# Resistance Calculation of planing hulls

Discussion in 'Boat Design' started by Sumit, Feb 20, 2019.

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1. Joined: Feb 2019
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### SumitJunior Member

Hello!
I have modeled a planing hull model in paramarine, but when I carried out the resistance by Savitsky method in Paramarine I wasn't able to generate the required characteristic curve of (resistance/ weight) vs Speed.
The curve I got is more of a displacement type hull.
I dont have much data about the model as i have modeled it with reference to a paper.
Data like VCG,Distance from amidships etc are unknown to me.
What could be the reason behind this behavior. Can you suggest me some changes i need to make

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### DCockeySenior Member

The location of the Center of Gravity is an important parameter for planning hull performance. The resistance vs speed curve will be can vary significantly with variation in CG location. If you don't know the CG location then you need to estimate where it may be and use a range of values to understand the effects of CG location uncertainty.

Papers about hull resistance usually have the CG location described somewhere or provide a reference where it is described. Alternatively if you know the static waterline and have the hull shape then you can determine the longitudinal CG location. If you have any static stability information and the hull shape you can determine at least an estimate of the vertical CG location.

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### SumitJunior Member

Thank you so much for the prompt reply. I will try to iterate the results with different values of CG.

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### JSLSenior Member

Using insufficient input data will not give you good output data. ie: (GiGo- Garbage In- Garbage Out)
A couple of points:
(a) if a planing hull is improperly loaded or designed, the output can sometimes resemble that of a displacement hull. Other items that can affect things are variances of deadrise, chine beam, etc
(b) Depending on speed, your LCG (and the LCB) should be around 53% to 58% of the LWL aft of the FP (stn. 0 or 10)

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### DCockeySenior Member

What is this recommendation based on? Is it independent of the shape and length of the bow?

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### JSLSenior Member

What is this recommendation based on?
If you read any books or papers on planing hulls (Levi, Lord, SNAME etc) they address the LCG matter. It is also the LCG location for most (but not all) displacement hulls.

Is it independent of the shape and length of the bow?
yes, I think so but I am not quite sure what you mean here.????

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### Mr EfficiencySenior Member

Most planing hulls will run satisfactorily with the COG one-third of the total boat length forward of the stern.

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### KramericaNew Member

47% sounds very far forward of the transom for a planing craft and would be more within the semi-displacement range.

Sumit, generally a monohedron monohull will be 40% of the waterline length from the transom. Warped hulls are used to move the LCB further forward than this.

It is better practice to have the LCG further aft (to a limit) as trim control can be utilised. If it’s to far forward, only movement of weights aft may correct this which may not be possible if the vessel is already being built.

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### SumitJunior Member

I tried varying the position of LCG over the above mentioned percentages, But still I'm unable to get the curve of a planing hull

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### SumitJunior Member

Thank you, I will try this.

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### SumitJunior Member

Would it be possible for anyone to verify my model which i have modeled in Paramarine?
It would be of great help.

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### DCockeySenior Member

Does Paramarine have a module to calculate the resistance of planing hulls? Based on the Paramarine website it appears to be intended for use in designing ships, not planing boats.

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### AlikSenior Member

Another lazy software user is asking which key to press!
Go and study theory behind planing hull first. Larsson's 'Principles of Yacht Design' or 'Savitsky planing' are the key words to Google.

Ilan Voyager likes this.
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### JSLSenior Member

Thanks Alik -good call

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### SumitJunior Member

What is meant by "distance normal of keel to center of gravity" ?
Is this similar to VCG?

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