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Spreader deflection/mast stiffness

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Post Options Post Options   Quote Bender Quote  Post ReplyReply Direct Link To This Post Topic: Spreader deflection/mast stiffness
    Posted: 09 Oct 12 at 10:58pm
Everything that I read suggests that increasing spreader length stiffens the mast laterally. 
Thinking about the physics, I understand that if I leave the shroud plates alone and increase the spreader length, I increase the shroud tension (because I'm stretching the wire) and so stiffen the mast. But if I increase the spreader length and then readjust the shroud tension to what it was, I can't see that the mast stiffness would change much at all! Confused
Any views?



Jerry Hone
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Post Options Post Options   Quote giraffe Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 7:43am
To some extent I think you are logically correct, however your rig is dynamic rather than static

when it is dynamic there will be higher tension on the windward shroud than the leeward shroud. a longer spreader will have greater deflection on the shroud and have a greater poking moment to make the mast stiffer.

The way i think about it is that increasing spreader length has the most significant effect, but if you reduce rig tension you will reduce the effect.
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 11:23am
The spreader works by deflecting the shroud and as the shroud is in tension this deflection causes a compression load in the spreader. This compressive force will act on the mast and try to deflect the mast.. In a dynamic situation ie. when sailing, the force of the wind will want to bend the rig to leeward. The spreader will now resist this by forcing the mast to leeward at the spreader bracket which has the effect of reducing the sideways bend of the mast above the hounds. This is why an increase spreader length powers the rig up. If this is done to extreme it can cause the mast to take an S shape when viewed from behind which is probably not fast. Also increasing the spreader length will increase the prebend and so if you want this to stay the same you should angle the spreaders slightly further forward if you increase their length. ( keep the same distance from  a batten put across the ends of the spreaders to the rear face mast and the prebend will be constant).
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Post Options Post Options   Quote Noah Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 11:41am
At a training session run by Adam Bowers this year he said that - contrary to popular opinion - heavier crews (on the wire) do not need longer spreaders, because it'll end up pushing the mast to leeward at spreader level and knackering the slot.
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 7:37pm
Adam Bowers is correct. Short spreaders reduce the leeward deflection of the mast and therefore prevent the slot on a two sailed boat from being closed, therefore those looking for more power should shorten spreaders. Pre-bend of the mast is controlled by spreader sweep, but remember that sweeping spreaders reduces the effective length, and so you may need to lengthen spreaders slightly as you increase sweep.
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 8:10pm
Are you boys quite sure you aren't passing on some very class specific info as general advice? I have a hard time imagining a contender rig being powered up by shortening the spreaders. I think you're also missing the crucial role of rig tension on a trapeze boat with spreaders, especially a singlehanded one.
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Post Options Post Options   Quote Noah Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 8:12pm
Oops! Missed the class in Jerry's signature. No Jib = no slot. Ignore the ramblings of a trainee muppet!
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 12 at 9:48pm
Doh!

Good job we have you to put us right Jim, otherwise I'd look like a right arse

You are of course right, and the effect on a boat such as a contender, is really dependant on sail shape and, rig tension, and spreader sweep.
If you look at the RS700 fleet, the general consensus early on was for the heavy guys to be running less swept but longer spreaders, however in reality guys at the top of the fleet are using setting that are rather varied. I for one have my sweep set to max. and the deflection set to one hole in from max. I did this simply to get my mainsail to set right upwind without too much fullness in the middle panels. At 84kg I'm at the heavier end of the 700 range (just), so need a bit of mast stiffness, but need a slightly flatter main than the fat boys thus the max. sweep.

In terms of setup I guess the contender is not too different.
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Post Options Post Options   Quote Bender Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 12 at 11:40am
Thanks for all of the input, but maybe I'm still missing the point... Contender wisdom is that masts are set to a "standard" rake and shroud tension is then set  to a setting appropriate to the crew weight, as once the crew's on the wire, the windward shroud tension will be reduced. Spreader rake and lower shrouds can be used to set prebend in the mast. None of that takes account of spreader length.
Now, let's assume I've setup my mast as above and the Loos gauge says 30 (whatever that number means!). I now decide to lengthen the spreaders. If I do nothing else, the shrouds now have further to travel and are stretched, the Loos might now indicate 31, and I will have changed the mast rake slightly as well. But I know that I need 30 set, so I tweak the shroud plates back to 30 and make sure the mast rake is back where it was. As the shroud tension is unchanged, the only effect that I can see happening is that the compression load on the spreaders will be increased due to (and only due to) the change in angle of the shrouds where they meet the spreaders. A quick calculation - 0.4m spreader 3 metres below the hounds if increased in length by 2 cm will increase the spreader compression from 13.2% of shroud tension to 13.9%. Am I really going to notice that?
Or maybe it's all a black art and physics has very little to do with it! Confused
Jerry Hone
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 12 at 11:46am
Think you need to recheck your sums... or maybe the theory behind them. Should be all about offsets on the spreaders from neutral. If the spreaders don't offset the shrouds at all they will have no compression on the mast at all. Usually the offset is only a few cm anyway, so in increase of 2cm must make a significant difference...
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