I wasn't critizing you, it was more a post to the general public that I had place somewhere in the thread. This turned out to be you post as you gave a typical tortured reasoning that is often expressed by persons who keep clinging on to Froude's law as a predictor for maximum speed.

Some reply to your new post.

Froude's NUMBER is not the same as Froude's LAW or FORMULA. I did not discuss or refer to Froude's NUMBER which I personally feel is a pretty uninteresting quotient anyway. Someone described it ones to me as the water surface based Mach number which is indeed it most similar cousin.





No it is isn't. Changing the symbols of othering in a formula doesn't change the formula it is still one and the same. Meaning

V = 1.34 * sqrt(L)

is exactly the same formula as.

sqrt(L) = V/1.34

As both describe exactly the same behaviour of a wave.

CHANGING the formula to V = 1.54*sqrt(L) is intentionally altering the formula to make an error and as such is a totally different formula and can not be called Froude's formula. This fast and loose actions are what caused the eternal "maximum hull speed" myth; which is pretty unscientific.

Compare it to this. NO-ONE changes newtons most famous law F=M*A to have it cover different freefalling objects like lead and feathers in the atmosphere better either. We are expected to improved our model and include other effects like aerodynamic forces to be able to use THE ONE AND ONLY ORIGINAL FORMULA.

Only ship designers are so scientifically challenged that they have to change Froude's formula in order to avoid the need to let go of a mental model that is far too simple. I'm sorry but that is just bad science.






Now you make the same mistake like many other people again. The statement :

A displacement boat travelling faster than the velocity of the waves it generates, is subjected to large resistance against its movement

IS SIMPLY NOT TRUE for a large portion of hull designs. ONLY FOR RELATIVELY HEAVY ship designs. For example a beach catamaran does indeed see a (non-linear) increase in its wave drag component when passing the wave-velocity but that still does NOT mean that the related increase in drag is a LARGE component in the total amount of incurred drag. In fact it isn't. I glarified it as such in my earlier post so you should have spotted that. And indeed you yourself argue again this very statement at the end of your posting.


My comment to ALL readers here is to stop using Froude's law (NOT Froude's number) in ways that are inconsistant with what it described and tell any professors who say otherwise to stop mystifying the situation.

Froude's law (NOT Froude number) does not say anything about any hull design. It only relates the wavelength of a wave on the watersurface above deep water on earth to its travelling speed. In some cases this travelling speed happens to coincide closely with the maximum speed a certain limited group of boats are experienced to achieve in real life.

An analogue example : The fact that some sports car can sustain an accelleration of 1 G in real life does not mean that Newtons law F=M*A = M*G describes the achieved accelleration of all automobiles cars ! Not even when we include some (fictional) Newton Numbers to "Correct" the formula for different automobile designs like. F = Newton number * M*G

So why are ship builders doing to Froude's law ?

Wouter