hull speed, what it means.

[Tedd]

What @sail sfbay said.

I'd also like to note that Froude number is analogous to Mach number in that they are both dimensionless numbers, useful mainly to engineers, that generate an unwarranted amount of opinion and speculation among laymen. They get misconstrued as "limits" in layman circles when they're nothing of the sort. A Mach number of 1 or a Froude number of sqrt(1/2π) = 0.3989 simply indicates a point of delineation between a regime where one kind of flow condition dominates and a regime where a different kind of flow condition dominates. For real-world objects, the transition is smooth and continuous, not a "barrier," and, while operating "beyond" that value may require careful and clever design, or even a complete re-think of the approach (such as changing from a displacement hull to a planing hull), there's nothing mysterious about it.

 

[sail sfbay]

Tedd...............My two cents is Mach Number and Froude Number are extremely valuable in refinery design calculations, which I have used throughout my chemical engineering career to develop reliable refinery equipment and piping design that worked without loss of performance, reliability, safety or violation of established industry standards like the API.

 

[ontherocks83]

All I know is that my Catalina 30 commonly exceeds its 6.7 knot hull speed with ease, often into the consistent 7 - 7.5 knot range without a problem.

 

[Will Gilmore]

They get misconstrued as "limits" in layman circles when they're nothing of the sort.

This is one of two things I find most frustrating about any conversation with regards to "Hull Speed". One is the concept that "Hull Speed" represents a limit and two, that it is theoretical. It is simply a definition, a description of a specific condition, based upon physical properties. I am sure the Froude Number and Mach are the same for that.

It isn't Theoretical Hull Speed, it's Hull Speed and it is defined very specifically. The theory is, that your boat or my boat should be able to achieve it at some point. To go faster, it needs to be surpassed, by definition.

Whoa! I sound kind of petulant. Sorry about that. Really, it's not that big of a deal to me.

-Will (Dragonfly)

 

[Capt jgw]

1.34x the square root of LWL has always considered to be an average. Some hulls may be faster, others slower. And also that number has been around a long time so likely based on an average that existed in bygone years. Some multihulls with their narrower beam to length ratio exceed this theoretical speed easily, also designs with bluffs bows and wide sterns. So more modern designs may have a faster hull speed. There still is a limit for any given hull, it just might be faster than the 1.34 number. Look at the new AC boats with their foils. If I wanted to go that fast I'd take an airplane.

 

[sailme88]

I believe the 1.34 formula is incongruent with fore and aft waterline. It applies to the parabolic waterline that occurs upon heeling..

 

[jssailem]

I believe the 1.34 formula is incongruent with fore and aft waterline. It applies to the parabolic waterline that occurs upon heeling..

Actually it defines the displacement hull speed using the Length of water line. It is possible for a boat to have a longer water line heeled than sitting upright. This can result in two “hull speeds” that are different in value.

The number results change but the formula remains the same.

 

[Tedd]

This can result in two “hull speeds” that are different in value.

Yes. Or, more precisely, the Froude number (and, therefore, the "hull speed") varies with the cross section at the water surface, so the "hull speed" varies continuously as the heel angle changes. Most dimensionless numbers used in flow analysis (Froude, Reynolds, and Prandtl) use what's called a "characteristic length" in the flow direction. The choice of this dimension is fairly obvious for certain shapes, such as diameter for a sphere or chord length for a wing section. For more complex shapes, and non-symmetrical shapes (such as a hull at a non-zero heel angle) the choice is somewhat arbitrary, and what matters most is that the same dimension is chosen in the same way for every calculation. A few choices could have been used for a hull. I suspect waterline length at zero heel was chosen partly for simplicity and partly for good correlation between theory and empirical data.

 

[srimes]

Actually it defines the displacement hull speed using the Length of water line. It is possible for a boat to have a longer water line heeled than sitting upright. This can result in two “hull speeds” that are different in value.

The number results change but the formula remains the same.

Yes, and why the old IOR boats had distorted hulls shaped for favorable ratings according to the rules.

Hull speed is a rule of thumb that describes when wavemaking drag goes up quickly and is totally relevant to fat displacement boats.