Jimi,

I have no other option then to say in so many words :"Your text book is wrong"

Numerous counterexamples are available, ranging from full sized high speed marine fregats (ever saw such a boat planing ?) to beachcats.

Your text book is providing you with a particular intepretation of Froude's Law that really only covers a particular part of the real world. Pretty much only heavy displacement hulls of low prismatic hull ratio.

What the book should have written is that a displacement hull travelling at a speed equal to 1,33 * the root of its waterlength will experience a relatively dramatic change in its growing wave-making drag. In some displacement vessels this sharp increase is so big that it prevents the vessel from travelling through the water much faster. Other displacements vessels were the total amount of wave-making drag is only a rather small of the total (beach cats) will no be held back by this increase. The reason for this is that a doubling of a 40 % drag component is really significant while a doubling of say a 15 % component is relative small compared to the total. Beach cats and Modern Navy frigates fall in the last category.

In effect the max hull speed law is nothing more then a speudo-scientific intepretation of the Froude law which only describes what happens to wave-making drag at a certain hull speed.





I don't care much for the title of professor. In my time at the university I've seen professors making the biggest of dumb mistakes. The simple fact that the Taipan 5.7 easily travels faster then 6.07 knots disproofs the max hull speed law in the way you have worded it. If prof Anders Endal did indeed word it in the very same way then he too is wrong. No-one will be claiming that a Taipan 5.7 travelling at 10 knots is in planing mode, you yourself can confirm that. Just like at how the hulls travells through the water at this speed.




Don't worry Jimi, I'm not made of cake. I can handle my fair share of abuse (not that you are given me anything of that at all). I've had discussions with many people were the discussion have gotten alot more heated. Also I'm not unfallable either, I fully expect to be put in my place when I say something that is scientifically wrong. However in this particular case I'm certainly not wrong.





That may be his reputation, I don't know. However, are you sure that you inteprete his writings correctly ? If his writings are exactly as you presented them then I fail to see the thruth in his reputation.





I understand, hell even my own universities textbooks were less then thruthful on this subject. I had maritime courses during my studies. Later on it turned out that they knew better but thought it too much to burden the young students with the truthful situation. I think this to be on the border of what is acceptable. It would have been alot better to describe the law not as a law but as an intepretation that was only valid on a limited numbers of cases. How difficult would that have been for young students to understand ? We must give the youngster some credit, they are smarter then we
often think they are.


Jimi, if you ever see a discussion about the jib slot effect then you can again expect to be taught a myth, much like this max hull speed law.

If you really want to learn then read Marchajs book or Frank Bethwaite book or the writings of Arvel Gentry. These writers have gotten it right on max hull speed and jib slot effect.


Good luck,

Wouter

I used to think this too. I now do not believe it to be true. There is some very good drag prediction software called michlet, specifically for long slender hulls like catamaran hulls. It appears to be well validated, and I have done a little experimental validation on it myself. I am happy with its predicted results. It is by using this sotware that I have come to those conclusions.

If you know of a better resource for drag prediction of slender hulls I’d like to hear about it, or better still experimental data.

So I will stick to my guns on this one when I say that on beachcats longer and lighter is always better for low drag at the speeds that count and that at 6 knots the wave drag is far from negligible.

In addition when you say that for a 107kg boat 5m is better than 5.5m I would say that that only holds true for speeds above 20knots, below 20 knots 5.5m is better. Although to be honest there is bugger all in it.

Gareth
www.fourhulls.com

I guess I just have to ask the guy himself, prof. Anders Endal, on what he thinks about the subject. No matter what, if we acutally are tought things that are wrong simply dued to the impression that we(the students) cannot handle the science behind the truth, then that truly is f.... up. I will ask him the next time I see him.
As to the book written by Frank Bethwaite, my dad (Stein) has read it and also thought it was great.
Thanks Wouter!

Well, this discussion has the potential to explode into a very large validation research. But such a thing will get beside the point.






The best experimental data I have is Bethwaite and the Miss Nylex C-class design article. Especially the last quantifies the different drag components. Here the wave-making drag (form drag)for the conditions in which the C-class races was given as 15 % while the wetted surface area drag (skin drag) was given as 22 % of the total drag of the whole boat.

To put things in perspective, the drag related to the rig itself was given as 30 %. The daggerboards at 21 %

I still value the C-class designers as the most scientificatlly oriented catamaran designers. It also squares nicely with what is known from other sources like Bethwaite and the design of mono-hulls. The model I described neatly fits all the experiences over a wide range of vessels.

Added to this the F16 experience itself. The F16 concept was developped fully based on the model I presented earlier. As of yet the F16 is doing exactly what it is predicted to be doing by the model and so in that experience I find no counterproof as of yet.

I don't know what kind of model your software is using. I would be interested in finding out what drag it predicts for a long flat plate dragged through the water. Good predictions are to be found for this situation to check the software itself.


Having said all this I spot a contradiction in your statements. You wrote :




First you say it is far from negligible but later you say "there is bugger all in it", implying that indeed it is negligiable !

Please clearify.


Regards,

Wouter


p.s. I'm afraid that I won't be able to discuss everything in detail, I simply don't have the time for that.

From experience, synthetic shrouds stretch too much and if you wiggle carbon back and forth too many times it breaks. I like dyform

Also have the rigger configure your wires for minimum weight where the wires connect to the mast.

You say that “ at these low speeds the drag from the hulls is almost entirely created by wetted surface drag” wheras the Miss Nylex data you present shows that there is a 60/40 split. I don’t know off the top of my head at what speed or Froude number this data is taken but I bet a cold beer its higher than 6knots. At 6 knots the split would be the other way around 40/60 with wave drag taking the higher percentage of drag. Surely the Miss Nylex numbers back up what I am saying.

That is why I say that wave drag isn’t negligible. When I say there is bugger all in it between a 5.5m and a 5m hull. I am saying that the wave drag has a big influence on total hull drag but changing from 5m to 5.5m length only is not enough of a change to affect the wave drag by much. A thinner longer hull is always better but reducing displacement is the real key.

If you want to find out more about Michlet it can be downloaded for free at http://www.cyberiad.net/michlet.htm

Here the wave-making drag (form drag)for the conditions in which the C-class races




Correct, that is why I wrote in the preceding sentence :"... for the conditions in which the C-class races ..." These speeds are above the Froude law based max. hulls speed number. The article names speeds around 10 knots (6 - 14 knots). This speed is the one that is most encountered along the track even when higher speeds are achieved on smaller sections.




No Grob, this is completely wrong and most fundamentally flawed scientifically. Wave-making drag INCREASES when the hull speed is lowered ? While the wetted surface drag decreases ?

I'm not going to explain away all nonsense arguments that are put forward. I'm truly sorry.




Now were are back at the "always" rules again. Ahh well, I've explained my side. Whether other people believe or not is up to them.

Best of luck Grob,

Wouter

Wouter,

If following a weight reduction we only reduce width or only reduce length or only reduce height, the boat will result slower. The right procedure would be to scale down the entire hull in proportion to the weight reduction, so that the original shape is maintained. You proved this point well, but forgot to define some necessary conditions for this to hold true:

Firstly, we need that the initial hull shape is already "ideal" for its weight. If it isn't, reducing only one dimension might actually improve the shape.

Secondly, the crew weight should be reduced in the same proportion as the boat weight, so that the original hull shape remains "ideal". If it isn't, there will exist a new "ideal" shape and a single dimension modification could work well (although different reductions in each dimension would work best to adjust the shape)

Lastly, the sail area should be scaled in the right proportion (not linearly, of course), so that the original hull shape remains "ideal".

Bill Roberts likes to use this type of rationale and posted something along this same line of thought in this forum some time ago.

Cheers,
Luiz

<img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" /> You are saying I am completely wrong and then agreeing with me, I do wonder about you sometimes.

Wouter it cracks me up when you get fired up but it appears youve thrown some good info my way, so before I accept all this writings of Arvel for gospell are there any of his conclusions to be wary of. ( Sorry to be off topic )
thanks in advance

Luiz,

You are mostly correct. Actually this is the basis under the F16 design in relation to the F18's. More info found at http://www.geocities.com/f16hpclass/F16HP_to_F18_analysis.html

I may write some things differently in the article if I had to rewrite it as my knowlegde has advanced somewhat over the last 5 years, but the general line of thought is still the same.

Bill Roberts often misapplies this method to components that are not really accurately described by these laws. Example; he uses them to calculate boatweight by scaling, but this is wrong as several aspects are already at their minimum wall thickness etc in the bigger boats. You can't scale such components to smaller dimensions. But displacement calcs etc are accurately described by these laws.

Also the is no need for the crew weight to be adjusted, this can be done by reducing the boat weight by an extra amount and adjusting width to compensate for differences in righting moment. Again this is what F16's have done. It is one of the reasons why the F16 width is 2.5 mtr instead of 2.6 mtr.

Have to go now.

Wouter

I guess my lack in mastering the english language in combo with my typing skills is to blame here.

the phrase :

No Grob, this is completely wrong and most fundamentally flawed scientifically. Wave-making drag INCREASES when the hull speed is lowered ? ...


Would have been better if written down as.


No Grob, this is completely wrong and most fundamentally flawed scientifically. It is illogical to claim Wave-making drag INCREASES when the hull speed is lowered. ...


Extra explanation

This would either mean that wave-making drag is high when the boat is motionless, which is to weird to consider seriously. Or for some weird reason is is zero when motionless then increases with speed only to decrease again after Froude's law max hull speed and possibly increase again with additional higher speed. This would be weird as that that would predict heavy monohulls to NOT be held back at the max hull speed rule of thumb as we know they are. Either way you run into a contradiction with well documented real life phenomena.


Also I've lost sight of my own "red line" now.

Basically my model is :

-1- Both wave-making drag and wetted surface drag increase with increasing boat speed.
-2- Wetted surface drag is a significantly larger component at low speeds (below Froude's speed)
-3- At higher speeds wave-making drag starts to become a significant factor when compared to wetted surface drag. The transition to this situation is to be found around the Froude speed. (and is what the Froude law actually says)
-4- Overall wave-making drag on cats doesn't seems to be a very large component of the total drag.
-5- Wave-making drag is not very dependent on hull length beyond a certain min prismatic hull ratio.
-6- Wetted surface drag is always strongly dependent on the total wetted surface area.


Wouter

You should never consider anybodies writings as gospel and that certainly includes mine as well. As I wrote earlier I've even seen professors make huge dumb mistakes. I don't consider myself to be something that even the greatest minds are not.

I can't name problem issues with Gentry so far but I haven't read all of his stuff yet. Always use your own mind and intellect, not forgetting common sense, to check anybodies writings.

Maybe one extra note that is interesting in the larger discussion is the fact that Texel measurement handicap system EFFECTIVELY uses a 2.5 power relationship between speed and hull length. The relationship between wetted surface drag and speed is a quadratic one (2 power). This means the wave-making drag component is approximated by 0.5 power relationship on top of the wetted surface drag component. Both are continiously increasing functions for increasing speeds. Texel handicap seems quite accurate in its speed predictions relative to speed. It is not a big argument but I do think it is another element supportive of the model as presented earlier by me.

Wouter

Why would you want anything lighter than an M20?
On the beach these boats have sandbags on the front beam to stop them from blowing away!
I talked to a top M20 sailors at Texel this year and he told me that the M20 was very scary in high winds, something about the cat controlling them instead of the other way around.
And how much performance does weight reduction affect performance in practice? If you see this year's Round texel results, very little. (F18 six minutes behind the first M20, and 1.5 minutes on the first Tornado spi (all sailed by crews with equal skills).

I think I have to go back to the initial question in the thread-starter to answer this one. First of all, as wind picks up, the advantage of a low weight boat decreases quite rapidly. I do not recall how the conditions were during the last texel, but were they not around 10 to 12 knots or more? And I think it is fair to say that low weight boats have their biggest advantage in lighter winds than 10 knots, take the A-cat for example.
Second, I initially asked the question more like a "what is theoretically possible question" more than a statement that low weight always is best for a beach cat. However, one do have to keep in mind that all high performance development classes use pretty much of their time (and money)to find new ways of saving weight. And as time goes by and new techniques and technologies are discovered, maybe there will be ways of making the mast for instance a couple of kilos lighter AND 500 dollars cheaper. And that is exactly why one should keep focus on, and ask these questions on catsailor. First because it stirrs people up and causes discussions, second it "provoces"(sorry, only word I could think of) the home builders and engineers on this forum to think of new ways of imroving beach cats. I don't know about the rest of you, but I myself am extremely keen on trying to find new ways to make the cat go faster <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

We sail the Super Taipan in 30kts with no problems at all. The only small issue is the fact that the kite is pretty big and the hulls lack bouyancy to drive it hard downhill. but we just go a bit deeper and get there almost as quick.
The advantages are very clear in the lighter wind ranges where the power to weight really comes into its own and the boat is super quick particularly downwind.

With a more bouyant (ie longer) hull and slightly less weight i would expect the boat to be quicker in all winds and in particular downwind in a blow.

The light weight is only a problem if you chicken out and start sailing too high upwind, as soon as you do that the boat slows up a lot and then the breeze starts to pick the tramp up and then you are in a world of hurt....

We keep pressing and drive the boat at its proper angles and it gets upwind very well. Some of this is due to having a jib which moves the COE of the sail plan lower than if it was mainsail only like a M20.

should read :

-5- Wave-making drag on cats, in an absolute sense, is not very dependent on hull length beyond a certain min prismatic hull ratio.

In this case the wetted surface drag is by far the most dominant factor and it is this aspects that will favour shorter extreme lightweight hulls over longer lightweight hulls.


Sorry,

Wouter

Drag consists of
1.Skin friction drag, which depends on the surface area in contact with water.
2. Form drag (or pressure drag), which depends on the shape of the moving object. Boats moving in the water surface and planes at transsonic and supersonic speeds create waves which create wave drag.

1. Skin friction drag:
Wouter, you are correct that reducing weight by reducing length reduces surface area more than reducing the same amount of weight by reducing hull width. Reducing weight of a rectangular box floating on the water surface by 10% in a 6 m boat by reducing length, reduces wet surface area by 9.37%. Reducing weight 10% by reducing width, reduces wet surface area by 3.7%.

2. Form drag (pressure drag)
Cross-section area of the hull and hull shape are of importance.

2.2. Wave-making drag is a component of form drag.
For displacement ships, the power needed increase speed above the (root of water-length * 1.34) increases abruptly. This rule is theoretically founded on the fact that max wave-speed depends on wavelength and gravity.

When increasing speed over theoretical max speed, you need to climb the bow wave, and this demands a lot of power.

One way around this limitation is the semi-planing mode. With a vertical flat stern the water detaches from the stern at a certain speed. Thereby, the stern wave is tricked into reattaching far behind the boat. Hence, the wave-making boat length is much longer than the water line.
The International 14 class guys use horizontal wings below the water surface to trick the water to behave as if the waterline length is much longer. (In addition, these wings prevent pitch-poling.)

Most of our beloved catamarans sail in the semi-planing mode.

(Some has maintained that cats like the M20 can sail in planing-mode, but there are different definitions of planing.)

Another way around the displacement speed, is to make the hulls so narrow that they only create very small waves.


By shortening the hull, you may as Wouter, has pointed out, reduce friction drag. However, increasing hull cross-sectional area to maintain volume, increases form drag. One part of form drag is wave drag.


Most sailboat classes have a maximum length.
HAVE YOU EVER SEEN SUCCESSFUL BOATS THAT ARE SIGNIFICANTLY SHORTER THAN THE ALLOWED CLASS LENGTH?

Stein

Stein,

I always considered the "semi-planing mode" to be akin to "a little bit pregnant"

In aerodynamics and hydrodynamics lengthening the object by adding a virtual object is possible, the aerospike rocket engine is using this principle very effectively, but it isn't happening with cat hulls. Look up aerospike rocket for more info.

For such a virtual hull to work it needs to excert a raised pressure onto the vertical stern itself. As in real life the sterns are only experiencing ambient airpressure it results that no vitual hull is effectively present. The theory of virtual hulls applied to cat hulls is now nothing more then fitting the errornous formula to "explain away" conflicting real life data.

I'm sorry.





I'm of the opinion that beach catamaran hulls are not of the "semi-planing mode" type if I could bring myself to ever accept such a concept, but rather that they are of the long narrow type with only relatively small wave-systems. I can actually SEE the truth of the last statement when I sail my own F16 while there are absolutely no indications of any planing.





When the reduction in wetted surface drag is bigger then the increase in form drag then going for a shorter fatter hull is more attractive from a drag point of view. Clearly this is not always the case, but just as clearly we can conclude that this isn;t an impossibility either.




To be really honest, I know of no boats that are shorter then their maximally allowed class lengths. In effect I have no data points of this kind. I put this down to nearly everybody believing that longer is always better.

The first real recognision of the fact that shorter make be less draggy, that I personally know of, was made in the F16 class. That is with one exception as Phill Brander once told me that such a thing was tried by a homebuild A-cat at one time. Not further data is know to me about this experiment.

But also my point is worded a little bit differently. Is wrote that when you really reduce the overall weight (boat+crew) for a given setup that it is best to also shorten the hull length by some amount over ONLY reducing the width of the hull. Most boatshowever are far to heavy to even consider such a move. Thereis of course a transition point somewhere. Also the only class that ever made a weight reduction big enough to have encoutered such a phenomenon are the A-cats. The F16's are still relatively young although the level racing with the F18's despite being shorter with a smaller rig does hint at the theory being right. They can only be as fast despite these "disadvantages" is the overall drag of the boat was reduced by at least the same amount. Now either the hull drag component is too small to matter in relation to other drag factors like the rig or the hull drag was reduced by at least 15 % as well.

Wouter

Here is the drag of a 5m 300kg (150kg boat+150kg crew on one hull) catamaran hull. Predicted using Michlet.


Blue Rw is wave drag, Red Rv is Wetted surface drag, speed is in m/s so double it to get knots. Drag is in kN.

As you can clearly see wave drag is far from insignificant, and is in fact larger than WS drag up to around 9 knots.

The next plot is the total drag (wave + WS) of a 5m hull compared to a 6m hull, both at 300kg and both optimised for minimum wetted surface area for there length.


The Blue hull1 is the 5m hull and the green hull2 is the 6m hull. Again you can clearly see
that there is no speed which favours the shorter extreme lightweight hull up to 20knots.

When I talk of wave drag at low speed I was talking about under 10knots not under the hull speed perhaps this is what is confusing you.

Gareth
www.fourhulls.com