# How much planing surface do I need?

Discussion in 'Powerboats' started by kengrome, Oct 13, 2007.

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### kengromeSenior Member

Here's my situation. I designed a small (one person) rough water planing power boat and I would like to have an idea if it will plane before I build a prototype.

I understand there's a formula for determining the required planing surface area for a boat to plane given a specified hp and weight. Is this true or is it far more complicated than this?

Thanks in advance!

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### GilbertSenior Member

Why not measure the area of a jetski and find out its weight? Then estimate the weight of your design and if your weight to bottom area ratio is comparable you should be good to go.

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### marshmatSenior Member

You are probably thinking of the Savitsky method. It is not exactly a simple formula but it does give a good indication of how the boat will perform for a given thrust vector and load case.
Dingo Tweedie posted an excellent Excel implementation of the method on here a while back, http://www.boatdesign.net/forums/showpost.php?p=141860&postcount=76

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### WillallisonSenior Member

Opinions vary - especially once you take service conditions into consideration, but for a relatively lightweight boat I'd aim for around 250kg per square metre of waterplane area.
Along similar lines to Gilbert's suggestion, a good approach would be to estimate the AWP of similar vessels that you know to be successful and aim for similar figures. The problem with this approach is that it's notoriously difficult to get reliable weight figures from manufacturers...

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### hmattosSenior Member

Planing area is very much a function of speed. We - see explorermarine.co.uk - build fast RIBs in Devon England and a 5.0 metre SPORTS model will plane at 10 mph. This is using 7.5 metres sq of planing surface to support 600 kg.
However, at 45 mph the same boat uses less than 2 square metres to support the same weight.
Not also that a low vee angle - deadrise at stern - supports much more weight when planing at lower speeds.

Good luck

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### WillallisonSenior Member

Yes - I probably should have been more specific...
As the AWP is very diificult to estimate at speed, it is the at rest AWP that is used. I don't believe there's any great science behind it - it's simply based on previous experience...

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