Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. DennisRB
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    DennisRB Senior Member

    Very good. If we understand these trends we will be able to design, or choose a design to build that satisfies our SOR better.

    Look at Schionnings nice new boat the Arrow 1200. http://www.schionningdesigns.com.au/arrow-1200

    I like it, but I do wonder why on only a 12m (39") crusing boat, the hulls need to have a 14.5:1 ratio? If it were something like 12 or even 11:1 I am sure it would hardly be heavier, but feel more spacious and not bog down as much in light wind with the inevitable cruising loads, and according to the data have negligibly more drag?

    I watched the bris to glad race last year and the very nice Schionning WL 48? That was in the race. I love the boat and know its fast, but damn was that thing sticky in the light wind!

    Here is something common on most designers websites.

    The Oram 44c is probably one of the fastest genuine cruising cats I can think of yet it's barely even performance cruiser according to this.

    Would the list would be more realistic if they swapped the L:B list with a DLR list. IE, heavy boats are slow and light ones are fast?

    Stanton, Free Flow have another list like this. Starting with under 10.5:1 is a "dog, go buy a mono" yet the FF40 has 10.5:1 hulls! No mention about what weight it has be to before it becomes a dog?


    Bruce number suggestions are given, but no bruce numbers are given for each design. Obviously, you can just keep adding sail to a heavy boat and expect it to go faster. Imagine what the rig on a lagoon 44 would look like to equal the bruce number of an oram 44?
  2. groper
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    groper Senior Member

    Dennis, you cannot seperate length to beam ratio or Displacement length ratio from one another. A more voluminous catamaran WILL be heavier all else being equal . You may not think the weight changes much, but it is very significant. The surface areas of all panels within a hull increase with an exponential relationship in general terms. ( radius of the hull squared )

    The other thing you don't seem to be considering, is the build method and materials. A lagoon 44 is not built from 100% sandwich core composite like a schionning or oram type duflex boat! the lagoon is a production boat and uses extensively solid glass laminates and solid molded components. Cores are more expensive to use and is more labour intensive adding to production costs. Production boat building is about making money, not producing the highest performing boat possible. It's almost 3x heavier, not simply because it's hulls have a lower length to beam ratio, but a multitude of other reasons I've only just briefly touched on some of them.
  3. DennisRB
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    DennisRB Senior Member

    Sure, I agreed with that already. The thing is, if the real reason that cats with wider hulls are slow is because they are heavy. Then why do the glossy brochures and websites insist on explaining how important L:B is instead of just getting right to the point and saying heavy boats are slow, and make the performance prediction list using DLR instead of L:B?

    You are right that I do not know exactly how much the weight will change with a wider hull. If the Arrow 1200 is 5.2t with 14.5:1 hulls what would your estimation be if it were built with 12:1 hulls? I know you have some experience with this sort of thing and might be able to take an educated guess. My guess would be well under 10%, but I am not experienced enough to make a worthwhile guess.

    I am absolutely taking this into consideration. That is actually my whole point. The lagoon is slow because not only are wider hulls heavier to build, as we all agree. But its slow because of low tech cheap building materials etc, plus all the "fluff" and that is where the real weight comes from.


    My point was if it were built the same way as the oram 44 I was comparing it to, but with hulls the same width as a lagoon, the boat would certainly be slower than the original Oram, but it would probably perform pretty well even with the wide hulls. It would probably weigh half as much as the original lagoon (which is still a fair bit more than the Oram 44) and would probably sail past the hump with relative ease.
  4. groper
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    groper Senior Member

    If it's length to displacement ratio is good, then it's well on its way to good performance regardless of its length to beam ratio! We already looked at how insignificant the other design ratios are in comparison to DLR, but remember you are changing more than 1 thing when you change the volume of the hulls. Seperation to length ratio reduces, weight increases, windage increases, waterline beam increases, just to name a few.

    The schionning arrow 12 does not weigh 5.2t. This number, is the maximum design displacement, including people, supplies, fuel etc... everything. I suspect the actual weight of the lightship would be around 3.5 tonnes for that design based on what Ive seen.

    If your looking for trends in increasing volume catamarans, look no further than schionnings website - compare the weights of his more voluminous designs with fuller hulls to that of the more performance oriented designs that are all built in duflex. By keep construction and engineering equal, you should get an idea of the increases in weight you should expect. This is assuming he prints empty weights of his designs as opposed to max displacements - been a while since I've read through his website...
  5. DennisRB
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    DennisRB Senior Member

    I have found it hard to compare weights/displacements of various designers as often the way it is written is hard compare. AFAICT usually, displacement is supposed to be the weight to the DWL which includes gear etc. And if your build ends up at the displacement right away you are in trouble! For Schionnings, often he has displacement then payload which can be subtracted to get the lightship.

    The Arrow 1200 is obviously supposed to be a very fast cruiser at that weight, which justifies such narrow hulls. Yet, its marketed towards sailing around the world. I do wonder if the chosen narrow L:B would actually be faster in the real word when someone actually lives on it full time and sails around the world. People tend to pile on the gear in these situations.

    I am pretty careful, but on a smallish cat like this I think I would struggle with all the cartons of beer I require taking it overweight. :D It seems to me one would need to be very careful with this design if actually circumnavigating to keep it extra light, which may not be a problem if you are good at it.
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    You won't get much out of the early parts.
    But you should be able to glean trends from the principal dimensions of the large collection of hulls in Chapter 7 and the experimental resistance data in Appendix A.
  7. DennisRB
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    DennisRB Senior Member

    Thanks Leo. I had not reached that far yet and my eyes were already glazing over :D
  8. Ad Hoc
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    Ad Hoc Naval Architect

    You have answered your own question, see the highlighted part :p

    You will never, ever, get the true actual hard data in a glossy sales mag, why..its a sales mag. A sales and marketing tool thus economic with the truth; true style over substance. But for some reason many people quote the values posted in such glossy sales mags as if it is a hard fact.

    To find real hard factual data, steer clear of the glossy sales mags and hunt down the black and white esoteric mags that Leo et al read for bedtime :D
  9. DennisRB
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    DennisRB Senior Member

    :p I am a fairly critical person, but some of this stuff is non intuitive so the BS radar has a hard time. I have been reading those papers and they are not the kind of infotainment most people would enjoy reading. But I'm slowly starting to see a little more clearly.

    Anyone of my friends/colleges that has seen me reading them asks if I am studying for an exam. No, just for the fun of it. :D
  10. oldsailor7
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    oldsailor7 Senior Member

    All of the preceding is pretty good reading, and lots of food for thought.

    On the face of it the Buccaneer 24 at 8:1BL and multiple chine, should just be a fat little cruiser.
    But it turned out to be a fast little racer as designed.
    Fitted with a taller mast and a fathead or square top main, with an asymmetric on a prodder it turned out to be a Giant Killer.

    Explain that. :eek:
  11. DennisRB
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    DennisRB Senior Member

    Its got a decent DLR :D Also the effective DLR of the "fat" main hull gets less as the wind picks up and weight is transferred to the more narrow amas. What is the L:B of the amas? That is why tris can be fast in light and strong wind. The main hull can be shorter and fatter so it will have less WSA than a cat. That is my understanding anyway.
  12. Richard Woods
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    Richard Woods Woods Designs

    I have only read the last couple of pages of this thread, so others may have already made the same comments that I do here.

    As always I won't give you graphs or equations because I know that some reading this find those things difficult to understand.

    So I'll just give some comments based on "on-the-water" observations.

    First, although I haven't sailed a Buc 24 (or even seen one) I have sailed against several Buc 28's and 33's so know their relative performance. I would be surprised if the Buc 24 was that much faster than it's bigger sisters, so it seems unlikely that it could be considered a "Giant Killer".

    Builders always try to imply that their boats are "fast" and one way they do that is to say it has a high L/B ratio, even though that, in itself, has a limited effect on speed. They do this instead of giving the "Slenderness Ratio" because they know people accept any sort of "technical" number, and a L/B ratio is easy to understand. So I'll carry on that tradition here.

    As an aside, some people still quote the "Bruce Number" for a boat. Even though it is a completely meaningless number. If you don't believe me, what is the Bruce Number of a model catamaran, length 1ft, sailarea 1sqft, weight 0.5lb?? How fast will it really be? You really have to wonder about the depth of understanding of naval architecture of designers who use such ratios.

    About 30 years ago I designed two trailable catamarans, the 24ft Strider and the 25ft Gwahir. The former had a L/B ratio of about 12:1, the latter 16:1

    In light winds, say under 10knots, I was surprised to find that there was very little difference in speed, in fact changing the helmsman changed who won. However in stronger winds the finer hulled Gwahir was massively faster.

    A few years later I raced a Strider against the fine hulled Firebirds with the same result. In fact in our first race we beat all six Firebirds on elapsed time in 10 knots of wind. Something we could not do in more wind.

    Others have also found that the Strider is a good boat in light winds and flat water. A Zimbabwe owner wrote "Cosmos who has never gone as well. She had no problem with the two Farrier 27's In one race we won by an hour !! that was in a race of 200 min !!" Then from a later race report "What great racing with four Striders on the water & Farrier 27 which we beat easily, and now T-L has launched his new Farrier that he scaled up 15% and the sails 20% up! It went well, but we still beat him over 7 races."

    So practise shows that fine hulled multihulls do indeed "stick" in light winds. That is because WSA is the dominant resistance at slow speeds. A fine hull has more WSA than a fatter one for the same displacement. The least WSA for a given displacement is a hemisphere, so the fatter the hull the better. Of course that's one reason why monohulls do well in light winds

    A wide shallow hull may have the same SLR and WSA as a deep narrow one. However a shallow hull means it is flat bottomed and that shape continues to the bow. Normally a hull is more seakindly (and hence more comfortable) if it is Veed forwards, as that helps cut through waves.

    If, like me, you have ever sailed a Farrier trimaran you will know they slam worse to windward than many bridgedeck cabin catamarans. (My last race in a F boat was the 3 Bridge Fiasco http://sailingcatamarans.com/index.php/articles/10-race-reports/270-three-bridge-fiasco-race but I also have done the VanIsle 360 in a F31R). Incidentally, the last multihull I raced was a Schoinning 38. See my May 7th blog for more details


    The upshot of all that is that I think it is unwise for a cruising catamaran to have too fine a hull. 12:1 seems optimum, 10:1 is the minimum and only racing boats sailing in strong winds should have finer hulls.

    Of course a fine hull limits interior room and load carrying, which are obvious drawbacks for cruising boats. As i said, cruising boats should have Veed sections forward, which means a hull is usually proportionately deeper rather than wider.

    Richard Woods of Woods Designs


  13. yellow cat
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    yellow cat Junior Member

    Hi Tommy,
    I own 2 (tanks) Prindle 16, and sail mostly now with my close friend on his Nacra 5,7.
    They have both the same purpose on this lake (lake Magog) . Midjuly, during our construction holiday (2 weeks) , we see alot of algea packs around (people cut them and push them away ... grrrr) , enough to get our rudders jammed, so you can imagine with dagger boards. There is this new Nacra 20ft on the lake , we hate him out of jealousy, but he is playing with his dagger boards quite a bit these days ... he still kicks the hell out of us in light winds and especially upwind , he is afraid to get out in 70 km/h winds, but he can't go upwind as much as my Prindle16. The 5,7 is a symetrical hull with a fat bottox, we reach often 35-36 km/h (my friend likes km/h ... on his Garmin) and we certainly feel passing over a hump at about 26 km/h on a sideback wind (grand largue) . We dont feel a hump going upwind because we get the cat to pitch a little (better with the Prindle) in order to go as close to the wind as possible. But nothing as close as we can do with a kite on the same Cats. I play a little with the angle of the mast, i.e. get the mast vertical as the cat has only one hull in water.
    We see that friction is the major part of the game since alot of hull surface is not planning, but still going fast (30 km/h on the water on 1 hull is not a slow uncle) .
    Foils will get the hulls out of the water , i.e. less friction and all dagger, and generally a hull has a tendency of plunging into water the faster it goes, the foils keep it out of the water. I am designing a large 60 ft cat, the more i look into it, the more i like the very fine cutting bow, a round belly (less wet surface, less friction) horizontal (no rocker) and a fat bottox (stern) with a removeable dagger board for ocean (gulf stream) in Bahamas kite and cat paradise.
    Recently, we experienced bugging outofwater mini barnacles ... i have to clean the d ... hulls each day almost, waxed hulls makes it easier, but... funny enough, especially on the Nacra's white hull. The Prindle white hull is in the garage , but the yellow (Prindle 18) one is outside and has less ??? I just bought some cold blood creature killer spray for spiders , etc , i want to spray but afraid it will spot the white Nacra hull.
    A good analogy is on Hovercrafts, they do have a past the hump challenge, and they are all friction reduction. Come to think of it, if one closes bow and stern trampoline(opaque) with a skirt, it could become an hovercraft ...
    The Wharram cat designs apparently are surprisingle fast ... check Erik Lerouge designs .. sleek stuff.
    But remember one thing, at 50 knots, is water as hard as sand ? and if it is (i used to bare foot) , would you ski, foil, or wheel on sand to go faster ... if only those waves would not be in the way ... one toy for each park ...
    Back to waxing ...
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