Hull speed of a 450 ft LOA FFG-7 class frigate is going to be about 27 kn. Published max speed for the FFG is 29 kn, so not planing.

However, a 40 ft cat (hull speed 8.5 kn) at 30+ kn is not acting as a displacement hull and if it's not planing, then it must be in the "fourth mode."

I still derives by far most of it's "gravitational suspension" due to archimedes law therefor it is still in its displacement mode.

Bethwaite invented some extra modes, but that is a different topic.

Wouter

Please, please don't go anywhere near mentioning HWSNBN!! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

It is very hard to say at what point a boat is planing, you will always generate some lift regardless of speed.

Interestingly if you plug a beach cat into the simple planing eqn given in http://www.hydrocompinc.com/knowledge/whitepapers/HC113-MinPlaningSpeed.pdf

You get a planing speed of about 30knots, and a VX40 gets on the plane at 40knots!

Is this the fourth mode <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Gareth

Hmmmm, can you prove that, or am I required to disprove it? Also, define "most." Also, I have yet to see a consensus as to when a boat is planing. Or stating it another way, I have yet to see a consensus on what defines a planing boat. However, if you look at the immersed lines of the leeward hull of the attached F16, you see some straight, flat runs. So you have speed well in excess of hull speed, some percentage (most? significant?) of boat weight and sail force counteracted by dynamic lift, flow separating cleanly from the transom and a portion of the hull of proper (if compromised) planing geometry. If not planing, then in that power-sucking semi-planing regime. But not displacement.

Hull speed formulae are boogus when looking at beach catamarans. The hullspeed formula that is so often thrown about is really a misintepretation of the Froude law which actually only links wave length to wave speed.

For some boats the Froude law coincided with the maximum speed they can attained given a limited drive due to some factor. Of course all sailboats are limited in drive their sails can produce because of stability considerations.

Now Froude's law applied to beach cats as well it just that the commonly encoutered misintepretation of it is not rightfully applied to these designs. Hence the parados that catamarans can sail faster then "their maximum hull speed".

I can tell everybody out there that debunking the "max hull speed formula" is like fighting the multi-headed hydra. For some reason this "error" is immortal and ever since its rise to fame it has dominated discussions where. I'm done with it. If you don't understand the true meaning of Froude's law and the true nature of the "max hull speed" formula that is derived of it, then there is simply no use in continueing the discussion. It is like arguing that the earth is round with a midevil christian fundamentalist who thinks it is actually a flat pancake because he can see with his own eyes that it is flat. Sorry about this, but that is just the way things are.





Read Bethwaite very carefully again and then see where his "modes" are just names he gives to a limited series of specific displacement craft. He doesn't include catamarans for example and later on in the book he actually writes that cats are "different" and don't seem to have the forced displacement mode (or whatever he calls it). So his model is only applicable to a specific type of sail craft and it falls down when looking at more different craft.

And that Stealth F16 is definatly still in displacement mode. It may have a component of dynamic lift somewhere. Either from the angled foils or from some pressure build-up under the hull, but 80 % of its weight is still carried by archimedes law. Therefor it is a displacement craft. I'm sorry

Wouter

I think it is a little simpler than that. Long slender hulls when exceeding hull speed have no benefit from cleaving the water surface, other than reducing surface friction, because the deck is in the air, which has less viscous drag than water. At these speeds, to be totally immersed starts to look good. Wave form drag is bad news. Every wave is literally energy running away from the boat. The "fourth mode" would be submarine mode. Large ships have recognised this with a practical compromise - the bulbous bow.

Who really cares

The Marketing boys have got people talking about the boats and so they are happy.



I just want a go on a Vx40

I also want a go on one, Simon. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I find this part of the discussion quite interesting. We just need some hard figures from the crews now to establish what kind of top speeds they have reached so far. Perhaps somebody has the opportunity to ask one of the crew?

Now, to derail the discussion. It's quite a huge and steep stern wave from that Stealth in the photo. Lower part of mainsail is very deep, large twist (and boom is characteristically out of column). Are they on some kind of reach?

I don't think anyone is talking about hull speed here, we have progressed to talking about planing speeds. I am certain from towing tests that I have perfromed that long slender hulls with elliptical profiles do plane. I had a video to prove it. But I wiped it, actually that sounds a bit like the alien autopsy story so forget I said it. <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

You only have to look at planing moths to realise that long slender hulls can plane (just depends upon how long and how slender).

I'm hoping to catch up with Will Howden in the next few weeks - he's been on Motorola a fair bit.

I'll see what he has to say !

I think you need to remember that these are peak speeds they are claiming, not averaged over a 500m course as the record attempts are. The VO70's are hitting very close to 40kts and the theoreticaly should be no where near those speeds...

Yes, water skis are long and slender (and badly designed for purely planing) but they defnitely plane. The problem is that hulls with rocker or a knuckle resist planing and literally suck themselves into a hole in the water. Some cruiser cats claim the stern upcurve as providing negative lift to resist forward pitching off the wind.!!!

From reading about this a long time there is one problem when people throw about the usual hull speed formula - finess ratio. The formula in its common usage has a ratio that is kind of universally used to describe a wide but common variety of displacement shapes. Unless you apply a different ratio to it you don't get anything meaningful for catamarans. This was in a Sailing World article a long time ago (I probably still have it somewhere) that addressed this very question. Using different finess ratios that more closely reflected the shape of the hulls the hull speed numbers became more reasonable, at least when they stay in displacement mode. When you put up a big headsail and get the newer designs honking on the flat section of the hulls things are a little different. But just because a cat may go faster than the traditional hull speed equation says it should does not necessarily mean it is planing, although it may ultimately do that as well.

waterskis do have some curvature to the tail rocker which is where the ski contacts the water whilst planning.
the fastest speed boards are flat in the tail, but wave boards have curved tailrocker.
also Bethwaite's book is incorrect on the statement that he makes about windsurfboards. he sure didn't see them as being possible holders of the speed record. the book is otherwise quite informative.
topspeed 40knots is an attention getter so the statement does work from that angle. something a little more creative would have been nice IMHO. they are preaching to the choir in our case anyway. obviously we all want to take one of those bad boys out for a ride.

Hi Rolf,

That pic was taken early 2005 as I recall, during a club race. We are beam reaching (just too shy for the kite) to our West Mark, in about 12-15mph of wind. It was just after this that I replaced the carbon boom with a stiffer ali one. We've got almost 180Kgs out on the wire!

That huge stern wave is more than likely caused by a bit of weed around the rudder - a permanent problem when our courses take us down into that corner of our sailing area!

In terms of displacement/planing arguments I'd hesitate to suggest that that pic proves anything!

Thanks John, everything explained..

What do yo do on the Stealth when you catch weed on the rudders? Park the boat to clean them?

Try and sail faster !

Acttually a good question Rolf.. Not so easy to just flip the rudders up !

I don't know about everyone else, but I take the opportunity mid-tack to lean over the stern and pull the weed off. Takes maybe 3-4 seconds, then another 5-6 seconds to get out of irons if it all works out! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Using correction factors like that is nothing more then fitting the errornous formulae to reflect the data. And there that formula become extremely limited in its use. Afterall you must already now the REAL max speed to even calculate the correcttion factor needed. Additionally you screw up the relationship and you magnify the error.

Max. hull speed is nothing more then Froude's law with a different (and often errornous) intepretation.

Froude's law says that a wave with a wave length of X has a speed of Y ;

Wave speed = 1.54 * (wave length)^2

For alot of (displacement) sail boats this coincides with their maximum speed their hulls can travel through the water as their hull get trapped between two wave tops with insufficient drive to overcome the rapid rise in hull drag.

A good number of other sailcraft design DO HAVE enough drive to overcome this rise or have reduced the magnitude of the increase in drag to such an extend that it no longer plays an important role. The standard example of that is the catamaran hull shape.

It is of no use to use a corrector factor and just IMAGINE that the wave system is actually travelling faster then it is in real life just so to assume that that this corrected wave system encloses the cat hull at much higher speeds than it does in REALITY. We can just as well think up a story then little goblins start pushing against the bow of the hull at 20 knots; it has the same amoun to reality value = none.

That is the eternal myth of the max hull speed formula. And it is indeed immortal as in 12 months time we'll have this discussion AGAIN. In the mean time you can read this law 10's of times on so called professional boating magazines because the bulk of those journalists don't know squad about science as well AND because this formula is pretty accurate for heavy yachts.

And this is the way the "max hull speed" formula survives.

Wouter