# Savitsky method in Maxsurf Hullspeed

Discussion in 'Boat Design' started by devudegoa, Feb 3, 2016.

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### devudegoaJunior Member

Sirs,

The question is with reference to the attached screenshot of Maxsurf Hullspeed generated Power vs Speed graph, which shows two curves related to the Savitsky method. As seen in the graph, the Savitsky planing curve intercepts the Savitsky pre-planing curve in two places.

(1) Savitsky Pre-planing = 592 HP at Speed = 14 knots
(2) Savitsky Pre-planing = 793 HP at Speed = 16 knots

I would like to know if it is safe to assume that the boat will start planing at the first intercept, i.e. 14 knots speed.

Thanks.

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### daiquiriEngineering and Design

Hi devudegoa,
no it is not safe to assume any fixed number as the point where planing begins.

First, because there is no precise definition of the term planing and when does it start. See this discussion for more: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/definition-planing-45248.html

Second, because these curves have an implicit uncertainity, and the uncertainity grows towards the lower end of the speed range. In other words, they are not really neat and precise curves. You should consider them as the median line of shaded areas like these:

It is a qualitative graph, the actual width and shape of shaded areas will depend on a particular hull.

Third, for a given hull the above curves are highly dependant on the displacement, CoG position and trim. So they are not a single curve but a family of curves.

So, your boat will start to plane somewhere between speeds bounded by two arrows. You can't say anything more precise than that.

Cheers

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### devudegoaJunior Member

Thank you very much for the reply. I will read the thread which you have mentioned. Regards.

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