Canting Keel Monos vs Multihulls

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

  1. RHough
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    RHough Retro Dude

    I did some thinking about almost the same thing today.

    Of course jehardiman is right within the very narrow frame that he placed his comment. He is also right about the ability of mono's to self right ... after all, they are very easy to knock down! :)

    Looking at a 90deg knockdown without considering the force needed to roll the vessel to 90deg gives a very simple answer; A multi knocked down to 90deg is likely to be past it's limit of positive stability and thus will capsize. This model assumes that the same amount of wind is needed to roll both boats to 90deg. Of course, that is not the case.

    If we look at two boats of equal displacement and moderate dimensions what is the likelihood that the boat will suffer a knock down?

    Using 20,000 pounds displacement a D/L of 250 gives a LWL of about 33 feet. A moderate SA/D of 17 gives a sail area of about 700 sq ft.

    On the mono side we have a 35-40 foot boat. To keep the design moderate lets give it a beam of 12 feet and a draft of 6 feet. Ballast/Displacement might be 40%? so 8000 pounds of ballast? What RM max would be reasonable to assume? 60,000 pound/feet? (I think that is very generous, RM30 will be close to 40,000)

    A cruising Trimaran that displaces 20,000 pounds might be 48 feet LOA and have a 34 foot beam. It will have an RM of about 300,000 pound/feet. (A Farrier 39 is 10,000 pounds at full load)

    The Multi has a 5:1 advantage in RM. Since our two boats share the same rig it takes much more wind to capsize the the multi. True, once it goes ... it's gone but it takes 5 times the force.

    If it takes a 50 knot gust to flatten the mono, it will take over 100 knots to capsize the multi (in flat water).

    I wonder what the load capacity of a 33 ft LWL 20,000 pound mono is compared to a 48 x 34 20,000 pound trimaran? I'll bet the load capacity of the tri, while small for a 48 foot boat, might be larger than the smaller mono that displaces the same amount.

    I may have to start re-thinking the mono vs multi question ... :)
     
  2. Crag Cay
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    Crag Cay Senior Member

    That's why these three aspects of 'stability' are evaluated seperately when discussing 'capsizes'. And also why the word capsize without any further qualification is meaningless.
    1. How easily can a boat be 'upset' and knocked down to 90 degrees? (capsize resistance)
    2. How well does it recover from a 90 knockdown?
    3. How does it behave beyond 90 degrees, (AVS, inverted stability etc)

    Obviously there are huge differences in the way a multihull and monohull behaves in all three situations, and the skipper has to decide which combination, for him/her, makes for the more seaworthy craft.

    The parallel is with cars: Are you safer in a car with better brakes/handling or more air bags?
     
  3. jehardiman
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    jehardiman Senior Member

    Crag is correct, I should have stated what I call a capsize. To me a capsize is when the vessel rolls into a stable inverted condition, resulting in an non-manageable condition of the vessel. A knockdown to masthead in the water is not a capsize if the vessel rises again, i.e. it is still a sailing vessel. As someone pointed out, lots of monohull racers have "washed the Windex" on occasion.

    There are also other aspects to what happens in a knockdown between the similiar weight hulls I proposed.

    1) Multis having more RM and roll inertia, place larger loads on the rig, resulting in a greater chance of dismasting.
    2) Multis having more roll inertia, have a greater chance of continuing to roll as wind overturning moment decreases as the heel. i.e. once it starts to roll, it is harder to arrest it.
    3) Monos, in heeling sooner, quicky reduce sail area to survivable amounts. That is why I specified a wind speed profile, not an overturning energy input. While the available energy is large, by heeling the hull reduces the amount of energy absorbed into overturning. A multi is an oak trying to resist the wind, a mono is the willow bending in the wind.
    4) However monos, in heeling more, sooner, and longer, are more susceptable to downflooding and wave effects while knockdown but not capsized. With a multi it is a moot point as once the maximum RM is reached the capsize is a given

    Lots of considerations when selecting a hullform, and reflecting this turn in thread back on the original post....A multihull is not the only hull form that should be considered for the sailing community.
     
  4. brian eiland
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    brian eiland Senior Member

    Surely, I think most all of us can agree with this statement
     
  5. Vega
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    Vega Senior Member

    Yes, I think you have to rethink again:p .

    I had asked you guys for a measure of the total area under the RM curve of those two boats (I was hoping not to have to make it, I am lazy). But you have not given me any choice and I have done just that.

    On the case of the Oceanis 50 versus the Lagoon 411, the area of the Oceanis is 23% bigger and that means that for capsizing the Lagoon it will be needed only 77% of the force required to capsize the Oceanis.

    Rhough, you are confusing two things: Max RM and the total force needed to capsize the boat. The Max RM of the Lagoon is massively superior to the Oceanis. Something like 20 996kg/m to 8 174 kg/m, but the force needed to capsize the boat is not given by that, but by the total area under the positive part of the RM curve. And that area is 23% bigger in the Oceanis.

    Regarding the RM that can be translated in sail power: the Lagoon has a RM of 20 272kg/m at 11º of heel and the Oceanis only 6 611kg/m at 30º of heel. This means that the Oceanis can not carry as much sail as the Lagoon and needs to reef a lot sooner (the Lagoon has less sail for safety reasons).

    That’s the reason why the Lagoon is a lot faster with stronger winds and also the reason that makes it an easier boat to capsize with strong winds. Most of the stability of the Oceanis is reserve stability (can not be used to sail), while a bigger percentage of the Cat stability can be used to sail. That makes the Cat a faster boat, but makes also the mono a safer boat.

    Regards
     
  6. Chris Ostlind

    Chris Ostlind Previous Member

    Reconsider?

    Well Vega,

    I hate to break bad news like this, but a canting keel monohull has just capsized in the Indian Ocean near the tip of Africa. Since this thread started with the discussion of canting keelers vs multihulls, I thought it proper to bring this issue to light here. You can read all the entries so far on the Velux 5Oceans site: http://www.velux5oceans.com/page/Home/0,,12345,00.html

    I find this remarkably poignant in light of your recent statement:

    I'm saddened by any boat that goes over and stays over, but with this technology, it appears that there is still one heck of a lot to learn before the statement above can be made in a blanket form.

    I realize you were comparing a non-canter to a cat at cruising design and trim, but it makes for bad business to create any of this type of commentary when the form (canters) are so very far from dependable status.

    Let's hope for a safe recovery of Alex Thompson by Mike Golding. In the meantime, here's a quote from another, well-known source on the problem aboard Thompson's Hugo Boss canting keeler from the Velux site:

     
  7. RHough
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    RHough Retro Dude

    I don't understand that at all. :(

    It seems to me that to capsize a boat, the heeling force must exceed the RM. No?

    The only use I have seen made for the stability curve in relation to capsize, is the ratio between the positive area and the negative area. The more equal the areas, the more likely the boat is to stay inverted.

    I think that max RM is a pretty good indicator of resistance to capsize. The Lagoon has more that 2.5 times the max RM. Given equal rigs (or windage of the rig), it would take 60% more wind to capsize the cat. If it took 50 knots to knock down the Oceanis, it would take 80 knots to start the Lagoon over.

    I'm probably wrong, just saying that the area comparison does not make sense to me. What units would that be? pound/foot/degrees?
     
  8. brian eiland
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    brian eiland Senior Member

    Chilling Report

    Alex Thomson explained what happened earlier this afternoon in a satellite phone conversation:

    "I was sailing on a broad reach in 30-35 knots of wind, with two reefs in and the Solent and it was pretty hairy, speeds up to 25 knots, got the boat under control. I was in bed at the time and the boat suddenly fell over into a big broach. So I jumped out of bed and ran up on deck to ease the mainsheet and when I eased the mainsheet nothing happened. By this stage the boat was right over until the spreaders were in the water and I was struggling to stay in the cockpit.

    "I couldn't understand what the issue was but I could hear the keel pump going on. So I came down below and I did the keel, and I did the keel on deck, but nothing happened. So I went up forward and I had a look into the keel box at the top of the keel and the keel wasn't attached to the rams anymore. It had broken off. And so the keel was down to leeward making the boat lean over even more and I knew then I was in serious trouble. So somehow I managed to get the sails down and contacted my shore team and race headquarters.

    "As soon as I saw the keel I went back up on deck to try and get the sails down. But I was sure that I wouldn't be able to get the sails down so I had visions of the boat being pinned on its side with the spreaders in the water until help could come, or maybe even the boat getting capsized, I knew immediately I was in trouble."

    Thomson continued: "Well I am 1,000 miles south of Africa. The closest land to me is the Prince Edward Islands, which are uninhabited. At the moment the conditions are quite benign, a force 4-5, bright sunny day, quite cold. But in 6 hours' time I am going to have a severe gale 9 with gusts up to 50 knots. I have managed to stop the keel swinging at the moment but it's a temporary fix but when the keel does start swinging in those conditions it's quite possible that the keel will swing. The top of it will swing into the boat and hole the boat, and the boat could sink.

    "Mike's about 70 miles away and we are trying to converge on each other as quickly as possible. We are both aware that the wind conditions, at the moment, we estimate we are about 6 hours from where we are to where we meet. And unfortunately in 5 hours it's going to get dark. So I am trying to get ready, get my life raft ready, get flares, get all the things that I need, get my survival suit on and be ready for when Mike comes and hopefully conditions will still be benign enough for me to be able to get off the boat, otherwise I would have to think about a night on the boat with the keel free swinging which is not a very pleasant thought. I have spoken to Mike and it is very kind of him to come and help. He is making his way here as quickly as possible and I just hope he gets here in time."



    Brian added:
    Now I don't know about you, but at this point I would much prefer being on a vessel that would not sink, even if I was upside down. and getting into a life raft with a big gale coming is not a comforting feeling either.

    Heres wishing the best of luck to the rescue effort!
     
  9. rayaldridge
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    rayaldridge Senior Member

    Wind-only capsizes are pretty rare. Most capsizes occur from wave action or from a combination of wave and wind. It's my opinion that a multi has a couple advantages in the conditions most likely to produce capsizes; big wind-induced waves with minimal or no canvas up.

    A monohull with a keel is more vulnerable to wave-induced rollover, I think, because the keel is down in denser more stable water when a boat is hit by a fast-moving aerated breaker and provides a lever over which the boat can trip. Multihulls, and particularly cats with boards up, are more raftlike. They tend to surf sideways in a breaking wave, a phenomenon I can personally report to be true.

    Consider, for example, a keelboat spilling sideways down the face of a big steep wave-- a scenario that has damaged and sunk several yachts. The keel will dig in and attempt to flip the boat upside down. At best, the rig will be driven masthead first into the trough, and few rigs will survive such treatment.

    Finally, any such discussion usually ignores the elephant lurking in the room. This particular pachyderm is the fact that the greatest loss of life aboard pleasure craft of all types is due not to sinking or capsize but to man overboard. The more stable platform of the multihull, it seems to me, would make it significantly easier to keep your crew aboard. It's also been my experience that multihulls are less likely to exhaust their crews, because operating at a significant degree of heel is more tiring than operating flat. A more rested crew is less likely to make the sort of disastrous decisions that leave boats on the rocks.

    Anyway, lots of good reasons to like monohulls better than multihulls, but safety probably isn't one of them.

    Ray
     
  10. jehardiman
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    jehardiman Senior Member

    Actually at the time of your posting, the vessel hadn't capsized...yet and still had not when he was taken off. However, Golding lost his mast soon after the pick-up. Maybe go back and get the good stick? Anyway...as a NA that has spent a good part of his life designing that not to break at sea, I find these type of failures worrisome as they are preventable. I'll be willing to bet I can name that failure type. Cyclic loading always tends to be underpredicted by ayalysis methods and there is always money for everything but a full scale test/ data collection program. It is just tireing to see so many people make the same mistake.

    Yes to capsize, the righting moment must be exceeded,but only for that brief moment, the real thing that or capsizes a vessel is energy, not force. The important thing to remember in a free system is that only energy can be transfered from one body to another. Force is just the rate at which that energy is being transfered.

    Lets take an example similar to the example Vega gave us. I have two vessels. The first (vessel A) has a righting moment curve that can be modeled as 21,000*sin(theta *(180/70)) where theta is the angle of heel. The second (vessel B) is modeled as 8,100*sin(theta *(180/120)). We model the wind over turning moment as 100*V^2*cos(theta) where V is an arbitary wind speed unit and the moment is reduced due to the heel angle.

    So at V =10, Vessel A heels ~11 degrees, Vessel B ~44 degrees. But as we increase wind speed to V = 15, Vessel A goes to 28 degrees, but vessel B only goes to 70. In this step up vessel A's heel doubles, while Vessel B's doesn't...Why, because being more upright means more force and energy is being given to vessel A. If we increase velocity to 16, A goes to 35 while B only increases to 72. Note that a 6% change in wind speed (13% increase in force) caused a 25% change in heel angle for vessel A. For vessel B, changes like that occur at low sind speed, not high wind speed. Finaly, at a windspeed of ~16.4, Vessel A fully capsizes to leeward from a heel of less than 45 degrees. See attached file. Vessel B is still riding with about 74 degrees of heel. Even if we double the wind speed needed to capsize Vessel A (i.e. 4 times the force), vessel B is still only knocked down to 86 degrees, not capsized. Vessel A has what can be called a percipitious righting curve, i.e. the vessel stiffens up until failure is immediate, catastrophic and unrecoverable.
    Actually, it's more the other way around especially in fairly moderate conditions such as SS5 through SS7. Monos, with there mass inertial stability are less of wave followers that multis with their waterplane inertia. While monos are more succeptable to crest breaking effects such as you describe, multis are more succeptable to bridging and phasing effects. Both are at about the same risk in etereme seas, with the advantage going to the mono due less windage for the weight as the sea is generally flattened in very high wind conditions. As for the man overboard issue, submarining (i.e. driving through a wave) or having a crest break on deck is the major cause of overboarding while underway, not deck motions.

    My underway experience has been just the opposite. Monos I have experienced in heavy weather in have had larger, but lower acceleration motions, while multis motions have been short and jerky. Heel IMHO didn't make a bit of difference in a well setup racing boat, though some crusiers I've been on would be difficult at large heel due to lack of handholds/footbraces and too large of compartments.
     

    Attached Files:

  11. Vega
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    Vega Senior Member

    RHough, I am going to try to explain, but remember that English is not my main language;) :

    Beginning from 1º of heel, when the heeling force exceeds the Rightening Moment on that point of heel, the boat heels a little bit more. Then if the heeling force exceeds the RM on the new point of heel, the boat heels a bit more and so on, till the point of heel where the boat makes the max. RM force. And that point is just a point like the others. The story goes on like before (when the heeling force exceeds the RM on that point of heel, the boat heels a little bit more) till the moment that the heel of the boat does not produce more RM and that is the AVS point. At that point the boat capsizes.
    The force needed to capsize the boat is the total force produced by all the heeling points. The RM curve is just that, all the RMs at all the heeling points. That's why the total force needed to capsize the boat is given by the area behind the positive part of the RM curve (all the RMs at all the heeling points). The area behind the negative part of the RM curve is the total force needed to put the boat upright.

    edit: I didn't see the pervious post. Jehardiman,that's much better, it looks that I should have said the "total energy needed to capsize the boat", instead of the "total force".
    I will maintain this post, perhaps my very aproximated explanation can be easier to understand.
     
  12. RHough
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    RHough Retro Dude

    Thank you both. I understand the argument. However, at some point the heeling force must exceed the maximum righting moment to capsize the boat. Thus in conditions where the heeling force from the wind is great enough to exceed RM max, the boat with the greater area under the curve is less likely to capsize.

    My question is how likely are winds of that force?

    If the wind speed required to generate that force is 2-4 times higher for one boat than another, the probability of capsize is lower for the boat with higher RM. If it takes X knots of wind to generate the force needed to capsize the boat, the probability of capsize is the same as the probability of wind strength X. The higher X is the lower the probability of capsize. Thus the boat with the highest RM max is less likely to capsize. No matter what the area under the curve is.
     
  13. Vega
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    Vega Senior Member

    Rhough, the boat will only capsize if the RM force generated by the last heel degrees before the AVS is exceeded. This, for a modern class A monohull, it will be the RM produced over 110º of heel. Nothing to do with Max. RM.
    Over 60º of heel (the point of heel where normally the Max RM is generated), most of the wind no matter the force, will pass over the sails. At 90º all the wind is passing over the sails and the monohull is still producing a considerable RM moment.

    This is not an argument;) . There are lots of boats (specially racing or cruiser-racers) that have a Max-righting moment superior to other boats that nevertheless will require more energy to be capsized (the first ones typically with an AVS of 115/120, the last ones typically with an AVS between 135/180.

    Of course, accepting that a knock-down, for a yacht is not a capsize.

    Regards
     
  14. Chris Ostlind

    Chris Ostlind Previous Member

    Correction

    Thanks, John, for the tidy correction notice as to the specifics of the knockdown/capsize reality on Hugo Boss. I have been a lifelong multihull sailor. With the exception of beach cats and some properly equipped larger boats, when you get a serious knockdown, you are very likely going to also get a turtle. Hence my reference to the condition of the Boss boat.

    With that clarified, I really do agree with John's assessment; that there always seems to be enough money to go get the broken boat (assuming it can be gotten) but not enough (or time) to properly check it out before putting the thing on the water for all-out racing.

    If the conditions back off and they get very lucky, they just might be able to salvage the boat to take a long look at the situation on board. From that, perhaps a solid learning experience will benefit everyone in the sailing community as to the design of the structure for the rams and the keel mounting. In any event, it does seem that there have been entirely too many of these "malfunctions" having to do with the engineering and eventual build with the canting keels on this type of boat.

    John, have you seen any indication that the rig had been cut away to reduce rolling moments on the boat? That might have been a way to subsequently recover her for analysis. OR, (and I'm speculating here with no specific focus) perhaps the cast of characters really don't want to recover the boat?

    Perhaps she's had just too many cycles, even though there was a recent total overhaul and partial redesign by Lombard Design and the MAG Ocea yard in France.
     

  15. RHough
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    RHough Retro Dude

    Vega, this is fun. I have much clearer picture of the capsize event. Thank you for you patience.

    I understand the concept of total energy required to capsize a boat.

    I still have a problem with relating the area under the curve to a measure of energy. The graph is force over degrees. I cannot see how to derive a total energy number with no time scale.

    Certainly a force below that required to exceed RMmax can be applied to a boat indefinitely with no risk of capsize.

    For me the probability of capsize is what is important. In 100,000 M of sailing how often do conditions that can heel the boat past RM max occur? Sailing a Laser in SF Bay in June those conditions occur daily. Yet there are sailors that survive for months without a capsize. It takes a set of conditions for a knock-down to happen; wind strength, wind angle, sail area, RM, and boat speed all factor in.

    If the wind strength is not high enough to produce force greater than RM max no combination of angle, area, or speed will produce a capsize.

    I think my point is valid; you are much more likely to encounter 50 knot winds and get knocked down in a mono, than you are to encounter 70+ knot winds and get capsized in a multi.
     
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