# Ships lengthening

Discussion in 'Hydrodynamics and Aerodynamics' started by Alexanov, Jun 6, 2020.

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### AlexanovSenior Member

How to predict speed of lengthen vessel if original vessel 1o9 m speed was 12 knots and we add 3om in parallel midbody? Engine and propulsion same as for original ship.

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### AlikSenior Member

I would do it this way:
- calculate resistance by existing method, say Holtrop,
- 'anchor' these results to original ship performance - say, by coefficients and correction factors
- calculate the new ship using Holtrop method and correction factors from old ship

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3. Joined: Feb 2003
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### AlexanovSenior Member

I think about something like that... thanks.

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What method/procedure did you use to calculate the resistance of the original 109m length ship?

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### TANSLSenior Member

If you use the Holtrop-Mennen method you must recalculate the following values:
• Waterline length
• Length between Perp.
• Moulded Draft at Lpp / 2
• Displacement
• Longitudinal C.of B. from AP
• Center of bulb area above keel line
• Waterplane area coefficient
• Prismatic coefficient
• Transom submerged area
• Hull wetted surface
• Bilge keels area
In other words, there is no way to extrapolate the results from the initial boat to the new boat. You must perform the calculation as if it were a new boat.

Last edited: Jun 7, 2020
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### AlexanovSenior Member

CFD calculations.

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Then why don't you simply rerun the CFD?

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### AlexanovSenior Member

Finally we decided to calculate CFD for short vessel at required speed and long one with the same power to get new speed.
Probably it will be most correct method.

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That depends if you validated your first set of CFD results.

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### AlikSenior Member

I would not trust any CFD only. Systematic series or regression methods, then CFD. Usually, if there is no tan test data, we do resistance curves by 2-4 methods (regression of systematic series), then CFD to finalize, say, stern shape.

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### AlexanovSenior Member

We compared resistance value from CFD and several other sources. Itâ€™s quite close. Usually resistance prediction for ships with length/beam > 7 works very well in many different methods.

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