I've always believed that catamarans are displacement boats. I sail a G-Cat and I cannot see ANY way those super-deep-V hulls could plane.

However, I also own a monohull and am on an E-mailing list with other owners, and we were discussing the hull-speed theory. I tried to explain how catamarans circumvent this theory with their super-narrow hulls, but all I got for my troubles were some nasty e-mails saying that these boats just get on a plane to go so fast.

So what I need is some really technical information that proves catamarans are not planing vessels, or a good smack upside the head if they really are...

Tell them to read Bethwait's book on sailing - he probably knows more about high-speed planing monohulls than anyone else in the world. He designed several of the modern skiff designs.
Also, here's the address of an article by
him.

http://www.geocities.com/f16hpclass/speaking_good_boat.html


The newer light-weight cats certainly feel like they plane, though.

Michael Coffman
T4.9#32

I agree with Michael that sometimes cats feel like they're planing. My cat has deep V hull--at least as deep as your g-cat, but symmetric. No way that could plane, right? Well, when flying a hull high in brisk wind, though, it sometimes feels like the side of the hull starts planing. Hmm. Turn a V on its side and you've got a broad surface to potentially plane on. Some of the lightweight boats with broader transoms may well and truly plane. I've seen photos of the Taipans that really make you wonder--not enough hull left in the water to really be in displacement mode. This is all very speculative. If your friends are like mine, they only want proven, studied facts. That's where you need the source Michael suggested.

Frank Bethwaite has laid down the "dogma" and explicitly states several times that cats sail only in displacement mode. To prove your point, refer your monohull friends to his book HIGH PERFORMANCE SAILING, chapter 20, especially sections 20.7 through 20.10. He lays down the super-narrow hull theory with low drag, etc. To quote:

"Slender-hulled craft such as catamarans sail always in one mode--displacement sailing."

I have the same thoughts as Eric, although no technical data to back itup, particularly as it relates to boardless cats; in my case, a Prindle 18 but also the Hobie 16.

It appears to me that the bow area is fairly deep but as you move to the stern, there is a full, flat area that could easily promote planning. Inasmuch as I move backwards on the boat on the broad reach, and the bow does raise, I do feel that the boat does pop onto a plane. I can feel the difference as the boat "lightens" to the touch and gets slotted in. It's almost exactly the way my Laser felt when I could get that boat flying, and there is no argument that the Laser is easily a planning hull.

Sheldon
P-18 (sometimes planning!)

Good posts -

A simple explaination may be it takes a certain amount of driving force or power along with designed planning hull area suffecient to raise the total weight of boat and crew and also overcomming the downward force on the sails themselves ,-up on the water surface overtaking its bow wave ,
Some flat hulled fishing boats accomplish this with as little as 5 h p outboard motors , but have props angled to help them ,-Verticle Sails exert a side and downward force at the C E on the sailplan ,-levered moreso typically 15 ft up .

We need a good planning cat design ,-with lightweight building techniques , powerfull sailplans w chutes the power or driving force is there but would need modification from conventional rigs.

The problem with attempts I,ve seen in hull design only is the changing heel angle of cats when sailed ,-attempts have been made to angle masts and sailplan to help creat verticle lift ,-via sailboard examples . The chutes really help here ,but we need much higher luff angles ,--per example of 18 planning skiffs set on longer poles -Also the problem of large compromise in lower wind speed ranges where this type of hull is much slower relative to existing design . -
As soon as the wind hit 15 or so ,popp the chute and ---gone ---speeds up to 40 -50 maybe ,--get your crash helmit on -padding ,-strap in and scary fun .

My understanding is even the big cats like 120 ft Platstation has a max speed per wave making displ. hull limits around 40 .
Some of the record sailing speed attempts are really interesting ,but on perfectly flat water in only one angle and direction in ideal conditions only ,-

Way back when-- built a very experimental 20 ft cat -12 ft beam with flat wide forward hull shape and angling mast -per forward set A frame support and mounting the mast base on a traveler , we had a huge spin also ,--it was fun but unbalanced and uncontrolable and needed much more refinment and work . It would also fall off plane when heeled too much ,--the need for more angled hulls which we see more of in todays designs ,-think 20 degrees or so dependant on beam .

-Hope we see one soon ,
maybe --someday --

have fun
Carl

The explanation is rather simple and is also described in Frank Bethwaites excellent book.

Yes, slender and long hull are predominately displacement hull. They may partly plane, but will never do so in an efficient manner. A small side step here : A planing surface is the most efficient when the area that is planing has the least amount or rim (edges) where energy can "escape". Ofcourse a squarelike hull has more are per length of edges than a long hull with long edges.

Planing on cats (long slender hulls) can be introduced partly but will never play a very big role and it must be regarded as refinement rather than a revolution. Boat like the Taipan are more likely to partly foil on their daggerboards, but then again the jury is still out on that one.

Okay, Back to the issue at hand WHY do cats go faster than what teh displacement formula allow them to go.

That is because the Froude law is often regarded as a absolute limit while is merely states that drag MAY significantly increase at that particular boat speed. It is just like sound barrier. 50 years ago everybody believed that that this was a absolute barrier untill somebody proved that it was just the shape the fusalages of the day that caused a jump in drag at those speeds, preventing the planes from going faster. They than altered the fusalage shape to the famous coke bottle shape and voila teh new plane with a very similiar engine puched through it (After the X-plane proved it could be done at all)

Froudes law states that for a given displacement (alot in monohulls , alot less in cats) the drag of the craft will increase significantly at hull speed. Later it was found that the creation of a wave system was largely to blame for this. Problem with cats is that at their calculated froude hull speed that they don't create a significatn wave system due to their slender hulls. Also the drag cats experience at Froude hull speed is rather small and a singnificant increase on a small drag total often amounts to only little drag just the same. Ergo, the Froude Law is applicable to cats but its predicted effects are simply to small to make an impact. The Frisian Engineeer Froude lived many centuries ago and didn't forsee the creation of long slender hulls. And ofcourse most sailors aren't maritime engineers and only repeat what others say in a dogmatic manner. Especially leadmine sailors suffer from this. Surfers, skiffies and catsailors just looked for escape routes and found them.

Wouter

Carl,
What about the Richard Roque designs -- aren't they supposed to be planing hulls? And what about that one-off Holder design that Randy Smyth was supposed to use one year in the Worrell back in the 1980's. I believe those were supposed to be planing hulls. Unfortunately, the boat broke (main beam, I think, wasn't it?) on the first leg, so it never got a chance to show what it could do. I thought it was a beautiful boat. I wonder what happened to it.

Hi Mary

Think I saw that boat in the classified section a couple years ago in FL.
Sailed by Randy in the 88 W 1000 i think -It was described as a smashed down hull shape --much wider and lower ,but think it suffered similar problems of heel not allowing it to plane at higher speed and being much slower below speed .
Flat large hull area creates drag -don't think we have seen a real successfull design yet .
It will take a great amount of experimentation to perfect such a revolutionary cat design ,-but we see elements required for it by observing other planning design -boards skiffs etc .
It will have to be a refined compromise between the two types ,-narrow enough to be fast ,-but enough flat forward hull angled area to plan at a reasonable speed .--
We have some existing hull design approaching this type of shape in the Inter line . flat wide forward sections ,but not enough yet to really plane , though some skippers on high speed reaches have felt this particular cat to get up and feel really skiddish across the top in flat water ,--it has 6 degree angled hulls ,-not quite enough of either area or hull angle but indications are it is getting close .

One fun aspect when one does hit the market is it will be slower in light air then take off in higher wind --kind of a rating nightmare like foilers are .a wind speed mod factor may double its base rating number ,-really put R C on the spot calling with B factor to use ,-then take its initial number ,-set in low wind -or high speed ,-there is nothing to gauge by
Class racing -maybe it will be the HT Formula 20 class of the future-

The boat ideally may be a 20 ftr or more -
larger beam or wings to add sail carrying power -
light weight ,
mast aft design -C E way back towards the stern like N-6/0s -more than 50 %
plenty of mast rake -
It may use small foil type forward boards to help balance the chute effects used only when set --A Cirrus design is showing us the way there .

longer pole and distance to aft set and raked mast .
flat forward planning hull shapes
Larger -dagger type retractable rudders -much larger that may replace the boards altogether -except the spin boards -
retractable to allow balance adjustment at different speeds and angles of sail and needed to balance the aft mast C E position ,-That needed to allow as much bow as possible to absorb forward and downward forces of the sail -
Sail area would be targeted to max. with beam at mid range as most are now which will produce some great spectacular crashes at the higher wind range -
If you desired a cat to break speed records you would place a very small sailplan on it and take it out in 30 knot winds

design and experimenting would be so much fun -
just need to hit the lotto -
-
If we really want a spectacular show on high speed cats for T V and media in a professional setting again this type of cat would be it ,-raced in the breezyest areas around the world -

have fun
Carl




-

Carl,
The Holder design for the Worrell was designed specifically for a downwind race. (That's what we were told at the time.) You are saying that planing-type hulls will not work if the boat is heeling. But what happens if, instead of tacking downwind as we all do these days, you sail a boat like that straight downwind, with spinnaker, and get it planing? Going fast and sailing a LOT less distance. Does that make sense? It would be nice to know what Randy's game plan was for that boat.

The biggest factor in cat speed is:

A) light weight - we have wide beams used as leverage so we don't have to carry lead to counteract the power of the sails. We can accelerate much more quickly to puffs AND we don't have to displace an inordinate amount of water to overcome the lead in a keel.

B) the hull length to width ratio of our cats is greater than 11 (i.e. 1 foot wide and more than 11 feet long). All hull shapes push a wave of water in front of them while they are in displacement mode. Generally sailboats don't have the power to climb up on top of this wave and therefore cannot ever move faster than the wave. Planning mono's can get up on top of the water reducing their drag enough to climb their bow wave. Cat's, or any other hull shape with the length to width ratio greater than 11/1 do not have to contend with the bow wave and are not limited by the speed of it.

Jake,
No comprende

"I agree with Michael that sometimes cats feel like they're planing. My cat has deep V hull--at least as deep as your g-cat, but symmetric."- ejpoulsen

G-Cats also have symmetric hulls- and not a single "round" hull section to them. They are super-deep-V bow-to-stern and use the bottom of that shape like a keel to go to weather. That keel, and a monstrous jib (since it attaches low at the bow beam instead of high at the bridle) really give the boat a surprising windward ability.

With "phatkid" crewing for me in some fairly high winds and flat seas, I've had the boat near 20 knots and it at no time felt like it even wanted to plane.
"Phatkid" has an aversion to trapezes though, so I never did get to crank in the main sheet and bear off to a broad reach

Thanks for all of the information- I think I'll check out a few books and soon be able to post a nice, informed reply to the man who claims:
"Whether a boat is a planing hull or a displacement hull is not a matter of opinion, but definition.... I only have a masters degree in engineering so I am not qualified to engage in the establishment of new science."

My understanding of part of the problem is that the basic formula folks use to determine "hull speed" of a displacement hull based on waterline length uses an approximation for the fineness ratio of the hull. That approximation is a good average for the boats the formula is usually applied to. There was an article awhile ago I think in Sailing World that expanded the fineness ratio input (actually calculating the value instead of using an approximation) to more closely represent beach cat designs, and if I remember right, the results more closely matched known capabilities.

My 18, sailed level and with the bows in (seems to go faster that way), undeniably in a displacement mode, can exceed the "hull speed" calculated by the usual off-the-cuff formula. No opinion. Newer and still faster designs may plane at the upper edges of their performance, but that seems in addition to what they can already do.

To get a really good precise answer I would suggest you contact Bill Roberts or Jim Boyer...

thom

Hi,

Bill Roberts addresses this very thoroughly in an article in Multihulls mag a long time ago. (I have a copy of it somewhere - maybe CatSailor can get a hold of it (or Bill) and re-print it?). In summary, though the hull speed is proportional to the square root of it's waterline length (as defined by the Froude Number), there is a coefficient or multiplying factor used which takes other things into account, such as fineness ratio, prismatic coefficient, etc. Most monohulls use 1.33. In Bill's article, he shows a graph of varying fineness ratios, and the coefficient to be used for these values.
Another excellent source for multihull theory is a SNAME (Society of Naval Architects and Marine Engineers) paper written by James Antrim (also published in September 1990 Marine Technology). This paper has a polar plot for a given hull design. For his 40 foot trimaran design he states "...at high speed, dynamic lifting effects comparable to planing are significant." Of course his design was capable of 22 knots in 15 knots true wind, so some dynamic lift was possible. I agree that it is less significant in most beach cats in moderate conditions, except possibly while under spinnaker, where planing type forces may develop.
Copies of his paper are available from SNAME. I highly recommend it!


Steve

Hi Mary

-On the chute straight downwind ,--you know this one -
we have to build apparent wind and attached flow across the sails,- sailing at some angle to use sails effeciently and creat lift . An airplane would never get off the ground if it placed its wings at a right angle to the wind . A boats sails are propelled by this same lift effect though set verticle.
A stalled sail blocking wind with huge turbulence behind would be limited to below whatever wind speed was present .
Ice boats and landyachts demonstrate this need for attached flow and building apparent wind best with their tremendous speeds well beyond wind speed having little drag except wheels or blades on ice .We have to go through water --or up on it to reduce drag .

2nd part --planning cat hulls will work when heeled but need to be designed accordingly . A conceptual stumbling block for many catsailors that fastiduisly allign hulls bow and stern thinking they go through the water symetrically ,--they never do unless they are perfectly level for and aft , and are not heeled but sailed perfectly level ,-in no waves ,--
When does this occur ? --
Now add angled hulls in the design mix ,-the for and aft sailing attitude dramatically changes hull symetry through the water ,-it is one reason we are seeing more forward bouyancy in hull design by some ,-to reduce bow down tendancies of narrow bow design ,-this has a drag increase trade off ,-but does help lead us to a planning type cat hull shape .
Key is setting hulls at enough angle to work ,-with less heel with both hulls in the water they would work off each other -most likely with a small amount of toe in allignment -
We have seen this with some C Class cat design with angled hulls .-They did this seeking more hull symetry through the water when heeled at a predicted or targeted average wind speed they expected to race in .

3 D computer simulation is really helping to understand effects and clearly seeing the underwater profile of hulls through water with different for and aft and side amounts of heel .
Many catsailors that are very visual thinkers and can potentially see this .

The aft set raked mast ,-longer spin pole and luff angle of spin to help creat lift ,-combined with forward spin lifting foils - to balance the chute and provide lift -per cirrus design features ,-flat angled forward hull sections ,-larger through hull type dagger rudders for adjustment balance and control ,-
light const. -proper rig and sailplan design ,--all these features would make a spectacular cat design .

To sail em we would see crash helmits ,-and begin to see pads worn maybe in combination with P F D , AND SOME SAFE WAY to keep us attached and in control of the boat at those high speeds bouncing over wavetops .

-fun stuff
thanks for all your great posts
and Catsailor Mary
congrates on another great event over the weekend -
Carl

Some time ago Bill Roberts answered many questions on this same issue. In summary, he said the following:

1-Cats sail in displacement mode, but the famous "1.34" factor does not apply. As Wouter said, this is old stuff.

2-A cat can plan.

3-In order to keep the balance of the forces, a cat would necessarily plan on the forward part of the hull, like a hidroplane.

4-A cat with a planing fore section is perfectly feasible, but the ride upwind is very uncomfortable. He tried it once, replacing only one hull. This way he could compare hull perfomance just tacking or jibing.

I insisted on the subject in message directly to Bill, proposing the concept of a variable geometry hull. The idea is copied from planes, that change the geometry of the wings according to the need: they extend flaps.

The hulls would display small retractable lateral flaps in the forward sections, so that upwind the bottom would be conventional but downwind the flaps would be extended and the bottom would be proper for planing.

Bill said that it would work and suggested to try with a powerful cat, like the ARC 22. But he also said that in his trials the speed improvement downwind was less then 5% so there is a possibility that the heavier weight and complications are not worth the effort.

I called the device "flapcat" but never tried it because I decided to jump directly into hydrofoils. The draft drawing is available for anyone interested in spending time and money developing a planing cat.

Cheers,
Luiz

-Hi Luiz

Bill's the man ,-his years of experience with S C and R C and Arc designs and ready ability with applied engineering formulas make him a logical choice for any contemplating a design to at least consult with on the concept and drawings.

You should contact Dave C and compare foiler notes -he built a foiler A class cat , and is always very interested in the subject.
-Sounds like your describing forward canted dagger boards that would help provide lift along with the chute.
This should not weigh too much .

On foilers I,ve only seen the H trifoiler in action -
The limitations seem to be they fall off plane in seas or gusting conditions and potentially break something at those high speeds .-
Is this foiler problem being overcome in current and new design -?

thanks for the info
Carl

Carl,

The idea is to temporarily increase the footprint of the forward part of the hull, to make planing easier - it has few to do with foils.

I will try to attach a drawing (maybe with the new on screen help I will finally succeed).



I have been exchanging ideas about foils with another great guy, with decades of experience with foils. When he is not too busy Aldis Eglajs clarifies my doubts.
I am not nearly bright enough to truly exchange ideas with him, but he still answers because I am building a boat that he designed - the Catri

See the "Flapcat" draft drawing in the attchment of my previous message.

When i first started sailing (NACRAs), an old-timer told me to pull my daggerboards halfway up when reaching. He said it would help me from being pushed over by a big puff, because the hulls would "slide" a little sideways, instead of the sailplan being pushed over the top of the boards. I've felt this happen, has anyone else? Although it's hard to call it planing, it certainly isn't displacement....

From what I understand this is to prevent what is called board planing. Powerful boards will create to much lift and help the boat rotate into a capsize.

Mike

Hello,

Ventilo claims that there new 'zip'o' 16 foot cat plains!?
It has a flat bottom and a wave piercing shape
see pics

Good concept Luiz

Looking forward to seeing the tri sailing .

There is a great book titled --Curious Yachting Inventions -
which took a look back at various nautical applications of ideas from the 1800s on .
Did a patent search once on a rig concept I had ,--as you look through old volumes and micro film photos and diagrams of other concept inventions in catagory it is AMAZING -and sometimes humorous ,--
It makes great reading ,--someone should write one on --Curious multihull Design concepts ,--
They could place my angle rig concept in there somewhere .

-Though thinking of it --it did get a very negative write up in a now long defunked magazine ,--calling it a Rube Goldberg contraption ,-which was an old newspaper funnies comic on making inventions to accomplish simple tasks in the most complex way possible ,--it was a mean spirited commentary on someone like many before experimenting with concepts .
-{ gee wonder why the mag. went out of biz} --

good life lessons early on ,-not too much phases me now, no one should allow other small minded people to with negative derogatory comment ,-public humiliation etc ,--its nothing ,--people like Edison had thousands of failures before finding solutions that worked ,-from the knowledge gained in each was able to make fantastic connections of concepts and ideas . We all need to take a page from his book and be persistent and patient with such things of life ,-and each other too !

Have fun
Carl

Carl,

No doubt the "flapcat" would work. The question is how much would be eaten from the downwind advantage by the lower upwind speed due to the additional weight in that position and, most important, if the additional cost and work are worth it.

My Catri 27 will still take some time. The molds took more time to build then expected, the construction is technically more sophisticated then I expected and I demand perfection (as expected...). But the boat is finally being laminated now.

The second Catri 27 made in the US is performing well in its first trials: sustained 27 knots with almost 20 of wind, under main and jib only. It seems that it will easily surpass 30. The performance is more or less matching that of an Open 60 in equal conditions.

The designer refuses to make any prediction about the top speed, so I guess I'll have to wait to test it myself...

(Before anyone steps in to say that the ARC 30 tops 35, remember that the tri carries an engine, folding gear, a head and other amenities)

In case you want to see the boat in action, I am attaching a photo of the first US Catri sailing in San Francisco.

Cheers,
Luiz

"Success is the ability to move from one failure to the next without a loss of enthusiasm."

[color:"#666666"] Winston Churchill [/color]

From what little I understand SOP (Seat Of Pants, for Mary):

Think about hull shapes.
An assymmetric cat doesn't work unless it 'bites' the water. Even on a heel, you've got quite a bit of 'v' hanging below. If you didn't, the boat would merely be pushed off sideways, with nothing to stop it, well, except the rudders. Thass displacement if ever it was.

Other designs involve fine bows with hemicircular sterns. Now in the case of these boats, it's a matter of the WWW factors and speed. At lower boatspeed, windspeed, rougher water (either/or) you're gonna want to sail the boat bows down, or in displacement mode, which as it turns out is most of the time. But if it's flat, or blowing stink, or you're really 'sheeting in and maxing out', you'll set the boat more towards its sterns and begin to plane to some degree.

Looking at the newer designs, the Inters in particular, are designed more as a planing boat, with rather flat bottoms. But regardless, when you're going slow, cats are displacement, and depending on the hull design, as you go faster, they begin to plane.

So the answer, to me, is...
BOTH.
Depends on the hull design and WWW factors.

sea ya
tami

all planing hulls displace at lower speeds.

catman,

I used to have a Laser, and when I'd sail it in heavy air, reaching and running off wind, as it planed, if I forgot and left the board down, Wow, over I went, it wasn't the wind, it was the lift on the daggarboard when the hull planed! Always let me know when I left it down! I'm sure many have experienced and know this.

I wouldn't say any of our current cats plane. They "lift at speed and reduce the amount of hull dragging in the water.

I sailed on supposed planning cat, the Planecat. On a smooth lake, it traveled through the water they way little fish do when a big fish is chasing them. It was noisy, it bounced, and it wouldn't track. To top it off we were outrun by a Hobie 16.

Opposite to this was a homebuilt 18 sq. It had very deep, very narrow hulls, with lots and lots of rocker. The idea was it had a great fineness ratio so it had a high theoretical top speed. The theory was as speed build, the leeward hull would lift, reduce drag, and go faster.

It was a nice boat to sail, smooth, good tracking and an OK boat to tack as long as your weight was aft. Actually it reminds me of the new Ventilo 16 with narrower, deeper hulls.

That's it! I think the Planecat is the Richard Roque design -- it was a 16-footer, wasn't it? I I couldn't think of the name earlier. And Randy Smyth won the Worrell 1000 on the bigger Roque design back in the 1980's, just edging out Roy Seaman on the Nacra 6.0, which Seaman designed for the Worrell. If this is wrong, let me know. I got this info from Rick. Randy expressed his opinion at the time about the Roque design vis-a-vis the Nacra 6.0, but I am not going to repeat it, because I don't want to misstate somebody else's opinions.

Beauti Luiz

beautifull tri -

The question most have would be how efficent or effective are the forward angled boards -foils - they look great
-Did they provide not only tracking -verticle lift ,-but horizontal lift on the hull ,-what is the ideal heel angle of these while sailing .

Are the anams 100 buoyant ,-that is like the 60s can you fly on one hull , or does the design reply on the lift of the angled boards and center hull remaining in the water .

Thanks
Carl

I haven't read this thoroughly (I'm in the middle of my work day), but this article seems to explain some of the physics:


http://members.xoom.virgilio.it/mfcatamaran/hull.html

FWIW, if I take the hull speed formula from the Annapolis Book of Seamanship and apply it to my Prindle, which I think is 16 feet at the water line, I get:

Maximum Hull Speed = 1.34 * LWL1/2
= 5.67 knots

I can go a lot faster than that with both hulls in the water in pure displacement mode. You don't have to lift a hull to exceed this kind of speed. The Annapolis Book of Seamanship says that the normal hull speed limitations do not apply to catamarans. I think that catamarans normally are displacement boats, even when flying a hull, but the oversimplification normally used for dinghy's just ain't applicable when analyzing the way our favorite displacement boats work.

I found some other interesting links that I need to read in more depth. Here's one on optimizing hull shape for drag and other characteristics:

http://www.basiliscus.com/CaseStudy/geometry.html

Ah well, back to work ;->

Jonathan

Unfortunately, the 1.34 "constant" is true for monohulls with a fineness ratio of about 3:1 or 4:1 (J/24 is ~2.8:1, based on ~22' waterline length:8'6" beam) but not for typically much finer multihulls. The beam of each hull on your Prindle is less than 2', which combined with a waterline length of 16', leads to a "constant" more like 3 or 4. Thus theoretical top speed under sail is more like the high teens. I know the speed record, under controlled conditions, for the Hobie 16 is 26.7 mph (or is it knots? I'm not sure) but anyway much much faster than the 1.34 constant would suggest. Naturally, more fineness leads to a higher constant.

sail fast

The simple truth is that Froude's law doesn't predict the maximum boat speed for lightweight boats with slender hulls. It is therefor nonsense to continue using that law in a fashion that is was never intended to be used.

Froude's law only correlates a rise in drag (wave making) to a specific speed at which the bow wave and stern wave length equal hull length, nothing more. In Heavy boats this wave system is so big that it prevents the boat from going faster. With ligtweight boats with slender hulls is only results in a bumb in the drag plot or an increase in overall drag.

Wouter

I`m not sure you can apply Froude`s law to a small light displacement catamaran - we may sail displacement boats that might not plane, but it`s not sensible to apply a law that was written based on observations of heavy dispacement boats.
Further to this, if you take the length to width ratio of the average beach cat with a rounded hull-shape ie symetric hulls, you will find a ratio of about 12,5 : 1. If you take this and multiply it by say 2metres you get 25metres - A keelboat of 25metre
length with a 2metre beam would have to lug a large lump of lead around to keep the mast pointing skyward, or would need a few hundred crew out on trapeze ! That`s why cats are faster than mono`s. So next time you sail faster than Froude`s law
says you should be able to, just smile ! Life`s a bit faster on a cat.

Carl,

It is beautiful, indeed - I expect mine to look even better due to some small details.

Answering your questions:
The Bruce Foils (inclined daggerboards) provide vertical and lateral lift, like those of the Open 60s. The only difference is that in the Catri the foils are designed to lift up to 90% of the boat's displacement, while in the latest Open 60s they are designed to lift about 20% (Aldis Eglajs' estimate).

The ideal angle of heel is small. The best atitude for speed is with the ama sinking 10% of the total displacement, the foils supporting the remaining 90% and the mainhull skimming the surface.

The floats' volume is about 110% of the displacement aided by up to 90% lift provided by the foils in high speed, totalling 200% "flotation".

In short: it flies two hulls, but goes faster flying low.

When the mainhull flies higher, the rudder's foil is raised and stops lifting, so the mainhull automatically returns to the ideal atitude. This is (part of) the stabilization system patented by the designer.

With less wind, the speed is smaller and the float can sustain 110% of the displacement - but then it is sharing the weight with the mainhull. Conclusion: the foils' lift enable the floats to be smaller (= lighter + less windage).

Sorry for writing too much - I just can't stop...

Cheers,

The boat I sailed on was a Planecat 18. It had square looking flat hulls. It was a dealer or factory demo. It was cold and only 3 people showed up.I was the only person willing to go sailing a second time.

My impressions of the boat were; slow in light air, fast but hard to control in medium air and weird things happened (it popped off plane or tried to spin) when it started to fly a hull (that's when the Hobie 16 passed us). With experience you would get used to it's quirks and anticipate what it was going to do but, there was nothing about it that inspired me to want to try.

Cats are displacement boats. That law is not general, and it is wrong to apply it to beach cats.

Beach cats sail through the water pushing it aside, therefore it is proper to say that they sail in displacement mode and they can be properly called displacement boats..

Cheers,

I heard about Carl Roberts planing cat but sadly never saw it. Carl's experiences with a cat's behavior when planing help visualize the problems of speed, lift and control. This may be a triangle- you can have any 2 of the 3!

It's tempting to think about dropping a planing surface under or in between the hulls when you are going fast already, but how does one get such a rig to sit back on this surface while planing at speed? Besides the force in the sails wants to blow everything sideways. Hydrofoil sailboats have the same sideforce to deal with: It looks as if vertical fins above or below the foils are necessary to deal with the wind pressure, converting it to forward motion. This is not so simple.... even when you have lifted both (light) hulls out of the water already! It makes you breathe hard and grin a lot, tho. 8^)

I think my initial post was unclear.

The following resource does not use the 1.34 constant, and seems to go into some depth about the actual factors used to calculate drag for catamarans:

http://members.xoom.virgilio.it/mfcatamaran/hull.html

After citing that resource - and I can't vouch for it's accuracy, but I can say that it looks detailed and careful - I then attempted to say that a straightforward application of the hull speed formula using the 1.34 constant - which is NOT advocated in the URL above - would give very low speeds indeed for my boat.

Jonathan