# area under gz curve

Discussion in 'Boat Design' started by s.Angel, Oct 19, 2011.

1. Joined: May 2010
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### s.AngelJunior Member

Hi

I want to Determine VCG of my jet boat.

Specifications:

Type: mono planing hull
Length:4.5 meter
Weight: 900 kg
Max speed: 40 knot

And my question:
Is there a standard for jet boats to the area under GZ curve ?

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### erik818Senior Member

S.Angel,
I don't know about standards for the area under the GZ curve, so I'll adress the issue of establishing VCG. The obvius way to do it during design is via the weight budget and CAD-computed CoG for the hull itself, so I assume you want to establish VCG for an existing boat.

Wich way to do it depends on what you already know about the boat. If you don't know enough about the boat you can always measure the location of the CoG.
- Prepare straps around the boat so you can lift it with a crane.
- The center of gravity is always located on a vertical line from the point where the lifting lines to the the boat come together, usually at the hook of the crane.
- If you repeat the lifting a few times, with the boat tilted differently, you will get several vertical lines from the hook through the boat.
- The center of gravity is where the vertical lines intersekt.

I don't know how you are going to register the vertical lines through the boat. When I had to suggest a similar measurement on a land vehicle it was sufficient to use a laser beam as the vertical line, and take a foto of the point were it hit the roof plus measure tilt and pitch of the vehicle. With the help of drawings (CAD model in our case) the CoG could be figured out.

Edit: If you have a horizontal surface in the boat, e.g. the sole, you can easily figure out the vertical center of gravity if you note where the laser dot is when the boat hangs horizontally and where the dot is when the boat hangs at some roll angle (that you have to measure).

Erik

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3. Joined: May 2010
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Location: roma

### s.AngelJunior Member

Thank s Erik for your attention.
I can find center of gravity of my boat, but i want to know position of optimum VCG for better stability. please help me

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### BTG YACHT DSGN-sailing is believing-

As far as I understand, you are asking about a complex problem... Optimum VCG for any powerboat is derived from a calculation of equilibrium trim angle at given boat speed. The routine I'm talking about is in Larsson's ,,Principles of boat design". Please refer to the book.

VCG is one of the parameters taken into account when determining this angle. In order to avoid bow pitching or too big transom submersion the VCG should be optimised. These are the guidelines. For detailed descrition please refer to ,,PoBD"

Regards,

Jakub

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