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#21
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Foam-filled mast
On Thu, 27 Oct 2005 16:22:26 -0700, Robert or Karen Swarts wrote:
Let us suppose the mast is 4" in diameter and 25 ft long, stepped 4 ft above waterline. The weight of water it displaces is 138 lb. Let us suppose half if that is the weight of the mast. That leaves 69 lb positive buoyancy. The center of buoyancy of the mast is about 16 ft above waterline. That will create a righting moment of about 1100 ft lb when the boat is flat on the water. That is not inconsequential, and could be a large help in preventing a total capsize. BS [snip] This all seems very doubtful. I am not sure if you understand how dynamic the sea-state is in conditions likely to cause a knockdown in the first place. Maybe you should ask a real naval architect (marine engineer) about this. I think it is a very bad idea which you will regret if you do it, but I am not an expert. --Mac |
#22
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Foam-filled mast
This is not an original idea with me. I have seen it recommended in a couple
of articles/books on deep water sailing/boats. There is no disputing the math, although other factors do come into it. BS "Mac" wrote in message news On Thu, 27 Oct 2005 16:22:26 -0700, Robert or Karen Swarts wrote: Let us suppose the mast is 4" in diameter and 25 ft long, stepped 4 ft above waterline. The weight of water it displaces is 138 lb. Let us suppose half if that is the weight of the mast. That leaves 69 lb positive buoyancy. The center of buoyancy of the mast is about 16 ft above waterline. That will create a righting moment of about 1100 ft lb when the boat is flat on the water. That is not inconsequential, and could be a large help in preventing a total capsize. BS [snip] This all seems very doubtful. I am not sure if you understand how dynamic the sea-state is in conditions likely to cause a knockdown in the first place. Maybe you should ask a real naval architect (marine engineer) about this. I think it is a very bad idea which you will regret if you do it, but I am not an expert. --Mac |
#23
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Foam-filled mast
Mac wrote:
On Thu, 27 Oct 2005 16:22:26 -0700, Robert or Karen Swarts wrote: Let us suppose the mast is 4" in diameter and 25 ft long, stepped 4 ft above waterline. The weight of water it displaces is 138 lb. Let us suppose half if that is the weight of the mast. That leaves 69 lb positive buoyancy. The center of buoyancy of the mast is about 16 ft above waterline. That will create a righting moment of about 1100 ft lb when the boat is flat on the water. That is not inconsequential, and could be a large help in preventing a total capsize. BS [snip] This all seems very doubtful. I am not sure if you understand how dynamic the sea-state is in conditions likely to cause a knockdown in the first place. Maybe you should ask a real naval architect (marine engineer) about this. I think it is a very bad idea which you will regret if you do it, but I am not an expert. --Mac It seems, if you are worried, that if the mast is reasonably tight, it would not flood immediately after being whipped into the water. It's speed of descent would provide a great deal of drag through water to decellerate the rolling momentum, and would provide some righting bouyancy until it filled. Sealing a mast may be impossible, but slowing its flooding, possibly with external halyards and a wad of foam or some pop bottles and even pumping it once horizontal with a small hose to the mast head, might be a better plan, strange as it sounds. Once on it's side, a keel boat will want to right itself, until it fills with water. Keep the properly designed companionway bottom board in place. On a trampoline / catamaran, you will want righting accessories like possibly a self inflating "Mae West" at the masthead. 200 lbs bouyancy might to, and that is about 1.5 cubic feet of balloon, 10 pounds of tennis balls? Many catamarans have servicable rope loops rigged, to enable overboard capsized crew to pull the boat upright once it has landed on it's side. They take a moment or three to turtle after resting on their sides, with the mast flooding and because of their geometry, wanting to continue rolling inverted. The floating mast is their only hope, however faint. The dynamics of weather tending to capsize the boat will be greatly diminished, once you come to a stop and the sails are horizontal on the water. The sail itself would prevent rapid sinking of the mast, if it did not simply tear, sheeted in. The mast may well break open. Dare I suggest floating line shrouds as an adjunct, or even a floating, sealed at the ends hollow wooden mast? Once turtled, a long enough halyard or spare detached from the sail head and "lowered" from it's submerged cleat now only a foot or so submerged, brought to the surface athwartships, can haul the boat back upright, if enough bouyancy and counterweight can be found amongst swimming crew with lifejackets, coolers, floating cushions, gas tanks, rescue assist vessels, and whatever. Flooding one completely upside down cat hull to make it sink so the boat is suspended by one floating hull also helps. Once it is a little more downwind than the floating hull, raising it is easier, since all you need do is counterbalance the submerged, reduced weight and especially, the mast, once horizontal at the waters surface, and which, we hope, is still trying to float, at least a little. Pumping the submerged hull will right the boat. Crossed hull pumping lines and a pump tied on to avoid it's loss, able to inflate the hull with air or extract water complete the scheme. Such serious righting arrangements are able and can be seen on serious cats if you ask their skippers. Flooding the hull may seem difficult, but an "air syphon" hose will do it easily and dependably, if a hatch can be opened aside from this air vent / pumping hose. One would hope the design permits a loaded cruising cat to still float, even with one hull full of water, purposely flooded. Small cats are a natural, larger ones will want bouyancy compartments sufficient to minimally float the boat even when capsized and flooding if the companionway hatches are not watertight. Just one more reason to keep those through hulls closed, eh? Terry K |
#24
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Foam-filled mast
On Fri, 28 Oct 2005 07:46:14 -0700, Robert or Karen Swarts wrote:
This is not an original idea with me. I have seen it recommended in a couple of articles/books on deep water sailing/boats. There is no disputing the math, although other factors do come into it. BS Well, there have been many bad ideas published over the years, I'm sure. But anyway, a mast filled with air and submerged will have more buoyancy than one filled with water. And that buoyancy will, all things considered, have a tendency to right the boat. I don't dispute that at all. I am just asking the question "What effect is there on the chances of righting your boat when your 'sealed' mast fills up with water which then can't easily escape?" Because I fear that the mast may stay submerged long enough to fill up. Anyway, I don't think I have anything new to say on the subject. Maybe you could put flapper valves on the mast so water can't come in, but can get out at the mast head, and air can't get out but can get in at the mast's base? ;-) --Mac "Mac" wrote in message news On Thu, 27 Oct 2005 16:22:26 -0700, Robert or Karen Swarts wrote: Let us suppose the mast is 4" in diameter and 25 ft long, stepped 4 ft above waterline. The weight of water it displaces is 138 lb. Let us suppose half if that is the weight of the mast. That leaves 69 lb positive buoyancy. The center of buoyancy of the mast is about 16 ft above waterline. That will create a righting moment of about 1100 ft lb when the boat is flat on the water. That is not inconsequential, and could be a large help in preventing a total capsize. BS [snip] This all seems very doubtful. I am not sure if you understand how dynamic the sea-state is in conditions likely to cause a knockdown in the first place. Maybe you should ask a real naval architect (marine engineer) about this. I think it is a very bad idea which you will regret if you do it, but I am not an expert. --Mac |
#25
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Foam-filled mast
On Fri, 28 Oct 2005 16:59:19 -0300, Terry Spragg wrote:
Mac wrote: On Thu, 27 Oct 2005 16:22:26 -0700, Robert or Karen Swarts wrote: Let us suppose the mast is 4" in diameter and 25 ft long, stepped 4 ft above waterline. The weight of water it displaces is 138 lb. Let us suppose half if that is the weight of the mast. That leaves 69 lb positive buoyancy. The center of buoyancy of the mast is about 16 ft above waterline. That will create a righting moment of about 1100 ft lb when the boat is flat on the water. That is not inconsequential, and could be a large help in preventing a total capsize. BS [snip] This all seems very doubtful. I am not sure if you understand how dynamic the sea-state is in conditions likely to cause a knockdown in the first place. Maybe you should ask a real naval architect (marine engineer) about this. I think it is a very bad idea which you will regret if you do it, but I am not an expert. --Mac It seems, if you are worried, that if the mast is reasonably tight, it would not flood immediately after being whipped into the water. It's speed of descent would provide a great deal of drag through water to decellerate the rolling momentum, and would provide some righting bouyancy until it filled. Sealing a mast may be impossible, but slowing its flooding, possibly with external halyards and a wad of foam or some pop bottles and even pumping it once horizontal with a small hose to the mast head, might be a better plan, strange as it sounds. Somehow I don't think anybody is going to be pumping a mast full of air in the conditions that cause a sea-worthy boat to get knocked-down. Once on it's side, a keel boat will want to right itself, until it fills with water. Keep the properly designed companionway bottom board in place. Are you under the impression that the bottom board will stay in place when this happens, or that water will only come up to the bottom board? In these conditions, the companionway (and all other hatches) need(s) to be completely closed. On a trampoline / catamaran, you will want righting accessories like possibly a self inflating "Mae West" at the masthead. 200 lbs bouyancy might to, and that is about 1.5 cubic feet of balloon, 10 pounds of tennis balls? Many catamarans have servicable rope loops rigged, to enable overboard capsized crew to pull the boat upright once it has landed on it's side. They take a moment or three to turtle after resting on their sides, with the mast flooding and because of their geometry, wanting to continue rolling inverted. The floating mast is their only hope, however faint. The dynamics of weather tending to capsize the boat will be greatly diminished, once you come to a stop and the sails are horizontal on the water. The sail itself would prevent rapid sinking of the mast, if it did not simply tear, sheeted in. The mast may well break open. By and large, it is the sea which capsizes boats, not the wind. Dare I suggest floating line shrouds as an adjunct, or even a floating, sealed at the ends hollow wooden mast? Once turtled, a long enough halyard or spare detached from the sail head and "lowered" from it's submerged cleat now only a foot or so submerged, brought to the surface athwartships, can haul the boat back upright, if enough bouyancy and counterweight can be found amongst swimming crew with lifejackets, coolers, floating cushions, gas tanks, rescue assist vessels, and whatever. Flooding one completely upside down cat hull to make it sink so the boat is suspended by one floating hull also helps. Once it is a little more downwind than the floating hull, raising it is easier, since all you need do is counterbalance the submerged, reduced weight and especially, the mast, once horizontal at the waters surface, and which, we hope, is still trying to float, at least a little. Pumping the submerged hull will right the boat. Crossed hull pumping lines and a pump tied on to avoid it's loss, able to inflate the hull with air or extract water complete the scheme. Such serious righting arrangements are able and can be seen on serious cats if you ask their skippers. I have heard of this business. I've never heard of anybody using it at sea after a capsize. Have you? Please be specific because I would love to read about it. Also, I have to point out that it would probably be impossible to do this until days after the storm subsides. Flooding the hull may seem difficult, but an "air syphon" hose will do it easily and dependably, if a hatch can be opened aside from this air vent / pumping hose. Flooding a hull seems easy, to me. Floating it afterwards would prove difficult. One would hope the design permits a loaded cruising cat to still float, even with one hull full of water, purposely flooded. Small cats are a natural, larger ones will want bouyancy compartments sufficient to minimally float the boat even when capsized and flooding if the companionway hatches are not watertight. Just one more reason to keep those through hulls closed, eh? Terry K I don't know, Terry. I am not opposed to these types of measures, but it is important to understand what the capabilities of a crew are really likely to be in the kinds of conditions we are talking about. The best bet is to make sure that the boat doesn't sink during the storm, and try to jury rig something to get you to the nearest port after the storm is over. After you feel comfortable that the boat can be kept afloat indefinitely (you hope), when being knocked-down repeatedly or even capsized, then you can think about other measures such as buoyant masts and air-pumps for floating hulls and so-on. Just my $0.02. --Mac |
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