# Sectional Area Curve?

Discussion in 'Boat Design' started by mecmec, Aug 17, 2012.

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### mecmecJunior Member

sir, ma'am. can anyone teach me how to get the SAC using only the particulars? ima newbie here and currently doing my thesis. thanks! is there any site i can go or pdfs i can downlaoad.

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### jehardimanSenior Member

An area curve for what? They are different for different types of ships. Get a copy of Hydrodynamics in Ship Design by SNAME/Saunders. Basic idea is to make the area under the curve equal to the displacement with the maximum section and PMB placed properly for the speed/length ratio desired. Also see Taylor's The Speed and Powering of Ships.

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### mecmecJunior Member

SAC of Body plan sir, i mean. iwant to know where to start on how to form my body plan. ihave calculated my particulars. thanks sir! ill try to find that book.

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### mecmecJunior Member

its for a passenger vessel

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### jehardimanSenior Member

Ok it is all explained in Saunders and in Principles of Naval Architecture (SNAME) of Basic Ship Theory (Rawston and Tupper)

The length of the SAC is the LWL of your ship in length units. The maximum height of the SAC is the area of the maximum (midships) section in length ^2 units. The area under the curve (length*length^2 = length^3) is the volume of displacement. For a passenger ship the maximum section should be ~53-54% of LWL aft of the bow. There should be little to no PMB. The area of the midships section should be 80-85% of the beam (B) * the draft (T) with some deadrise and a good turn of the bilge. The fore and aft shapes of the SAC should approximate sine curves with the tangents of the SAC being ~19 degrees at the ends.

Getting a lines plan out of the SAC is a little bit tricky...you need to look at a lot of good ships to determine this. First decision is to make the forward floors flat, U or V.

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### TANSLSenior Member

The attached figure shows several SAC, for various depths, in a modern passenger ship.
The coefficient of the midship, in modern passenger ships can easily reach values ​​of 0.95.
Modern passenger ships often have no deadrise.
In my opinion, it is not possible to obtain reliable SAC based on the main dimensions. You can only deduct them ( ?????) by comparison with similar vessels.
I speak from my experience, knowing that I am not in possession of the truth.

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7. Joined: Aug 2004
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### jehardimanSenior Member

Depends on how you interpret "passenger vessel" For modern large (40,000 ton) cruise condos that need to get into shallow island ports they are flat with high midships coefficient, but that is because of thier CONOP, not good general design practice. For a mid size (5,000 ton) inter-islander passenger ferry that operates a point route with moderate depths, better economy can be found with some deadrise and slacker bilges. Then there is the whole lifecycle costs game in what skill to economy the building yard will play.

As is said about blind men and elephants...it is ones experience only if you aren't looking elsewhere. A naval architect's perception needs to be much broader than only what passes across his desk or else he will fall into self laid traps.

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### TANSLSenior Member

Hi jerhardiman.
I do not understand the tone "bitter" in your answer. Did I say something that bothers you, that makes you feel guilty, and you try to defend yourself with a good offense?.
I talked about my experience in passenger ships and I understand as "passenger ship" the same as you.
Regardless of the navigation zone, the size, the product life cycle, and any other consideration of any type and although I can be wrong, I dare to say that in the last 30 years should not have been built many passenger ships with deadrise. But that does not matter because, due to the short cylindrical body of existing vessels, its effect in propulsion can not be very important (in my opinion).
I respect you, but I disagree with your explanation of the SAC, and I would like you also respect me and will avoid opinions about me. Thanks.

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### jehardimanSenior Member

I'm sorry if my last post seemed bitter or antagonistic, it was not intended to but you have to start from a SAC if trying to hit a tonnage mark. In pre-design all you need is a cargo block, limiting principle dimensions, a SAC, and a midships section type to start a ship design. A thesis study is not, and should not be, constrained by what was done in the past.
But TANSL, where we differ is you say "not many" with deadrise, yet the vast majority of "passenger vessels" in the last 30 years are well under 1000 tons, i.e. fast ferries and the like, and most of those have deadrise, cats or not. While I agree that most new large cruise ships have flat floors, that is not optimum and driven by the need to enter shallow ports, not economy at speed. When you consider that most all the high speed liners (1920-1960) when economy was at stake had some deadrise and powering data shows that deadrise reduces drag, removing the draft restriction should drive an increase in deadrise. A "passenger vessel", not constrained by cargo block or draft, should, by good practice, have some deadrise built into it.

Copying a niche craft is not where a thesis should be going...it should diverge into new areas...but it needs to start from the solid middle, not the edge of the envelope.

Anyway, mecmec, see my post in this thread...http://www.boatdesign.net/forums/boat-design/lines-plan-29882.html

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### mecmecJunior Member

so very thanks jehardiman! ill study all of this for sure! hehe thanks for the sources youve given me.. thank you so much! gotta work now, so many things to read and study!

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### mecmecJunior Member

i wanna learn more sir. am i on a right track? is the body plan first i should draw? imma lil bit confused.. XD

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### mecmecJunior Member

what do you mean by "The area under the curve (length*length^2 = length^3) is the volume of displacement."

i have calculated my displacement already sir.

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### mecmecJunior Member

sir, is there anyway i can start drawing my bodyplan? im stuck with my set of particulars.

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### TANSLSenior Member

You can get as many answers to your question as the answer people. Each one will expressed his opinion and all, sure, will be correct.
I would start by obtaining the body plan of a similar vessel and, modifying it gradually, achieve the necessary hull.
If you notify us what boat you want to project, it is possible for someone to have, and borrow you the lines to get started.

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### mecmecJunior Member

can i borrow ur lines of a passenger? if you wouldnt mind. if not, its ok.. still thank you for the infos

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