# oil tanker and freighter stability

Discussion in 'Stability' started by Shallowatereds, Sep 26, 2008.

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### ShallowateredsJunior Member

What do you think the primary reason behind the seaworthiness and stability is for these types of vessels: sheer size or careful hull design? Or, is it a combination of the 2? The hulls arean't much more than long blocks with pointy ends.

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### Kay91600T Master

Stability is never an accident, nor is it constrained or helped by size. You can make a fully loaded tanker as unstable as a fully unloaded tanker any day of the week, and in any sea conditions.

There is a LOT of information available on web about ship stability. If you have any trouble finding it, come back here and Ill gladly post up some links for you to review.

K9

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### marshmatSenior Member

Well....
Firstly, welcome aboard bd.net and here's hoping you find this a useful diversion from real work.

I think the answer to what you're trying to figure out lies in the concept of relativity and similitude. (Not the theoretical physics kind, the "A is twice as long and 8 times heavier than B" kind.)

When you scale a boat by a factor of two in all dimensions:
- Distances are doubled
- Areas increase by a factor of 4 (quadratic relation)
- Volumes and weights increase by a factor of 8 (cubic relation)
- Initial stability increases by a factor of 16 (quartic relation)

So a big freighter, especially a big square-bottomed freighter (as most are), is proportionally a lot more stable than a small one. At least, at first. Its large weight and large righting moment, compared to the surface area it presents to wind or waves trying to tip it, make it very hard to capsize.

But what happens when you roll one a bit too far- and how far is too far? Tip a loaded container liner 50 or 60 degrees and it'll probably just keep on going over. There is a tradeoff between initial and ultimate stability- it is very hard to get both, at least not without a heavy, deep keel to bring the centre of gravity really low down.

Big ships aren't designed to handle being capsized- they're designed to carry as much cargo as possible, thus they have high initial stability. And the NA firm will have prepared a detailed book of tables and graphs indicating exactly what will happen to the stability curve under all conceivable loading conditions, so the captain can ensure the ship is loaded properly and is kept within her limits. All this works just fine as long as they can stay out of the kind of weather that will push them over the edge, and the ship isn't loaded in such a way as to compromise its intended performance.

If you were to scale a freighter hull, a cruising sailboat, and a Coast Guard launch to the same displacement, you'd find the freighter to be a miserable sea boat compared to the other two. Seaworthiness in these things comes, to a large degree, from careful loading and planning, sheer size, and the ability to go fast enough to dodge problematic conditions, while smaller boats subject to the same conditions must consider stability and seaworthiness in a very different light.

For more detail, if you want it, I'd refer you to any of the real experts (ie, not myself) on here or to any introductory text on yacht design.

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### ShallowateredsJunior Member

Thanks for the answers. I'm a long-time boater, and have built a couple of boats. I'm sketching out designs for a large trawler to start building relatively soon, in the hopes that it's ready when I retire.

I like the hull shape of an oil tanker, and believe that scaled down, it would make a very roomy live aboard. Something along the lines of 60x12, rear pilot house, 75hp John Deere swinging a big prop, hull speed in the 10-11kts range, long and clear fore deck, etc... I can do the topsides, interior, systems, etc.. but am certainly considering bottom design options.

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### marshmatSenior Member

Ahh, OK. So you're looking to live aboard in something roomy, that can still be a quick and reasonably comfortable sea boat?

Are you familiar with the Dashew boats? They're pricey- far too pricey for my tastes- but Steve is probably one of the most meticulous, most pragmatic designers out there right now. Even if it's not what you're thinking of, their 64' and 83' motoryachts are definitely worth looking at just for inspiration.... http://www.setsail.com/dashew/fpb64update-92208.html

A square-bottomed tanker hull scaled to 60' is likely to have a fairly snappy roll and a low angle of vanishing stability. If draught is a bigger concern than ocean ability, it could work, but I think if you want good seakeeping and speed in this size you might be better off looking at the kinds of hull shapes found on pilot boats, Coast Guard boats, the Dashew motoryachts, etc. That is, boats designed from the outset with this size range in mind.

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### ShallowateredsJunior Member

This is a Mirage N47...

I'd like to design and build a 60' version.

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### GuillermoIngeniero Naval

Well....That boat doesn't look to me like an oceanic trawler at all. Rather like a creeks creeper one.

Cheers.

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8. ### lazeyjackGuest

may I respectfully suggest you look at a few dutch brokerages, get some ideas of beautiful steel craft,, soften the lines, get rid of the sharpness
Always remember it costs as much to build a boat that is not so hansome, pretty beautiful, as a stunning eycatcher that will sell at 4 times the price when it comes time to quit

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### ShallowateredsJunior Member

Yup. I don't want to roam the oceans. I want to poke around the eastern seaboard, down thru Florida, across to the Abacos, down the Exumas, over to Puerto Rico, US & BVIs, and on to Grenada and such.

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### FAST FREDSenior Member

"down thru Florida, across to the Abacos, down the Exumas, over to Puerto Rico, US & BVIs, and on to Grenada and such."

I suggest you get an origional copy (the second edition is trash) of Bedee's book Passagemaking.

While many small sail boats have easily made the trip the Mona Passage is about 180 miles of REAL ocean. Not ever a place for the boat you posted , as even with a good weather window, a thunder storm could be disaster , never mind a Northerner.

"Something along the lines of 60x12, rear pilot house, 75hp John Deere swinging a big prop, hull speed in the 10-11kts range, long and clear fore deck, etc"

Most cruisers will choose to run at SL (the square rt of the lwl) times .9 to 1.1 for fuel costs. On a 60 ft lwl that's under 8K to be efficient & cheap.

If you prefer SL x 1.34 ("hull speed") your fuel consumption will at least doubble, not many folks chose this.

11K ??, install BIG, BIG tanks!! And have a thick thick wallet.

FF

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### GuillermoIngeniero Naval

Just an small correction: Beebe, not Bedee.
And the book: 'Voyaging Under Power'
Have a look at the book on-line.

Cheers.

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### ShallowateredsJunior Member

I'll pursue some other design ideas. Thanks.

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### GuillermoIngeniero Naval

A couple of final notes:

Tankers use to run at very low speed/length ratios and so their prismatic coefficient is pretty high. An efficient motor passagemaker will probably be run over twice the S/L ratio of a tanker, and its prismatic coeficient needs to be consequently much lower.

Also tankers have a very high displacement when compared with the small one of a sixty footer. However, if they both had the same hull shape, say the yacht is shaped like the tanker, the yachtâ€™s displacement would be very light, its beam and draught being only a fraction of what is needed for a proper sea going motor yacht.

This is only to talk about a couple of considerations a designer needs to take into account when designing a vessel. Clearly one shape does not fit all. Horses for courses.

Cheers.

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### PARYacht Designer/Builder

The primary reason for the square sectional shapes used in these vessels is their reaction in rolling seas, which is the least of any shaped used. It also has the additional benefit of being able to carry the maximum load, particularly when compared to more slack bilged shapes.

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