Today was my first time on a 580 sport with spinnaker. I usually sail a 500 on which you can't control the mast rotation. We had about 7-10 knots wind, so nothing serious.
With spinnaker up while beam reaching I observed excessive bending of the mast during wind speed increase. At some point I thought that the mast was going to break...
The mast rotation was set to minimum, so the spinnaker top pulled sideways relative to the mast. I had the mainsail traveller about 30 cm out and the mainsail sheet pretty tight.
Is this causing the excessive mast bending? Should I trim the mast rotation to maximum rotation, so the spinnaker top pulls the mast top more aft instead of sideways? Or I have overlooked something else?
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Peter
Vancouver BC
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Nacra 580 sport: how to prevent excessive mast bend under spinnaker?
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Posted: Aug 07, 2011 - 02:15 AM
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Posted: Aug 07, 2011 - 04:31 PM
The question is, how much bend is excessive? Those masts are 30 ft of extruded aluminum and can bend quite a bit without a problem. If it was bending like 2 feet sideways at the top then you are bending too much.
Sounds like you described your problem above. You need more mast rotation to bring the major axis (the longer part of the mast cross section) in line with the spinnaker force. Not letting the mast rotate under spinnaker with heavy mainsheet tension would cause it to bend sideways (along the minor axis). The mast is VERY flexible along this axis so it will bend quite a bit and could definitely break. Set your rotator so that it will allow the mast to rotate until the mast track is in line with the shroud and that is as much as you should limit it unless you are trying to depower (then you should probably keep the spin in the bag).
Another thing to do is to make sure your diamond wires are tight! Enough to 'just' noticeably bend the mast backwards, that will help stiffen the mast up quite a bit on the minor axis.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Aug 07, 2011 - 09:19 PM
pbodifee,
three simple rules when the spin is up, max rotation, downhaul off and main sheet on. The main sheet is your backstay. Don't use it and you will be buying a mast.
What does pbodifee stand for, or are you just nameless?
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Philip
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Posted: Aug 07, 2011 - 10:11 PM
Why no downhaul when under spin? Just curious.
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Rob
OKC
Pile of Nacra parts..
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Posted: Aug 07, 2011 - 10:35 PM
Likely because it adds more stress on the mast (and could break the downhaul)? I would also think that the little dewrinkling downhauls on the older cats wouldn't apply. If you are running a spin you shouldn't need the high power downhaul to depower the main on a broad reach. If you do you shouldn't be running the spin! :)
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Aug 08, 2011 - 12:15 AM
Philip,
pbodifee is my usual username on web sites and stands for Peter Bodifee, my name
Dave,
From my angle it looked like almost 2 ft at the top.
Thanks for the explanation. When at home I started to draw some force vectors on a piece of paper after reading on this forum that the mast is designed to bend 'somewhat' along the minor axis, which is useful to flatten the sail going upwind in stronger wind conditions. From that I concluded that my mast rotation was not good, but wasn't sure. Initially I also let the main sheet go, as my crew member was too inexperienced (first time flying spi) to depower on time. The bending effect really scared me, thank god we had light wind, otherwise I would have broken the mast. After realizing that letting the main go was not a good plan either, I had it in the cleat and went out on the trapeze as well. Guess what happened... next wind gust I yelled let go, but he was too concerned not to loose balance by holding on to the sheet: capsize.
I was too slow in getting in, so I was 'launched' and landed between the tramp and the boom in the water with the wire still hooked. That caused another 'bend' (more permanent) in the boom. 
(On the 500 you don't have that problem, as there is no boom) Fortunately a lot cheaper part then a broken mast....
This is a boat of a volunteer-run club, and the members who are allowed on this one are mostly very experienced and make sure the rigging is in good shape. So I am not too concerned, but I will double check next time. We did briefly stopped on the water and I checked the diamond wires, and they seemed to be pretty tight.
I think I learned a lesson here: don't do things if there is not enough experience onboard. I have quite some cat experience (hobie, prindle, nacra), including flying spi as a crew on a F18, but this was my first time at the helm under spi on a new boat type. I should have taken first some trips with an experienced crew on this boat type, who could have pointed out my mistakes.
Thanks all for your excellent feedback.
Peter,
Vancouver BC
Edited by pbodifee on Aug 07, 2011 - 10:45 PM. -
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Posted: Aug 08, 2011 - 11:57 AM
Well if you have to replace the boom yourself, the cheapest way would be to get an 8 ft length of 3/32" thick 2" diameter 6065 aluminum tubing (metals supermarket I think sells that locally to you). Should run about $50 max. Cut it to the same length as the rectangle section. Pull off the old hardware and remount on the new tube using stainless rivets. Now you have a nicer boom than stock without having to buy a new one.
Or you could just find a used one in the classifieds....
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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