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keel stepped/deck stepped masts
On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote:
my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. ============================================ I think this is one of those cases where theory and the real world break down, probably because of faulty assumptions supplied to the theory. In the real world of squalls, knock downs, luffing sails and accidental jibes there are many asymmetric side loads generated which are trying to force the mast out of column. That's when the extra support provided by the deck becomes the most useful. |
keel stepped/deck stepped masts
QLW wrote in message ... Steve, As I suspected, my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. If the mast were stepped on a poorly supported deck then all of the thinking changes...but that's a deck problem not a mast problem. Good reasons for either stepping the mast on the keel or on the deck can be argued, but compressive strength is not one of them. I think you're talking slightly at cross-purposes here. Ignoring bendy masts, keel stepping (and its corollary, deck support) doesn't add to strength in compression (as such), but it increases the bend stability of a mast under compression. Bend disturbances will occur due to inertia effects in a seaway, and the various sail tensions in different sailing conditions. This is not a design flaw, it's a design case. To keep the mast stable under compression, these bending moments must be resisted, either by using a large enough cross section, or by constraining movement with stays and deck support. With appropriate support, smaller cross sections can be used. Most vessels designed to withstand extreme conditions (ignoring racing) prefer straight masts. Keel stepping either adds to rig strength, or can be used to reduce weight aloft. An engineer will correctly say it makes no difference to the (pure) compression strength of a cross section. But as part of a rigging system, all other things being equal, it does add strength. JimB |
keel stepped/deck stepped masts
QLW wrote in message ... Steve, As I suspected, my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. If the mast were stepped on a poorly supported deck then all of the thinking changes...but that's a deck problem not a mast problem. Good reasons for either stepping the mast on the keel or on the deck can be argued, but compressive strength is not one of them. I think you're talking slightly at cross-purposes here. Ignoring bendy masts, keel stepping (and its corollary, deck support) doesn't add to strength in compression (as such), but it increases the bend stability of a mast under compression. Bend disturbances will occur due to inertia effects in a seaway, and the various sail tensions in different sailing conditions. This is not a design flaw, it's a design case. To keep the mast stable under compression, these bending moments must be resisted, either by using a large enough cross section, or by constraining movement with stays and deck support. With appropriate support, smaller cross sections can be used. Most vessels designed to withstand extreme conditions (ignoring racing) prefer straight masts. Keel stepping either adds to rig strength, or can be used to reduce weight aloft. An engineer will correctly say it makes no difference to the (pure) compression strength of a cross section. But as part of a rigging system, all other things being equal, it does add strength. JimB |
keel stepped/deck stepped masts
On Wed, 14 Apr 2004 18:37:56 -0400, Wayne.B
wrote: On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote: my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. ============================================ I think this is one of those cases where theory and the real world break down, probably because of faulty assumptions supplied to the theory. In the real world of squalls, knock downs, luffing sails and accidental jibes there are many asymmetric side loads generated which are trying to force the mast out of column. That's when the extra support provided by the deck becomes the most useful. I'm not so sure about that. If, when there are sideloads in squalls, knockdowns, etc., the deck stepped mast failed by remining in one piece but slipping out of it's step, then I'd agree that a keel stepped mast would solve that problem ... if it didn't snap at the deck. But most of the mast failures I've see are when it snaps somewhere aloft, like at the spreaders. How it's stepped doesn't make a difference when it breaks up there. I haven't seen many keel stepped masts break at the deck either. If it did, that would indicate there was enough sideload at that point to maybe knock it out of the step if it were deck stepped. Steve |
keel stepped/deck stepped masts
On Wed, 14 Apr 2004 18:37:56 -0400, Wayne.B
wrote: On Wed, 14 Apr 2004 03:44:54 -0500, "QLW" wrote: my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. ============================================ I think this is one of those cases where theory and the real world break down, probably because of faulty assumptions supplied to the theory. In the real world of squalls, knock downs, luffing sails and accidental jibes there are many asymmetric side loads generated which are trying to force the mast out of column. That's when the extra support provided by the deck becomes the most useful. I'm not so sure about that. If, when there are sideloads in squalls, knockdowns, etc., the deck stepped mast failed by remining in one piece but slipping out of it's step, then I'd agree that a keel stepped mast would solve that problem ... if it didn't snap at the deck. But most of the mast failures I've see are when it snaps somewhere aloft, like at the spreaders. How it's stepped doesn't make a difference when it breaks up there. I haven't seen many keel stepped masts break at the deck either. If it did, that would indicate there was enough sideload at that point to maybe knock it out of the step if it were deck stepped. Steve |
keel stepped/deck stepped masts
Nahhhh. an engineer will say that the SHORTEST section (ie. deck
stepped) will have the best resistance to buckling deflection and harmonic vibrational excitement (pumping). Especially, If the deck stepped mast is stress connected to the compression post via a bolted flange it probably gives the best 'stability' of all cases in comparison to a 'pin-set' deck step or keel stepped. Side loads are a 'problem' vs. buckling failure and the deck stepped is better able to support the side loads 'at the deck' (if the deck cross members are properly engineered); plus - the overal length is shorter which decreases the vulnerability of buckling due to the shorter overall unsuported length. A keel stepped is always vulnerable to movement as it enters the deck, no matter how tight you 'think' the 'wedges' are in place (elasticity of the structure). Always straight? ... not if the rig is properly tuned! A single spreader rig (of 'normal' cross section typically needs a 1" forward 'prebend', double spreader 2" of prebend to dampen oscilations / reduce induced harmonics (pumping)... all of which changes the natural frequency of the mast, etc. to a much higher frequency, hence better *dynamic* compressional load handling ability. A dead straight stick (unless it has a 'bodaceous' cross section and HUGE moment of inertia) will more easily be vibrationally excited by harmonics induced from the rigging/sail interaction. Prebend is a 'norm' for a finely tuned rig. In article q7tfc.17450$4N3.2083@newsfe1-win, JimB wrote: QLW wrote in message ... Steve, As I suspected, my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. If the mast were stepped on a poorly supported deck then all of the thinking changes...but that's a deck problem not a mast problem. Good reasons for either stepping the mast on the keel or on the deck can be argued, but compressive strength is not one of them. I think you're talking slightly at cross-purposes here. Ignoring bendy masts, keel stepping (and its corollary, deck support) doesn't add to strength in compression (as such), but it increases the bend stability of a mast under compression. Bend disturbances will occur due to inertia effects in a seaway, and the various sail tensions in different sailing conditions. This is not a design flaw, it's a design case. To keep the mast stable under compression, these bending moments must be resisted, either by using a large enough cross section, or by constraining movement with stays and deck support. With appropriate support, smaller cross sections can be used. Most vessels designed to withstand extreme conditions (ignoring racing) prefer straight masts. Keel stepping either adds to rig strength, or can be used to reduce weight aloft. An engineer will correctly say it makes no difference to the (pure) compression strength of a cross section. But as part of a rigging system, all other things being equal, it does add strength. JimB |
keel stepped/deck stepped masts
Nahhhh. an engineer will say that the SHORTEST section (ie. deck
stepped) will have the best resistance to buckling deflection and harmonic vibrational excitement (pumping). Especially, If the deck stepped mast is stress connected to the compression post via a bolted flange it probably gives the best 'stability' of all cases in comparison to a 'pin-set' deck step or keel stepped. Side loads are a 'problem' vs. buckling failure and the deck stepped is better able to support the side loads 'at the deck' (if the deck cross members are properly engineered); plus - the overal length is shorter which decreases the vulnerability of buckling due to the shorter overall unsuported length. A keel stepped is always vulnerable to movement as it enters the deck, no matter how tight you 'think' the 'wedges' are in place (elasticity of the structure). Always straight? ... not if the rig is properly tuned! A single spreader rig (of 'normal' cross section typically needs a 1" forward 'prebend', double spreader 2" of prebend to dampen oscilations / reduce induced harmonics (pumping)... all of which changes the natural frequency of the mast, etc. to a much higher frequency, hence better *dynamic* compressional load handling ability. A dead straight stick (unless it has a 'bodaceous' cross section and HUGE moment of inertia) will more easily be vibrationally excited by harmonics induced from the rigging/sail interaction. Prebend is a 'norm' for a finely tuned rig. In article q7tfc.17450$4N3.2083@newsfe1-win, JimB wrote: QLW wrote in message ... Steve, As I suspected, my Engineer Friend went on in great detail to explain why stepping the mast on the deck or on the keel has no effect on the strength of the mast in compression. While some small benefit could conceivably be gained by helping to keep the mast in column, he claimed that would only occur in the case of a flawed design. If the mast were stepped on a poorly supported deck then all of the thinking changes...but that's a deck problem not a mast problem. Good reasons for either stepping the mast on the keel or on the deck can be argued, but compressive strength is not one of them. I think you're talking slightly at cross-purposes here. Ignoring bendy masts, keel stepping (and its corollary, deck support) doesn't add to strength in compression (as such), but it increases the bend stability of a mast under compression. Bend disturbances will occur due to inertia effects in a seaway, and the various sail tensions in different sailing conditions. This is not a design flaw, it's a design case. To keep the mast stable under compression, these bending moments must be resisted, either by using a large enough cross section, or by constraining movement with stays and deck support. With appropriate support, smaller cross sections can be used. Most vessels designed to withstand extreme conditions (ignoring racing) prefer straight masts. Keel stepping either adds to rig strength, or can be used to reduce weight aloft. An engineer will correctly say it makes no difference to the (pure) compression strength of a cross section. But as part of a rigging system, all other things being equal, it does add strength. JimB |
keel stepped/deck stepped masts
I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece but slipping out of it's step, then I'd agree that a keel stepped mast would solve that problem ... if it didn't snap at the deck. But most of the mast failures I've see are when it snaps somewhere aloft, like at the spreaders. How it's stepped doesn't make a difference when it breaks up there. Mast failure (usually at midsection) is usually due to some rigging failure that permits the mast to move 'out of column' and permits catastrophic buckling failure when the compressional loads get off center. Doesnt matter if its deck stepped of keel stepped, if the rigging support fails and the mast deflects catastrophically .... the latent compression load finishes the job. |
keel stepped/deck stepped masts
I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece but slipping out of it's step, then I'd agree that a keel stepped mast would solve that problem ... if it didn't snap at the deck. But most of the mast failures I've see are when it snaps somewhere aloft, like at the spreaders. How it's stepped doesn't make a difference when it breaks up there. Mast failure (usually at midsection) is usually due to some rigging failure that permits the mast to move 'out of column' and permits catastrophic buckling failure when the compressional loads get off center. Doesnt matter if its deck stepped of keel stepped, if the rigging support fails and the mast deflects catastrophically .... the latent compression load finishes the job. |
keel stepped/deck stepped masts
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