Assymetric Cat Hulls

Discussion in 'Multihulls' started by basildog, Apr 8, 2009.

  1. Ikarus342000
    Joined: Mar 2007
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    Ikarus342000 Ikarus342000

    Assymetrical Cat Hulls

    The big confusion

    As you know I seldom join in a discussion in a forum besides the K-designs forum.
    As I was reading the different opinions and responses I was confused. Because to many issues mixed together. To many meanings without explanation.
    The main reasons why asymmetrical catamarans are disappeared is more a social issue as any other. Why so ? Simple asymmetrical cats are no good load carriers. What do I mean by that.
    Easy told. The normal buyer today looks more for comfort as for sailing quality, point.
    When you look at the catamaran development there is in my view a backward development.
    The first successful catamaran after Herreshoff was designed and built by Woody Brown 1947. The Manu Kai, a 40 ft asymmetrical catamaran. The boat was made from 3/8 to ½ inch plywood.
    1955 the Waikiki Surf was built.
    Some particulars of the boat, length 40 ft, beam 13 ft, sail area 1050 sq ft, displacement empty 2000lbs ! (904 kilogram). Load displacement 3000 lbs. Compare this to catamarans with the same length today. The Waikiki Surf was sailed 1955 from Honolulu to Santa Monica in 15 days, distance 2700 miles.
    Compare this particulars to a production catamaran of the same size today.
    Here three examples:
    Catamaran A; length 40 ft, beam 21 ft, displacement empty 16299 lbs.*
    Catamaran B, length 40 ft, beam 24 ft, displacement empty 22687 lbs.
    Catamaran C, length 36,6 ft, beam 21,6 ft, displacement empty 8150 lbs. This one is smaller but at least has a reasonable weight. The boat is built in the infusion technique (Polyester/multi axial glass and Divinycell). But also this boat is still three times more heavy as the “Waikiki Surf”.

    If you need a load carrier an asymmetrical cat is a nono. But if you like a fast catamaran, then the asymmetrical catamaran comes to its own.

    There are reasons why you see no mass produced asymmetrical catamarans. Designing an asymmetrical catamaran is more complicated. Making the molds is more complicated. Besides the catamaran with asymmetrical hulls will have less inside space and less loading capacity for the same size of boat. So the prospective boat buyer and his wife visit a boat show. The look at different boats and in the end his wife says, boat Y has more inside space and cabinets lets buy this one. Sailing quality is not mentioned at all and they will buy boat Y. With all the “comfort” like the deep freezer , air-condition, micro wave etc the husband decides to have the bigger engine alternative. The boat will be anyway to heavy to be good sailing catamaran and anyway his wife doos not like to sail in a wind with more as wind force 4 and he will motor. And that is it.
    I will not discuss anti vortex panels (winglets, bilge boards) again. This has been done on this forum before.
    I think also the contribution about asymmetrical dagger boards doos not belong in this discussion. Only so far. You do not need molds to build a good asymmetrical dagger board. When for example the
    Clark Y section is used you start with a flat lower surface. The rest can be build up with say 4 mm plywood. The leading edge can be a piece of spruce. The whole covered with a layer of glass fabric and voilà a dagger board is ready.
    Section: Clark Y ,Lift coefficient ( LC ) at 6 degree = 1,1 ; drag coefficient = 0,038.
    Compare, Section: NACA 0012, at 6 degree LC = 0,5 ; drag coefficient = 0,032. The numbers indicate very clear that the asymmetrical Clark Y section has more as double a much lift as the symmetrical NACA 0012 section. The slightly bigger drag is not important because the asymmetrical dagger board can be 40 % smaller for the same effect.

    This brings us now to asymmetrical hulls versus symmetrical hulls.
    The above comparison of the two wing sections, the difference of created lift force, simple shows by numbers why an catamaran with asymmetrical hulls will go better to windward as a catamaran with symmetrical hulls. Because we can look at a hull as a wing standing vertical in the water. This is only valid for slender hull section as we use for multi hulls (valid till about an aspect radio of 1 : 8).
    Asymmetrical hulls should have an aspect ratio of 1 : 14 or better. Clearly if you like to carry a lot around such a boat will have a big wetted surface compared to a boat with 1 : 8 hulls. I presume that you are familiar with the course theorem.
    Of course you can argue that on even keel the forces are canceled out. Correct, this is valid for symmetrical and asymmetrical hulls. When an asymmetrical catamaran heels around 3 degree the force generated on the leeward hull is sufficient to sail very well to windward. The symmetrical hull will need boards or keels to go as well as the asymmetrical boat to windward.
    Asymmetrical hulls will have about 4 % more wetted surface compared to symmetrical hulls. But symmetrical hulls need extra devices to go well to windward, also keels, dagger boards, center boards. If properly dimensioned the area of these should be 2 % of the sail area This boils down to an extra 2 % of the under water area multiplied by 2, because we have to calculate with both faces of the boards, and the result is the same. Low aspect ratio keels are no issue for me. To be effective the need to be bigger as the mentioned 2%. Why spoil one of the nice things about multi hulls, its low draft, to reach coves and perhaps beach the boat which is impossible with other boats?



    Broaching I.e. sea behavior

    Asymmetrical cats are easy to handle at sea. No boards to handle when tacking. The boat sails like on rails. Almost no yawing in a diagonal following sea. In a heavy following sea no danger for broaching. See explanation further down. Nothing for nothing, coming about is a bit tricky. But when you have the knack for it, not more difficult as with any other catamaran.
    [​IMG]
    Fig. 1A

    In heavy weather and a following respectively diagonal sea asymmetrical hulls have been found highly resistant to broaching. See figure 1A to understand this. When the boat is beginning to broach and reaches a yaw angle of about 10 degree, the lift effect in the leeward hull increases. The windward hull is at a negative angle of attack and produces almost no lift. The lift of the leeward hull is accompanied by more drag. The excess in drag of the leeward hull over the windward hull multiplied by the overall beam of the boat generates a torque to counteract the broach. The superiority of asymmetrical hulls under this conditions has been proven in praxis many times.







    To the remark of pounding at anchor

    What is in fashion and common sense boat design are contradictional in some cases. A typical example are the negative sloped transoms with steps. They look good in a brochure with a picture the boat in a bay with nice blue water and beautiful people sitting on the steps. Under sail the situation changes. Look at picture 1. You can observe the back flow of the water. In fact this transom works as a water brace. This costs at least 1 knot of speed in this case.
    http://www.boatdesign.net/forums/images/attach/jpg.gif
    Fig. 1
    http://www.boatdesign.net/forums/images/attach/jpg.gif
    Fig.2

    At anchor or in port. figure 2 shows a transom of a catamaran with U-cross section.
    http://www.boatdesign.net/forums/images/attach/jpg.gif
     Fig. 3
    http://www.boatdesign.net/forums/images/attach/jpg.gif
     Fig.4

    Figure 3 is the transom from a trapezium cross section and figure 4 from an asymmetrical catamaran. Which transom would pound to you think ? More so, which transom will have the better flow in a sea way ?

    Why dit asymmetrical catamaran disappeared
    Easy told. The normal buyer today looks more for comfort as for sailing quality, point.
    When you look at the catamaran development in short there is a backward development.
    The first successful catamaran after Herreshoff was designed and built by Woody Brown 1953. The Manu Kai, a 40 ft asymmetrical catamaran. The boat was made from 3/8 to ½ inch plywood.
    Two years later the Waikiki Surf was built.
    Some particulars of the boat, length 40 ft, beam 13 ft, sail area 1050 sq ft, displacement empty 2000lbs ! (904 kilogram). Compare this to catamarans with the same length today. The Waikiki Surf was sailed 1955 from Honolulu to Santa Monica in 15 days, distance 2700 miles. Later Rudy Choy designed his successful range of asymmetrical catamarans. These where relative big boats.

    If you need a load carrier an asymmetrical cat is a nono.

    There are reasons why you see no mass produced asymmetrical catamarans. Designing an asymmetrical catamaran is more complicated. Making the molds is more complicated. Besides the catamaran with asymmetrical hulls will have less inside space and less loading capacity for the same size of boat. So the prospective boat buyer and his wife visit a boat show. The look at different boats and in the end his wife says, boat Y has more inside space and cabinets lets buy this one. Sailing quality is not mentioned at all and they will buy boat Y. With all the “comfort” like the deep freezer , air-condition, micro wave etc the husband decides to have the bigger engine alternative. The boat will be anyway to heavy to be good sailing catamaran and his wife doos not like to sail in a wind with more as wind force 4 and he will motor. And that is it.

    Happy sailing

    Bernd

    * by purpose I give no names of the manufacturer or type of the boat for obvious reasons. I retrieved the data from there web pages.
     

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  2. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    Asymmetric hulls

    I agree with you 100% and some common sense on this issue is long overdue. I worked for Rudy Choy for some years and built some of the last classic asymmetric hulls. Rudy felt that asymmetric hulls resulted in superior steering in downwind surfing conditions and the fact that he got away with sometimes very small rudders was a testament this this.
     
  3. Catamaran Dan
    Joined: Apr 2009
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    Catamaran Dan Junior Member

    Interesting conversation on lateral stability that I have been thinking about in respect to my older Crowther Spindrift. I have been toying with the idea of cutting out one of my boards to free up my side of the boat. I would leave the guest quarters with the operable board. (Would not want the guests to get too comfortable and overstay their welcome.) This boat was designed in 1974 I think as a 37 ft. long 20 ft. beam and be light at 9500 lb. The problem is it weighted about 14,000 when complete so 5 ft. was added to the sterns to get the transoms out of the water. It could have used another foot or two. I am going to watch very closely this summer on the performance with one or both boards down. I can say for sure the boards on my boat do not stay within millimeters of straight!!!! I have also considered adding some additional volume to the back half of the hulls on the inside to help set the boat up where she should float. This would then make the hulls asymmetrical, but only in the back half. I am not a racer but the thought of having the bridge deck floor a little more level would be nice, and it would gain some much needed clearance under the outboard pods. The back 30 % of the hulls are not U shaped as the front 70% is, and I know the rudders leak over the top anytime they are off center so adding to the inside a flat section would help in steerage too. Anyway that job may never get done as I would rather be sailing than building, but maybe some day....Great board though with complete explanations of ideas. Glad to be a new member.
     
  4. Calculator
    Joined: Apr 2008
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    Location: Denver, CO

    Calculator Calculator

    Asymmetrical Cats

    Regarding the comments by Ikarus, I have two comments to offer:
    1) The Clark Y airfoil has its chord line defined such that, at zero degrees angle of attack it has considerable lift. In fact, the zero lift AOA is -3 degrees. Symmetrical foils such as the NACA 0012 have zero lift at zero angle of attack. So comparing the Clark Y to the 0012 foil, both at 6 degrees AOA, is not correct. You should compare the Clark Y at three degrees to the 0012 at six. I recognize that you can play the hull orientation of the asymmetrical hulls to optimize hull lift and induced drag as the boat heels.
    2) The elegant sailing catamarans Rudy Choy was building back in the 60s had relatively short waterlines. I have specifications for five of these boats; the 36 foot Imua, 36 foot Makai, 42 foot Imi Loa, 43 foot Allez cat, and 44 foot Patty Cat II. The average Loa/Lwl for these five boats is 1.25. The average for nearly 920 catamarans in my database is 1.05. Each Choy boat had combined bow and stern overhangs of around eight feet. So these boats had very small waterplane areas and thus, the small load carrying capability. I think it was more this than the fact that they were asymmetrical hulls. Unfortunately, I don’t know of an asymmetrical-hull catamaran that had a waterline length near or equal to its overall length. As for maneuverability of the asymmetrical cats, I watched two of Choy’s 43 foot catamarans chasing each other in tight figure eights just before the start of the 1964 (I think) Transpac. Their ability to tack readily was very apparent.
     
  5. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    pitching

    Hello Gary and Bernard

    A thought - I remember reading a Crowther interview printed in the early seventies. He talks about Sea Bird being the fastest multi at the time. Lock (he worked down the road) showed me Rudy's book and said it was a great read. I reckon I can see a bit of Myers/CSK influence on the early Spindrift designs. In the design explanation Lock calls the hulls full in the ends - no one would say that now.

    Then comes the seminal moment in Aussie cat history for Lock. Pennant, 45 ft long Spindrift, and Bagatelle, 45 long bulb bowed big sterned sister, race each other. Inside Sydney harbour they are pretty equal and then outside Pennant is left for dead as she pitches in the Sydney slop and Bags runs away. The race was then on for higher prismatics - big bows and sterns. They were higher resistance on the flat but much better platforms for rigs offshore.

    So how did CSK do it? Was it that their boats were relatively long for the rig height? Was it a function of the rigs of the day being more suitable for a low drag hull? Or would they have been faster with more pitch resistant shapes? Or as CSK were the only Pacific cats racing each other was it a case of group think? Even Rudy mentions that Pattycat 1 got done by Harris' cats

    I would be very interested in your thoughts.

    Cheers

    Phil
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    hi catsketcher
    I presume you are mixing the two kiwi Gary's up a little - in that I don't think Gary Dieking was referring to the Hugo Myers' design Sea Bird (it is in the CSK thread) but was agreeing with Bernd here on the asymmetric thread on the superiority of asymmetric hulls.
    Back to Sea Bird, which had fine, but symmetric, double ended hulls. I think your view of conformist group think surrounding the CSK designs and sailors was probably dead right and maybe the reason Buddy Ebsen snootily ignored the "different" Sea Bird in the Transpac. Anyway Sea Bird went on to winning following Transpacs and established a world record daily run that stood for many years of 365 miles. Bob Hanel extrapolated on the 44 foot cat with his larger 54 foot Double Bullet - which incidentally came to NZ at the Bay of Islands 30 years or so ago - they also had rudder problems I think. David Barker (Sundancer and Sunreamer designer) went up and had a sail on the big cat before he designed his big cats. I think I've got the chronology right there. Maybe he was halfway through at that stage. Double Bullet was a powerful brute but much heavier than Barker's designs. And incidentally, earlier on again, he also worked with CSK in Hawaii - but he too, put symmetric hulls on Sundancer. So the cross linking between CSK knowledge and Lock Crowther with his Spindrifts and Barker with his cats, is/was apparent. Regarding pitching: there was comment years ago about this problem with CSK cats in the UK - which ended up with them being beaten by broader stern UK cats and tris. There was a fine bowed/stern version 45 Spindrift built in Auckland which was incidentally also named Double Bullet - it cracked up near the Three Kings and was lost, crew saved.
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    apologies for spelling your name incorrectly Gary Dierking.
     
  8. Guest62110524

    Guest62110524 Previous Member

    built many boats, and amworking on one of this mans designs(not started build yet,alloy , 6mm bottom Survey scantlings
    http://www.bloomfieldinnovation.com/Multihull-Sailboats.html
    he was chief Crowther designer for 10 years, asym hulls , and his yachts have many Aussie speed records
    on page one of this thread there is written so much plain misinformation from a guy who has done almost no sailing whatsoever and no building at all
    it is this sort of misinformation that is totally offputting to would be builders and buyers
    http://www.rawnerveracing.com/index.php
    By way do Orams cats have asym hulls? and do they have one or two plates:)
    I have seen one on web page that has ONE motor!! the great advantage of a cat berthing is with two, you dont even need touch the wheel, guess this was the owners way of trying save money but I sure as heck would bot like to berth in any sort of wind with one in a crowded marina
    Imagine being in irons? in some crowed channel with one motor
     
    Last edited: Jun 21, 2010
  9. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Stuart Bloomfield

    Gday Whoosh

    Amongst many cat afficianodos there is often a big line drawn in the sand between Lock and Brett Crowther designs. Stuart worked for Brett and not for Lock but has co-designed some fast boats - Raw Nerve etc.

    Bob Orams boats are symmetrical and often have one board. Some Shuttleworths have one board. My little trailer sailer sails just as well on either tack with one board. My 38 ft cat has one motor too. Makes for lots more room inside (no twin wells, motor under helm seat) and cheaper. If you steer the motor you will be surprised at the manouvrability. Even so you are better than tris and monos. Here in Oz many of us stay away from marinas. Cost for one outboard was $5000. To go two diesels was $30000 with folding props etc. I went the outboard.

    cheers

    Phil
     
  10. Guest62110524

    Guest62110524 Previous Member

    fair nuff catsketcher
    My first boat was a Prout sheerwater with single plate, back in 64(apart from my P dinghy)
    Since then I have sailed a 40, liked it, no plates small keels
    when all said done, its about fun, and doing it cheaply
    but for a serious cruising cat, twins would be a must, IMO
    This will be my first build cat It iis extremely well designed, even though i dislike building with stringers I am looking forwards to it
     
  11. Guest62110524

    Guest62110524 Previous Member

    THANK YOU VERY much for your excellent post, I learnt heaps why youir feedback is lo i have no idea have some, we need you here, maybe catch up En France?
     
  12. Guest62110524

    Guest62110524 Previous Member

    comments abt this boat at sea?
     
    Last edited: Jun 21, 2010
  13. masalai
    Joined: Oct 2007
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    masalai masalai

    I do not think it is asymmetric, and the build looks fairly light and fast and the forward cockpit is one of Bobs ideas but I cannot quite recognise it
     
  14. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Surely you're joking

    Masalai

    Its one of your own beloved Masalai - an Oram Mango or somesuch - the sheer, the single daggerboard, the sheet hulls all say Oram to me.

    Bit naughty of the photographer to call the Jpeg "ugly cat" Someone loves this boat. Simple rule I always followed - always find something nice to say about someones boat. Call them a name if you like but don't insult their boat. I've been on big steel monstrosities and commented on the lovely woodwork in the wheel house or the well thought out wiring.

    PS - I think the thread is now way off topic. Sorry for my part

    Phil
     

  15. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Silly me - so It is... Bob will not be pleased....
     
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