Canting Keel Monos vs Multihulls

Discussion in 'Multihulls' started by brian eiland, Aug 31, 2006.

  1. Alan M.
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    Alan M. Senior Member

    The first sentence is a perfect example of blind prejudice.

    What could anybody recommend as an "all ocean all weather" vessel? - A nuclear submarine maybe, not much else.
     
  2. rayaldridge
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    rayaldridge Senior Member


    If submarining or breaking crests on deck is the major cause of man overboard (though I don't know where you're getting that information) then the advantage goes to the multihull, at least in cruising boats. The lighter multihull, with its larger waterplane to displacement ratio, will be more likely to ride over those crests, and submarining is generally only a danger in boats that are being driven too hard (as in racing.)

    The U.S. Navy disagrees with your assertion re seakindliness. I'll refer you to a paper called "Seakeeping Criteria and Specifications" by Hadler and Sarchin, Naval Ship Research and Development Center, 1973. From the paper: "The region from 4 to 10 degrees of average roll shows a marked deterioration in (crew) performance, progressing from some fatigue through the requirement for additional manpower to perform a motor task. At more than 10 degrees, normal functions such as eating, sleeping, and moving around the ship range from difficult to impossible."

    To be fair, wind pressure against the rig stabilizes roll to an extent not seen in a Naval ship, but in a seaway, even with sails up, roll will be greater than 4 degrees, in a monohull. The multihull will roll less in any conditions.

    The idea that the quicker motion of the multi is more tiring than the deep roll of a mono is an idea I've only seen put forward by monohull purists.
     
  3. Vega
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    Vega Senior Member

    Do you think John Kretschmer is a monohull purist?

    Look at what he says about cruising cats and sea motion:

    “…… It takes awhile to adjust to the lack of heel, a result of high initial stability, and the ensuing fast, jerky motion. These cats in general have a lot more motion than a monohull. Also, as designers create more interior volume in the saloon, they lose wing clearance, or bridgedeck freeboard, between the hulls. This can result in a great deal of pounding and slamming underneath, especially when sailing upwind in big seas. During a Force-9 gale in the Venezia two years ago, the water action between the hulls was at times so violent it created a 10-foot-high geyser through the bridgedeck scuppers. Beating across the Yucatan Straits in the St. Francis was slow going and in the aft cabins you felt like you were inside a kettledrum. Indeed, as cats tend to slap at every passing wave, the noise requires some acclimation.”

    John Kretschmer is a boat test writer and a delivery skipper. He has experience with all kinds of sailingboats and about 8000 miles skippering Oceangoing cats, and has tested several cats.

    http://www.sailnet.com/forums/buyin...atamarans.html?highlight=catamarans multihull

    Some years ago someone “sold”the idea, to the Azores fishermen, that motorcats were the best fishingboats for their activity. They have to face big seas and they make a lot of miles. Motorcats are more stable than the typical fishing boat, waste less fuel and have more space for working. It seemed perfect and some of them tried.

    They all got back to traditional fishing boats, they could not stand the jerkier motion of the cats while they were working.

    I guess that these fishermen are also “monohull purists”:rolleyes: .
     
  4. Vega
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    Vega Senior Member


    Rough, I think that you are forgetting something. Let’s imagine a possible real situation:

    Both boats are sailing with 25K of wind, approaching its destination and rounding a cape in a route that will lead to the Port.

    Wind is 70 degrees true and (according to the posted Polar) the Oceanis is making 9.5K and the Lagoon 12.2 k. The Oceanis at 25k has to be reefed (it has Max RM 2.5 times smaller than the Lagoon), let’s say he has the second reef in and a partially rolled head sail. Let’s say that he carries 47m2 of sail. The Lagoon, is going a lot faster, because with its huge RM he can still carry all of its sail (94m2). In fact both boats are carrying the amount of sail that their RM permits them to carry safely. The safety margins are the same.

    Turning the cape both boats are unexpectedly struck by a huge gust of orographic wind, say, 45K. The Oceanis suffers a knock-down and the Lagoon capsizes.

    What I think you are forgetting in that analysis of yours, is that the wind will not be making the same force in the monohull and in the cat, because, given its bigger RM, the cat is capable of carrying the double of the sail that is carried by the monohull, and that is what makes it faster.
    In the described situation, the monohull carries half the sail of the cat and that means that the wind force on the sails is half of the one made on the cat. For having the same force experienced by the cat on its sails, the monohull would have needed a 90k wind.

    There is also a big difference: if the big Max RM of the cat is overcome, the boat capsizes, if the Max RM of the mono is overcome the boat will broach and can (or not) suffer a knock down.



    If you look at the GZ curves, you will see that after the big Max Rm of the cat, there is almost nothing left. On the Mono, it’s the opposite, much smaller Max RM, but most part of the boat stability comes after Max RM.

    Completely different stability curves and completely different sailboats.

    Jehardiman has made a very inspired the comparison. He has compared a mono sailboat with a Willow and a cat with an Oak, referring to Monohulls versus Sailcats sailing characteristics.

    In this post he says what I am saying in a much more concise way:


    This doesn’t mean I find cats unsuitable to Oceangoing (or that I don’t like cats), but it means that I think an oceangoing monohull can be substantially smaller (and less expensive) than an oceangoing multihull (for having the same safety, in what concerns capsizing).
     
  5. rayaldridge
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    rayaldridge Senior Member

    Maybe he is a monohull purist. Still, I can't find in the statement you quoted any evidence that he believed the motion of a cat to be more tiring than the motion of a monohull, which is what I was talking about, though perhaps I didn't make that clear. Furthermore, the judgement he made was of a specific cat design-- apparently one in which bridgedeck clearance was too low. Would it be fair to judge seakeeping ability for monohulls using a poorly designed monohull?

    Consider also that while the motion of a motorcycle is faster and jerkier than the motion of a rollercoaster, the latter is more likely to induce nausea. If you had to choose between the motorcycle and the roller coaster to ride for three weeks straight, which would you pick?

    I have a great deal of respect for Kretschmer, having favorably reviewed a couple of his books. That doesn't mean he can't be influenced by his personal biases, and the vast majority of his sailing has been aboard monohulls, which may mean that he has a firm preference. One has only to look at the absurd nature of Don Street's take on multis to see that even the most knowledgeable among us can be influenced by preconceptions.
     
  6. rayaldridge
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    rayaldridge Senior Member

    I can't disagree with this. The smallest monohull to circumnavigate is 12'. One would have to be insane and very very lucky to cross an ocean in a cat that small.
     
  7. RHough
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    RHough Retro Dude

    I think the Lagoon has fallen into the trap of thinking that a cat will react the way a mono will. The skipper is operating his boat much closer to the limit than he thinks. As you point out and as we all know, once the cat starts to go, the probability of saving it is very low. As the gust hits, the mono, running on it's auto pilot gets knocked down and the soup is spilled. The lagoon must immediately turn down wind and release the mainsheet.


    For equal weight boats (my original premise) the multi is longer, 45 vs 35 feet. It would be a very poor design indeed for the multi to require More sail area in any wind over 10-12 knots. My multi has greater area than the mono to equalize light air speed, at 12 knots and up it reduces sail at the same wind speeds that the mono does. Just because my mono is capable of carrying twice the sail in 25 knots breeze, it does not have to in order to match or exceed the mono's speed. SA/D is equal at all wind speeds 12+.

    In the situation you describe, my 20,000#, 45 foot multi, is carrying the same sail area as a 20,000# mono (while sailing at a higher speed). When the gust knocks the mono flat, all it does to my multi is increase the speed. In the mono the soup goes on the deck due to heel, in the multi the soup goes on the deck due to acceleration. :D

    You are absolutely correct. That is one of the reasons that multi's have a bad reputation ... there is very little feedback (as far as heel is concerned) before the multi is dangerously close to it's limit. I do not think it is fair to malign the type just because it requires a different skill set. Sure if you sail it like a mono, it may bite you.

    In this, my friend we agree 100% :cool:
     
  8. Alan M.
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    Alan M. Senior Member

    This is nothing like a real life situation. The Oceanis sailing at 9.5 kt in a 25kt breeze, with 2 reefs and a part furled headsail? You must be dreaming. And then to say that in the same breeze the Lagoon would only be sailing at 12.2 kts? With all of it's sail up? More like 22.2 kts.......

    The cat would reef sooner and deeper than the mono. It is something cat sailors are aware of - reef early and often - unless the boat has a very small rig on it - like a charter boat for instance, in which case the gust wouldn't bother it anyway.

    A more realistic scenario - the Oceanis is sailing at 9.5 kt in 25kts true, with all of it's sail up. The Cat is sailing at 15 kt with 2 reefs in and part furled headsail. The gust hits, the Oceanis heels hard, probably loses rudder control, rounds up and stands up. The cat accelerates to 20 kts and the skipper eases the mainsheet and slows it back down.
     
  9. rayaldridge
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    rayaldridge Senior Member

    Vega, I read that Kretschmer piece you referenced. Apparently he is not a monohull purist, because he had this to say about multihull comfort:

    I was on the dawn watch thinking I had a read on the apparent wind. So I was surprised to find out in scanning the instruments that the true wind had actually increased to a steady 25 knots. When did that happen?

    My cup of coffee on the fiddle-free cockpit table was just where I had placed it minutes ago, and my three crew members remained sound asleep in their separate staterooms. The Autohelm 7000 plugged along in automaton-helmsman style. And in general, this Fountaine-Pajot Venezia 42 maintained poise as she tracked over the Atlantic, easily coping with the six to eight-foot seas rushing between her hulls. It was easy to get lulled into a contentment.

    The Venezia's speed was a consistent seven to eight knots with an occasional low, double-digit spurt when surfing down a wave. My 20-year-old, 44-foot steel ketch could keep pace in these conditions, so I thought I'd be clutching my cup of Starbucks as we rolled through 30 degrees. Having sailed Next Wave nearly 5,000 offshore miles, I knew any dreams of screaming along at 20 knots, or even 15, were just that, dreams.



    It should probably be mentioned that the boat he's talking about is an overloaded condomaran. Well-designed cruising multis, kept light and well-sailed, certainly are faster than the vast majority of monohulls of similar length.
     
  10. Vega
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    Vega Senior Member

    A situation like that happened recently in the Med. It was a cruising cat, cruising home with a very experienced crew. They were cruising home after a race in the Balearic Islands.
    If I remember correctly, not to much wind, a sudden strong gust (orographic wind) and a capsize that unfortunately had tragic results.
     
  11. Chris Ostlind

    Chris Ostlind Previous Member


    Vega,

    We all get your position on this argument and it is supported by some rather irrefutable issues. Excellent work on your part to that end.

    Perhaps, in the interest of fairness, you could provide a data supported, as well as an anecdotal, set of references as to monohull sinkings once knocked-down? I have a sneaking feeling that the process would more than qualify as "tragic"

    Isn't the ultimate question as to suitability actually about getting your cruising butt back home?

    Let's see now, a boat at the bottom of a very large sea or a boat still floating with full use of the electronics for signaling and access to a berth and food stores....... Hmmmm? I'm going to have to think on that for awhile and get back to you.

    Next topic should be relative comparitive value of the state of the art in life raft technology vs the inherent value of the upside down multihull. You know, the stability thing, the relative comfort thing, the data supporting the survivability thing... all that messy stuff about staying alive so you can do it all again next year.
     
  12. PI Design
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    PI Design Senior Member

    I have joined this discussion very late, and haven't read through it all, but I thought I 'd air my opinion. My father is fortunate enough to own both a Prout 34 and a Dragonfly 920 Extreme. Between them, they offer everything he could want from a yacht. The Dragonfly is unbelievably quick and offers enough accomodation to cruise comfortably around northern Europe (sailing by day, moored at night). Its speed is a safety factor. You can cross the English channel in 2-3 hours. That leaves very little time exposure to bad weather. I haven't sailed it in very bad weather and wouldn't particualy want to, but the motions in normal weather and superb - I defy anyone to feel tired or seasick.
    The Prout is a really solid boat. It can withstand the fiercest storm (I have sailed it when when monohulls have been sending out Mayday's) and does not heel to any noticeable angle. It does have an unusual motion and nearly everyone I know has felt queasy the first time they've been on it. However, subsequent sailings feel fine - it seems to be a matter of the brain experiencing a new phenomonon and then accepting it (but I wouldn't stay below deck for too long whilst sailing). The big advatages over a monohull are the speed and the accomodation. Even the super heavy weight, ultra cruising orientated Prout cruises easily at 8-10 kts, whilst having the interior space of a 50 foot monohull. It can be sailed easily by husband and wife, or take extended family for weeks on end. To have a monohull of equal speed and space would require a boat so big (and expensive) as to be unwieldly for everyday use.

    Anyway, those are my personal experiences.
     
  13. yipster
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    yipster designer

    Vega,
    allthough it may look different i'm sensing a great interest in cats at your side
    i only sailed beachcats with a wetsuit and safetyjack and those do flipover easy
    with cats i think its eighter that or for a big cat go big (above 16 m)
    i'm no expert in eighter but like van peteghems lagoons and
    in the med i saw them also for rent, you could try a few...
     
  14. jehardiman
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    jehardiman Senior Member

    Don't know if the rig was cut away. Given what I know about the moving part problem and what I've heard about the canting keel failures (most seem to be fracture of the ram ends or foundations) here is my take. I believe that the analysis of the maximim dynamic load was done correctly and the ram ends designed with a proper factor of safety. However, from my own experience I have found that the maximum load is actually just a mean, with a random variant force due to keel flutter, roll, pitch, wave orbital, wind gust, etc,superimposed. Now the designer wants the ram to be light and strong so he selects a high-strength material....and unknowingly starts down the path to failure. As has been discussed in other threads, some high strength metals are susceptiable to low-to-moderate cycle fatigue. At high loading factors, l suspect that the materials being used are having fatigue cracking problem, which leads to loss of section...which increases the load, and we are off to a cascade failure. IMHO this is why we are seeing failures in even benign conditions. We now use massive dead soft steel fittings in my office for moving parts to avoid this problem.

    Energy is force times distance or moment times angle, it is not time dependent. Energy units are lb-ft. In the graphs if you intergrate the monent required to heel the vessel to some angle theta over some small differential theta, then the area under the curve of righting moment is lb-ft-theta, and as theta is unitless, the answer is in lb-ft = energy. Work or power is energy applied over time, i.e. lb-ft/sec. It always requires the same energy to heel the vessel to some theta, it would require ten times the power to do that in 1 second as opposed to 10 seconds.

    THE most important factor in prevent a capsize is the crew. The very first time I took my future wife sailing we capsized a 470. As the boat heeled in a heavy gust I spilled the main and drove up as she hiked out....and pulled in on the jib sheet to keep her balance...and ended up swimming as I stood on the hull looking down at her. I don't understand why she still married me. The closest I ever came to swimming home was in about 10 knts of breeze and SS 1+. While running with the chute up, we got into a crossing with a powerboat....we stood on....they nearly ran us down. With mainsheet and spinniker sheet running, we stood her on her heel and pivioted as the A**H*** in the powerboat stared at us from his compainonway. Still not enough off her and laid her on her beam ends with the chute still pulling...foot in the water. I walked on the cabin side and mast to blow the halyard while the helmsman did chinups and the other crew nearly drowned tangled up in the guy and lee lifelines. IF there had been a running seaway, we might have lost her. All vessels are potential losses at any time. Alertness, awareness, seamanship are the closest things to safety...righting moment, wetherliness, etc. are all just insurance.

    As someone that writes maneuvering guidance for nuclear submarines, I will catagorically say that there is NO BEST "all ocean all weather" vessel. There are only better choices given the selection criteria. My prejudice about selecting a mono over a multi for myself and my family to conduct a world voyage is neither blind nor prejudiced. it is a reasoned technical one. I am open to all well reasoned technical arguments. (N.B. screaming "HERETIC!!" is not a well reasoned technical argument).

    While it depends on the particular hull, all the studies I've seen and conducted, show that water on deck/slamming is a greater problem with multis than monos...becuase the waterplanes are selected to minimize motions and accelerations. You see, you can't have it both ways. Either the waterplane is so small the hull doesn't rise to the wave and you get wet depending on cross deck effects, or the waterplane is large and the hull profiles the wave causing large motions and accelerations.

    The choice of the Hadler paper is interesting (I have seen it, but today we use other criteria for motions) because that paper is not generally applied today due to the atmosphere it was written in (i.e. the lab was pushing an agenda..see the backstories behind AGOR-16, ASR-21&22, SWATH, SES, NATO North Sea Helo projects, and some other I can't remember from that time frame). That paper was an attempt to justify a hull form selection, not define absolute criteria as anyone who has worked backdeck in the offshore field will tell you. Because Hadler was associated with AGOR-16 and ASR-21 and if you know what happend to them, you can see why he was out of the loop soon after that paper. It must always be remembered that politics and funding lines drive a large amount of study...a sword that cuts both ways.
     

  15. brian eiland
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    brian eiland Senior Member

    I'd be interested in your comments on this hull design
    M-Hull, M hull, M Ship
    http://www.boatdesign.net/forums/showthread.php?t=5687

    Opps, pardon me. You've already contributed to the discussion
     
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