 0 is another problem.

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### latestarterSenior Member

TANSL, the problem Alexsey is asking about is with the SF at the end, not in the middle where the max BM occurs.
The end should have a theoretical value of zero.
By amending the weights in column 4 to match the deadweight of 11777 as I suggested in post 46, it will reduce the difference from 193 to 15. The 15 is due to rounding errors.

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### TANSLSenior Member

@latestarter, with all due respect, I believe that SF in the extremes can not be null. I hope I'm not mistaken again.

Edited : I have to correct my statement. At the ends, where there is no weight nor buoyancy, of course, BM, SF and everything is null. I had not taken this into account, which is totally obvious.
Of course there may be, and there are always, rounding errors, but when the SF must be zero, it can not be close to zero.
Another obviousness that must be checked: in the equilibrium position, the resultant of the weights must be equal and opposite sign that the result of the buoyancies.

Last edited: Sep 3, 2017
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### latestarterSenior Member

You are right if we are considering a real ship, obviously the overhangs will create a shear force and bending moment, as they extend beyond the waterline.
I have assumed this is a question set for students learning the basics of weight distribution, buoyancy and the resultant SF and BM calculations.
It is a simplified situation to focus on the essentials. There is not enough data provided to do a proper analysis.

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### TANSLSenior Member

Although it is a theoretical exercise, for purely academic purposes, the concepts are the same and, unless the curve of bending moments is totally horizontal, SF can not be zero. The SF is null when the slope of the BM curve changes sign and this occurs at the very ends (there is no BM) and when the curve reaches a maximum or a minimum. These are the concepts, the same for a real boat as for an academic exercise.
Cheers

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### AlekseyJunior Member

For me is important, that senior members advise to me if my calculation of SА and BM is OK and correct

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### AlekseyJunior Member

In this book only integrals which is not so easy for me

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### TANSLSenior Member

Forget the book and calculate, first, the curves of weights and buoyancy distribution. Then check that the resulting (the sum, ie, the integral) of both curves have the same value.
Note that I do not speak of calculating weight and buoyancy in a series of points but in calculating the curve that passes through each of those points, that is, the distribution along the length of the boat. The curve of the buoyancy can be calculated by means of the curve of the areas of frames (the portion of frame submerged in each point of the boat)

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### AlekseyJunior Member

For this I need to use Bonjean tables ?

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### TANSLSenior Member

Yes, it is a procedure, if you have calculated them previously.

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### AlekseyJunior Member

I have Bonjean tables in stability booklet

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### TANSLSenior Member

If the boat has a certain trim or you are considering the boat in the wave, you must calculate the draft in each frame to take the area of each of them to that draft.

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### AlekseyJunior Member

Is there some description with example how to do this? May be some simple book without integrals ?

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