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#1
posted to rec.boats.cruising
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Conventional wisdom
As I discuss in another post, the conventional wisdom is that external
keels are better than internal keels. Statistics may show otherwise but it is hard to tell. With the exception of that Woods Hole boat, all the keel failures I could find involve external keels. Basically, if your external keel fails, you die. This just seems like bad engineering to me. You hang thousands of pounds waaaaaaay down from a few bolts in a way that it is subject to tremendous repeated changes in torque in a seriously corrosive environment and you then bet your life on it? Uhhhhhhhhh, does this make sense? Is anybody surprised at the deaths that have occurred from these things? Now, an encapsulated keel is supported over its entire surface via glass cloth that is stronger than steel and does not corrode. Even in the one case in Woods Hole where one was ruptured, the damage was such that nobody died and it did not sink (or not fast enough to matter). In my 28' internal keeled S2, I once hit a truck with my keel going 6 kts, yes, a truck. Following the so-called Storm of the Century, I was motor sailing into Steinhatchee, FL when WHAM, we stopped dead with the rigging shaking like crazy and we all fell over. I recovered mmy balance and went below to check for damage and found none. A little while later a coast guard boat came out and told us to watch out for a truck that had washed out to sea in the storm, we told him we had found it for him. I dove under her the next day and found the impact area but there was no real damage. Conventional wisdom is simply WRONG in this case. |
#2
posted to rec.boats.cruising
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Conventional wisdom
wrote in message ... As I discuss in another post, the conventional wisdom is that external keels are better than internal keels. Statistics may show otherwise but it is hard to tell. With the exception of that Woods Hole boat, all the keel failures I could find involve external keels. Basically, if your external keel fails, you die. This just seems like bad engineering to me. You hang thousands of pounds waaaaaaay down from a few bolts in a way that it is subject to tremendous repeated changes in torque in a seriously corrosive environment and you then bet your life on it? Uhhhhhhhhh, does this make sense? Is anybody surprised at the deaths that have occurred from these things? Now, an encapsulated keel is supported over its entire surface via glass cloth that is stronger than steel and does not corrode. Even in the one case in Woods Hole where one was ruptured, the damage was such that nobody died and it did not sink (or not fast enough to matter). In my 28' internal keeled S2, I once hit a truck with my keel going 6 kts, yes, a truck. Following the so-called Storm of the Century, I was motor sailing into Steinhatchee, FL when WHAM, we stopped dead with the rigging shaking like crazy and we all fell over. I recovered mmy balance and went below to check for damage and found none. A little while later a coast guard boat came out and told us to watch out for a truck that had washed out to sea in the storm, we told him we had found it for him. I dove under her the next day and found the impact area but there was no real damage. Conventional wisdom is simply WRONG in this case. You assume all external keels are built the same. |
#3
posted to rec.boats.cruising
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Conventional wisdom
I guess its time to repost this page:
http://www.cliffisland.com/boat.html BTW, the boat is an early J-35 (fully cored), and apparently it had grounded previously. |
#4
posted to rec.boats.cruising
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Conventional wisdom
On Jun 10, 3:31 am, jeff wrote:
I guess its time to repost this page: http://www.cliffisland.com/boat.html BTW, the boat is an early J-35 (fully cored), and apparently it had grounded previously. Yuck. It looks like the hull failed, not the ballast keel/sump joint. I suspect the "outsideness" of the ballast was not a contributing factor. -- Tom. |
#5
posted to rec.boats.cruising
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Conventional wisdom
The probably chief cause of keel bolt failure is: they are not
removable for inspection. Stainless steels are 'very' subject to 'crevice corrosion' and fatigue failure, mostly a combination of the two modes of failure. Worse, the maximum stres on a keel boat is 'cantilever stress' ... very hard to predict since the loads and actions are 'variable to unforseen'. Aiarcraft have the almost same problem with cantilever stress ... but those wing root bolts are REMOVEABLE and thus are able to be periodically inspected. So, keel bolt failure is a functional design failure due to the bolts not being able to be removed and periodically inspected. Encapsulated (iron) keels are not a panacea, as if the cavity is penetrated and water enters, the rust (ferric) that forms is less dense than the original metal (iron) and the encapsulated keel will 'push' itself apart ... then the balllast will simply fall out if the rust formation is that great. So, from the above it seems that a encapsulated keel with solid lead internal ballast would probably be the 'best'. |
#6
posted to rec.boats.cruising
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Conventional wisdom
"RichH" wrote in message ... The probably chief cause of keel bolt failure is: they are not removable for inspection. Stainless steels are 'very' subject to 'crevice corrosion' and fatigue failure, mostly a combination of the two modes of failure. Worse, the maximum stres on a keel boat is 'cantilever stress' ... very hard to predict since the loads and actions are 'variable to unforseen'. Aiarcraft have the almost same problem with cantilever stress ... but those wing root bolts are REMOVEABLE and thus are able to be periodically inspected. So, keel bolt failure is a functional design failure due to the bolts not being able to be removed and periodically inspected. Encapsulated (iron) keels are not a panacea, as if the cavity is penetrated and water enters, the rust (ferric) that forms is less dense than the original metal (iron) and the encapsulated keel will 'push' itself apart ... then the balllast will simply fall out if the rust formation is that great. So, from the above it seems that a encapsulated keel with solid lead internal ballast would probably be the 'best'. I think the best keel is a drop keel and water ballast just like that which the Macgregor 26 series of fine and respected sailboats uses. You have NEVER heard of one single solitary Mac26 with a keel falling off of it, have you? -- Gregory Hall |
#7
posted to rec.boats.cruising
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Conventional wisdom
On Jun 10, 7:50 am, RichH wrote:
The probably chief cause of keel bolt failure is: they are not removable for inspection. As I said before, just looking at the pictures in the link I think the failure was of the hull bottom (or structural keel of the vessel) and not the keel bolts. Hull failure is not uncommon in hard groundings on lightly built, fin keeled boats but I think that is a separate topic. Stainless steels are 'very' subject to 'crevice corrosion' and fatigue failure, mostly a combination of the two modes of failure. Worse, the maximum stres on a keel boat is 'cantilever stress' ... very hard to predict since the loads and actions are 'variable to unforseen'. The "Received Wisdom" is that stainless is not the best choice for keel bolts because it is subject to crevice corrosion and wasting in an wet, low oxygen environment. Fatigue failure can be engineered around. I haven't looked into it but my guess is that the time to ductility exhaustion of the typical set of ss keel bolts would be on the order of forever and a day. Cantilevers and sheer stress are pretty well understood for bolts and beams. And, in the case of keels where weight isn't generally a concern designers can be, and generally are, very conservative. ... So, keel bolt failure is a functional design failure due to the bolts not being able to be removed and periodically inspected. Kinda Zen statistics, but my feeling is that most keel bolt failures, as opposed to keel failures, are on fairly new racing boats and probably result from over aggressive designs or constructions errors. In any case, keel bolt failure is very rare. Yes, it would be good if the bolts were easy to inspect and yes, stainless isn't the best choice for them, but in practice, the fleet is holding up very well. ... So, from the above it seems that a encapsulated keel with solid lead internal ballast would probably be the 'best'. Well, you haven't sold me yet. Keel bolts can be made of things other than stainless. Even stainless bolts seem to be holding up well. If the bottom of the boat fails the point is moot anyway. -- Tom. |
#8
posted to rec.boats.cruising
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Conventional wisdom
On Jun 10, 3:01 pm, " wrote:
On Jun 10, 7:50 am, RichH wrote: The probably chief cause of keel bolt failure is: they are not removable for inspection. As I said before, just looking at the pictures in the link I think the failure was of the hull bottom (or structural keel of the vessel) and not the keel bolts. Hull failure is not uncommon in hard groundings on lightly built, fin keeled boats but I think that is a separate topic. Stainless steels are 'very' subject to 'crevice corrosion' and fatigue failure, mostly a combination of the two modes of failure. Worse, the maximum stres on a keel boat is 'cantilever stress' ... very hard to predict since the loads and actions are 'variable to unforseen'. The "Received Wisdom" is that stainless is not the best choice for keel bolts because it is subject to crevice corrosion and wasting in an wet, low oxygen environment. Fatigue failure can be engineered around. I haven't looked into it but my guess is that the time to ductility exhaustion of the typical set of ss keel bolts would be on the order of forever and a day. Cantilevers and sheer stress are pretty well understood for bolts and beams. And, in the case of keels where weight isn't generally a concern designers can be, and generally are, very conservative. ... So, keel bolt failure is a functional design failure due to the bolts not being able to be removed and periodically inspected. Kinda Zen statistics, but my feeling is that most keel bolt failures, as opposed to keel failures, are on fairly new racing boats and probably result from over aggressive designs or constructions errors. In any case, keel bolt failure is very rare. Yes, it would be good if the bolts were easy to inspect and yes, stainless isn't the best choice for them, but in practice, the fleet is holding up very well. ... So, from the above it seems that a encapsulated keel with solid lead internal ballast would probably be the 'best'. Well, you haven't sold me yet. Keel bolts can be made of things other than stainless. Even stainless bolts seem to be holding up well. If the bottom of the boat fails the point is moot anyway. -- Tom. In spite of the beliefs of most people here, the Mac 26 really is a much safer boat than most heavily built cruising boats. If one integrates safety over the life of the boat, I think you would find the Mac 26 to be far safer than a heavily built boat with a deep keel. The deep keeled boat may be safer in a certain unusual situation (being out in a hurricane in deep water) but the Mac 26 can more easily avoid such weather by going into a shallow entrance that the deep keeled boat cannot. |
#9
posted to rec.boats.cruising
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Conventional wisdom
wrote in message ... In spite of the beliefs of most people here, the Mac 26 really is a much safer boat than most heavily built cruising boats. If one integrates safety over the life of the boat, I think you would find the Mac 26 to be far safer than a heavily built boat with a deep keel. The deep keeled boat may be safer in a certain unusual situation (being out in a hurricane in deep water) but the Mac 26 can more easily avoid such weather by going into a shallow entrance that the deep keeled boat cannot. Truer words have rarely been spoken. The Mac 26 is a very safe boat as evidenced by its unparallel safety record. And, the Mac 26 can get to that shallow entrance a lot faster than any sailboat other than perhaps a racing multihull. -- Gregory Hall |
#10
posted to rec.boats.cruising
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Conventional wisdom
"I haven't looked into it but my guess is that the time to
ductility exhaustion of the typical set of ss keel bolts would be on the order of forever and a day. Cantilevers and sheer stress are pretty well understood for bolts and beams. And, in the case of keels where weight isn't generally a concern designers can be, and generally are, very conservative". Ductility exhaustion? What are you doing drawing wire? Sorry but once 300 series stainless gets loaded above its endurance limit it typicallly only lasts approx 1 million load cycles - doesnt matter if its rigging, keelbolts, chainplates. If the endurance load factor (at about 30kpsi) is exceeded, 1 million cycles is about all you get You bet that cantilever stress is well understood thats why bridges, aircraft wings, etc. dont fall off. That sailboats constantly have to have rigging replaced, on some - keels & rudder shafts, etc. keep falling off ... would tell any prudent engineer/designer that 'something is wrong' in the 'typical design'. |
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