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H2 View Drop Down
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    Posted: 23 Nov 17 at 4:43pm
Please do not roast me - I am going to fess up and admit that I am not 100% sure how spreaders work!

So my mast on the H2 is one of those lovely carbon super spar things, we have an adjustable forestay but no lowers. I picked up an idea that if you are at the top end of the weight range you might want to make the spreaders longer and angle them more forward (less sweep). Can I ask why?


Edited by H2 - 23 Nov 17 at 4:45pm
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JimC View Drop Down
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Spreaders use the tension in the shrouds to push the mast about. So if the spreaders are deflecting the shrouds aft then the mast is pushed forward, and if the spreaders are deflecting the shroud forwards then the mast is pulled back. So effectively the further forward the spreaders, the straighter the mast is and the stiffer it behaves. With sideways deflection its much the same principle. However the difference is in static trim the spreaders are pushing against each other, tending to lock the mast upright. When the crew weight comes on it tends (except for trapeze boats) to increase the tension in the windward shroud and decrease in the leeward, and tries to push the mid mast to leeward.

When you get to multispreader ris with checks and things there are all sorts of complications.

Does that make sense? I could maybe do sketches later if it helps.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 5:17pm
The mast is held up in a triangle of fore-stay and shrouds. Where the mast is held at the hounds is determined by the relative lengths of the shrouds and forestay (rake and rig tension). 

Where the mast sit further down within the pyramid is determined by the spreader rake. They push the mast about relative to the shrouds (and the hounds and the mast step).   

If you rake the shrouds forward, it pulls the middle of the mast back (relative to hounds above and mast step below). This straightens the mast. Straightening the mast pushes fullness in to the sail. Fullness means more power, up to a point where the depth is too much and you stall / can't point / close the slot. 

If you rake the spreaders back, if pushes the mast forward in the pyramid and relative to the hounds and step. This stretches out and flattens the sail. 

Spreader length or 'poke' I find harder to define. To me, the wider the spreader the more stiff the mast feels as you're widening the base of the triangle at the spreaders and making the mast more stable. So for a given rig tension it will stop the rig bending off as much under load. 

If you're light weight, you may want to rake back the spreaders to de-power and increase pointing / reduce drag. Spreaders width I think is more of a personal preference on how you want the rig to react to load, but typically in gusty places or lighter sailors it's better to go narrow to allow the mast to bend off when a gust hits. 

It's then weighing up downwind where you'll want straighter stiffer mast to upwind where you're looking to minimise drag and not stall in gusts.  

The other important thing to mention is rake. Raking the mast back will create more mast bend assuming you keep everything else the same. I find it better to set spreaders for what I generally think is good, then use rake to depower the rig before a race (but in the 200 we have to go in to iron to adjust rake on the water). Rake is usually far easier to adjust on the day and I feel has a larger impact. But that depends how easy all these things are to adjust on your boat though. 


Edited by mozzy - 23 Nov 17 at 5:26pm
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Posts: 750 		Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 5:28pm
OK so if as a general rule I want more power from the rig then its logical to make the spreaders longer AND move the tips toward the front of the boat so that the mast has less bend. Lets do that first then come to mast rake! Sound right?

Edited by H2 - 23 Nov 17 at 5:28pm
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JimC View Drop Down
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Posts: 6661 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 5:33pm
Yeah, roughly speaking. The big caveat is that you lose gust response if the spreaders are pulling the mast back, and you can end up with power you can't use. especially if sailing in gusty unpredictable conditions.
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Posts: 750 		Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 5:37pm
Originally posted by JimC

Yeah, roughly speaking. The big caveat is that you lose gust response if the spreaders are pulling the mast back, and you can end up with power you can't use. especially if sailing in gusty unpredictable conditions.

Got it - but I will try not go that far and besides the mast is very good at bending in gusts and depowering already so I would rather trade some downside for more power especially as I am built like an ape and love hiking my nuts off!
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Posts: 440 		Post Options Post Options   Quote ian.r.mcdonald Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 5:47pm
informed and detailed responses to a sensible question. Nice one guys!
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Posts: 81 		Post Options Post Options   Quote Wiclif Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 6:12pm
I’m a bit late here, but when I was learning about spreader effects I drew the mast with stays and a gentle bend (as if it had no spreaders) viewed both from the side and from the stern, so you have two separate drawings.

Then add the spreader in a choice of positions, but not connected to the stays. Then imagine the stay attached to that spreader and you can then see what effect the spreader has.

If the spreader is too long then there will be an “S” bend in the mast in its lateral bending. This is usually considered slow, but I did have this in one boat with this setup and it was fast. My golden rule is, if it’s fast then use it, it doesn’t matter what the text books say. I am not necessarily recommending this setup though!

Rig tension is another variable. Most people will set the rigging up ashore to a known rig tension. Whenever you start bending the mast to what your setup would be in a blow it is quite possible to halve the rig tension on the shrouds.
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Posts: 1345 		Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 6:53pm
I sailed H2 101 a couple of years back in a lot of wind and found the rig to be quite brutally on/off but I am pretty sure that this could have been improved by letting the forestay off and making the mast a bit more dynamic.

I suspect that I would concentrate on keeping the lower mast pretty straight athwartships by lengthening the spreaders, since it is not like a two sail boat where you are trying to open the slot In the gusts.  That way you could concentrate on calibrating the fore and aft bend for a given wind strength.  

One thing that I did not quite get is the absence of any control of the lower bend, but I guess that all H2s have the same set up.

If I had one I am sure that I would get great reward from buddy tuning, to help with set up and calibration.

It might also be worth talking to HD to establish how they envisaged the mast/spreader/forestay setup.
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 17 at 7:06pm
Deck stepped mast and no lowers is a little unusual with a modern rig, clearly the designers had a good reason and I'm all in favour of simplicity though.

Edited by Sam.Spoons - 23 Nov 17 at 7:06pm
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