Upper spreader angle - contributing factors

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EdS
Posts: 188
Joined: Mon Aug 06, 2012 12:06 pm
Boat Number: GBR 1524
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Upper spreader angle - contributing factors

Post by EdS » Tue Mar 19, 2013 9:02 pm

I am afraid I have another stupid question.

How do you determine the correct angle of the upper spreaders and what would influence your decision?

My understanding of the reason for caps and therefore the effect of the top spreaders

- Support the top section fore and aft with the spinnaker up
- provide lateral support to the top of the mast for the mainsail
- If you tension them you must force some bend into the top mast
- Cunningham does this as well.

Thanks in advance
Roaring Forties GBR1524

Dan H
Posts: 71
Joined: Wed May 18, 2011 11:02 am
Boat Number: 1515
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Re: Upper spreader angle - contributing factors

Post by Dan H » Thu Mar 21, 2013 12:49 pm

Ed, pretty broad church here I think.
Conventional wisdom is that lowere spreader lengths and deflections are pretty ballpark in classes where sails are built for spec masts, this seems to be true to a lesser extent in 14s as very rarely are 2 boats totally the same, most good sailmakers will have your boat in for measurement rather than just giving you something on spec. As such you see quite a big range in lower spreaders in 14s.

As for the upper spreaders, you seem to be on the right level. As soon as you have a decent upwind amount of vang and / or cunno on, the leeward shroud will be pretty much slack if not loose, so all the windward one is doing is limiting the tip dropping off, which would then makes the main panel buckle a bit more. In the light airs when the leeward cap still has some tension, then it will be doing a bit of prebending, which may or may not be a good thing depending on your rag. All a function of sweep andgle and to a lesser extent length.
Some guys were playing with having uppers with no sweep - this means that you retain the same amount of tip support regardless of cunno/vang i.e f/a bend. Seems that idea never gained traction - inline upper spreaders also mean no support downwind against the kite. Don't think that is a structural problem, but it makes the main very deep towards the head. Probs fine when punching deep in light to mod VMG but not ideal when trying to make it up to a bouy on a reach.
My feeling is that there are a lot of top spreaders that are longer than they maybe need to be - i.e upwind when compression is an issue they don't see much load, so no real gain for increasing the staying base. Maybe its a downwind thing when they do see a decent amount of load.

Then of course there are a few ships both sides of the pond with no top spreaders - generally have higher hounds and stiffer sections so that the tip fade doesn't "talk" too adversely to the column below. Nice weight and windage saving for a little loss of control.

Dan

EdS
Posts: 188
Joined: Mon Aug 06, 2012 12:06 pm
Boat Number: GBR 1524
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Re: Upper spreader angle - contributing factors

Post by EdS » Mon Apr 08, 2013 8:14 pm

Dan,

So you are suggesting the sail makers suggest a suitable lower sweep angle?
In which case given the large number of fixed spreaders, this must be challenging.

Thanks,

Ed
Roaring Forties GBR1524

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