# Force diagram for a vessel loaded with concrete block cargo

Discussion in 'Stability' started by Adeyele, Jun 19, 2012.

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Dear all,

Please i will like to know what will happen if a vessel will hog or sag? when loaded with a concrete block to thier maximum load draft.

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### daiquiriEngineering and Design

In short - it will sag.

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Many thanks.

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### DCockeySenior Member

It depends on the longitudinal distribution of the cargo compared to the longitudinal distribution of the buoyancy. There is an unstated assumption in the answer above.

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### daiquiriEngineering and Design

It's a cargo vessel, loaded with a concrete block. And it brings the vessel to its maximum loaded draft. In how many positions can it be placed to load the vessel in such way?

Cheers

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The boat is about 31M LOA. The cargo is loaded to abt 21M of the ships lenght, lets assume it was on even keel, and it was loaded to max load draft.

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### gonzoSenior Member

It depends on the structural design. All vessels deform. As long as it is within acceptable limits, there is no problem.

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### DCockeySenior Member

The original question didn't provide any information about the vessel. I agree sag is more likely but there are exceptions, particulary when the question is "academic" and the instructor is trying to get the student to "think". In another post Adeyele said he was a student.

Given that information the answer would be sag.

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### klaasJunior Member

Well get a buoyancy curve and do the math's. That is.
Calculate the resulting load, create a shear force diagram and then calculate the bending moment.

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### mydauphinSenior Member

Concrete blocks are not really that heavy per volume. They also vary in weight depending on manufacturer and water content of concrete in them. So weigh a few blocks of the one you want to carry. There might be as much as a 20% difference.

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### DCockeySenior Member

Sounds like a good exercise for a student interested in learning. But not as easy as asking for the answer on the internet.

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### klaasJunior Member

That's true. However I am happy to explain how, and help somebody solving the question themselve, especially when they are still studying, for free.

Othwerwise I am happy to provide my services and answer the question straight at my regular consultancy rate.

By the way density of normal concrete is abt 2,3 (non compacted) -2,4 (compacted) tons/m3.

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### DCockeySenior Member

Yes, there is a big difference when a student or anyone seeking to learn asks for assistance with how to answer a technical question compared to asking the answer itself.

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### TANSLSenior Member

Infinite positions but to be prudent, let's say that "a lot".
Having said that, I have to add that our student's question, in my opinion, it makes no sense. It is absurd to argue about the answers that each one, assuming various scenarios, can give.
B.R.

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### davhillJunior Member

Given the information provided, and assuming the load is evenly supported, the hull will neither hog nor sag. Load is given as "a concrete block" which implies a single rigid rectangular form of consistent density; it is loaded in the center 2/3rds of the hull length, e.g. between apx 1/6 from each end. Hull is loaded to capacity so actual density is unimportant .. but rigid condition IS critical to the question. If the hull begins to sag, the load distribution changes to point loads at 1/6 and 5/6 length.. which halts the sag and produces a hogging force.. but when keel resumes contact with the (rigid) load, the weight distribution is restored and hogging ceases. The forces cancel each other out. No hog or sag.

This problem would be more difficult to solve if the load was "concrete blocks" instead of a single piece... but as given is quite simple.
Dave

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