# Choosing Froude Numbers

Discussion in 'Boat Design' started by msbealo, Sep 28, 2007.

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### msbealoNew Member

Hello.

I've been reading The Principles of Yacht Design and I have a question regarding the choice of Froude numbers. Essentially, how do you choose the Froude number to which you optimise the remaining design parameters?

In figure 5.18 we have the equation for the residuary (wave) resistance equation derived from Delft experiments. We then go through the various contributions to the equation and it's seen that we have to choose the speed at which the yacht shall have it's optimum performance. It is stated that normally hulls are designed for maximum performance beating upwind in a breeze.

If I'm designing a 28 foot medium-to-light boat, what do I need to think about in choosing the Froude number to optimise the design for?

I hope this makes sense, all help appreciated.

msbealo

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### Raggi_ThorNav.arch/Designer/Builder

How much time you spend in different conditionsm
I think that most people optoimize for max speed while thesy spend most of their time at moderate speeds

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### msbealoNew Member

Ok,

So with a Froude number of 0.40 our boat is sailing in good wind and is sitting between it's bow and stern wave and is technically achieving it's hull speed. However if want to optimise our hull for going upwind in a light breeze we should be looking at F=0.3. Principles suggests designing for a Prismatic coefficient of 0.56 which corresponds to F=0.35.

However, why not 0.34 or 0.36? I guess is depends on the designed average wind conditions, but how? Is this the real area of guess, or am I missing something?

On the other hand, should a yacht be designed to go as fast as possible when the wind gets up, or should it be designed to go as fast as possible in lighter winds?

Mark

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### Raggi_ThorNav.arch/Designer/Builder

I am no expert on this, but I think, If you design a boat for Americas Cup you know where it will be sailed and you have statistics for the conditions. Probably the same for Volvo/Whitbread and Transat and Transpac etc. For a given design you can estimate the speed for different conditions, wind, waves, and directions. You can optimize for the Cp that maximize average speed.

For a pleasure boat or cruising boat it's quite different. Maybe you want to optimize for maximum speed, just for fun. Maybe you want to make a ghoster, that can sail away from all others in light conditions?

Another thing, it seems like it's a known fact that a Cp on the high side will cost you less in light winds than a too low Cp will cost in strong winds.

Just my thoughts

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### Man OverboardTom Fugate

Msbealo,

I dont have my 'Principles of Yacht Design' here at work, can you post the equation?

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### jehardimanSenior Member

You have it backwards....by already selecting the LWL and displacement to length ratio, you have already selected the optimum Froude number by default. By selecting a LWL you have fixed (mostly) where the humps and hollows in the powering curve will occur. By fixing the L/D ratio you fix the ratio of power to speed for a given LWL. What will happen is that the boat will climb the humps until it runs out of power...and that is the speed it will go...period. You can only design for a speed based upon Froude number if LWL and D are the dependent variables, not the given ones.

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