Spreader deflection/mast stiffness
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Technique
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URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=9918
Printed Date: 27 Jun 25 at 6:38pm
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Topic: Spreader deflection/mast stiffness
Posted By: Bender
Subject: Spreader deflection/mast stiffness
Date Posted: 09 Oct 12 at 10:58pm
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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!  Any views? ------------- Jerry Hone RS Aero 7 - 2965 |
Replies:
Posted By: giraffe
Date Posted: 10 Oct 12 at 7:43am
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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. |
Posted By: Oatsandbeans
Date 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). |
Posted By: Noah
Date Posted: 10 Oct 12 at 11:41am
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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.
------------- Nick D-Zero 316 |
Posted By: craiggo
Date 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. |
Posted By: JimC
Date 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. |
Posted By: Noah
Date Posted: 10 Oct 12 at 8:12pm
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Oops! Missed the class in Jerry's signature. No Jib = no slot. Ignore the ramblings of a trainee muppet!
------------- Nick D-Zero 316 |
Posted By: craiggo
Date Posted: 10 Oct 12 at 9:48pm
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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. |
Posted By: Bender
Date Posted: 11 Oct 12 at 11:40am
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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!  ------------- Jerry Hone RS Aero 7 - 2965 |
Posted By: JimC
Date 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... |
Posted By: sargesail
Date Posted: 15 Oct 12 at 11:10pm
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I don't think it's got much to do with compression: rather by increasing the length of the spreader you increase the angle between between mast and shroud at the point where they meet. This therefore means that there is more resistance to the side force on the mast above the spreaders and therefore less sideways bend in the upper section of the mast. For many boats this has a significant effect on available power but also "tunes out" some of the automatic gust response. If I sail someone elses 2K I find I need to play the main less but lack the power. As for the spreader lengths and tensions: if you must have the same you can can achieve the effect by shortening the halliard or forestay rather than adjusting rake by moving the position of the shrouds in the shroud adjusters. Hope that simplifies things?
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Posted By: Oatsandbeans
Date Posted: 16 Oct 12 at 9:41am
| I don't really go with that theory. Try looking at it this way. If you have a mast with shrouds but no spreaders as the wind loads up the sail, the topmast will bend to leeward. When this happens the mast between the hounds and deck level will bend to windward, with the mast forming a smooth arc. If you now put a set of spreaders on they restrict the movement of the mast under the hounds to move to windward. This has the effect of reducing the movement of the tip of the mast to leeward. As you increase the length of the spreader it pushes more on the mast thus increasing the stiffening effect on the upper mast section. If this is overdone you can get an S- shape develop in the mast sideways, which normally means that you have overdone it on spreader length. (Note Just because Adam Bowers, allegedly, once said that " increasing spreader length doesn't stiffen up the mast sideways" doesn't make it right!) |
Posted By: JimC
Date Posted: 16 Oct 12 at 10:26am
I don't believe he said that at all. Going back to the original post where he was mentioned I suspect what he meant was that in one particular class in one particular set of circumstances lengthening spreaders beyond a certain point isn't a good idea for heavier crews because of the effect on the jib slot. |
Posted By: Oatsandbeans
Date Posted: 16 Oct 12 at 10:55am
| That was my original comment on not overdoing it on the the spreader length or you will get the mast moving to leeward at the spreaders which is not fast. But generally the relationship between spreader length and sideways stiffness of the rig is well established. |
Posted By: sargesail
Date Posted: 17 Oct 12 at 11:01pm
Whatever - the net effect is the same. Lengthening the spreader increases the force available for the same rig tension to support the mast in a sideways direction, and this is most keenly felt in the section above the hounds where the relative lack of support allows bend to produce gust response by opening the leech. I do agree that if the mast were displaced to leeward in its mid section this might produce an unwelcome narrowing of the slot.
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