I am in the process of replacing the chain plates on my CD28, following the examples of Fred’s Fenix and David Cruickshank’s Alaina.
On the Fenix web site, Fred stated that the lower shrouds carry more load than the upper and he used 1/4 cable (factory was 3/16th). If I change to 1/4, then there would be one size for all the cables. However, there would be more stress when tensioning a heavier cable. While the new chainplates can easily handle the additional load, what about the tang on the other end and the added compression on the deck (there will not be a new compression post like on the Finx). My gut feeling is to keep the factory’s 3/16 wire size, but I was hoping to solicit some discussion.
Fred also stated “…the new chain plates will be farther aft. When there are fore and aft lower shrouds the shroud attachment should have "drift". General rule for drift : one inch for every foot of height. The stock placement drift for the aft lowers was 14" but the shrouds are 20 feet above deck, the drift should and will be 20â€
Chainplates & Rigging
Moderator: bobdugan
Ok either way
First of all, hats off to you. I like chainplates that you can see!
I believe that the heavier wire will transmit the same tension as the lighter. The impact of the wire's own weight should be so small that you can focus on other considerations (like cost and commonality).
As far as Fred's advice on the location of the lower shroud chainplates, he sounds pretty sure about it when you quote him as saying "...should and will be 20"...".
The last time I might have had some clue about working this out mathematically is shrouded in the mists of time. I'd split the difference, call it 17", and say you based it on the consensus of experts.
I believe that the heavier wire will transmit the same tension as the lighter. The impact of the wire's own weight should be so small that you can focus on other considerations (like cost and commonality).
As far as Fred's advice on the location of the lower shroud chainplates, he sounds pretty sure about it when you quote him as saying "...should and will be 20"...".
The last time I might have had some clue about working this out mathematically is shrouded in the mists of time. I'd split the difference, call it 17", and say you based it on the consensus of experts.
I would start by asking what exactly you want to do with the boat. Are you trying to get it ready for a trip across the Atlantic or coastal cruising? There are many examples of your boat out there with aging stock size wire and chainplate placement that have not had any problems.
Regarding wire size, you can tension any wire to any tension that you want provided that the wire's weight is not a significant portion of the tension which it is not in this case. Increasing wire size will increase the stiffness of the shrouds which does have an effect on what carries the load. Unless you are planning on extended voyaging, I can see no reason to increase the wire size.
Regarding the placement of the chainplates, I agree that they are spaced too closely from the factory. As designed, it would almost function the same as simply having one wire leading to the same point as the cap shroud. However, this begs the question, why fix it when it works just fine? By changing the location, you do change the loading on the wire slightly due to the changing angle it presents to the mast but the change is extremely small. The larger change would be stiffening the mast fore and aft. You don't need to move the forward chainplates if you move the aft ones but you might as well.
Regarding wire size, you can tension any wire to any tension that you want provided that the wire's weight is not a significant portion of the tension which it is not in this case. Increasing wire size will increase the stiffness of the shrouds which does have an effect on what carries the load. Unless you are planning on extended voyaging, I can see no reason to increase the wire size.
Regarding the placement of the chainplates, I agree that they are spaced too closely from the factory. As designed, it would almost function the same as simply having one wire leading to the same point as the cap shroud. However, this begs the question, why fix it when it works just fine? By changing the location, you do change the loading on the wire slightly due to the changing angle it presents to the mast but the change is extremely small. The larger change would be stiffening the mast fore and aft. You don't need to move the forward chainplates if you move the aft ones but you might as well.
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joelcunningham
- Posts: 60
- Joined: Sep 15th, '10, 22:22
- Location: CD 25 #793 1981 "Omega"
Keyport, NJ
Questioning Cape Dory?
No expert here, but consider some basic physics. While the lowers can't help but contribute to holding the mast up, their primary function is to keep it straight. Little boats don't have a four lower configuration because short masts are less prone to bending. The pyramid configuration of the four lowers is extremely stable, being four times triangulated. The lighter wire is used because of the shorter length and the reduced leverage at this height. Finally, I would find it hard to believe that Cape Dory and Alberg got this wrong.
Re: Chainplates & Rigging
I'd like to learn more about the basis for that statement. Why then are the orignal equipment forestay, backstay and capshrouds heavier guage wire? Why then are the forestay, backstay and capshrouds typically pretensioned higher than the lowers? Like Joel, I've always thought the lowers were there primarily to keep the mast from bowing.hilbert wrote:On the Fenix web site, Fred stated that the lower shrouds carry more load than the upper
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fenixrises
- Posts: 450
- Joined: Feb 13th, '05, 08:01
- Location: SunShine S2 11c
- Contact:
Hi all,
I would suggest "Skene's Elements of Yacht Design", Chapter XVII Spars and Rigging, Page 163(in my edition), as a starting point.
A typical single spreader rig with fore and aft lowers means the boat has four lowers, two to a side and two uppers, one to a side.
Since there are two lowers to a side the load is split between them, hence they could be of a smaller diameter.
There is most likely a part of yacht design software that does these calculations quite readily but I do not have that software. I started with "Skene's" over 30 years ago. Through experience I pretty much know the normal rigging size for standard rigged boats from 25'~40'.
I rebuilt FeNIX to do something that the original designer, Carl Alberg, and builder, Cape Dory probably never envisioned for a CD 28. It is easy to add strength to a boat when it is sitting in the driveway. But quite difficult to make repairs 1,000 miles from the nearest land.
The stock boat was well rigged. 1/4" uppers are overkill for a 28' boat, even a heavier one like a CD28. But larger diameter wire stretches less for a given load and length.
Westsail 32's are designed to displace 18,500lbs. I have yet to meet one that is less than about 23,000lbs in cruising trim. There is one here in Samoa. The owner said it's current displacement is 13 tons, that is 26,000lbs. It was weighed when last hauled. Westsail in the early years touted it's 32 as having overstrength rigging. It had 1/4" uppers and lowers. By calculation the wire could have been 7/32", 1/32" less than 1/4".
Don't forget that SS wire rigging comes in 304 and 316 SS. 304 is stronger but more prone to corrosion. Over the years many Westsail owners have bumped the rigging size upto 9/32". While 7/32" 304 would be strong enough, 9/32" 316 gives some additional peace of mind at sea.
I met a boat in Tahiti As memory serves it was a 36'~38' possibly by Caliber. The fellow bought the boat in Mexico. The boat was surveyed and additionally the rigging was checked by a professional rigger before the new owner left for the Marquises. All was given a thumbs up for ocean passage making. He got a good deal on the boat. It was very well equiped, out fitted and came with many spare parts.
Halfway to Nuka Hiva he nearly lost the mast due to rigging failures. It took 45 days to make the 3,000 mile crossing under power and reduced sail. Then he had to wait in Nuka Hiva for over two months for replacements to be shipped. All told his cost was up near $6,000 for replacing the all the rigging and chainplates. About $1,000 of the cost was for shipping. The rigging was five years old and the chainplates were original.
If you are a weekend coastal sailor most stock boats are properly designed and rigged for the normal conditions you might face. The Pacific Ocean is about 8,000 miles across. Conditions can be mild to severe. That 8,000 miles equals more sailing than the avereage weekender might do in his boat for the entire time of ownership. In either case frequent rigging checks, repairs and replacements are a must for safe and carefree sailing.
Don't forget Fred's two cardinal rules:
1. Keep the mast pointing at the sky. You have a sailboat. No mast means no sailing. Maybe OK if you can see the beach, not so OK if the beach is hundreds of miles away.
2. Keep the saltwater on the outside of the boat. Or as Bill Cosby's God asked Noah "How long can you tread water?" Small drips and leaks can be annoying. Big drips and leaks can be deadly unless of course you can tread water for a really long time.
Take care,
Fred
I would suggest "Skene's Elements of Yacht Design", Chapter XVII Spars and Rigging, Page 163(in my edition), as a starting point.
A typical single spreader rig with fore and aft lowers means the boat has four lowers, two to a side and two uppers, one to a side.
Since there are two lowers to a side the load is split between them, hence they could be of a smaller diameter.
There is most likely a part of yacht design software that does these calculations quite readily but I do not have that software. I started with "Skene's" over 30 years ago. Through experience I pretty much know the normal rigging size for standard rigged boats from 25'~40'.
I rebuilt FeNIX to do something that the original designer, Carl Alberg, and builder, Cape Dory probably never envisioned for a CD 28. It is easy to add strength to a boat when it is sitting in the driveway. But quite difficult to make repairs 1,000 miles from the nearest land.
The stock boat was well rigged. 1/4" uppers are overkill for a 28' boat, even a heavier one like a CD28. But larger diameter wire stretches less for a given load and length.
Westsail 32's are designed to displace 18,500lbs. I have yet to meet one that is less than about 23,000lbs in cruising trim. There is one here in Samoa. The owner said it's current displacement is 13 tons, that is 26,000lbs. It was weighed when last hauled. Westsail in the early years touted it's 32 as having overstrength rigging. It had 1/4" uppers and lowers. By calculation the wire could have been 7/32", 1/32" less than 1/4".
Don't forget that SS wire rigging comes in 304 and 316 SS. 304 is stronger but more prone to corrosion. Over the years many Westsail owners have bumped the rigging size upto 9/32". While 7/32" 304 would be strong enough, 9/32" 316 gives some additional peace of mind at sea.
I met a boat in Tahiti As memory serves it was a 36'~38' possibly by Caliber. The fellow bought the boat in Mexico. The boat was surveyed and additionally the rigging was checked by a professional rigger before the new owner left for the Marquises. All was given a thumbs up for ocean passage making. He got a good deal on the boat. It was very well equiped, out fitted and came with many spare parts.
Halfway to Nuka Hiva he nearly lost the mast due to rigging failures. It took 45 days to make the 3,000 mile crossing under power and reduced sail. Then he had to wait in Nuka Hiva for over two months for replacements to be shipped. All told his cost was up near $6,000 for replacing the all the rigging and chainplates. About $1,000 of the cost was for shipping. The rigging was five years old and the chainplates were original.
If you are a weekend coastal sailor most stock boats are properly designed and rigged for the normal conditions you might face. The Pacific Ocean is about 8,000 miles across. Conditions can be mild to severe. That 8,000 miles equals more sailing than the avereage weekender might do in his boat for the entire time of ownership. In either case frequent rigging checks, repairs and replacements are a must for safe and carefree sailing.
Don't forget Fred's two cardinal rules:
1. Keep the mast pointing at the sky. You have a sailboat. No mast means no sailing. Maybe OK if you can see the beach, not so OK if the beach is hundreds of miles away.
2. Keep the saltwater on the outside of the boat. Or as Bill Cosby's God asked Noah "How long can you tread water?" Small drips and leaks can be annoying. Big drips and leaks can be deadly unless of course you can tread water for a really long time.
Take care,
Fred
You should always have an odd number of holes in your boat!
I have never heard keep the mast pointing at the sky, that is a good one. I might add to these, keep the boat off the land and keep the people on the boat.fenixrises wrote:
Don't forget Fred's two cardinal rules:
1. Keep the mast pointing at the sky. You have a sailboat. No mast means no sailing. Maybe OK if you can see the beach, not so OK if the beach is hundreds of miles away.
2. Keep the saltwater on the outside of the boat. Or as Bill Cosby's God asked Noah "How long can you tread water?" Small drips and leaks can be annoying. Big drips and leaks can be deadly unless of course you can tread water for a really long time.
I'm Confused
Fred
I don't know if you were quoted properly or not, "that the lower shrouds carry a heavier load than the uppers"?
But, assuming you were, how does your posting above, wherein you describe the number of shrouds explain that? Forgive me as I must be missing something?
Continued success and cruising.
Darrell
I don't know if you were quoted properly or not, "that the lower shrouds carry a heavier load than the uppers"?
But, assuming you were, how does your posting above, wherein you describe the number of shrouds explain that? Forgive me as I must be missing something?
Continued success and cruising.
Darrell
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fenixrises
- Posts: 450
- Joined: Feb 13th, '05, 08:01
- Location: SunShine S2 11c
- Contact:
Hi all,
Darrell according to "Skene's" on page 169 there is a table titled
"Load Distribution on Shrouds from Tests"
For the mainmast of a masthead rig with one set of spreaders and two lowers per side the lowers carry 60% of the load, the uppers 45%. Now that's more than 100% so a safety factor is built into these numbers. Yes 60% / 2 = 30% each and one at 45% is one at 45%. So 45% > 30%. The combined load on the lowers is greater than the load of the upper and the load on the upper is greater than the load on an individual lower. As I said the lowers carry more load than the upper.
BTW the man that developed the Cap Horn self steering system was thwarted on his bid to do a single handed non-stop circumnavigation on his Alberg 30' The reason, his starboard lower chainplate attachment inside the hull failed. He chastized himself for not rebuilding this part during his preparations.
Interestingly enough the primary problem the fellow that I met in Tahiti had was a failure of the starboard foward lower shroud. After arriving in Nuka Hiva he removed his chainplates for a visual inspection. The chainplates were similar to the ones on SunShine. That is they are SS flat bar that pass through the deck and are bolted to either knees or bulkheads. The chainplate for his starboard upper shroud was cracked and only 1/8" of material was holding the upper shroud. Quite fortunate not to have lost the entire rig.
I have heard that in a conventioal rig the forward lower shrouds are the most higly loaded. I have no scientific proof for this statement.
A side note: The 25' Pacific Seacraft that I built in 1979 and last saw here in Samoa is still here 4 years later. The same person owns the boat, he still lives aboard and works ashore. By his own admission that means he has lived aboard here in Samoa for 12 years, WOW. The boat is still pretty ragged looking. But it seems to have gone through the tsunami w/o a problem.
Take care,
Fred
Darrell according to "Skene's" on page 169 there is a table titled
"Load Distribution on Shrouds from Tests"
For the mainmast of a masthead rig with one set of spreaders and two lowers per side the lowers carry 60% of the load, the uppers 45%. Now that's more than 100% so a safety factor is built into these numbers. Yes 60% / 2 = 30% each and one at 45% is one at 45%. So 45% > 30%. The combined load on the lowers is greater than the load of the upper and the load on the upper is greater than the load on an individual lower. As I said the lowers carry more load than the upper.
BTW the man that developed the Cap Horn self steering system was thwarted on his bid to do a single handed non-stop circumnavigation on his Alberg 30' The reason, his starboard lower chainplate attachment inside the hull failed. He chastized himself for not rebuilding this part during his preparations.
Interestingly enough the primary problem the fellow that I met in Tahiti had was a failure of the starboard foward lower shroud. After arriving in Nuka Hiva he removed his chainplates for a visual inspection. The chainplates were similar to the ones on SunShine. That is they are SS flat bar that pass through the deck and are bolted to either knees or bulkheads. The chainplate for his starboard upper shroud was cracked and only 1/8" of material was holding the upper shroud. Quite fortunate not to have lost the entire rig.
I have heard that in a conventioal rig the forward lower shrouds are the most higly loaded. I have no scientific proof for this statement.
A side note: The 25' Pacific Seacraft that I built in 1979 and last saw here in Samoa is still here 4 years later. The same person owns the boat, he still lives aboard and works ashore. By his own admission that means he has lived aboard here in Samoa for 12 years, WOW. The boat is still pretty ragged looking. But it seems to have gone through the tsunami w/o a problem.
Take care,
Fred
You should always have an odd number of holes in your boat!
Fred,fenixrises wrote:Hi all,
Darrell according to "Skene's" on page 169 there is a table titled
"Load Distribution on Shrouds from Tests"
For the mainmast of a masthead rig with one set of spreaders and two lowers per side the lowers carry 60% of the load, the uppers 45%. Now that's more than 100% so a safety factor is built into these numbers. Yes 60% / 2 = 30% each and one at 45% is one at 45%. So 45% > 30%. The combined load on the lowers is greater than the load of the upper and the load on the upper is greater than the load on an individual lower. As I said the lowers carry more load.
Thanks, that explains it completely. Sorry, I didn't have access to Skeene's, or I wouldn't have bothered you further for an explanation. Actually, I was furthering Joel and Stans question regarding the belief that the lowers main function was to keep the mast straight, and carried less of the load than did the uppers, which I must say, I had thought previously as well. I just didn't grasp how quoting the number of uppers and lowers (regardless of the wire guage) explained that those of us who had erroneously believed that were mistaken.
The uppers carry 45% of the load and the lowers carry 60% of the load. I assume that this is true regardless of the number of lowers in the rig, no?
Darrell
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Neil Gordon
- Posts: 4367
- Joined: Feb 5th, '05, 17:25
- Location: s/v LIQUIDITY, CD28. We sail from Marina Bay on Boston Harbor. Try us on channel 9.
- Contact:
>>I was furthering Joel and Stans question regarding the belief that the lowers main function was to keep the mast straight, ...<<
Am I missing something here or isn't the purpose of the whole rig "to keep the mast straight"? Build a mast from carbon fiber and it might stand up, under load, all by itself, no uppers or lowers required.
Am I missing something here or isn't the purpose of the whole rig "to keep the mast straight"? Build a mast from carbon fiber and it might stand up, under load, all by itself, no uppers or lowers required.
Fair winds, Neil
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
Heresy.
Well, I read so much of Skene's as I could find on-line and I have my doubts. His discussion is pretty impenetrable, but it appears his model assumes all of the force on the mainsail from the height of the boom to the height of the spreader bases creates a point load at the spreader bases, and all of the force on the mainsail from the height of the spreader bases to the height of the masthead creates a point load at the masthead. In other words, he seems to be treating the mast as if it had a hinge at the spreader bases. My suspicion is that our masts were not intended to work that way, but, instead, were intended to use the rigidity of the mast itself to transfer a lot of the force (if not most of the force) from below the spreader bases up to the masthead.
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fenixrises
- Posts: 450
- Joined: Feb 13th, '05, 08:01
- Location: SunShine S2 11c
- Contact:
Hi all,
Neil. Freedom rigs use carbon fiber unstayed masts. The masts are built by Tillotsen/Pearson and offered a lifetime warrenty against breakage to the original owner. When FeNIX and I were in St Helena we met a couple from South Africa aboard a Freedom 33. The boat is a cat/ketch with carbon fiber masts.
On an earlier passage the main mast snapped at the partners because the rubber collar that cushions the mast at the partners shifted and at sea the owner had no way to re-set the collar.
While in Brasil he unstepped the masts. He moved the mizzen mast to the forward position(the masts in the 33 being identical) and cut off the lower broken part of the main mast. That mast was then installed in the mizzen position.
He further modified the rig by converting it to a junk ketch. The sails were made from blue polytarps, the same kind you get at Home Depot. When I met the boat he was headed to Brazil for the second time and was quite happy with the performance and ease of handling of the modified rig.
I seriously considered buying and modifying a Freedom 33 the same way but the ones I saw for sale were out of my price range.
Stan and everybody. Generally speaking a stayed sailboat mast is considered a column under compression. Envision a soda straw. Put the bottom on a hard flat surface. Load the top of the straw until it starts to bow under load. Add spreaders and rigging to stop the bowing. Keep adding weight to the top and eventually the walls of the straw will collapse.
Another way to envision this is to set up the mast horizontally. It would have a step and rigging attachments the same as in use. In this demonstration the mast only needs rigging on one side and no fore of aft staying. Then add weight to the downside of the mast at the masthead. Keep adding weight until the mast extrusion itself fails or some part of the rigging fails. If I remember correctly LeFiell had a test rig like this in their shop, complete with strain gauges at all load points.
This is not an exact analogy because the load on a mast starts from the righting moment of the boat vs. the sail and rigging loads.
But other than the mainsail load along the sailtrack on the aft edge of the mast and the point load of the gooseneck the loads are transferred to the mast at the attachment point of the rigging. The mast is being compressed by transferring the transverse and longitudinal loads from the rigging and sails to the mast step.
This is a complex engineering calculation. The loads are dynamic as well because of the boats motion and the variabilty of the wind strength and sail area.
Long ago masts were designed empherically. In others words build it and sail it. If it broke make it a bit bigger if it didn't maybe make it a bit smaller.
Then the basics of engineering and the math to support it came along. Static load calculations could be done. A bit extra was included for a safety factor. For non cutting edge sailing machines this serves pretty well. Now we have computers and all manner of software that can do the calculations without resorting to "test to destruction" methods. But sometimes the engineers still like to do these tests.
I saw a Discovery show about Boeing and one of the new planes they designed. Their software calculated that the wing would fail when bent to a certain amount and it would take a certain amount of force to bend the wing that far. They company set up a test plane and started to bent the wing. It failed. The failure point and amount of load required were within a small percentage of error of what the software said would be required.
If you have the money and expertise you can do this yourself or probably find someone to do this calculation. I suspect many spar makers have this type of software.
For the kind of sailing we do and the type of boats we have, Skene's supplies the information needed to be safe. While not cutting edge and probably insufficent for go fast racers and high speed multihulls our boats are adequately served by the older methods.
Take care,
Fred
Neil. Freedom rigs use carbon fiber unstayed masts. The masts are built by Tillotsen/Pearson and offered a lifetime warrenty against breakage to the original owner. When FeNIX and I were in St Helena we met a couple from South Africa aboard a Freedom 33. The boat is a cat/ketch with carbon fiber masts.
On an earlier passage the main mast snapped at the partners because the rubber collar that cushions the mast at the partners shifted and at sea the owner had no way to re-set the collar.
While in Brasil he unstepped the masts. He moved the mizzen mast to the forward position(the masts in the 33 being identical) and cut off the lower broken part of the main mast. That mast was then installed in the mizzen position.
He further modified the rig by converting it to a junk ketch. The sails were made from blue polytarps, the same kind you get at Home Depot. When I met the boat he was headed to Brazil for the second time and was quite happy with the performance and ease of handling of the modified rig.
I seriously considered buying and modifying a Freedom 33 the same way but the ones I saw for sale were out of my price range.
Stan and everybody. Generally speaking a stayed sailboat mast is considered a column under compression. Envision a soda straw. Put the bottom on a hard flat surface. Load the top of the straw until it starts to bow under load. Add spreaders and rigging to stop the bowing. Keep adding weight to the top and eventually the walls of the straw will collapse.
Another way to envision this is to set up the mast horizontally. It would have a step and rigging attachments the same as in use. In this demonstration the mast only needs rigging on one side and no fore of aft staying. Then add weight to the downside of the mast at the masthead. Keep adding weight until the mast extrusion itself fails or some part of the rigging fails. If I remember correctly LeFiell had a test rig like this in their shop, complete with strain gauges at all load points.
This is not an exact analogy because the load on a mast starts from the righting moment of the boat vs. the sail and rigging loads.
But other than the mainsail load along the sailtrack on the aft edge of the mast and the point load of the gooseneck the loads are transferred to the mast at the attachment point of the rigging. The mast is being compressed by transferring the transverse and longitudinal loads from the rigging and sails to the mast step.
This is a complex engineering calculation. The loads are dynamic as well because of the boats motion and the variabilty of the wind strength and sail area.
Long ago masts were designed empherically. In others words build it and sail it. If it broke make it a bit bigger if it didn't maybe make it a bit smaller.
Then the basics of engineering and the math to support it came along. Static load calculations could be done. A bit extra was included for a safety factor. For non cutting edge sailing machines this serves pretty well. Now we have computers and all manner of software that can do the calculations without resorting to "test to destruction" methods. But sometimes the engineers still like to do these tests.
I saw a Discovery show about Boeing and one of the new planes they designed. Their software calculated that the wing would fail when bent to a certain amount and it would take a certain amount of force to bend the wing that far. They company set up a test plane and started to bent the wing. It failed. The failure point and amount of load required were within a small percentage of error of what the software said would be required.
If you have the money and expertise you can do this yourself or probably find someone to do this calculation. I suspect many spar makers have this type of software.
For the kind of sailing we do and the type of boats we have, Skene's supplies the information needed to be safe. While not cutting edge and probably insufficent for go fast racers and high speed multihulls our boats are adequately served by the older methods.
Take care,
Fred
You should always have an odd number of holes in your boat!
Fred
I do understand that our masts are a column under compression, and don't question your statement that Skeene's says that the uppers carry 45% of the load and the lowers carry 60% of the load.
Your "straw analogy" is right on for me. But, as I said, assuming that is so, I am wondering if the number of lower shrouds (since there can be one, or several) is irrelevent to that statement?
Neil,
I think you are confusing "straight" with plumb, or vertical. We could hold a mast plumb or vertical with only top or upper shrouds, but, if you put that mast under load (compression) you will need lowers to keep it "straight"
Darrell
I do understand that our masts are a column under compression, and don't question your statement that Skeene's says that the uppers carry 45% of the load and the lowers carry 60% of the load.
Your "straw analogy" is right on for me. But, as I said, assuming that is so, I am wondering if the number of lower shrouds (since there can be one, or several) is irrelevent to that statement?
Neil,
I think you are confusing "straight" with plumb, or vertical. We could hold a mast plumb or vertical with only top or upper shrouds, but, if you put that mast under load (compression) you will need lowers to keep it "straight"
Darrell
It still doesn't make sense to me that you normally pretension the capshrouds more than the lowers if the lowers are carrying the greater load.
I can see that a lateral load puts a stayed mast in compression, but it also puts the windward shrouds in tension and it's that tensive force we are talking about.
I also see now that Skene divides the force on the mainsail into three sections, not two as I previously thought. However, the size of those sections (and the amount of the resulting assumed lateral force) depends in large part on how high the spreader bases are located on the mast. Skene seems to assume spreader bases significantly above the halfway point between the boom and the masthead. On a CD 28, the spreader bases are significantly below the halfway point.
Additionally, although Skene says half of the "jib's" lateral force should be attributed to the point where the jibstay attaches to the mast, it is unclear whether his model is based on a masthead or a fractional rig. It also does not appear that the possibility of a big overlapping genoa was considered at all.
In short, I remain suspicious that Skene's 60/45 guidline was true for the rigs of his day but is not true for the (relatively) more modern rigs on our boats.
I can see that a lateral load puts a stayed mast in compression, but it also puts the windward shrouds in tension and it's that tensive force we are talking about.
I also see now that Skene divides the force on the mainsail into three sections, not two as I previously thought. However, the size of those sections (and the amount of the resulting assumed lateral force) depends in large part on how high the spreader bases are located on the mast. Skene seems to assume spreader bases significantly above the halfway point between the boom and the masthead. On a CD 28, the spreader bases are significantly below the halfway point.
Additionally, although Skene says half of the "jib's" lateral force should be attributed to the point where the jibstay attaches to the mast, it is unclear whether his model is based on a masthead or a fractional rig. It also does not appear that the possibility of a big overlapping genoa was considered at all.
In short, I remain suspicious that Skene's 60/45 guidline was true for the rigs of his day but is not true for the (relatively) more modern rigs on our boats.