# Speed predictions...

Discussion in 'Boat Design' started by Tevens, Feb 15, 2011.

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### TevensJunior Member

why are the predictions given from VPP programs always faster than the prediction i calculated from a spreadsheet process?

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### DCockeySenior Member

Several possible causes:
1) Differences in how the models behind the VPP and spreadsheet are formulated including assumptions, approximations and equations.
2) Differences in input data.

Where did the spreadsheet come from? Do you understand how it works and the assumptions approximations behind it?

Same questions for the VPP you are using.

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### TevensJunior Member

i mean generally, when using VPP's wwhich aspects does it not analyse very easily. Or on the flip side what does it do really rather well?

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### sorenfdkYacht Designer

How do you expect anybody to answer your questions, when we don't know what VPPs you are referring to and you don't tell us anything about this spreadsheet of yours?

"Because your spreadsheet is faulty" is as good an answer as any to your original question.

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### Tim BSenior Member

It's a massive generalisation, but usually the following are reasonably well modelled:

Hull resistance is usually pretty good for not-too-exotic boats
Hull sideforce is not quite as good
Foils can be well predicted within a certain range of applicability
and righting moment should be spot on from the hydrostatics.

On the other hand, sail forces are rarely well-modelled. The flow around a 3D Sail upwind is not easy, and it tends to get harder from there. If you consider the fact that a sail will stretch to a final shape, then it's even harder.

A lot of VPPs also assume that there is no pitch due to forward speed of the yacht, and this means that the angle-of-attack will be slightly wrong. However, this can't be corrected without some pretty time-consuming mathematics.

Tim B.

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