Sink or Swim??

Discussion in 'Multihulls' started by Richard Woods, May 5, 2009.

  1. BeauVrolyk
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    BeauVrolyk Sailor

    Ray,

    I have read a great deal about that Fastnet and talked to a few of the folks who were crewing board boats on the race, friends of mine. There are a lot of conflicting stories. One, that tends to explain the results observed about rolling over without a mast, which for some reason the authors didn't care to discuss, is much simpler than the issue of a larger polar moment of inertia caused by having the rig in place.

    {BTW, I do agree that resistance to the beginning of rolling or pitching is increased by a tall heavy mast. However, for the same reason once a boat, multi or mono, starts to roll or pitch if it has a large polar moment of inertia it will want to continue to go. What this means is that if a quick steep wave hits a boat with a large polar moment, the wave will wash up and over the boat without rolling or pitching her much, I think it was Ancient Kayaker that talked about this with respect to his narrow canoe. In the case of his canoe, his body weight (substantially above the roll center of the canoe and heavier than the entire canoe) causes the canoe to have a massive resistance to initiating a roll. However, if Ancient Kayaker starts to fall over, the canoe will let him keep rolling and doesn't have the ability to stop him. In Ancient Kayaker's post he attributed the "stability" of his canoe to it's narrow beam, which is partially right, but the main reason is the massive (relatively speaking) polar moment of inertial compared to the heeling force.}

    Now back to Fastnet. What was not reported accurately (IMHO) in the book was the simple fact that the boats without masts were almost utterly disabled, meaning that they couldn't motor or sail. In this instance, as we've all observed, a monohull will most likely lay abeam to the seas. In this position the boat has the absolute highest probability of rolling again. The few boats that managed to deploy warps or a sea anchor didn't get rolled again, mast or no mast (to the best of my memory). This ties directly to my earlier post about rolling sideways or end-over-end. Boats, regardless of the number of hulls, tend to be much more resistant to flipping if bow or stern to the seas, than they are with their beam to the waves, simply because they are longer than they are wide. Of course, as I said in the earlier post, this is particularly true the more narrow a boat is. As most monohulls are narrower than most multihulls, the monohull will roll more.

    Putting these two facts together, the decrease in polar moment of inertia due to the missing rig weight aloft and the lack of an ability to keep the bow into the seas, I'm not surprised that boats that rolled once and lost their rigs rolled again more easily. However, if one could get the bow into the waves, I'm also pretty sure that a boat without a mast would fare better due to much less windage aloft (tipping her over backwards).

    One final thought on this topic. When waves are relatively small but very very steep, as they were in the Fastnet Race (reports were that waves were only about 15 to 20 feet high and many boats were rolled over by microbursts of extremely high wind, not large waves.), then the question is what happens to the boat as it initially impacts the steep frontside of the wave. The boat must rise very quickly and can, in some cases, punch through the breaking top of the sea. Here, a larger polar moment would help. But, in extremely large seas that are breaking at the top, such as those found in hurricanes where trough to peak distances can exceed 90 feet, there is plenty of time for a small boat to respond to the wave face until it gets to the very vertical wave top which is actually breaking. In these seas things are quite different. It is here that weight aloft can be quite bad as the weight of the rig is either off to the side or behind the boat as it struggles to get over the wave, and that weight is decreasing the stability of the boat (no mater how many hulls it has) dramatically. This is why, in my earlier post, I pointed out that the point at which a boat is struck by a really big wave's top is one where the wave face is probably already at 65 to 75 degrees from horizontal. In these conditions, the momentum of the mast's weight, which provides a very short term increase in stability through its polar moment, won't help much as the boat is already heeled dramatically.
     
  2. BeauVrolyk
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    BeauVrolyk Sailor

    Well, duh... let me think... maybe the foam? Did I get it right? B-)) LOL!

    But, that's not really the question that matters, as there's a lot more "stuff" in both boats that either lead or foam.

    The question that matters is: "Will the Lagoon 440 sink if flooded?" and "Will the Santa Cruz 40 sink if flooded?" I know the answer to the second question to be a resounding "Yes"! But, do you know the answer to the first question? I don't. The builder's web site dodges the question entirely.

    Beau
     
  3. Freenacin
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    Freenacin Junior Member

    I'm pretty sure it's a requirement for CE certification for multihulls that they can stay afloat when flooded. Also quite sure the Lagoon 440 has CE certification.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Capsized, I believe, not flooded..
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    - quite true: we of the paddling persuasion speak of primary stability - the righting moment as the boat first begins to heel - and secondary stability, the latter term describing an increase of righting moment as the boat continues to heel further; I’m not sure if the same terms are used for larger craft.

    My old plastic kayak has heaps of both despite being tiny; I can lean over the side and observe waterlife. My first canoes with wide flat bottoms rode high and felt unstable as hell but my current one, although narrower, sits lower in the water and feels more stable, although it has much lower righting moment and is therefore technically less stable. Too twitchy for a beginner probably but it feels solid to me; I have slept in it while afloat, until some powerboat guy woke me up - thought I’d had a heart attack! I imagine a racing shell has the same feel, maybe more so. Let’s not even think about sailboard stability ...

    Some kayak designs have positive righting moments out to amazing heel angles, but it doesn't mean they are great paddling boats. Basically, it all goes to show that mere numbers do not tell the entire story. No wonder they call boat design an art.
     
  6. BeauVrolyk
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    BeauVrolyk Sailor

    The CE certification language is:

    Buoyancy and Flotation: The craft shall be constructed to ensure that it has buoyancy characteristics appropriate to its design category according to section 1.1, and the manufacturer’s maximum recommended load according to section 3.6. All habitable multihull craft shall be so designed as to have sufficient buoyancy to remain afloat in the inverted position.

    Both the Lagood and the Beneteau are CE certified, as are ALL boats that are sold in the EU, by law. It says nothing about floating when flooded, only about floating when inverted. A topic that the CE organization was concerned enough about to mention specifically.

    Beau
     
  7. Fanie
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    Fanie Fanie

    Why can't it remain afoat even when flooded ? I would think that is a given.
     
  8. BeauVrolyk
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    BeauVrolyk Sailor


    AK,

    Sailboards, the newer wider ones, have a lot of stability. Indeed, the board is really hard to flip over. Of course, the sailor probably falls of pretty quickly.

    I used to row a little and rowing shells have tremendous stability when their oars are in the water. To tip a shell when the oars are being held by the crew and the blades are submerged is really difficult. However, if you lift the oars out, it's easy. The polar moment of inertia of the weight of the oars helps a little, but not nearly as much as the clamp the blades have on the water when they're submerged.

    Another great example of stability created by polar moment of inertia is the pole a tight rope walker uses to balance with. The rowing shell can't stay upright without its oars and most tight rope walkers can't stay upright with out their poles.

    The difficulty with increasing polar moment is that it usually doesn't work too well in a boat. If a boat resists moving with the water, either by pitching or rolling, it does so by using energy. This slows the boat. A great way to slow a sailboat down (multi or mono) is to put a big weight in the bow (sort of like an anchor and its chain) on in the stern (sort of like a dingy and outboard on davits) or at the top of the mast (sort of like running lights, anchor lights and radio antenna). The added weight in the ends increases the polar moment of inertial in pitching and causes the boat to bury its bow more deeply in waves as it encounters them. This slows the boat tremendously. For the life of me I can't understand way folks sail for miles and miles with an anchor on the bow and a dingy on the stern. Sure, it's easier, but it is sure slow.

    The polar moment of inertia goes up as the square of the distance from the center of rotation. So ten pounds 20' above the center of rotation of the boat up the mast is not twice the resistive force of ten pounds 10' up, it is 10 times 10 pounds of force, or 100!! This is why it is so bad to put the radar antenna way up in the rig, or the anchor on the bow or the dingy on the stern. Sure, you can see a little further with your radar higher. But the resistance to pitching and rolling, or the polar moment of inertia to be correct goes up as the square; that's a big deal.

    The same effect is true in rolling side to side. It takes energy if you're boat is resisting the roll. Fortunately, for most situations, the energy being used is not being absorbed from the forward motion of the boat, as it is when a boat pitches. So, most designers don't worry about it much. I would think that in a long kayak there would be an effort to get the weight out of the ends so that the bow and stern didn't dig in and slow you down.
     
  9. BeauVrolyk
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    BeauVrolyk Sailor

    I'm completely certain that one could build a boat that would remain floating when flooded. It's a simple matter of putting enough foam in it to float all the weight. This is obviously a lot less foam for a multihull than a monohull (which has a weighted keel that can be has heavy as over 1/2 the weight of the entire boat). The reason that most designers don't add that much foam or sealed air space is because it takes up valuable space within the boat that could be used for other stuff. Also, the buyers of these boats usually don't worry too much about sinking, as it's pretty darned rare. So, the designer and builder will leave the space open for storage of supplies or other uses.

    This is true for both monohulls and multihulls. There are lots of monohulls with positive buoyancy, but they are usually small daysailing boats not cruising boats. While I have a number of friends who have told me their multihulls are able to float when flooded, they typically read it off the flier or are told that by the salesperson, and that information usually does not take into account all the junk that a normal cruising sailor puts on his/her boat when they go to sea.

    The important question is: will the boat float while flooded when fully loaded to go sailing in the manner that one is going to use the boat?

    It should be easy to figure out. The manufacturer of a supposedly non-sinkable boat should be able to trivially tell you how much "reserve buoyancy" the boat has, and that will tell you how much heavy stuff you can bring (meaning stuff that is heavier than water) before you will sink the unsinkable boat once it floods. Oddly, I can't find reserve buoyancy or even any "unsinkable" claims for most of the production cats, although I'm sure that they'll turn up in this forum if they exist.

    Beau
     
  10. Freenacin
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    Freenacin Junior Member

    If it can float upside down (and these French production boats have escape hatches, so it's not due to air trapped in the hulls), it can float right way up surely?

    And CE certification requires it to be able to float while carrying the designed maximum load.

    Monohull sinkings might be rare, but just recently there have been several sinkings of reportedly well maintained cruising boats which are not in any way attributable to weather. Also one sank in the Sydney-Hobart race in very good weather conditions.

    As for cats compared to mono's in severe weather : http://lists.samurai.com/pipermail/passagemaking-under-power/2005-April/001034.html
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    That is the way I rest in my canoe: I put the double paddle across the gunnels and put a leg over it, then relax. I don’t, however, normally go to sleep ... not very comfortable.

    Regarding the storage vs floatation question, kayaks commonly have sealed buoyancy compartments that can also be used for storage. As well as regular clipped hatches, circular screw-down hatches are used, which can also be fitted with water-proof luggage bags. That would be clumsy scaled up for a large boat, but bagging stuff to provide buoyancy in the event of a capsize or flooding must be common practice in boats used for serious cruising. I am planning to incorporate small airbags in the buoyancy chambers of my tiny sailboat in case a collision holes them, as well as foam.
     
  12. BeauVrolyk
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    BeauVrolyk Sailor

    AK,

    A lot of us put things that are giant versions of your bags into our boats. During this discussing about boats sinking folks have generally disregarded this technique, which has been around for a long time. But, it's a great one. A lot of bigger boats come with watertight bulkheads that should keep the boat afloat if the doors are closed. But, I think one of the earlier posts pointed out that the Titanic had these and it sunk, which is true. For this reason, a lot of "serious" deep water boats have closed water tight tanks that are kept filled with air, which was not the case on Titanic.

    In one of the boats I raced, we used the rubber bags that divers use to float sunken boats as a way of "filling" the ends of a boat we didn't trust. It cost us a few hundred dollars, and we never broke the boat to test the bags, but we slept better knowing that both ends were filled with giant air bags.

    Beau
     
  13. ThomD
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    ThomD Senior Member

    "It should be easy to figure out. The manufacturer of a supposedly non-sinkable boat should be able to trivially tell you how much "reserve buoyancy" the boat has, and that will tell you how much heavy stuff you can bring (meaning stuff that is heavier than water) before you will sink the unsinkable boat once it floods."

    That will stop you from sinking, but survival is another mater. There are several examples of people surviving in tris for weeks even months, in flooded capsized boats. The boats float with so much bouyancy that the occupants can find areas to sleep in above the internal waterline. Conversly, a dismasted mono, with zero water on board can tumble the occupants so violently that they sustain broken limbs etc... Often the worst injuries occur after secondary capsizes, multiple further capsizes, something that virtualy does not happen with multis.

    A flooded positive bouyancy mono would be a pretty bad environment. It would have very low stability. The best option for extreme survival is the Jester/lifeboat type with no/little entry point for water in a capsize. But it could still be violently unstable, dismasted, in heavy weather.
     
  14. BeauVrolyk
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    BeauVrolyk Sailor

    I just re-read the CE spec - I did not see any reference to a "maximum design load" anywhere in the spec when talking about the boat being flooded. I've built things to these specs and I don't think I've ever seen it. If you could point me at the section or quote it I'd appreciate it. Please note that every Monohull sold in Europe is CE approved, and clearly they sink if holed.

    If you're talking about the boat being able to float with the load it was designed to carry when it is NOT flooded, then I agree, there are words to that effect that say the boat is to have adequate freeboard and buoyancy to safely carry the designed load. But, that wasn't the point now was it. We were talking about flooded.

    The escape hatch is not positioned to let the air out, indeed it is positioned in most designs, including the Volvo 70s (monohulls) which have the same problem floating upside down as multihulls, to keep the air in. The POINT was that the CE spec does NOT say that they should be unsinkable. That was the issue we were discussing. Further, the reason that the CE spec specifically mentions multihulls is because they will be upside down for a long time, and as a result water will come in around hatches etc... In contrast a monohull may be upside down, and leak in that position, but it is assumed it will roll over right side up and therefore doesn't need a specific

    With respect to numbers of boats sinking. Isn't the correct issue what percentage sink, flip, disappear, as opposed to the occurrence of a few within a period of time? Given that the vast majority of cruising boats, and racing boats for that matter, are monohulls; wouldn't you expect a lot more accidents to happen to them?

    In contrast during the Pac Cup there has been one boat lost that I know of, a multihull, in all the years of racing no monohull has ever been lost that I know of. This just represents the problem with using one's perceptions of what's going on as opposed to some well researched data.
     

  15. Freenacin
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    Freenacin Junior Member

    The escape hatches are positioned to let people in and out. Generally they are quite close to the design waterline. The point I was making is that the boat will remain afloat with the escape hatches open. And again, if it floats upside-down fully loaded, you'd expect it would also float right-side-up.

    And the point wasn't that some boats had sunk. It was that they had sunk in very benign conditions.

    To capsize a racing multihull at least takes a strong gust of wind and an innattentive crew. To capsize a cruising multi needs a mix of very strong wind and very poor sailing.
     
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