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keel stepped/deck stepped masts
I have heard ppl say they would only have a keel stepped mast on an
offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. |
keel stepped/deck stepped masts
It's a mechanical engineering issue. A mast (called a column by mechanical
engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. For offshore work, the ultimate compression strength of the mast is important for situations like full knockdowns and capsizings. The additional compression strength also comes into play if you lose a stay or a shroud, and might just give you the additional reserve strength that would keep the mast from coming down. Tom Dacon "Parallax" wrote in message om... I have heard ppl say they would only have a keel stepped mast on an offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. |
keel stepped/deck stepped masts
It's a mechanical engineering issue. A mast (called a column by mechanical
engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. For offshore work, the ultimate compression strength of the mast is important for situations like full knockdowns and capsizings. The additional compression strength also comes into play if you lose a stay or a shroud, and might just give you the additional reserve strength that would keep the mast from coming down. Tom Dacon "Parallax" wrote in message om... I have heard ppl say they would only have a keel stepped mast on an offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. |
keel stepped/deck stepped masts
On Thu, 8 Apr 2004 10:19:19 -0700, "Tom Dacon"
wrote: It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. For offshore work, the ultimate compression strength of the mast is important for situations like full knockdowns and capsizings. The additional compression strength also comes into play if you lose a stay or a shroud, and might just give you the additional reserve strength that would keep the mast from coming down. To that lucid engineering perspective, I would add the following observations: 1) Dismastings MAY be better with a keel-stepped mast in the sense that a deck-stepped mast will tend to fail in its entirely, whereas even if you can salvage ten feet of keel stepped above the deck, you may be able to rig a jury rig and keep going. Also, if a deck stepped mast goes, you must IMMEDIATELY cut the shrouds still attached or the mast will poke a hole in your hull...and this under very likely less than ideal conditions. 2) Pluses of deck-stepped include no partners to leak water...no mast boot, no Spartite, and, usually, more room and less obstruction in the cabin. 3) Keel stepped masts are frequently heavier, but that weight can translate to the mechanical advantage and lower CG mentioned above. Also, deck flexing is avoided. It's a matter of taste and intended use, mostly. I prefer keel stepped on fiberglass boats, but see no objection to deck-stepped on steel boats, mainly due to materials used and likely function of the boat. I suppose the ideal compromise would be an aluminium deck stepped mast on an aluminum boat! G For the record, I have a keel stepped mast I am quite happy with. YMMV. R. |
keel stepped/deck stepped masts
On Thu, 8 Apr 2004 10:19:19 -0700, "Tom Dacon"
wrote: It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. For offshore work, the ultimate compression strength of the mast is important for situations like full knockdowns and capsizings. The additional compression strength also comes into play if you lose a stay or a shroud, and might just give you the additional reserve strength that would keep the mast from coming down. To that lucid engineering perspective, I would add the following observations: 1) Dismastings MAY be better with a keel-stepped mast in the sense that a deck-stepped mast will tend to fail in its entirely, whereas even if you can salvage ten feet of keel stepped above the deck, you may be able to rig a jury rig and keep going. Also, if a deck stepped mast goes, you must IMMEDIATELY cut the shrouds still attached or the mast will poke a hole in your hull...and this under very likely less than ideal conditions. 2) Pluses of deck-stepped include no partners to leak water...no mast boot, no Spartite, and, usually, more room and less obstruction in the cabin. 3) Keel stepped masts are frequently heavier, but that weight can translate to the mechanical advantage and lower CG mentioned above. Also, deck flexing is avoided. It's a matter of taste and intended use, mostly. I prefer keel stepped on fiberglass boats, but see no objection to deck-stepped on steel boats, mainly due to materials used and likely function of the boat. I suppose the ideal compromise would be an aluminium deck stepped mast on an aluminum boat! G For the record, I have a keel stepped mast I am quite happy with. YMMV. R. |
keel stepped/deck stepped masts
On 8 Apr 2004 09:52:55 -0700, (Parallax) wrote:
I have heard ppl say they would only have a keel stepped mast on an offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. I used to be of the school that favored keel-stepped masts as be more sound. Some things that have changed my mind. Pacific Seacraft builds very strong boats with deck-stepped masts, as do some other builders. I have heard stories, uncorroborated, of keel-stepped masts carrying away the coach roof during dismasting. There are probably well and poorly designed keel and deck stepped masts. A good solid step and a well engineered compression post are necessary on a deck-stepped mast. Jack __________________________________________________ Jack Dale Swiftsure Sailing Academy Director ISPA and CYA Instructor http://www.swiftsuresailing.com Phone: 1 (877) 470-SAIL (toll free) __________________________________________________ |
keel stepped/deck stepped masts
I think one of the 2002 issues of Wooden Boat Magazine had a design article
by Ted Brewer in which he gave some reasons for preferring keel stepped masts. Don't recall what they were though. "Parallax" wrote in message om... I have heard ppl say they would only have a keel stepped mast on an offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. |
keel stepped/deck stepped masts
I think one of the 2002 issues of Wooden Boat Magazine had a design article
by Ted Brewer in which he gave some reasons for preferring keel stepped masts. Don't recall what they were though. "Parallax" wrote in message om... I have heard ppl say they would only have a keel stepped mast on an offshore boat. Why? A well built deck stepped mast is as strong as a keel stepped one and easier to put up or down. If either loses a stay, it is coming down in a hurry? I fail to see the reason for this odd preference. |
keel stepped/deck stepped masts
On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote: I have heard stories, uncorroborated, of keel-stepped masts carrying away the coach roof during dismasting. =========================================== That can happen, and also the interior can be trashed by the mast butt whipping around below decks. For that reason, it's very important (and required by the ORC regs), that the mast butt be mechanically fastened to the mast step in a secure manner. I've had some personal experience with this issue since my old Cal-34 started out as deck stepped. As others have pointed out, almost any kind of rigging failure with a deck stepped mast results in a total dismasting with the entire rig in the water trying to hole the boat. Been there, done that, and it's ugly. When I went to re-rig the boat we did a lot of research. Ben Hall personally told me that keel stepped masts are about 25% stronger than a comparable deck stepped mast because of the extra support at the deck. Ben has engineered and built a lot of masts and should know. Personally, I would never go offshore again with a deck stepped mast. |
keel stepped/deck stepped masts
On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote: I have heard stories, uncorroborated, of keel-stepped masts carrying away the coach roof during dismasting. =========================================== That can happen, and also the interior can be trashed by the mast butt whipping around below decks. For that reason, it's very important (and required by the ORC regs), that the mast butt be mechanically fastened to the mast step in a secure manner. I've had some personal experience with this issue since my old Cal-34 started out as deck stepped. As others have pointed out, almost any kind of rigging failure with a deck stepped mast results in a total dismasting with the entire rig in the water trying to hole the boat. Been there, done that, and it's ugly. When I went to re-rig the boat we did a lot of research. Ben Hall personally told me that keel stepped masts are about 25% stronger than a comparable deck stepped mast because of the extra support at the deck. Ben has engineered and built a lot of masts and should know. Personally, I would never go offshore again with a deck stepped mast. |
keel stepped/deck stepped masts
"Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. He is currently building a 90' cat with an unstayed rotating mast but that is an intirely different problem. No compresson loads there, but at one time he was considering a stayed mast and must have done the thinking on it. |
keel stepped/deck stepped masts
"Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. He is currently building a 90' cat with an unstayed rotating mast but that is an intirely different problem. No compresson loads there, but at one time he was considering a stayed mast and must have done the thinking on it. |
keel stepped/deck stepped masts
On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote: I have heard stories, uncorroborated, of keel-stepped masts carrying away the coach roof during dismasting. Me, too. But if the choice is a big hole in the roof or a big hole at the waterline....G There are probably well and poorly designed keel and deck stepped masts. A good solid step and a well engineered compression post are necessary on a deck-stepped mast. Absolutely. This is key in anything, and from what I've seen of them, I wouldn't sneer at a Pacific Seacraft for a second. I simply indicate a slight personal preference that, if not met, would in no sense be a deal breaker. Of course, I always like stowing stuff in hammocks secured to the keel-stepped mast, too...G R. |
keel stepped/deck stepped masts
On Thu, 08 Apr 2004 19:14:39 GMT, Jack Dale
wrote: I have heard stories, uncorroborated, of keel-stepped masts carrying away the coach roof during dismasting. Me, too. But if the choice is a big hole in the roof or a big hole at the waterline....G There are probably well and poorly designed keel and deck stepped masts. A good solid step and a well engineered compression post are necessary on a deck-stepped mast. Absolutely. This is key in anything, and from what I've seen of them, I wouldn't sneer at a Pacific Seacraft for a second. I simply indicate a slight personal preference that, if not met, would in no sense be a deal breaker. Of course, I always like stowing stuff in hammocks secured to the keel-stepped mast, too...G R. |
keel stepped/deck stepped masts
In article , QLW says...
"Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
In article , QLW says...
"Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
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
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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keel stepped/deck stepped masts
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keel stepped/deck stepped masts
Stress-wise a 'cantilever' connection is very weak and vulnerable ....
and requires much more 'meat' to make it work. It is also subject to 'stress risers' (things that make it 'weaker' by geometry) at the root and requires some pretty precise shape/form to make it work well. In article , Wayne.B wrote: On Thu, 15 Apr 2004 12:38:06 GMT, (Steven Shelikoff) wrote: 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. ===================================== The issue of whether or not the mast fails as one piece or multiple pieces is separate from the structural considerations. Here's a different way to view the situation: A keel stepped mast is cantilevered at the deck and thus derives extra support. A deck stepped mast is essentially pivoted at the deck rather than supported by it. |
keel stepped/deck stepped masts
Stress-wise a 'cantilever' connection is very weak and vulnerable ....
and requires much more 'meat' to make it work. It is also subject to 'stress risers' (things that make it 'weaker' by geometry) at the root and requires some pretty precise shape/form to make it work well. In article , Wayne.B wrote: On Thu, 15 Apr 2004 12:38:06 GMT, (Steven Shelikoff) wrote: 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. ===================================== The issue of whether or not the mast fails as one piece or multiple pieces is separate from the structural considerations. Here's a different way to view the situation: A keel stepped mast is cantilevered at the deck and thus derives extra support. A deck stepped mast is essentially pivoted at the deck rather than supported by it. |
keel stepped/deck stepped masts
Relativisticly the keel stepped is perhaps better if you have rig
failure and NO LOAD on the rigging. But if you have extreme deflection at the deck interface and create deformation such as a 'teeny notch' where it goes through the deck ..... step out of the way, its coming DOWN. In article , Bill wrote: About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton, a French naval architect who lives in this area (or at least did at that time). Tanton insisted that the mast be keel stepped. The issue he was concerned about was mast pumping. If a deck stepped mast starts to pump it can theoretically jump out of the step. How the mast gets into this mode, I don't know. Now, this boat was for blue water, and this friend has sailed around the world at least once. As far as I know, he still has the boat. I also had an Aluminum vessel that was French designed and built. The mast was deck stepped. Bill "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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
Relativisticly the keel stepped is perhaps better if you have rig
failure and NO LOAD on the rigging. But if you have extreme deflection at the deck interface and create deformation such as a 'teeny notch' where it goes through the deck ..... step out of the way, its coming DOWN. In article , Bill wrote: About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton, a French naval architect who lives in this area (or at least did at that time). Tanton insisted that the mast be keel stepped. The issue he was concerned about was mast pumping. If a deck stepped mast starts to pump it can theoretically jump out of the step. How the mast gets into this mode, I don't know. Now, this boat was for blue water, and this friend has sailed around the world at least once. As far as I know, he still has the boat. I also had an Aluminum vessel that was French designed and built. The mast was deck stepped. Bill "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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that time). Tanton insisted that the mast be keel stepped. The issue he was concerned about was mast pumping. If a deck stepped mast starts to pump it can theoretically jump out of the step. How the mast gets into this mode, I don't know. Now, this boat was for blue water, and this friend has sailed around the world at least once. As far as I know, he still has the boat. I also had an Aluminum vessel that was French designed and built. The mast was deck stepped. Bill "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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
About 15 years ago a friend had an aluminum vessel ~40' designed by Tanton,
a French naval architect who lives in this area (or at least did at that time). Tanton insisted that the mast be keel stepped. The issue he was concerned about was mast pumping. If a deck stepped mast starts to pump it can theoretically jump out of the step. How the mast gets into this mode, I don't know. Now, this boat was for blue water, and this friend has sailed around the world at least once. As far as I know, he still has the boat. I also had an Aluminum vessel that was French designed and built. The mast was deck stepped. Bill "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. "Steve Christensen" wrote in message ... In article , QLW says... "Tom Dacon" wrote in message ... It's a mechanical engineering issue. A mast (called a column by mechanical engineers) that's supported only at the ends is less strong in compression than a column that's supported at two points at one end. The support at the mast step, for a keel-stepped mast, allows the mast to take more compression before failing than a deck-stepped mast can. Because the stays and shrouds take sailing loads almost parallel to the mast, the mast column comes under significant compression load. While I like the idea of a keel stepped mast, I'm skeptical about the reasoning above. I'm not an engineer but I have a good friend that is...and he has a lot of aircract and boat design experience...so I'll run this thread by him this afternoon and get his input before saying more. I hope your friend agrees with the above post, since this IS the accepted wisdom wrt rigs. Deck stepped masts get less support than keel stepped masts. Therefore the deck stepped mast must be larger - and heavier - in cross section to make up for it. It's always an option, but it adds weight aloft. Steve Christensen |
keel stepped/deck stepped masts
On Thu, 15 Apr 2004 15:44:47 GMT, Rich Hampel
wrote: 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. Exactly. Which is why I don't think it makes all that much difference. Steve |
keel stepped/deck stepped masts
On Thu, 15 Apr 2004 15:44:47 GMT, Rich Hampel
wrote: 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. Exactly. Which is why I don't think it makes all that much difference. Steve |
keel stepped/deck stepped masts
On Thu, 15 Apr 2004 13:39:15 -0400, Wayne.B
wrote: On Thu, 15 Apr 2004 12:38:06 GMT, (Steven Shelikoff) wrote: 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. ===================================== The issue of whether or not the mast fails as one piece or multiple pieces is separate from the structural considerations. Here's a different way to view the situation: A keel stepped mast is cantilevered at the deck and thus derives extra support. A deck stepped mast is essentially pivoted at the deck rather than supported by it. That's exactly how I am viewing it. But the support at the deck of a keel stepped mast is not going to do a damn thing to keep the mast from breaking if the rigging fails in rough weather. If anything, I'd think you'd have a slightly better chance of salvaging a deck stepped mast after a catastrophy and jury rigging it up again since, if it does come down with a pivot at the step, you've got a chance it may still be in one piece. Of course it all depends on what breaks and how it comes down. The only mast I've ever had come down due to rigging failure was on a deck stepped boat and everything was salvagable. Lose the rigging on a keel stepped mast and you're pretty much guaranteed to break it. Another difference between them is that, when the boat is parked at the slip, you can take the rigging down and not have the mast fall over if it's keel stepped. But then it's also a little tougher to get the mast up and down when you want to if it's keel stepped because you have to lift it up a bit. And the last difference I can think of is that you don't have to worry about a compressed compression post and attendent deck sag with a keel stepped mast. Steve |
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