Assymetric Cat Hulls

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

  1. Ikarus342000
    Joined: Mar 2007
    Posts: 32
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: FRANCE

    Ikarus342000 Ikarus342000

    Asymmetrical versus symmetrical hulls

    A final note from my side asymmetrical versus symmetrical catamaran.
    To Mr. Ormans comment. Asymmetrical hulls are not more difficult to build as symmetrical ones. Boats are build on bulkheads as first reference. If you loft ( if at all required, because in the computer age bulkheads are shown on drawings to scale 1 : 1) it will take about 10 minutes more as for symmetrical ones. Planking will be anyway the same.
    If you like a fast boat for its given waterline length the asymmetrical boat will be a good choice.
    If you need a load carrier and a lot of luxury, stick to a symmetrical design.
    If seaworthiness and easy of handling (no boards to handle on a tack, because you need non) is an issue for you, consider an asymmetrical design.
    On boats till about 8m. If you like to beach your boat. Asymmetrical boats are more handy. Sailing them through a surf is a lot more easy with such a boat because of there ability to compensate for broaching. If you like to sail into sallow coves the same. Catamarans with keels are here badly restricted.
    To Phils question. Easy told. CSK cats have had a good balance between waterline length to weight for optimal i.e. low frictional resistance. So they need relative small sails to be fast and can carry this sail area also in much higher wind forces because the CE of the sails is low. Pitching occurs when hulls are to symmetrical longitudinal. Which means have the same volume (form factor) forward and aft from the CG. Its like pulling on a spring, before the spring stops swinging takes long. Use a shock absorber in between and the swinging is dampened out. By giving the hull a bit more volume towards the transom, which is handy anyway normally you need more volume there any way for the cockpit and the engine, the pitching can be minimized.
    I think so far all is told. Beware of the so called experts who do a lot of armchair sailing. Make your mind up what you need and expect from your boat. An egg producing wool/milk/show doos not exists.

    Have a nice day

    Bernd
     
    1 person likes this.
  2. Chris Ostlind

    Chris Ostlind Previous Member

    No harm, no foul, Bernd... you are entitled to your opinion in these matters.

    I would, however, point to the fast catamaran designs of the last 20 years and ask you to explain the reasons why the world's best designers have not seen fit to draw asymmetrical hulls for virtually any of the boats that are truly fast catamarans.

    Not Maxi-Cats, Extreme 40's, Formula F28's, One-offs, various Beach cats, A-class cats, C-Class cats, Tornadoes.... and the list goes on and on. All of these boats are remarkably fast for their competitive purposes and not one of them is asymmetric in form. All of them show much higher Cp values, as well as fully symmetrical hull forms. There's got to be a reason why this is so.

    It would seem to me that the guys who have access to the absolute best sailors for testing, as well as the best VPP and hull analysis software, would have long ago jumped ship to the form which would ensure them of racing success. They have not done so and it has been many years since the design of fast multihulls has seen the regular use of asymmetrical hulls as a standard form.

    There is a very good standard boat out there in the form of the existing Extreme 40' class, as raced at various venues around the world. I would like to see the concept of a speedy assy. hulled cat going up against one of these boats to see just which one has the real goods when it comes to going fast, however it is sailed. If the budget to field such a boat is too high, then perhaps a well-known target, such as the Olympic Class Tornado, would make for a truly proper opponent for such a contest. Maybe someone like Randy Smyth could pilot the assy. hulled machine and Glenn Ashby could have a go with the Tornado? Then, have them switch boats to verify the results.

    The question is... how do we arrange such a demonstration?

    I am certain that the assy, hulled variant will give a good accounting of itself, but I do not think it will beat the boat with the symmetrical hulls. Just my take on the matter.
     
  3. Guest62110524

    Guest62110524 Previous Member

    i posted it for comment, , one big sea breaking green would stove in the front of the cabin
     
  4. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    NOT yet and not likely.... ask Bob Oram

    My build will be like quite a few recent builds with flat toughened glass laminated windows to the front - similar to that in your car...
     
  5. Guest62110524

    Guest62110524 Previous Member

    have you ever seen a big dumping green sea? , compare that with this
    there are seas near manie that can whash over the bridge of a large freighter
     
    Last edited: Jun 21, 2010
  6. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Have you seen a cork in a washingmachine? don't ask me ask Bob :D:D:D then look at some of the cyclonic damage in the last cyclone through the Whitsundays - some images here http://www.thecoastalpassage.com/ - matchsticks or scrap metal for many, couple of holes but still floatable for one of Bobs after being bashed into the rocks by a heavier mono that dragged its anchor.... What the heck is that ? and look here http://www.icecat.com.au/index.htm for a challenging voyage completed...

    More from TCP - - - "[April 3 09] Ausmaid rolled and dismasted. Crew rescued yesterday off Sunshine Coast. Vessel has been recovered and towed to anchorage. 6 metre swell reported. The previous day 13.5 metre Schionning cat Cheetah, motors out of Mooloolaba and crosses Wide Bay Bar later in day. According to the crew they faced 5 metre rollers at the bar and PJ advised the skipper, "don't look back!" when asked what it was behind him...."
     
  7. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Gday Whoosh

    As to the idea of a flat front to the cabin you have to weigh up different safety and comfort problems.

    A flat front plus forward cockpit (as in the photo) means that the crew always stays secure so that is a big plus in the safety department. Stoving in windows in cats is very rare so it may be fine to increase the chances of having a window pushed in if you drastically reduce the chance of you falling overboard in bad conditions.

    Aussie designers are also coming around to getting rid of the very slopey cabin fronts. They look fast but are a bugger to walk on and let in huge amounts of heat in the tropics. Some of the really slopey fronts have sail like shades built over them (which negates any of the windage and wave advantages of the slope).

    I have only had 1 wave over my 38 footer in 10 000 miles. That was not a big wave but a little wind against tide 4 footer that I sailed into off Hook island in the Whitsundays. Cats usually jump up so fast that it is very hard for a wave to get on deck. My cat is a compromise between the slopey front and the Oram front. Near the mast I have a flat area about 300mm each side with a flat front on the cabin behind. This is great as I also have an opening window to let the air in. This idea seems similar to your cabin. Oram just increases the flat front size. Grainger has also done the same in some of his new cats. He was probably the worst offender in the slopey cabin front stance but I guess his clients wanted something different.

    In tropical Australia I would not like to live in your cabin without changes (making the flat area bigger with an opening window) . Oram has lived in boats for a while and so his ideas are the result of his real livaboard experiences. In Germany you may be fine with a very slopey cabin but there are as many good reasons not to have one as there are to have one- it all depends on how the boat will be used. I always am careful to reject an idea if someone experienced or clever has done it. If I can learn from them it will probably save me time and money.

    cheers

    Phil
     
  8. Guest62110524

    Guest62110524 Previous Member

    My dear chap
    i live on Sunshine Coast!!
    When I registered I was over there
    we (my fault) are getting off topic
    All of my sailing has been done off NZ, and around the Pacific, but in 70,s I was on North Sea tug, Chippy T, annd gales were oft , I always remember the smash of seas on the sturdy hulls i also have sailed in conditions on the wind where you count the seconds from the top til when you land in the trough, a strange silence as the boat is in air
    see my gallery, and yes the sun is a darn big problem
    The cat I have been asked to build in 15 tonne, she will not get chucked around(by comparison)
    Brian has a very light boat
    I maintain, and thsi should be in seamanship thread, that very light cats are ok in the hands of skilled sailors like you an Chris and others For people, who have little exp they can be downright scary
    Brian do you know how you have the bight in the sheet? do you know what to do when the wind gusts?
    Ok cork or no cork, you are en passage from here to Fiji or NZ fIJI, or Tonga and one of those storms comes up, like the last one where that light cat just gradually broke up if you had your cat in 70 knots you would be terrified believe me if I had a 40 foot well built mono, totally wt and with vanishing angle of 130 and up, I would just go below , have a beer and wait I believ in your cat you can never do that, you would be rigging drogues, , you would be exhausted:))
    And Brian, I am not knocking you here, but wait and see, the power of the sea is unbelievable
    I have seen Grangers with flamin lounge room doors, range sliding things, no sills,
    But like I said, this is wrong thread . but in one race starting from Spaiin a cuppa years back, several multis never made it first day out, big multis, capsized
    Ok pick you weather fine, sail around Morten Bay fine
    But go offshore, and be prepared
    yes can see how that plumb cab front would give more space
    i will never get used to boats built of powder and glue:)
    i would say here that I think the owner is going into high lats, he has been to the Antartic in his al al mono, so strength is or paramount importance as an adventure charter venture Shape strength is a wonderful thing
    Finall( Brian, masa you comp to front of my car! god almighty, air weighs zilch, my car does not land in a brick wall of water, I mean who are you trying to kid? yourself? come on thsi is serious stuff, your life and the lives of others are right there on the flamin briny
     
  9. Ikarus342000
    Joined: Mar 2007
    Posts: 32
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: FRANCE

    Ikarus342000 Ikarus342000


    Hi Chris

    You got me to respond again, merte (I don`t translate).
    I have partly explained why asymmetrical designs have almost disappeared. It looks like they are also forgotten for racing purposes. I belief it is the same as with cars. In the infancy years of the car development there where a lot of electric cars with better properties as petrol driven cars. Then the petrol engine was taking over and all development was focused only at petrol engines. Now electric cars are coming back and some with for instance acceleration figures which are impossible to match with petrol cars.
    You are right, there are no asymmetrical racing class at the moment. I do not know what you mean by they have higher Cp (CL ? ) values. I think no one has measured them and then compared to values of asymmetrical hulls.
    Hull analyze programs have no algorithm for a-symmetry. The results will be wrong. The programs must be altered. The best test can only be done by sailing them or in water tanks. You know that the water /air transition makes a part of the theories and analyze programs obsolete. Otherwise see car analogy.
    Yes it would be interesting to perform such a demonstration as you propose. We would need somebody or an organization which would be willing to sponsor such a project. How to arrange it I have no clue you living in Australia I suppose, I am in France. But I am in for it.
    Some experience, I was sailing my DUO 425 ( asymmetrical biplane cat) sometimes against wind surfers in the Netherlands and Italy. By the way always in the higher wind force region ( above force 7 ). They could in not one occasion catch me, I beat them always or they more often as not changed course when the found out the could not hold my speed. Other owners of Duo`s have had the same experience.
    As some of this forum members will know, asymmetrical catamarans are only a small part of my design work. My main emphasis is to design affordable boats, sometimes asymmetrical then symmetrical depending on the design envelop. I am not a prophet for the one or the other. I was responding because of the misunderstanding concerning asymmetrical catamarans. There is no best design, only a best for a purpose which we can strive to achieve.


    Cheers Bernd
     
  10. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Obviously you do not know much about cats, Bob told me that one, 'Mango' he sailed out through a bar with a 5ft break, lifted over with no problems and NOT A DROP OF WATER in the forward cockpit, but the forward hatches in the bows were open and slushed a good deal of water into the hulls. (these boats handle short/steep seas better than most mono's) - In returning from the Whitsundays regattas he had a relatively fast passage - outpacing and outpointing most of the "famous" racing maxies with delivery crew of 7 against himself and wife crewing 'mango' in cruising trim. - so if you are in the mood to apologise to Bob he will be able to assure you of the superior seaworthiness and performance of his designs...

    I would not be stupid enough to go out in my 'live-aboard' with a 70knot gale - - but for the sheer pleasure have been out in a fully rigged 16' Hobie-cat "surfing" in Vanuatu when a cyclone was blowing 40+ knots - The clubhouse windspeed did not measure above that... Sailing solo all round the harbour certainly got the cobwebs out... - - and I have been in a mono on delivery in 40knots off the Abhrolis, also I have experienced many other designs in seas around tropical waters - and feel most lightweight cats will outrun cyclones, and I do not intend to sail in cold waters where different types of storm-fronts may be encountered.... Caution is a catchword worthy of remembering...

    Well said and put Bernd - I think the popular Hobie series of surf-cats would also be considered to have asymmetric hulls and the added "banana" also helped in rough water performance - manoeuvrability...
     
  11. Ikarus342000
    Joined: Mar 2007
    Posts: 32
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: FRANCE

    Ikarus342000 Ikarus342000

    Hi Masalai

    You are absolute correct, the Hobie 16 and 14 are asymmetrical cats. Our experience with the SC 435 and DUO 425 are the same, they are great in a surf. And when you have found out how to maneuvering is great to.

    Bernd
     
  12. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 168, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    I don't see the need to set up a trial race between a Duo 425 and another beach cat. In these youtube days there must be a video somewhere (although I haven't looked) of a Duo overtaking a windsurfer.

    Also, since the design has been around sometime at least one owner must have raced one (possibly even in the Round Texel Race) against other beach cats

    Race results are always the best indicator of real performance, much better than tank tests, drag theory or even designers opinions.

    Having said that, I'm not sure how many "Mr Average" beach cat sailors actually go sailing in a F7. Windsurfers do, and those I know say they do windspeed in those conditions. So if the 14ft Duo can sail steadily and consistantly at over 30 knots that is very impressive, especially when compared to say, a Tornado, which cannot.

    More important to my mind is sailing performance in a F3-5, ie the conditions most people want to go sailing in.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. Chris Ostlind

    Chris Ostlind Previous Member


    Richard,

    Maybe it's happened that a fresh F16 has cracked it off with a Duo 480 already.

    Maybe there is a collection of video clips somewhere, (I looked and found zip) but unless we know the situation, how does it matter? The F16 might be piloted by some hotshoe like John Pierce and the Duo was steered by a guy like myself... or the other way 'round, even. The water and wind conditions might seriously favor a particular boat type. I would think that if one wanted something like meaningful results, then the setup would need to be a whole lot more controlled.

    To keep this from getting too serious.... If Real Madrid beats ManU one time, are they, in fact a better team...;-) or should we be looking at a series of contests on a widely diverse collection of pitches and then get a feel for which team shows itself to be better? Having a daughter who plays college soccer, I tend towards the latter arrangement. Just don't ask me to deliver a long ball to a sprinting midfielder when the keeper is out of position. ;-)
     
  14. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 168, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    You are right of course. That is why so many people prefer using a PY handicapping system over one based on "science"

    Having said that, the Dart 18 and Hobie 16 are similar boats in concept (in that neither were designed for ultimate speed) and also in actual speed.

    However, in the UK the Dart was rated faster than the Hobie 16. In Holland it was the other way round. Most acknowledge that was because, on average, the best Dutch beach cat sailors sailed Hobies whereas in the UK they sailed Darts. So even with thousands of boats sailing regularly the statistics "lied", as always.

    Then to football: Isn't the best team the one David Beckham has played for? that means Real Madrid is the better team, oh no wait a minute....

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. Pat Ross
    Joined: Jul 2006
    Posts: 53
    Likes: 1, Points: 8, Legacy Rep: 11
    Location: Florida

    Pat Ross Corinthian 41 Tri #12

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.