Miss Nylex data as promised !

And please note that the theoretical hull speed (measure where wave making/ form drag is supposed to be dominant) = sq.rt. (25) * 1.54 = 7.7 knots and the craft is sailing at 10 knots. Theoretical hull speed formula isn't wrong it is just misapplied to craft where the displacement is much much smaller in relation to the waterline length than ballasted yachts. I think the miss Nylex data show this very well

Just filling in on a promise

Wouter

For "normal" keelboats of about 3:1 length:beam ratio, isn't the coefficent 1.34 rather than 1.54? Further, using a coefficent of approximately 4 or 5 accurately predicts the top speed of superfine hulls, say, greater than 10:1 or 12:1. Do you happen to know the fineness ratio for Miss Nylex?

Thanks Wouter

Andrew and others,

I will go t a movie in a short while and I type this up while I for the time that I need to get into the car. Just finsihed dinner you see and I feel like typing up a post in which I try to explain something very important to readers who possibly tune out at the first sign of science and will make the same mistakes in the future. I hope, and I seriously do that, that you will be one that reads the post throught and help educated all others in the future.

So here goes and please guys,gals stay with it because you are going to learn why froude's law is not what it seems.

There are two things in the area of sail boat design that are so abused as the froude law and the jib slot explanation. General intepretation of both have been proven wrong a million times already and still despite abundant signs that something weird is going, scores of sailor keep propelling the certified falsehoods.

In Froude's case the problem is two fold.

-1- Froude's law does NOT say anything about hull design or the speed potental of a hull design. Simply does NOT.

-2- Froude's weighting numbers (the 1.32, 1.54 3, 3 or what ever number is used) are all over the place, There is one for each particular case or situation and all, except 1.32, are meaningless in the physical sense.

Sharp minds will have observed by now that even I have been confused into a mistake. I used 1.54 myself over 1.32 as Andrew correctly informs us. This is the result of reading up on Marchaj and forgetting about the text books I had on fluid dynamics. It actually took my about an hour, off the net, to find the correct number 1.32. On the internet you can find numbers ranging from 1.1 to 5; not very helpfull.

Big claims, these two are, so here is why :

Froude's law is actually and complete about the speeds at which waves of different wave lengths traveller over the surface of a body of water. That is all. The word boat or hull does NOT appear in the definition of the law.

Froude's law : V = 1.32 x sq.rt. (wavelength)

So Froude's law says that

-1- a wave with 10 ft between two following crests travels at 1.32 * sq.rt(10) = 4.17 knots
-2- a wave with 16 ft between two following crests travels at 1.32 * sq.rt(16) = 5.28 knots
-3- a wave with 20 ft between two following crests travels at 1.32 * sq.rt(20) = 5.90 knots
-4- a wave with 100 ft between two following crests travels at 1.32 * sq.rt(100) = 13.20 knots

etc

Because Froude's law is derive directly from an observation on how wave on a surface behaves the number 1.32 is the only one with a physical meaning. That is as long as the density, viscocity and gravity of the fluid under consideraion do not chance. Once we start abserving waves on the surface of oceans of honey then the number will change and will be higher. Note therefor that changing the number to 2 or 3 (as is sometimes done to predict multihull speed) is physically jibberish. The water and gravity do not chance when a boat of a different design is let loose on it.

So why do we sailors come to use this Froude law and how did it ever acquire a god-like status.

Simple; Because the drag of a heavy water craft moving in displacement mode suddenly chances right around a certain hull speed. And some engineer noticed how for some craft this hull speed is the same as the speed at which a wave with a wave length equal to the observed hull length would travel. This engineer then immediately proclaimed the news and declared that all boats moving on the watersurface would hit a speed limit at 1.32 x sq.rt. (hull length). And of course the claim became that Froude's law proof this.

It must be said that nearly all boats at his time did hit a speed limit at what was called critical hull speed. Not because a powerful physical proces was a work here but mostly because at that time only heavy displacement hulls were or had been build. Of course Uffa with his Laser 1 design smashed the beauty of this law somewhere in the 1930's and polynesians had been booting it since the start of history. However up tight western elites refused to face the music and they kept the misconception from dying. Of course catamarans and dinghy are not real boats, only lead mines are right ?

The following intermezzo (between ) is not really needed for the remainder of the post but I'll stuck it in here so people understand what Froude's law CAN say hull design.

()

PART of the drag of a hull travelling on the surface is caused by Form drag and a large portion of that is wave-making drag. Actually drag here means "losing energy in the creation of a wave system". Of course a loss of energy can only be compensated by an engine adding energy to the situation (motor, sails, wave surfing) OR by the boat itself transferring kynetic energy to the wave system, (slowing down).

So what is so special about Froude's law ? Well as long as hull travels through the water at low speeds the wave system around the hull is characterized by short wave lengths. So the first crest at the bow raises the pressure on the ships hull cause a drag force while a similar crest further allong the hull (near the stern) pushed also against the hull and produces a force in the direction of the way travelled (a driving force). Of course this works only on hulls than widen from the bow and narrows to the stern again, a flat plate will not see this happening. What does this mean ? That a good portion of the energy lost in the wave system near the bow is given back near the stern. The energy is recouperated and the wave system BEHIND the stern is smaller than the system along side the boat.

Now what happens when a hull travels at great speeds, above critical hull speed. Now the wave system around the boat has a LONGER wave length then the hull just in order to keep up with the hull. This means that the loss of energy is still present at the bow, but the first crest that could be used to recouperate the energy is now way past the stern and out of reach.

So what happens at critical hull speed is nothing more then second crest of the wave system moving aft of the hull. Right at this point the hull drag starts to climb very rapidly with every small increas in hull speed.

Where does factor 1.54 come from ? Well when the second crest is only 1 or 2 feet behind the stern than still a significant amount to the enerfy can be win back but so hull can in reality exceed the speed calculated by 1.32 * sq.rt (hull length) a little. To compensate for that engineers sometimes use a modificated factor, 1.54, to predict maximum boat speed FOR A HEAVY yacht. Note how this number has lost it physical meaning ! 1.32 was exact and gave the only speed at which a wave of certain wavelength can travel. 1.54 how is sort of an average taken over a variety of hull designs that allow a certain boat to pass the boundery by different amounts.

()

So what does this say about all other numbers like 2, 3, or even 2.5 as coined by Marchaj in his volumious works ? That these are all, plain and simple, BS and highly deceiptive. These give a false sense of security as their predictive value is neglectable.

Let me give you a counter example :

Example begins ***

I (dumb engineer of questionale scientic skills); have discoverd that Froude's law accuractely discribes the maximum speeds of all things on earth ! Take a look at the proof :

Waves : max speed = 1.32 * sq. rt. (wave length)
yachts : max speed = 1.54 * sq. rt. (water line length)
Jet liners : max speed = 48.0 * sq. rt. (overall length)
Human beings sprinting : max speed = 7.90 * sq. rt. (body length)
1 kg lead falling : max speed = 4.94 * sq. rt. (width of the crossection)
1 kg of feathers falling : max speed = 2.76 * sq. rt. (width of the crossection)

And so on.

I think you get the picture here. As long as you think up a NEW weighting factor for each case where max speed prediction is wrong then you can make it right again. So when going from Kitty hawk to jetline the weighting factor was adjusted, ohh , at least 100 times.

This leads us to a very serious conflict. If you make a new design hull or boat you must first measure the TRUE maximum speed before you can calculate the correct weighting factor that you'll need to predict the maximum speed of the very same craft. That is a bit foolish, isn't. Measuring true max speed in order to be able to predict it.

And that is what happens to boats. Each time a design is found to break the back of the "Froude's law gives max hull speed" the tunnel vision crowd comes up with a new modification factor to bring balance to their falsehoods.

But is should be clear now that NOTHING interesting happens at say 3 x sq.rt. (hull length). The transition in the shape of the wave system has passed long ago and there are no serious changes in any important physical sense. There simply is no approaching a clearly defined physical barrier like the bow-stern wave system, the sound barrier or heat barrier. The law suggests that there is but there simply isn't.

Why don't cats move faster then ? They do ! Put a C-class solid wing style rig of equal area to normal rig on a tornado and it will move significantly faster. Put a Sun fish style rig of equal area to a normal rig on the Tornado and it will move significantly slower. The factor causing the max speed limit to chance is efficiency of the rig, not any property of the hull design. So what will the "falsehood crowd" have us do ? Assign 3 different weighting factors to all three situations and have us believe that the constant hullshape is causing all the chance.

I believe no-one would accept this in his or her right mind, However we all do except it when comparing say an Tornado to a ORMA open class tri. Or when comparing a cruising catamaran to a cruising multihull ?

And That is why the application of Froude's Law is one of the biggest frauds in the designing of boats. Be it sail boats or powered boats. Best example ? Navy ships like fregats, they move MUCH faster than critical hull speed and NO they do not plane.

Ohh that is another falsehood. "All boats exceeding critical hullspeed given by Froude's law must be planing to do so" WRONG ! Counter examples, Navy ships and catamarans, Standard answer of the lead mine public. Cats are not real boats and other similar excusses to keep clinging to their favorite falsehood.

No, 1.32 is the factor that describes waves in the surface plane between normal water and air.

1.54 is a modified factor thought up by boat designers as a rule of thumb for heavy displacement boats.

Actually

That factor of "4 or 5" hits a rather large area of maximum speeds that are seen on a particular set of current designs.

Note how a C-class cat Cogito and a Hobie wave alone give rise to a weighting factor spread of 6 to 4 = (15 to 30 knots)

And when we compare the Hobie wave with a modern displacement Moth (not the foilers) than the same hull length gives rise to a similar large swing in weighing factor.

But better still compare a Hobie 16 to a 49-er skiff. Hobie 16 has a finess ratio of 10: 1 or better and the 49-er has about 5:1. Still they move about as fast with the same sail area. (excluding the spi on the 49-er)

Stuff like this shoots large holes in any Law like you suggest. If there are so many exceptions then what exactly is the law.

And if we all start putting C-class style solid wings on ours boats than you may well have to up your ratio range from "4 to 5" to "5 to 6". And this brings my back to an earlier part of my post. That factor range of "4 to 5" was thought up to MAKE it work for current designs that have finess ratio of 10:1. That is works to some extend has more to do with the aerodynamic limits of soft sail rigs than on hull design or waterline length as the modified Froude law suggests.

Wouter

Thanks Wouter

Agreed, actually. Most beachcats far outrun their bow waves, and few (or none) actually plane to do so. Btw, Steve Clark has stated Cogito's top speed in her current configuration to be 23 knots, a speed which she reaches downwind in 10-12 knots of air. As the wind increases above this level, I believe she sails deeper but at the same hull speed.
sail fast

Wouter,

Great stuff, does the article say how they were able to seperate out the form drag and skin drag? i.e. how they measured it, two hulls in the water or one, I guess this is towing tank data.

Gareth

The article doesn't say how they seperated from drag from frictions drag. However the typical route is to calculated the wetted surface area and derive the friction drag from that by use generally available flat plate data. Then they measure the total hull drag and deduct the friction drag from it. All that remains is called form drag. Form drag is mostly a rest term that includes the drag related to several factors.

The article does say that only hull was in the water for the given data. Of course the C-class cats are fully powered up and flying a hull at 8 knots or less. Test data is of 12 mph winds = 10.5 knots.

Tank test data I don't know. Could be, quite early in the C-class history universities were brought in to help, however the Australians always did things their way and for all we know they towed a real sized hull through the water behind a motor boat and had an unster in the towing line giving the tension in the line.

Article is not clear on that. The article does give two series of data for miss nylex; an initial estimations and a corrected or modified one. I used the second series of data for the post I made.

Wouter

Wouter
could you publish the actual numbers, or give me a reference for the article so I can track it down.

Thanks

The webpage containing the article has been removed. Luckily I archieve data like this. Anyways I can scan the page containing the data and mail it too you if you give me your mail adress.

Wouter

Thanks, its gareth at fourhulls dot com

Wouter,
Mostly I agree with your discussion of wave making resistance. However, your post leaves the impression that Naval Architects and Yacht Designers routinely misunderstand and misapply the the parameter usually called "Speed Length Ratio". This is definately not correct. This parameter is used within the professional community as a comparison of vessel speed to wave speed and nothing more is implied. It is used along with prismatic coefficient, block coefficient, metacentric height, righting moment and a host of others to describe the characteriasics of a given vessel. It is not used to predict performance as you implied.

Kevin

I agree with your comments. But to my defense I would like to add that I said that Sailors and Sailing Elites are the one that are abusing the weighting factors. I'm well aware that Naval architects etc do not, how else can they design proper navy vessels like fregats that I used as an example against the abused froude law.

There is one more thing. I don't understand why engineers need an arbitrary parameter (weighting factor) to make a "... comparison of vessel speed to wave speed and nothing more is implied ..."

By definition the outcome of a (particular) quotient (Hull length / wave length of a given speed) is called a (particular) RESULT and not a PARAMETER. Scientists and engineers that are slack about the proper use of terms like this often give rise to misunderstandings that more often than not lead to persistent falsehoods in the larger (less educated) communities.

But I will go with you on the point that if anybody thinks that my post suggests that naval architects and Yacht designers wo are worth their salt abuse the froude law than that is not the thing that I wanted to express in my post.

Wouter

Caught pretending to be something you are not again weezy?
Don't you ever get tired of making a fool of yourself?

Here is an english/engineering lesson for you.

Webster's Dictionary definition of Parameter:
1 a : an arbitrary constant whose value characterizes a member of a system (as a family of curves);
also : a quantity (as a mean or variance) that describes a statistical population
b : an independent variable used to express the coordinates of a variable point and functions of them
-- compare PARAMETRIC EQUATION
2 : any of a set of physical properties whose values determine the characteristics or behavior of something

3 : something represented by a parameter : a characteristic element; broadly :
CHARACTERISTIC, ELEMENT, FACTOR
4 : LIMIT, BOUNDARY -- usually used in plural

To simplify for you;
The term PARAMETER may correctly refer to single measurable values, such as maximum weight
, a ratio, such as length to width
, the resultant value of a very complicated equation
, the material used for manufacture, such as aluminum.

The one who is "slack" is you.

P.S.
Formula Class rules are full of parameters.

I think Webster's Dictionary definitions proof my case just fine.

Your intepretation of what Webster actually means does not comply with what is written.

Wouter

Designers use "speed length rario" to compare hulls of different sizes going at different speeds. Just a convinient way to non-dimensionalize the wave making aspect of flow. For instance if you were going to build a 1000 foot ship to operate most efficiently at 20 knots you might want to try a 10 foot model in a towing tank first to check our wave making aspect of the design. You would need a way to scale the speed of the 10 foot model to correspond with the 1000 foot real thing. This only holds for scaling gravity waves and other aspects of the flow (e.g. boundary layer growth) will have different scaling rules that apply. Where I am agreeing with you is that wave making for typical sailing catamaran hull forms is probably not worth a lot time trying to minimize. The resistance curve of our hulls does not have the pronounced "hump" that full displacement hulls have when transitionong from speed length rations 1.0 to 2.0. As the Miss Nylex data suggest, it's better for us to worry about foil shapes and windage if looking for things to improve.
For the record, I'm not a Naval Architect - my degrees are in engineering mechanics. But my day job consists of supervising submarine design for the US Navy. So, over the years I have worked with many Naval Architects and Fluid Dynamicists. Don't want to leave a false impression here.

Kevin

To me you don't have to present your credentials. I think everybody is entitled to speak out.

With respect to

Now you are talking about the "scaling law of Froude" That thing is NOT described by speed = parameter * sq.rt (hull length). The scaling law has the same fundamental basis but is a different entity.

I was only commenting on the use of "speed = parameter * sq.rt (hull length)" Which is a bastardize formula derived from the equation "speed = 1.34 * sq.rt (wave length)" by pseudo scientists.

We must take care to keep talking about the same thing

Wouter