# Savitksy method catamaran

Discussion in 'Boat Design' started by TobiasJohan, Oct 17, 2014.

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### TobiasJohanNew Member

Hi
I wrote the savitksy method in excel for a monohull according to Savitsky's 1964 paper. I have to adapt it now for a catamaran.
what do I have to change?
Thank you

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### DCockeySenior Member

Two hulls instead of one, or analyze one hull using half the total weight.

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### AlikSenior Member

There are two approaches:

1. Split-hull catamaran, this is monohull sliced at CL and separated by the tunnel. In this case, can use Savitsky procedure if one just bring it back to monohull. So chine beam would be double of demihull beam. We assume that this type of hull is used mostly for high speed, and interaction of hull is negligible due to flat inner topsides of the tunnel.

2. Symmetrical catamaran. We do it this way: calculate one hull using its parameters and 1/2 of total displacement of the craft, and then double the resistance. At slower speeds we need to add interaction factor that can be taken from Sherman, Doubrovsky, Molland, Morabito and others.

3. Important for 1 and 2 is additional friction/resistance in the tunnel, due to wet surfaces. We used to evaluate this as 'additional friction' and use observations of tunnel flow on our cats. Though on properly designed cat (unless it is heavy lading craft with load) this factor should be kept to minimum, so the cat should not ride on tunnel surface.

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### TobiasJohanNew Member

Thank you.
I have an asymetric catamaran.
The interference factor is negligable-tunnel width is 2b
Morabito state that the lift equation changes?

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### AlikSenior Member

I am looking at this from practical side. Yes, Mike told me about this finding, but we used to calculate split hull just as I described, unless someone will come with better verified method. Till now it works, all other effects are within accuracy of calculations.

We as naval architects are not required to make predictions with highest possible accuracy. We should predict the resistance to make sure it is not lower than actual (+ accuracy only), within reasonable margin. This is to deliver the contract speed. Note that other components of propulsion system design are defined with some level of accuracy and credibility. Say, thrust curves of the jets, propellers, engine power output lines considering de-rating factors, etc. So You know standard deviations in engine power declarations? Have You met the engine models that deliver 20% less power than declared?... So the accuracy of resistance predictions does not need to higher than that for other components.

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