But lets get one other thing straight too. When I'm in the market for a cruising multihull the catri will be on the top of my list.

Just like Luiz i dig the simplicity of the system and it looks like a great fast multihull trimaran with pananche.

For more info go to :

specs : http://www.aegeanmarine.com/Specs.html

pictures : http://www.aegeanmarine.com/SF_Sailing.html

principle : http://www.foils.org/catri.pdf

The only thin I don't understand is why the design hasn't opted for a squaretop main yet. One would think that such a new design would also incorporate innovations in other area's.

wouter

Wouter, would you confirm that your understanding of engineering principles and physical laws state that present craft designs are limited in speed to about 23kts for b class cats, that would apply to all conditions (on water) except one wave events?

Wouter,

I am still not in position to give a definitive evaluation of the Catri concept, for mine is still being laminated now. However, I can pass the information received from people sailing the boat in Sweden and in the US, under the light of pieces of information received from the designer.

As you correctly stated, the concept is to lift only 90% of the displacement with the foils, but the boat does not always sail this way.

The new owner of the first Catri 26 prototype, from Sweden, told me that he likes to sail singlehanded with one or two rreefs in heavy winds. Under this conditions, he says that the boat sails totally airborne (supported by foils only) and is the only boat around that can rival the windsurfers.

The trick is that the boat is designed for a bigger crew and the displacement is lower when singlehanding. This enables the boat to be 100% supported by the foils.

He also said that, contrary to your expectations, the self-stabilizing system (rear outrigger foils) continues to work under this conditions and the boat remains remarkably stable and easy to sail.

The reasons appear to be:

1) there is far more lift in the forward bruce foil then in the rear foils - just look at their sizes.

2) the position of the bruce foils in front of the mast give better leverage. (with the added benefit of the "shared lift" - the same concept Bill Roberts uses in his latest ARC 21)

3) The reefed sails in heavy weather generate the same power - but in a lower position up the mast. This configuration requires less righting moment (reduced vertical load on the foils) and is less prone to pitchpole (due to the smaller leverage arm of the sail's horizontal thrust).

I think this is enough to understand why the design features of the Catri help keep the boat controlable and with a bow up attitude all the time.

About the square tops, the same guy ordered a new set of sails - with a square top main. I don't think the square top will make that much difference in speed - I expect the difference to be relevant only in match racing conditions.

The designer says that the eliptical top is better suited for heavy weather/high speed conditions, while the square top is better in light weather.

He has doubts about the behaviour of of square tops under heavy weather and for him this weights more then the light weather/match racing advantages (for safety reasons). This is a minor issue for him, though.

The 2 ft length difference between a Tornado and the Catri 22 is almost irrelevant. The Catri is a lot wider, so the righting moment is bigger and it can carry a lot more sail then the Tornado. Also, it is relatively less draggy in top speed conditions.

It seems more appropriate to compare the Catri potential with totally airborne hydrofoil boats, like the Rave, for example. Then the 32 knots can be seen as a conservative figure.

I see the Catri as a conventional tri in light winds that automatically converts into a hydrofoil tri in stronger winds. A very smart combination!

Thanks for discussing my dream boat - your expertise is always welcome!

Best regards,
Luiz

Luiz,

I saw the pics of you catri being produced on the website, looks good.

Hopefully you can sail it rather soon.

>>The new owner of the first Catri 26 prototype, from Sweden, told me that he likes to sail singlehanded with one or two rreefs in heavy winds. Under this conditions, he says that the boat sails totally airborne (supported by foils only) and is the only boat around that can rival the windsurfers.

The thing is that the arrow principle is used to set the angle of attack for the bruce foils. When the hulls become totally airborn the angle of attack can only increase trying to lift the boat even higher out of the water which in turn would increase the angle of attack and so on. The only two things that can stop this run away proce is to lift the rear of the boat to but it doesn't look like the rear foils are designed to do that. They look to horizontal to me. The other thing that can stop this runaway proces is the reduction of wetted foiling area. downside of this approach is cavitation as the all the weight of the craft then needs to be carried by an ever decreasing foil area. This can only be done by increasing the pressure difference over the foil which is mostly done by increasingly lowering pressure on the top side of the foil bringing is quickly closer the vapour pressure of the water. Once this limit is reached cavitation sets in.

There is no additions information available of the catri 26 so lets look at it's closest brother the catri 27



If you look closely you can just at the rear of the amas see the little arrow tail foils , in the drawing they are above the waterline. These foils will not lift the back of teh boat out of the water. At max they will lift the stern of the ama level with the waterline. Only the little horizontal foil on the rudder could lift the back of the boat free of the waterline but it is awfully close to the water surface which is not good for a hydrofoil and besides if the mainhull is lifted free of the watersurface the rudder foil would be lifted free of the water too. See how it is set higher than the keel of the mainhull. Also in the explanations of Eglais the rudder foil is intended for smoothing out the sternwave, I'm unsure to what extent it is also intended as a lifting foil.

In short my take on things is that the rear of the boat can be lifted no further than just in contact with the watersurface. The front of the boat is not limited in this respect and therefor any increase in lift in the front will be accompanied by tilting teh craft on its rear which will have the side effect of increasing the angle of attack on the main foils which in turn will cause them to produce more lift and trying to lift the front even further which lead again to even bigger angle of attacks .... Ergo the runaway system. Ofcourse the reduction of area will try to stop this but at any rate the increase in angle of attack will not make it likely the foil produces lift at its drag minimized setting.

This is just the reason why full foiling craft have activily controled attitude foils. Eglais system is so neat because it doesn't have any of those. Downside is ofcourse that acts based on certain garanteed conditions and being level with the watersurface looks to be one of them.

Now the catri 26 maybe designed differently, which I don't know, I can think of a few way in which to do that. But given the catri 27 I would like to see a picture of the full foiling catri tri, meaning with all of it's hulls noticably clear of the watersurface.

>>>The trick is that the boat is designed for a bigger crew and the displacement is lower when singlehanding. This enables the boat to be 100% supported by the foils.

from a weight perspective yes, but not from the perspective of the intended way the foiling system is designed acts to become a stabil, controllable and low drag system.


>>He also said that, contrary to your expectations, the self-stabilizing system (rear outrigger foils) continues to work under this conditions and the boat remains remarkably stable and easy to sail.


That may be so, but than I would like to see where the control system is. It maybe a unpowered passive system but I can't recognize it in the catri 27 design from the detail supplied on the websites. So please explain to me why it does stay stabil.


>>The reasons appear to be:
1) there is far more lift in the forward bruce foil then in the rear foils - just look at their sizes.


This is no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.


>>>2) the position of the bruce foils in front of the mast give better leverage. (with the added benefit of the "shared lift" - the same concept Bill Roberts uses in his latest ARC 21)


This is no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.



>>3) The reefed sails in heavy weather generate the same power - but in a lower position up the mast. This configuration requires less righting moment (reduced vertical load on the foils) and is less prone to pitchpole (due to the smaller leverage arm of the sail's horizontal thrust).

Yes this is again no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.

When I look at the claim made on http://www.multihull.de/catrimarine/

About the Catri 26 c (the prototype you refer too) having 50 square mtr sailarea per ton weight all I can say is :

Tornado = 135 sq.mtr. / ton = 72 sq.mtr / ton when including crew
Hobie 16 = 130 sq.mtr / ton = 65 sq. mtr. / ton when incl crew
F16 = 168 sq. mtr. / ton = 71 sq. mtr. / ton when incl crew

The catri 26c therefor needs to have 50/72 = 70 % of the drag to power ratio than the Tornado. In this respect the extra width of the catri will definately help by increasing the power in this quotient. But the drag of the catri may not increase accordingly when going from the ligher tornado to the heavier and bulkier catri.

Lets discuss this further but first :

>>I think this is enough to understand why the design features of the Catri help keep the boat controlable and with a bow up attitude all the time.


Well it was not pitchpoling that I was talking about. I'm talking about maintaining a low drag to power ratio. I do see the power thing in this but I have my doubts about the amount of drag. I do do see it is reduced with respect to comparable tris, but I'm not convinced about it being small enough to achieve 30 + speeds. The other thing was full foiling with the catri system.

But now lets get to the drag.

Once again because I have no further info on teh catri 26 I refer to catri 27. This baby weights in at 1700 lbs without crew. 1700 lbs = 770 kg's. This weight would normally be translated into displacement which in turn causes froms of drag like wetted surface drag, wave making drag, suction on hull etc. Roughly speaking. The whole idea of foiling is to reduce the displacement and thus reduce these forms of drag. The cost of this is of course the same drag forms that are being created by the foils itself and the induced drag of the foil itself. This is a new form of drag.

Induced drag can be explained in the following way. When a foil is place in streaming gas or liquid than two main forces are created. One is lift which is the result of the increase in pressure on the the foil and a larger reduction in pressure above the foil. The second is a force parallel to the current (often called induced) drag; which is also the result of the named pressure differences. Picture this. When a horizontal plate is absolutely flat an parallel to the current then any pressure difference that may exist over the plat will only experience a vertical force (lift) there will hardly be any horizontal force. Now tilt the plate relative to the current, ergo incrase the angle of attack. Now the pressure difference will cause both a vertical force as a horizontal one. Increase the angle even more and the horizontal force will grow because its projected area will grow and the vertical force will decrease because its projected area will decrease. For a given pressure difference that is. Ergo, For a given angle of attack the ratio between the produced lift and the produced drag is a constant. Mostly this constant decrease when increasing the angle of attack.

very good high aspect foils have ratios of about 30 ; Slender gilder plane wings and such.

Low aspect foils and curved plates = sails will have ratios between 5 and 10.

Medium aspect foils (or low aspect onces with end plates) will have ratios between 10 and 30. Mostly around 20 or so.

For now I wouldn't place the Catri foils into the efficient high aspect group but rather in the medium group. Very high rations are not easy to achieve and older glider planes used to have rations of 20 themselfs.

With other effects like the nearness of the watersurface distrupting ideal operation of the foils I would not place the catri foil past a ratio of 20.

So a craft of 1700 lbs will have a induced drag of say 1700/20 = 85 lbs. when full foiling just as a result of producing the lift needed to lift the hulls.

What am I trying to say here ? That foiling may reduce drag with respect to a comparable craft liek the farrier F27 but that this is not the same a having low or no drag. The total drag may still be substantial. Now foiling may well be more efficient in carrying weight than displacement when looking at weight to drag ratios but that may well be a long way from making a 1700 lbs foiling craft less draggy than a 375 lbs Tornado when not even looking at lightweight boats like the M20 of just 242 lbs. So how can it be that a tornado is claimed to be limited to 23 knots when a much much heavier boat is easily capable of blasting past 30 knots = no less than 30 % faster.

This can only happen when foiling is so increadibally much more efficient than displacement; it is this assumption that I have trouble with. Not that it is more efficient but that it is that much more efficient.


>>About the square tops, the same guy ordered a new set of sails - with a square top main. I don't think the square top will make that much difference in speed - I expect the difference to be relevant only in match racing conditions.


Well the biggest advantages of squaretops are their improved gust responses and their improvements in controlling twist.


>>The designer says that the eliptical top is better suited for heavy weather/high speed conditions, while the square top is better in light weather.


The designer is overlooking the gust response of squaretops. Also squaretops twist off better in strong winds. Pin head often displays the behaviour of twisting of the middle part while bringing back in the top of the sail. Not something you want.


>>He has doubts about the behaviour of of square tops under heavy weather and for him this weights more then the light weather/match racing advantages (for safety reasons). This is a minor issue for him, though.

Well gusts responses and the ability to power down the top of the sail are very much inprovements in contrability = safety too.


>>The 2 ft length difference between a Tornado and the Catri 22 is almost irrelevant.

That is why I compared the two.

>>The Catri is a lot wider, so the righting moment is bigger and it can carry a lot more sail then the Tornado.

And it is alot more heavy too. I mean the catri 23 C is 750 kg's = 1655 lbs. with crew 900 kg = 1986 lbs = 2,81 times heavier than a Tornado with crew. Increase in sailarea should be about 1,6 times mostly as the result of the jib. ratio mainsails = 1.33


>>Also, it is relatively less draggy in top speed conditions.

Is that so ? 90 % foiling means that still 90 kg's is in displacement compared to 320 kg's of the tornado. Alot less I admit but then the Tornado doesn't have the induced drag of that 810 kg's that is being carried by the foils. Not to meantion the aerodynamic drag of the cabin, extra beams and extra ama. And at 30 knots these aerodynamic effects are very noticable, ever stuck your hand out of the car while driving at 30 knots ?

For now I don't see strong indications for the claim :"is relatively less draggy" when comparing the catris to tornado's.

Currently it all revolves around a claim that a swedisch team achieved 32 knots as indicated by the GPS. This comes close to a cicular reasoning where these 32 knots must mean that it is alot less draggy which in turn must mean that is can achieve 32 knots. Besides how many claims did we see that people achieved 30 knots on cats by virtue of looking on their GPS.

I mean if cats can only achieve 30 knots by having help from a way maybe that catri was boosted to 30 knots by the same phenomenon ?

>>It seems more appropriate to compare the Catri potential with totally airborne hydrofoil boats, like the Rave, for example.

This I don't agree with fundamentally. The systems used on the rave are significantly different from the catri system. for example the catri system links sideways resistance force directly to lift produced. While the rave system can largely produce any amount of lift independent of the side resistance needed.

>>Then the 32 knots can be seen as a conservative figure.

Well, then I'm looking forward to seeing the catri enter the weymouth speed trails and set a record.


>>I see the Catri as a conventional tri in light winds that automatically converts into a hydrofoil tri in stronger winds. A very smart combination!


Well that is it beyond a doubt.

Ofcourse my sceptis in the much faster than a tornado discussion must be viewed seperately from my admiration of Eglais designs. As you know Luiz when we discussed the Catri before you purchased the plans, I expect a great deal from this design and it definately rates high on my list.

And really, if I ever go for a cruising platform the choice will be between a Catri and a conventional monohull capable of making long journeys.

I'm looking forward to more innovation from Eglais and his design team.

with kind regards,


wouter

Steve,

You introduce alot of conditions in your question :

Let me see :

-1- my understanding of engineering principles and Physical laws. (some would say that this is non existant, but how dependable are they)

-2- Present craft designs, does this include or excludes prototypes and experimental craft ? Does this limit the question to boats like the Tornado H16 or may M20 and M18 for example also be included ?

-3- B class catamarans. This means that Supercats and sorts are not allowed in me answering your question. Especially the big boys like ARC 27.

-4- all conditions on the water (trailered over the freeway is out) , excluding being aided surfing down a wave.


Now before I answer your question and I will I would like to state ones again what I said in teh post that sparked this Speed trails thread.

What I said was :"
...
In light winds cat can reach speed closing to 2 times the windspeed when under spinaker on their optimal course. After that the ratio just drops. Often teh ratio is only 1.5 or less.

Normal beachcat normally sail no faster than the windspeed with a maximum of just over 50 km/hour = 30 knots."

So I say that a beachcat (in the upper spectrum of the windspeed range) can be expected to sail at about the windspeed. This means that say a Tornado in about 20 knots of wind would go about 20 knots and and so on. This would be consistant with the average 23 knots at the speedtrails if these were held in somethin like 20 to 25 knots.

Than I continued with the comment ;"with a maximum of just over 50 km/hour = 30 knots" which is there to indicate that the rule of thumb of topspeed = windspeed in the stronger winds is limited. With this I indicated that a beach cat will not go 40 knots in winds of 40 knots.

I also consider for example the ARC 27 and M18 beachcats despite them being rare and I do define sailing as :"being propelled by natural energy sources". The last includes favourable waves but excludes currents as this first will increase waterspeed but the last doesn't. For these two reason I named the limit of 30 knots for all-out (water) speed of a beach catamaran. And I have no doubt that this speed is reach from time to time under favourable conditions.

By this I answered the question "What is the fastest recorded speed on a beach cat and how fast do cats go compared to the windspeed."

The question wasn't what was the highest average speed recorded over 500 mtr. at an official speed trail ? Nor did it limit the answers to only B-class cats, solely windpower or even mainstream cats.

But to answer part of your question (the main answer will come shortly):

I am totally convinced that even a design made to the B-class specs is capable of reaching higher speeds than 23 knots due to solely windpower. Do this mind experiment : If a standard tornado (singel trap ?) reach the top average speed of 23 knots at an official speed trail than a modified tornado platform of say 125 kg's (carbon) with a rig completely optimized for reaching (shorter mast with fuller cut main and fuller cut jib with double trapezes will undoubtly go faster than 23 knots. On AVERAGE and is very likely to show momentary bursts of speed even higher than that.

But to get the really max out a B-class boat I would lengthen a nacra 5,7 hull and make the hulls out of carbon, use 18 sq. beams to make it 3,05 mtr. wide and put a 8,5 mtr. Taipan (4.9 or 5.7) wing mast with a large high aspect jib on it, optimize the sails for reaching, find a light crew and cream that standard Tornado recording.

Now lets introduce your last limit "present craft design"

This one is more tricky as the better designed boats are often not compliant with B-class rules or when they are they are often fully optimized for windward/leeward sailing. The last means these designs have sacrifized (topspeed) reaching speed for pointing ability and high VMG's. This quickly reduces the choice to a few boats.

Tornado (new version), Nacra 5,7 (with raised boards), Taipan 5,7, Dart 20.

I think that an exellent crew (Booth, Bundock, etc) on of these boats with optimized sails for reaching may well be able to crank that record a few knots higher on wind power alone.

So the answer to your question :"would you confirm that your understanding of engineering principles and physical laws state that present craft designs are limited in speed to about 23kts for b class cats, that would apply to all conditions (on water) except one wave events?"

Would be "NO" I would not confirm that. The way you have phrased your question ofcourse has limited the extra gain that can be made that is true. Therefor 23 knots will be close. But when you allow more freedom in designing there is no engineering or physical reason why a displacement catamaran should be unable to reach considerable higher speeds. It will get alot harder with every knot that is true but not impossible.

I hope this answers your question fully as well as those of others.

To give a real life example. Nobody thought that 16 footers would be able to considerably faster than a H16 ot Nacra 5.0 until a lone lightweight Aussie Taipan was overtaking a whole fleet of 18 and 20 foot catamarans in a 1999 US distance race.

Wouter

Under the B-class rules I gave an example of a heavily modified nacra 5.7

But I forgot to mention that B-class rules allow a different setup from a sloop rig that will only really work on in strong winds and reaches and that is having two masts, one behind another.

I would make two masts of 8 mtrs and fit 2 rather high aspect mainsails to them of 11,5 sq.mtr. each. Thus totalling the 23 sq.mtr. limit of B-class cats. Each main will have a short foot of about 1,75 mtrs and be place right after one another.

Now I have totally slashed the heeling moments and pitching moments of the rig thus I can produce a whole lot more power than the standard sloop rig before I'm limited by righting moment or dive tendencies. The two rigs will be very efficient due to their aspect ratio and the slot between the two rig. (slotting is always used on modern C-cats). I could even use wing sails, B-class rules allow it.

Now I hear you say : what about the interaction between the two sails ? And I say that is just what I was after. Most single sail rigs are unable to redirect the wind all the way from the angle of entry to the centreline of the craft. That is why we travel our mains out on reaches. Now by placing two rig behind one another I can double the angle through which the wind is redirected and milk noticably more energy from the air current. It is similar to powering up your uni-rig with a jib on a reach but now the jib is being replace by a more efficient setup.

These two effects: lower of centre of effort and milking more energy from the same air current combined with a considerably lighter craft than a standard Tornado must go alot faster by the same principles that limit the tornado.

And best of it all is that it is still a B-class cat.

It will be a dog upwind, that is definately true and even downwind will it not be "all that" but speed records are never set on those courses anyway.

Just a useless idea that is only fun discussing it.

Wouter

Wouter,

Thanks for the extensive analysis. What you say makes perfect sense. I think we can summarize in one question:

What drag/weight ratio is more efficient - displacement with narrow beam hulls (cat) or hydrofoils?

I am sure that low weight multihulls with supercavitating hydrofoils are faster then beach cats - but this is no news.

I also believe that the Catri will be faster then beach cats, but only marginally, because of its more conventional hydrofoils and stabilization system - they are very practical but not so suitable for airborne sailing. The size itself will play a role in it, too.

(by the way, Playstation looks like a beach cat to me - this pushes the displacement speed limit to over 40 knots...)

Unfortunately I do not have enough knowledge to write about the Catri, but I am sure that itsi design has more smart features then it seems at first sight. A real specialist in multihulls told it to me after a thorough analysis.

Now some details I feel able to talk about, and that might interest:

1) The original Catri 26, the US Catri 27 and the Brazilian Catri 27 are the same hull but for cosmetic details. The US 27 has a different **** with longer transom and the Brazilian 27 has a rear seat and increased **** size (at the expense of rear cabin volume). The last is partly due to my request - I like ample ****...

2) The rear outrigger foils have a very interesting geometry, that is hard to see in the drawings, but can be seen in the pictures. There is a narrow triangular section under the outriggers supporting the rear foils and locating them slightly under the bottom curve. This feature and their angled down position, enable them to effectively lift the outrigger's transom out of the water - but for the triangular support.

3) As can be seen in the drawings, the outrigger itself is inclined forward, relative to the mainhull waterline. When the boat heels, this causes the bow to lift - and the rear foils to enter the water. The modified pitch attitude creates the initial angle of attack.

4) When the bruce foils are working, when the lift exceeds the needs, the foils themselves are raised out of the water. This has two effects: the Bruce foil's span is reduced and the angle of attack is increased - for all foils, in this last case. If the Bruce foils raise too high, since there is a limit for the angle of attack, the reduced span reduces the lift so it sinks a bit, and reduces the angle of attack. For me this seems to be a stable equilibrium situation. If we look at the rear foils, when the bruce foils are up, the rear ones start working with greater angle of attack, so they will lift the transom higher. This reduces the angle of attack. I have not analyzed it deeply, but both systems seem to be in stable equilibrium, especially when seen together.

The few informations I could get from the designer are that this kind of analysis is plain vanila and wave interaction is the real secret.

5) Eliptical x square extremities - the truth is that I have no clue why the designer prefers eliptical shapes both for sails and for foils.
My best guess is that in that side of the world they are not using equations that take infinite span as an initial condition (like we do) and found means to analyze the actual behaviour of the extremities in variable wave/wind conditions, as in the real world.

I don't understand completely how the Catri system works (and can not discuss all the parts I understand), but the reports from the guys who sailed it are amazing. One said something like "...yes, I know we can surpass 30, but after sailing at 25 I don't know if I really want to"

Anyway, I will learn more about it when the boat is ready...

Can't wait!
Cheers,

Luiz,

(by the way, Playstation looks like a beach cat to me - this pushes the displacement speed limit to over 40 knots...)

That is true.


1) The original Catri 26, the US Catri 27 and the Brazilian Catri 27 are the same hull but for cosmetic details. The US 27 has a different **** with longer transom and the Brazilian 27 has a rear seat and increased **** size (at the expense of rear cabin volume). The last is partly due to my request - I like ample ****...


Okay, noted.


2) The rear outrigger foils have a very interesting geometry, that is hard to see in the drawings, but can be seen in the pictures. There is a narrow triangular section under the outriggers supporting the rear foils and locating them slightly under the bottom curve. This feature and their angled down position, enable them to effectively lift the outrigger's transom out of the water - but for the triangular support.


I will look for it in the pictures


3) As can be seen in the drawings, the outrigger itself is inclined forward, relative to the mainhull waterline. When the boat heels, this causes the bow to lift - and the rear foils to enter the water. The modified pitch attitude creates the initial angle of attack.


I guessed as much


>>>The few informations I could get from the designer are that this kind of analysis is plain vanila and wave interaction is the real secret.


Wave interactions ?


>>5) Eliptical x square extremities - the truth is that I have no clue why the designer prefers eliptical shapes both for sails and for foils.


Probably as the result of engineering analyses on the spitfire fighter plane. Eliptical end pieces give a better transition to normal pressures on finite foils. But this approach to wing tip design has often been discarded for other considerations. Long time we had rectangular ending accepting the more pronounced tip vortexes, and currently we have the shark fin trend. I can understand the elliptical shapes for the boards but not for the sails as the reasons for a sqauretop sail or more on teh area of sail control than on efficiency.


>>Anyway, I will learn more about it when the boat is ready...

>>Can't wait

I'll bet Good luck.

Regards,

Wouter

Wouter, thanks for your reply. I was trying to pose the question in a way to apply to beach cats up to b class size (obviously lwl makes a difference, perhaps not as much as most of us think)
of sloop or cat rig, that would normally be sailed (raced) on all points of sail, without implying that you were the ultimate authority of what was possible. I mostly understand your reply to the 1st question, and your reply to my question has helped to me to think more critically regarding catsailing factors. Steve

Well my appologies I came across a bit annoyed. Hope you did seperate the info from the wording at some instances.

Regards,

wouter

Wouter said, "But to get the really max out a B-class boat I would lengthen a nacra 5,7 hull and make the hulls out of carbon, use 18 sq. beams to make it 3,05 mtr. wide and put a 8,5 mtr. Taipan (4.9 or 5.7) wing mast with a large high aspect jib on it, optimize the sails for reaching, find a light crew and cream that standard Tornado recording."

According to the Stealth web site, John P. is doing just that with his new Berserker. Should be exciting to see...

g

Problem is that I fell for my own trap; dumb, dumb, dumb of me !

The example I gave of the B-class cat is wrong.

How could I be so stupid, I've been working on a comparable concept for years now. I should have known better.

The example I gave was :

"I would lengthen a nacra 5,7 hull and make the hulls out of carbon, use 18 sq. beams to make it 3,05 mtr. wide and put a 8,5 mtr. Taipan (4.9 or 5.7) wing mast with a large high aspect jib on it, optimize the sails for reaching, find a light crew"

First of all I one makes a platform alot lighter one must never lengthen the hulls at the same time. The first will reduce both wavemaking drag and wetted surface (friction) drag but the second will go a long wave in limiting the reduction of wetted surface drag again. Okay the wave making drag will be much more reduced by lengthening the hulls but that is only adcantagious when wave making drag is absolute terms is bigger than wetted surface drag at the topspeed. This is by no means certain, in fact it is likely that frictional drag accounts for a bigger portion of the platform drag than wave making does. So the design should aim on minimizing the frictional area to the max. This will only be done by making the craft lighter and maybe even shorter hulled.

Than the "making is wider to 3,05 mtr." Boy that was even more dumb to say. In the article of the C-class miss Nylex it was found that an earlier C-class named Quest 3 had beams that accounted for some 10 % of the total drag (incl rig and crew) and no less than 18 % of the platform drag. C-class cats are 14 foot wide = 4,27 mtr. This is a huge chunck of drag. What is the best way of reducing this darg component ?NOT HAVING (so much) BEAM, ofcourse. So from a efficiency point of view the best way to arrive at a low drag platform is to stay at 2.5 mtr wide and have less beam as well as smaller diameter beam = less frontal surface area.
Some fairing on the hulls before the beams will also do alot to minimized the drag that caused by water flowing over the decks.

But what about righting moment I can hear some think. Well the trick to high speed is having low drag. Not just alot of power. Beside my own ramblings on the webpage :

http://www.geocities.com/kustzeilen/heeling_pitchpole.html

Show that a craft on a reach or shy broad reach (best course for topspeeds) is in far more danger of pitchploing than capsizing. So, the craft doesn't need more width at all. It will not be able to convert that width in power anyway because it will only pitchpoling more easily. In fact there maybe room the reduce width even a little bit to just over 2 mtr. (=O.5 mtr less = 1,5 ft less)

I would defiantely still put a wingmast on the craft as that has a better lift to drag ratio which is totally vital in achieving high performance.

I would still make the mast a little shorter to be able to get more thrust for a given pitchpole moment limit.

And I would still find a light crew.

So there you have it. Even I after looking and calculating at lightweight boats for so long fell for the predictable mistakes.

A laps of intelligence that I thought I should set straight. This boat will be a dog upwind even more but it certainly would go fast on reaches.

Wouter

Wouter,

I wrote>The few informations I could get from the designer are that this kind of analysis is plain vanila and wave interaction is the real secret.

You wrote>>Wave interactions ?

Dynamic analysis of the foils lift when sailing with waves.
Waves affect the way foils work and the foilborne stability in many ways:

For example:

Waves change the angle of attack more or less cyclically. I think this "more or less" is a math nightmare in all but ideal (theoretical) senoidal waves. Even in the simplified theoretical case, the analysis is not simple - spanwise, each part of the foil works with a different angle of attack.

As a consequence, the angle of attack in the working foils depends not only on the (relatively easy to calculate) pitch atitude, but also on wave's geometry (length and height).

Since there are many foilborne sailing configurations (and partially foilborne too), with different foils put to work in each configuration, it is necessary to take the relative position of the working foils and the sudden change to a different combination of working foils under different combinations of wave/wind generated transitions.

The dynamics of a foil working/stoping to work/working again also has to be considered. In other words, how fast are vortexes (and lift) created and disappear in the real world - this is not an instantaneous process.

This is enough difficulty for me...

Cheers,
Luiz

Dear Wouter,

The topics surely grew more and more interesting about the "ideal 19 ft cat ".

In fact, you are quite right about the relative importance of hydrodynamic drags. At maximum speed, where the rig propulsive force reachs a top level around 90 lbs ( 12 % of the whole crowded boat ), hull friction drag accounts for 55 to 60% of total drag ( appendages 25% and divergent wave-pressure resistance for less than 20% )...so reduction of wetted area is REALLY the main concern !

About the beam aerodynamic drag, it's more doubtful ! Intensive wind tunnel tests for Ollier's big cats didn't show spectacular levels, the induced vortical drag from the rig is the main aerodynamic component, hull drag with lateral huge separations came after, and beams followed...The water that hits regularily the connexions over the deck bridge is far more critical than aerodynamic fairing ( water has a 800 times superior density !!! )

Sincerely Yours,
Frenchie

Frenchie,

What do you mean exactly by "around 90 lbs ( 12 % of the whole crowded boat )"

You mean the envisioned boat needs to weight 90/0,12 = 750 pounds ? Or do I misunderstand your comment.


>>About the beam aerodynamic drag, it's more doubtful ! Intensive wind tunnel tests for Ollier's big cats didn't show spectacular levels


But didn't the Ollier cats feature faired beams ?

>>the induced vortical drag from the rig is the main aerodynamic component


Of course but there is not much you can do about that when faced with a given amount of drag. The weight of an engine is significant in most cars but without it it wouldn't be a car. Only making the craft less draggy may result in less engine needs and then one can optimize the rigs accordingly.


>> hull drag with lateral huge separations came after, and beams followed...The water that hits regularily the connexions over the deck bridge is far more critical than aerodynamic fairing


Ahh, but I didn't write "aerodynamic drag" did I ? I refered to the total drag of the beams which, as you say, can be devided in two main components (other the weights and sorts) Aerodynamic drag and hydrodynamic drag.

You seem to have acces to test results that I haven't seen yet. Are these public domain or can I access them upon request somewhere ? I would very much appreciate that.


Sincerely Yours,

Wouter