FYI: I have a nacra 5.8 with a standard pin-head main and factory (north american) jib, no spin.
But I have a general question about how I can determine what the maximum tension load (in lbs) would be on my shrouds in the most taxing conditions (blowing hard, heavy crew, double-trapped, etc)? I want to do some mods on my shrouds but I need to make sure that all the lines & hardware can meet the required 'working load'.
help??
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How do I calculate shroud loading?
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Posted: Jan 02, 2011 - 06:09 PM
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Posted: Jan 03, 2011 - 12:45 AM
Wow, that is NOT an easy question to answer. There are so many variables that I'm not sure anyone would have ever made that calculation. You would have to figure out a design wind case and the worst case scenario for sail shape, boat trim, crew weight and position. And then the worst case loading would likely be dynamic in nature not a static load, so you need to start taking into account movement. Only the boat designers using 3D modeling tools are likely to have a handle on this.
Probably the simplest way of doing it would be to figure out the static righting force applied by the weight of the boat and two crew members on the wire with one hull just out of the water and doubling it. This is the maximum amount of force that the sail can impart and that you can handle without capsizing. To do this you figure out where the center of gravity of the top hull is, figure out the boat weight and mulitply that by the distance to the center of the bottom hull. Then you estimate where the crew center of gravity would be on the wire and multiply their weight by the distance to the center of the bottom hull. Add these three numbers up and divide by the distance form the upper shroud to the center of the bottom hull. Double it and you should be relatively close to the max working load. Note that there is probably another factor of safety applied to make up for fatigue/dynamic loading as well as losing some of the wires in a wire rope shourd. This is probably another factor of 2 but I couldn't tell you for sure.
I strongly suspect that most boat design is just done by trial and error, and there are a limited number of sizes that shrouds can come in. Hence some boats have more problems with rigging and component failures than others. Probably the newer designs have been modeled extensively but the people who did those calculations are unlikely to tell you how they did it (proprietary designs and all).
Dave.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Jan 03, 2011 - 12:08 PM
this may be to simple but what the heck...your current proper shrouds will handle all those conditions you have unless there is damage and its about to crap out. there might be some data on loads for the shroud wires whatever dimension they are out there somewhere. start there and you know all the other harware associated can handle at least that. i have been shoping line and noticed they always list load capacity so mayby they do that with wire also. good luck!
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Posted: Jan 03, 2011 - 02:19 PM
I agree with both the above. No way you are really going to get an accurate calculation of the loading. But if you find the rating on the shrouds designed for the boat you can make sure what you are doing meets or exceeds that. Assuming you are not adding a lot of additional loading to the boat through other modifications.
don't forget if you are using lines, they need to be derated at the knots if you are using knots.
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Posted: Jan 03, 2011 - 05:48 PM
If you are going to go with dyneema/vectran line instead of wire rope DO NOT use any knots on those, only use proper eye splices and thimbles. Knots can derate your line strength by up to 80% depending on how badly they are tied (normally 50% is quoted).
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Jan 04, 2011 - 09:45 AM
thanks for your responses! i also got to thinking that some of the load from the main is taken by the mainsheet & blocks as well as some of the load from the jib going to the forestay, making this even more complicated and dynamic.
my present shroud wire = 5/32" 1 x 19 s.s. with a breaking strength of between 2,800-3,300lbs. if working loads were half of this, it would still seem to be an ample margin of safety. but i could see how waves and gusting conditions could generate some large dynamic loads.
i was interested to see this catapult cat with adjustable shrouds. it doesn't appear they are using them to cant the mast while sailing, but i suppose it could be used that way.
http://www.catapultcats.com/sailtech.trap.html
http://www.catapultcats.com/sailtech.capsize.html
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Aquacat 12 (sold)...'87 Nacra 5.8 (sold)...'03 Nacra Inter18 (sold)
Venture 15 (sold)....'89 Nacra 5.8 (sold)...'91 Nacra 5.8NA (sold)
'99 Nacra Inter20 (sold)
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Posted: Jan 08, 2011 - 10:52 AM
Why do you want to reinvent the wheel? The stock 5.8 is a real hardy boat and can handle some pretty gnarly conditions. I use to try an break my 5.8 (for sport) by pushing to the limits and could never do it. Never had a problem with shroud failure!
A lot of your load is going to be on your diamond wires and that is the weak point of this rig. If you're going to make any changes I would look to upgrade these to the same gauge as your shrouds and fore stay and keep them tight and leave everything else alone. I can tell I went through of few sets of these when it was honking (15 sustained gusting 20/25, 2 to 3 foots seas) which it normally is where I sail. They 1/8" that are on there now could not take the extra "umph" of the square top and IMPO is the only weak point of the rig. When I did go to the larger wire I never had a problem, the rig felt sturdier, and there was less flex in the mast.
Joel at Murray's can hook up with these!
Edited by JohnES on Jan 08, 2011 - 08:57 AM.
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John Schwartz
Ventura, CA
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Posted: Jan 08, 2011 - 04:03 PM
If you want to break the boat, take it out on a gnarly day with 3 big guys 2 on the wire and one working the helm. The maximum force of the boat is pretty much limited by how much righting force you can provide. With only 2 people on the boat you shouldn't be able to put enough force on it to break it without first flipping the thing. Unless of course one of your wires has frayed a bit or something has degraded and just breaks (which is usually what happens).
D
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Jan 11, 2011 - 07:55 PM
Been there done that with three guys - two who were very experienced 5.8 sailors. We had that bad boy humming. Beam to broad reach in about 20 to 25 kts riding down the fronts of some 3 to 4 foot waves.
To this day my brother in law still notes this as one of the best days he had ever had on the water!
When we got back in I had noted one of the diamond wires was stating to fail - they were new a few weeks earlier!
Edited by JohnES on Jan 11, 2011 - 05:56 PM.
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John Schwartz
Ventura, CA
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Posted: Jan 12, 2011 - 12:03 AM
Sure way to break a 5.8. I was talking to a guy in Aus who did the same thing. Flew the hull for like an hour with 2 on the wire and ended up riding the wreckage to shore and just escaping a lightning storm. Sounded like an amazing time! Somehow he broke shroud and the forestay.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Jan 13, 2011 - 02:40 AM
gentlemen, i'm not trying to reinvent the wheel or break my 5.8......just the opposite actually. i'm exploring the idea of making a canting mast by shortening my wire shrouds and installing blocks and line on the lower few feet. i just need to make sure my line and blocks can handle the loading.
in this video, compare the lower section of the shrouds when the camera is in front of the multihull to see how the mast is vertical and the shrouds are different lengths.
http://www.youtube.com/watch?v=AP985PjMGts&feature=related
i know they have hydraulics involved but if they can do it, why not us???
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Aquacat 12 (sold)...'87 Nacra 5.8 (sold)...'03 Nacra Inter18 (sold)
Venture 15 (sold)....'89 Nacra 5.8 (sold)...'91 Nacra 5.8NA (sold)
'99 Nacra Inter20 (sold)
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Posted: Jan 13, 2011 - 09:05 AM
I wrote three paragraphs of crap about calling a rigger, what about this and that, and then I thought isn't this what back stay adjusters do?
http://www.harken.com/rigtips/back.php
Edited by kgatesman on Jan 13, 2011 - 07:07 AM. -
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Posted: Jan 13, 2011 - 10:15 AM
Arch,
If I read correctly, you want to cant your mast. It's done with some frequency in the Farrier trimarans. The 31-1D is the more famous example.
The way it's done is to have textile shrouds with multi-purchase thimbles. Erik Précourt was in business doing that but he's recently gone. There are other people building such shrouds but I don't have experience with them. I have an F-27 trimaran and I have Précourt rigging, although I don't cant.
The shrouds themselves are Dynex Dux, and they run to a thimble which has Amsteel multipurchase line. I've done my own lowers on one side, it's really not rocket surgery. The canting Farrier rigs just take the tail or bitter end of the lower adjustment, run it aft to a block at the rear beam, turn it toward the pit, and thence to a cam cleat, it's really just that simple.
BUT... that said... one of the fastest Farriers in the country, GAMERA (Matt Scharl of the Great Lakes) has gone back to stock rigging, eschewing the canting rig. I figure if he of all people doesn't think it's worth it then it's not worth it, to cant, anyway. And, in fact, Smyth has never bothered with a canting rig on his F25C. These guys are FAST.
I have textile shrouds but I've never messed with canting (see above), and the main reason I've stayed with the textile shrouds is because of the ease of rigging since I rig each time I sail. Textile shrouds are super light and easy to deal with. If I left my mast up all the time I'd prolly go with Dyform stainless.
My bottom line advice... it's not worth your time and effort to bother with canting your rig AFAIC. If you want to lighten and ease mast raising type stuff then consider textile rigging but be aware that the fibres do take some time to settle. People confuse this with creep but it's just fibre alignment. My F27 took quite some time to settle in and I have a set of textile rigging for my 18Sq that hasn't settled in yet (mainly cause I've not been sailing her
If you leave your mast up and want to lighten, just go with Dyform wire
sea ya
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Posted: Jan 13, 2011 - 10:24 AM
kgatesman
running backstays are to control mast rake (fore/aft) and for extra mast support, not for cant (port/stbd)
Backstays are generally problematic on multihulls because we have large-roach mains. There are ways around that but for this application not worth the effort
Better served with extra downhaul purchase and better mast rotation control, both for depowering -
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Posted: Jan 13, 2011 - 07:04 PM
Thanks Tami. What I was thinking if you are trying to pull the mast left and right, one of these spliced into each stay would be very strong and built for a similar purpose. Maybe it doesn't work the way I think, my slow boats didn't have a backstays.
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Posted: Jan 13, 2011 - 10:38 PM
thanks tami, i respect your advice. it happens to be similar to what jack at performance sports (nacra) recommended....that it's not worth the effort and all the hassles for the minor speed enhancement. i've had my cat for a while and am looking for something to tweak. i'm sure you understand that feeling, right?
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Aquacat 12 (sold)...'87 Nacra 5.8 (sold)...'03 Nacra Inter18 (sold)
Venture 15 (sold)....'89 Nacra 5.8 (sold)...'91 Nacra 5.8NA (sold)
'99 Nacra Inter20 (sold)
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Posted: Jan 14, 2011 - 09:17 AM
arch
your tweaking would be well served by the purchase of an updated mainsail.... that'll damn sure fulfill that tweakin' urge or at least supplant it with raw fear. Now's the time to buy, don't wait till spring
You wouldn't cant mast in buoy race but over the course of a distance it'll make a better overall angle to a mark. (I corrected my earlier statement, here's correction: you move the mast to windward to reduce heeling moment, exposing more sailplan to wind, and pressing less on the boat.)
edit2:
Thanks kgatesman, that's my boat
Edited by tami on Jan 14, 2011 - 07:25 AM. -
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Posted: Jan 15, 2011 - 02:23 PM
Plus one on this!!!
I went from a Dacron pin head to a Pentax Square top and it was a night and day difference in the performance of the boat.
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John Schwartz
Ventura, CA
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Posted: Jan 15, 2011 - 07:35 PM
Agree wholeheartedly. Went from the stock sail on my 1982 Nacra 5.2 to a Whirlwind designed dacron square top. Blew my socks off (literally) the first time I took it out and has me searching for a 8:1/16:1 downhaul to reign it in! Downwind performance went up dramatically also even without a spin. Well worth the money!
D.
Edited by Wolfman on Jan 15, 2011 - 05:37 PM.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Jan 15, 2011 - 11:32 PM
The easy way would be to figure out what size wire you have on the shrouds now and then look up safe working load for that wire. You can guess that the factory didn't make them too small (warranty, liability) or too big (cost, wind drag). I loved my Whirlwind mainsail. Especially the part where it actually went up to the top of the mast not hanging down three inches.
