:Technical Translation of Bill's Post:

the idea behind a square top is to reduce the air flowing downward (i.e. parallel with the mast and toward the water) from the top of the sail. As most of you already know, there are different pressures between the windward and the leeward side of the sail. The windward side is more normal to the regular air pressure around the boat but the leeward side actually creates a vacuum. This force literally sucks the boat forward.

On a pinhead sail, because the sail is so narrow and the leach is at such a forward angle near the top, it is easy for the higher pressure air on the windward side to curl around the sail to the low pressure side at the very top. When it does this, this higher pressure air tends to fall downward (i.e. gravity) in the vacuum on the lee of the sail and disturbs airflow and pressure for a significant portion of the top of the sail.

Enter the square top. The head of the sail is now horizontal and in line with the direction of air flow. At the highest point, the leech is not so close to the luff and the angle of the leach is more verticle. Furthermore, the point at which the air detatches from the back of the sail is also farther away from the draft where the largest difference in pressures reside. (The draft is generally the deepest part of the sail shape and is normally vertical and in the front 1/3 of the sail) . The result of all this is that the wind can detach from the back of the sail more cleanly without as much tendancy to curl around to the leeward side. The top of the sail can generate power more efficiently because the vacuum is not as disturbed.

So you are actually translating Bill? Or is this you? Either way, it is a major improvement.

This gets into the suck versus push thing. My father, who was the best sailor I have ever known, said it was push, not suck.

Not fair, you got the easy one :P

I gotta wait till tonight to translate.... plus I've got a mindsplitting headache and our friend Wouter tends to make them worse

I'm trying to translate Bill's text (I hope I got it right!). I had no idea that the square head was doing all this.

Hi Steve,
Relative to your question about the top speed of the big cats: The most important design characteristic of these around the world racing catamarans is safety at speed. This means that these boats have to have the capability of sailing at speeds of 35 or 40 knots with adequate safety margin. At high speeds how does a catamaran get in trouble? It pitchpoles! What makes a catamaran pitchpole at high speed? Ans. A rapidly rising hull drag characteristic. How do you delay this rapidly rising hull drag characteristic. Ans. Make the boat loooooong! Then the theoretical max boat speed is well beyond the speeds the boat will see in actual use and the probability of pitchpole is greatly reduced. Also the pitchpole resistance or restoring moment of the hull is significantly increased with longer hulls. It is a conservative approach to a situation where peoples lives could be in danger and it greatly increases thr probability of success of the whole effort.
Fineness Ratio: Hull fineness ratio is simply LWL divided by max hull width at the waterline. The equation in many books for the hull speed limit of a displacement hull is:
Vel max = 1.4 X sqrtLWL. This equation seems to work for keelboat monohulls that have a hull fineness ratio of around 4:1 or so. In this form it sure doesn't work for beach cats. What I have found is that for displacement hulls the 1.4 coefficient varies with hull fineness ratio. For beach cats this coefficient varies between 4 and 5 depending on hull design. This says that a 20ft Tornado will have a max top speed of ( Vmax = 5 X 20**.5) or
22 knots. This relationship has served me well in analytical studies.
As far as the width of the M20 goes, the 9.5 or 9.7ft width is correct. It is disassembled to trailer. The boat has a 34ft tall mast and all the sail area in the mainsail so the center of effort is much higher than in the Tornado sail plan. The boat is 130 pounds lighter than a Tornado so the platform righting moment is well down from the Tornado, like 40% less. Add to this that the M20 crew lever arm is shorter than the Tornado and one can quickly come to the realization that the the M20 system is much overpowered (overturning moment/righting moment ratio) relative to the Tornado. If the boat had more righting moment, the crew could drive the sails harder and develope more sail thrust and make the boat go faster. It is that simple, f = ma.
Wind Force and Wind Power:
Wind force is a function of velocity**2 and has the units of pounds force.
Wind power is wind force times velocity and has the units of ftlbs per second. Wind power is a vel**3 function.
Sometimes people use these terms interchangeably but in the context of what they are saying you can usually understand what they mean.
Bill

Anyone got a sc20tr for sale?

Hello Bill-

Along time ago [20 years approx] we had a conversation about you cutting the nose of a SC20 and then stretching it out about 4 feet. You then took it out and tried it on both tacks with a noticeable difference. I am really curious as to where the theory and practical experience merge into the final design. I would think the R/D would progress until max velocity/stability is reached. Of course I know back then we didn't have computers at home. I believe there was a RC24 then the SC22. Was the RC27 part of this development from that experiment?

thom

Thom,
I did stretch a SC20 hull to 23ft once as part of a hull fineness ratio study/test. I put the boat together with a standard 20 hull on the other side. I ran speed tests on this system with speedos many times. I remember that when the boat would go 20 on one tack; I could come about and load the longer hull and the boat would go 24 or 20% faster.
The ARC22 has the bows from the 23ft test hull, finer entry and reduced bow wedge angle. The 22ft length was chosen because by moving the transom forward 2ft one could have a 20ft hull with lower drag the 22 bows all designed for ocean sailing. That boat could be built today out of the 22 molds.
Bill

Like this thread offshoot better ,-

Ya ,-think the whole disco era is responcible for numerous problems today ! Maybe the conformity and ready acceptance of regulation and incompetent govn controls begane there -
{joking too }

dance till your stupid ,--kind of sums up that era .
big heads big hair platform shoes and obnoxious music -

do like some of the "current music out ,--Big Yellow Taxi ,-some things have integrity and a timeless quality -art music theatre , invention etc,-
this is born of creative human freedom at work ,
We see it in excellent cat design too.

As Solzhenitzyn said in 78 --"This ascension will be similar to climbing onto the next anthropologic stage. No one on earth has any other way left but -- upward. "

Hope it continues ,its up to us !-
Carl

World Split Apart -Harvard Commencement speach 78 -

http://www.columbia.edu/cu/augustine/arch/solzhenitsyn/harvard1978.html

Gosh, Carl, and I thought the engineers were hard to understand. Guess I need an interpreter for you, too.

Don't forget the dictionary...

thom

Yeha Carl, anything you said was over my head there.

Do I need to continue with the 70's theme and be high on some kind of drug to understand it?

I was only half following the engineer-speak, but the disco/Solzhenitsyn post seemed perfectly clear

This is one of the coolest threads we've had here in a while.

Also it's good to have Bill Roberts back. He's been conspicuous by his absence(sp?).

David Ho
TheMightyHobie18 1067

OK - so who's translating for Carl?

I don't have the time to continue this further.

Tip vortex, flow from high pressure side to low pressure side, Leech vortexes and the shape of the top of the mainsail are all linked to the same processes at the end (tip) of a lift creating section.

A sqauretop main may reduce leech vortexes but will increase the magnitude of tip vortexes. They are all linked.

But the main point still remains that squaretops mains of equal size than pinhead sails aren't that much faster in top speed, they mainly behave better and quicker in gusts and lulls. And yes, this is the outcome of real life tests using identical platforms and well known sailors. Of course with a squaretop main you can put more area in the main but that is that brute force approach again which is skewing the picture.

Now, I much give up my participation in this debate for reasons of time. Till the next time Bill.


Wouter

>>To Wouter: The specs I have seen and a witness say that the M20 is about 9.5' beam. What is your statement that it is 8.5' beam based on? If 8.5' beam for the M20 is an error, wouldn't that substantially change your analysis of the boat?

I confirm that 9,5 feet is the right number and not 8,5 mtr. I have seen quotes from 2,6 = 8,5" to 3,0 mtr = 10" in the past but Texel just remeasured the M20 and it found 9,5". (I know it is not used in the system, but it is measured anyway)

Does this change the analysis. In relation to 8,5 mtr. boats "may do so, yes", I relation to the 10" wide Tornado, no.

This dependent on which reference we agree upon.

My argument was based on choosing a reference platform and modifying it. Than to make conclusions on what to expect as the result of the mods.

Take a tornado and lets make it lighter en less wide. This will lead to a boat likely to preform worse in trap condition. If the Tornado is made lighter but keeps the same width than it is has a good change of performing better.

However all is determined by the magnitude of the changes. 1 inch width reduction and 100 lbs weight reduction will have a different outcome than say 2 feet width reduction and 1 lbs weight reduction.

And here is where the main point of my argument is to be found. All of these aspects are linked ; the net effect of a certain reduction here is partly determined by a reduction (or increase) somewhere else. So when modifying important ratio than the ratio between the ratio itself is also important.

In this sense the statements made by Bill about width and weight are a subset of the more general case that I expressed. Bills way of improving speed is correct but it is not the only way of improving it. And there is the casue that started the debate between him and me.

In simplere analogue terms.

Bill argument is analgue to claiming that to go faster in a car you must use a bigger engine and preferable place it further back to put more weight in the driven wheel to avoid slipping.

My argument said that you could go faster with an even a smaller engine than before and also keep it in the same place as it was when you are succesful at sufficiently reducing forms of drag or resistance to acceleration. Examples of this are lighter wheels and a lighter flywheel in the driving system (not a lighter car ! mind you) or even reduced frontal area to reduce aerodynamic drag which reduces the amount of power available for accelerations/speed.


In the first example you increase the important factor that caused speed or accelleration. (Brute force methode)

In the latter example you analyse and try to optimize the ratio between important factor in such a way that the overall governing ratio is improv ed. Of course the first example implicitely does the same trick but without explicitely refering to it or even knowing about it.

On the starement of :

>>recently stated that the power (force, KW, hp) in the wind varied as the square of the velocity of the wind. My old DOE manual for home windmills gives a formula for wind power that has a cube on the wind speed. And that is the understanding most my landsailing buddies have. Did I misunderstand your statement?


The energy contained in a GIVEN (and constant) volume of fluidum is relative to the windspeed squared.

The energy contained in a volume of fluidum flowing through a given (and constant) stationary area is relative the windspeed to the 3rd order or cube.

Of course both windmills and sails are more closely described by the second case. When the wind blows harder than a bigger volume of fluidum is blow past the energy harvester which are sails and windmill wings. This volume increases lineairy with windspeed and it constant volume enery containment has increased with the sqaure. Together they from a 3rd power. as X time X squared is = X to the 3 rd power.

I hope this explains things

Wouter

>>Fineness Ratio: Hull fineness ratio is simply LWL divided by max hull width at the waterline. The equation in many books for the hull speed limit of a displacement hull is:
Vel max = 1.4 X sqrtLWL. This equation seems to work for keelboat monohulls that have a hull fineness ratio of around 4:1 or so. In this form it sure doesn't work for beach cats. What I have found is that for displacement hulls the 1.4 coefficient varies with hull fineness ratio. For beach cats this coefficient varies between 4 and 5 depending on hull design.

This formula of Vel max = 1.4 X sqrtLWL is directly derived from the Froude law and make a link between the wave length of the created (bow - stern) wave system and the waterline length of a given hull.

This formula givens the speed at which a given length hull needs to travell throught the water in order to have a single wave spanned along it hulls. With a crest at its bow and a crest at it stern.

In heavy vessels this wave system is a huge boundery as drag suddenly increases rapid at this speed and hardly any engine and no sailing rig is powerfull enough to overcome this rapid increase in drag.

Please note here that the factor of 1.4 (in this form of the formula) is a constant determined by the way waves on a water surface behaves. It has a very clear physical interpretation, A free wave travelling at the given speed has the same wavelength as the hull waterline length. The adjusted factor of 4 to 5 HAS NOT. This adjusted factor is nothing more than an imaginairy factor that someone dreamed up to make the froudes law useable for situations where it really doesn't apply. Such a case are catamarans. What has been done is that the end result has been determined and a factor is calculated from that that gives this end result at a given speed. This is a circular reasoning.

In catamaran design the situation that Froudes law describes (wavelength of bow-stern wave = waterlinelength) is found at the speed which the formula Vel max = 1.4 X sqrtLWL gives. However because of the special qualities of the catamaran design (maily very light weight) this wave system is much much much less significant from a overall drag point of view. In fact the drag is there (make no mistake about that) but it's magnitude so small in comparison to the other forms of drag on a catamaran design that its effects are not noticed on cats. Cats simply power through into the force mode state and continue to accellerate much longer till they reach much higher speeds where the other forms drag have grown to such a magnitude that they are big enough to stop further acceleration.

Simply put adjusting froudes law to cats is useless. Froudes law works very well in heavy displacement vessel (Heavy crusing cat design MAY be included here) but in general it is misapplied in light weight vessels like sport cats and some navy ships. Continiously adjusting the factor to make the formula work is fooling oneself.

Bills eaxmple is correct with the excepting that bill doesn't account for the reduction in overall drag due to the same reduction in weight.

The following statement

>> The boat is 130 pounds lighter than a Tornado so the platform righting moment is well down from the Tornado, like 40% less.

is simply misleading.

It the combination of Platform and CREW that is important here and not the platfrom alone. When looking at this combo the righting moment has only been reduced by 13 % and not 40 %.

See the math (in SI units) ; assumption centre of effort of the crew id 1 mtr (3 ft) up from their feet.

Tornado + 150 kg crew : 170 kg's * 1/2 * 3.0 mtr. + 150 kg * (3 + 1) mtr = 855
Lightweight Tornado + 150 kg crew 170 - 60) kg's * 1/2 * 2,9 mtr. + 150 * ( 2,9 + 1) = 744,5

744,5 / 855 = 87, 1 %

Wouter

>>I wouldn't know where to start though... Wouter crams a lot of jargon into his blogs :

Sorry,

I will try to do less of that in the future.

Wouter

Ohh, I'm quite exited to see how you will do it Maugan.

I'll be watching this thread closely over the next couple of days.

Wouter