I've got the popcorn, looks like it will be a good cat fight!!! oops, did I say that?

I think somebody needs a little prozac! Heck maybe we all do... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Clayton

You are the one saying it's impossible. IMHO the onus of proof is on you. I'd personally be suprised if they often do such a speed but definatly wouldn't say it's impossible.

Tiger Mike

I can tell you that I went faster than I have every been on a cat before in the Tybee...I think it was day 5 to Fernandina. Frank and I were double trapped and wrestling to reach deep (no spinnaker) in a pretty good patch of breeze, Frank is watching the GPS and calling out speeds 23 mph!...22mph! 23.5 mph!...24 mph!...and then a gust hit and about jerked the boat out from under us. Frank dropped the GPS to work with the sheet and I eased the traveler thinking we were doomed. My eyes stayed glued on the bows (I remember wanting to close my eyes but I couldn't look away) but to my amazement they stayed dry. I don't know how fast we actually went, but the GPS recorded a max speed of 29.9mph (26 knots) - and I might actually believe it. I don't think it's that unreasonable to expect one of those beasts to be able to touch 40 knots - unlikely ... maybe ... but all this is simply conjecture.

It is obvious to see. Going from 26 knots to 40 knots sees a 250 % increase in drag. This is why the record breaking boats are foiling or full planing hulls (as crossbow was).

All that drag gas to be overcome by a 250 % bigger thrust while at these angle of attackt leads to 400 % higher capsize moments. These VX40 boat were pitchpoling in 15 knots already. The are lifting a hull in as little as 6 to 7 knots, What do you think 20 to 30 knot winds will do ?

And this thrust has to be produced by soft sails which pretty much max out at an angle of attach of 25 degrees. Below 15 degrees the shape collapes unless you start using landyach setups which have done away with squaretops as you simple can't control them at these small angle of attacks. The VX40 feature very large squaretops.

The energy has to come from somewhere ; if you do the math you come out at required windspeed of at least 20 knots (and that is a very conservative minimum). Crossbow, yellow pages, Minard and even long shot (planing and foiling craft with way less drag) needed winds of AT LEAST 30 knots to break the 40 knots threshold. Why should VX40 do it in anything less ? Even the Maxi cats sustained their records in 30 knots winds or more. Any guesses as to what the seastate is at 30 knots ? Again, crossbow, longshot, yellow pages and the rest of them all made their runs on special locations with extremely flat water conditions.

And we must not forget that by far most of these record breakers were all using asymmetrical rigs and boards. I think longshot is the exception. This means that they were fully tweaked to run very fast on one tack ONLY. We all know that the VX40 is fully symmetrical. This is a serious hinderance to high speeds.

Being lightweight or not is not really an issue in these record speed attemps. Optimizing lift to drag ratios is. That is why these record attemps either use solid sails are very rigid windsurfer rigs.

It quickly becomes a problem of aerodynamics. And that is only when you have solved the hydrodynamics part first. 40 knots on a displacement hull is not a receipy for succes. The amount of drag goes through the roof. So you'll need planing hulls, bruce foils or full foiling. But even foils get into cavitation problems above 45 knots.

Enineering wise going from 26 knots to 40 knots is a major undertaking where you optimizing everything to achieve that single goal of high speed. Accepting pay off in all other areas. Areas like pointing, low wind speed, symmetry. All areas where the VX40 DID NOT skim on.

Therefor the idea of a VX40 doing 40 knots as max speed is just nonsense. I will be impressed if they do 30 knots. Using some carbon or kevlar doesn't change any of the engineering problems that took several teams (like crossbow etc) decades to solve. It is not about materials it is about hulls and rig design.

If you don't believe me then that is alright, go to college and do maritime engineering yourself for about 4 years, then spend about a decade to develop your own models and validate them with experimental data and then see whether you still believe these boats do 40 knots, just like that.

Wouter

Your post doesn't give any reason why 40 knots on a VX40 is impossible. Improbable maybe, you appear to be trying to confuse people with bad science.


When the big multihulls go fast they do so in high winds with reefed sails, the centre of effort in the sail drops enabling you to handle more power.

we have all pitchpolled at low speeds it doesn't mean the boat can't handle higher windspeeds. Are you trying to perpetuate the myth that catamarans are unsafe in high winds?



Speed records are set in flat water conditions why should the VX40 be subject to different rules.

Are you saying that square top sails are no good at high windspeeds, thats not correct, look at the windsurfer sails that break the speed records in very high windpeeds.

I am not saying its going top be easy but I don't see that max speeds of 40knots are impossible.

In previous threads you were arguing that windsurfers were slower than cats, whats it goin to be.



Are you saying that you can only argue with you if you have this level of expertise, you certainly do not. I would imagine that these boats have been through some analysis of the type you describe and it is not inconceivable that this is where the 40 knot number comes from. The designers are quite reputable I guess they would want to stop any wildly inacuurate statements made about their boats as it would damage their own reputation.

Gareth
www.fourhulls.com

blah blah blah...

I see a bunch of jargon up there and "geek speak" however one thing I know is...

the naval architects that designed the VX40's are better than you are Wouter. Which is why they are designing 40' racing cats and you're sitting there naked in front of your computer.

I think Wouter made a pretty fair argument. I think the windspeeds required by record breaking planing windsurfers make their mechanics pretty irrelevant.
I dont think it would be too hard to figure the drag on the displacement hulls at 40 kt and figure that that much force a reasonable distance up the mast would have turned it over (one way or the other - )
I also know that anyone who sticks their neck out with a logical argument inviting discussion should be encouraged. That is why I come to this forum.
Basicly, if your post knocks someone else and contributes nothing, don't continue, it just makes more scrolling for all of us. Thanks for scrolling.

well could the brainiacs in the group figure out how much more oomph it takes to push it from 36knots to 40?

If you don't know this, then you shouldn't be knocking Wouter's "geek-speak."

Drag goes with velocity squared, so...

(40/36)^2 = 1.234 or 23% more drag. Therefore 23% more thrust and at least 23% more stress throughout the rigging.

Yeah, it's only 4 knots.

BTW, I doubt that it's nav archs that design the speed boats. I'm guessing that more engineers are involved, specifically aeronautical. The nav arch curriculum simply doesn't get into the specialized aerodynamics and hydrodynamics involved in speed sailing. Not that it can't be learned.

Fortunately for me, I don't have to rely on Wouter speaking geek for me to knock on him.

He provides plenty of ammunition without it.

For an efficient displacement hull 30 % more drive (thrust) is needed. With the higher speed this means that 52 % higher energy input (=more horsepower) is needed. For a sailcraft this also means major improvements in the rig efficiency as the rig needs to harvest 52 % more energy from the same conditions BEFORE it can be translated into higher speed. A significant portion of the initial energy harvesting goes into kinetic energy of the craft; ergo into accellerating the craft to a high speed. This is why Macquarie Innovation needs a long track where it can pick up speed before it enters the measurement track of 500 meters.

You don't just hit 40 knots in a gust guys; you take a long time getting there.

I know some of you think :"Well it is only a short hop from 36 knots to 40 knots, how hard can it be ?"

Well, just find 50 % extra efficiency somewhere and your in.

That is not too much to ask, right ?

Wouter

That is a conservative approach.

For displacement hulls a power factor of 2.5 is more appropriete.

Wouter

They were doing 36 knots in 15 knots wind ?

Yeah, Right !


BULLSH!T

Wouter

top speed 40 knots just sounds like bad marketing to me. in sales it's best to be a vague as possible. limiting the abilities of the boat with the words top speed doesn't make much sense.
they said in the brasil article that the boats were at peak performance in something like 15 knots of wind, so the boats obviously are not really made for sailing comfortably in the kind of winds that you would need to go 40 knots. if you weren't on a speed course you would need to be in some radical open ocean stuff surfing down a giant wave to go 40. somehow i doubt they race those things like that. i have my doubts that they would ever hit 40 even at maalaea where we have pretty flat offshore conditions with sick high winds all summer long.

question- why isn't there a speed division of cat sailing? if i made a speed sailing cat it would have deep v hulls similar to a gcat or sizzler.-that statement should start a controversy, but i really think that's a fast shape.

That mix of units is giving me a headache. <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />
power <> energy <> unitless efficiency
and
kinetic energy <> acceleration




Except that at 30+ knots, a 40ft boat is well beyond displacement mode. I know, I know, a long, slender sailing cat hull "planes" differently than a short, wide power hull, but no way can it be considered displacement.

I also doubt that general displacement principles strictly transfer directly to catamaran hull forms. I believe this has been a limiting factor in the development of catamarans over the years. Generally, classic displacement hull profiles are too radical IMHO, dictating too much rocker, and bow and transom lift resulting in the classic hobby horsing seen in early cats designed using these principles.
Not being an marine architect I can't back my suppositions with technical explanations, only experience and Logos.

So you are saying that a Navy Frigat doing 35 knots with its twin 10.000 HP gas turbine engines at full throttle (expending just over 1 liter fuel per second) is in planing mode ?

Speed itself doesn't determine wether you plane or not. The hulls shape has to be designed to go up on a plane otherwise you pretty much stay in displacement mode.

Remember the very old assie 18 skiffs ? Before the introduction of the planing hulls ? Same sailarea and still they couldn't get up to a plane.

Wouter

Who honestly gives a toss whether they can do 40kts or not?

I'm gonna check'em out in rotterdam this weekend and head for Texel next week, hope to see as many of you there.

Yea, why not? Hope that SF Bay competition is successful. www.sanfransiscospeed.com