How can I obtain the longitudinal strength of a ship？?

Welcome to Fluther.

Look, I’ve been around boats and sailed for nearly all of my life. I love boats (though I’ve never built one). But I have no idea what it is you’re asking here. I’m not the only sailor in the forum, and there are certainly better engineers (though I doubt if there are any naval architects, which is what you’re really interested in, I think). So maybe I’ll be surprised.

Are you talking about the ability of the ship to withstand the sheer type forces that can break its back – its keel, for example, when it grounds on a reef (as your other question asks)? Or its ability to withstand torsion (twisting) of the hull in heavy weather? Or its tension strength – which is kind of a silly question, I suppose, because what forces act to pull a ship’s bow from its stern? Or how heavily it can be loaded? Or what, exactly?

Aside from those questions about your question, while we frequently love a technical question that’s outside of our normal experience and expertise, this one just seems so far outside that I’ll be surprised if you get another answer on the topic. Especially since you’ve given us no details to go on, such as the size of the ship, whether it’s a modern steel-hulled ship (and then, what type? cruise liner, bulk carrier, container or other cargo ship, tanker or warship), or an old-fashioned wooden (or steel) sailing vessel, or what.

What, exactly, are you looking for here?

As to a general answer, I guess I would say “consult a naval architect or marine engineer with a very specific question about what it is that you’re trying to learn”.

how long is a piece of string? this question, and the other like it, makes no sense.

Honestly, what I would do is call the manufacturer. It certainly can be calculated but there are a lot of variables that need to be considered like loading, hull type strength of materials,,, it’s not a straightforward calculation and it would be really easy to screw up so don’t risk it. Maybe start by stating why you need this information could help.

Sounds like an Engineering homework problem. If so, I would calculate based on geometry and material thicknesses, plus obtain strength of the material used.

http://screenshot.net/zkxwxh7

You will need at least:
1) assume a material and material thickness. It’s a hollow rectangle, right?
2) use Mc/I
3) look up loading conditions in Roark
4) they are looking for max F?

This will take a while to solve.

Okay, now I see the screenshot. But that doesn’t illustrate what your question asked. The illustration seems to be looking for a solution to F at the quarter-point in the vessel’s length. And that shouldn’t be too awfully difficult to calculate – assuming the ship is floating in water. Since the ship will displace its weight in water, we can figure what G is, and F will be exactly equal to that. (At least, F and G will balance under static conditions when the ship floats.) How to figure F at a given point on the hull is a new question that I’d have to think about.

But the ship’s strength, as you requested, will depend upon the combined factors of the ship’s design (the ship shape, if you will pardon the pun) and method of construction, the type of fasteners used, the thickness of the hull and condition of the materials, plus the workmanship, minus whatever allowance has to be made for decay, erosion and imperfections in construction. That’s not what the illustration shows.

The illustration seems to be a relatively simple force vector diagram (and even that is incomplete, as there will be lateral forces acting on the hull in the water, too, as well as the surging hydraulic action of waves, currents and tides).