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  #1  
Old 02-21-2017, 09:32 AM
Maciek188 Maciek188 is offline
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Replacing mast spreader and diamond wires by second set of short stays on 33' cat

Looking at modern racing multihulls it's easy to notice luck of spreaders end diamond wires on theirs rotating masts. Instead is employed second pair of stays, attached in the middle of the mast. Would like to do the same on my 33' light, racing catamaran, like Hobie 17 just twice the size in any direction, with 48' carbon fiber rotating mast. Pair of additional stays would be much lighter and less wind resistant then mast spreader and set of diamond wires. My question is would this be enough to keep mast from bending? Did anyone tried this before?
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Old 02-21-2017, 11:28 AM
John Perry John Perry is offline
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Good idea, but those lower shrouds are generally combined with a 'baby stay' forward of the mast, would that be a problem on your boat?
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Old 02-21-2017, 01:45 PM
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Richard Woods Richard Woods is offline
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jib sheets?
possible inversion unless you fit a baby stay as John says

Check with your rigger. You are only really saving the spreader weight, offset by extra chainplates/reinforcing

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Old 02-21-2017, 02:41 PM
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Corley Corley is offline
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You might need to reinforce your chainplates but if you follow the same system as the big multihulls they terminate in the same position. It does theoretically at least remove some of your ability to tune mast bend with spreader tension. That may be a non issue though in your application.
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Old 02-21-2017, 04:33 PM
Stumble Stumble is offline
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You would probably need to have a mast designer just work up a whole new mast. I doubt that a conversion would be worth the trouble. Particularly if the rig is carbon, it isn't all that good idea to just start cutting holes in the laminate, and if it isn't carbon then not only does the switch cut some weight, the localized stiffness opens up a lot of additional design freedom if built from scratch.
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Old 02-21-2017, 06:29 PM
Maciek188 Maciek188 is offline
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Baby stay is out of question for couple reasons. It will not work with self tacking jib, which works very well by itself and I intend to keep it that way. My mast (which I don't plan to replace) is very light, straight, carbon fiber wing mast 4" x 12.5" x 48' ,very stiff front to back and require only to add stiffness side to side. Big racing multihulls don't have baby stays probably for the same reason. I don't plan to drill any holes in the mast, just to use existing attachment, originally for spreader, to attach second set of shrouds to the amas. They will provide side to side stiffness instead spreader and diamond wires. Above change definitely will improve boat performance provided by loss of weight and air drag. Posting this post with hope that someone already made this kind of changes to his boat and would like to share the results.
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Old 02-21-2017, 07:46 PM
catsketcher catsketcher is offline
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It may be a little more complicated than you think.

I like my non spreader rig on my cruising cat. The lowers are not as tight as diamonds on equivalent boats AND it means I get an inner forestay which is a boon. I get to put a storm jib on this stay in heavy winds.

But on my little cats I use diamonds and it is mostly to do with the platform and prebend. On the spreaderless rig the inner forestay puts prebend in the mast. Without an inner forestay your mast won't get this prebend.

This means that the mast may invert. I would guess that the racing rigs are designed from the start to ensure they do not invert but your rig uses the sweep back of the diamonds to stop inversion. Inversion can easily lead to mast breakage.

On top of this is the platform. A cruising cat like mine is very stiff and the mast stays in column even in a big wind. On a flexible cat like yours, riggers use diamonds because the twisting of the platform is not a good base to have differently angled stays originate from. With diamonds, your mast will stay in column regardless of platform twisting.

So I would keep the rig as it is.

cheers

Phil
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Old 02-21-2017, 10:41 PM
Stumble Stumble is offline
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How are you going to induce prebend without diamonds? Short of locking your mast in place shrouds won't do it, because as soon as the mast rotates the prebend tension changes.

Secondly diamonds provide a lot of fore-aft stiffness. Or they do if you have swept back spreaders. This would be impossible to replace with just shrouds.

I am curious what rotating masts you are looking at without diamonds, because all of the performance boats I can think of use them (A-Cat, Extreme 40, MC 32).
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Old 02-22-2017, 12:19 AM
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Quote:
Originally Posted by Stumble View Post
How are you going to induce prebend without diamonds? Short of locking your mast in place shrouds won't do it, because as soon as the mast rotates the prebend tension changes.

Secondly diamonds provide a lot of fore-aft stiffness. Or they do if you have swept back spreaders. This would be impossible to replace with just shrouds.

I am curious what rotating masts you are looking at without diamonds, because all of the performance boats I can think of use them (A-Cat, Extreme 40, MC 32).
It's pretty common on really large multihulls like MOD70's and the like but then the rotating wingmasts are built with an optimized shape to start with and are very stiff.

eg. https://trienthusiasts.files.wordpre...7590739682.png
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Old 02-22-2017, 02:44 PM
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Richard Woods Richard Woods is offline
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I agree with Phil (as always) Your mast has a very narrow chord, usually that makes it tricky to tune and it can twist. Of course as it is carbon you might have very thick walls where necessary, but the aerodynamic challenges won't change (except the carbon mast has a different flex)

Richard Woods
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  #11  
Old 02-22-2017, 02:51 PM
Maciek188 Maciek188 is offline
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I don't plan to induce any prebend on my mast. This is very stiff wing mast which does not bend. It adds 50 square feet to main sail area and allows me to go in and out of the marina instead using engine. Whole structure of the catamaran is build out of carbon fiber only and is very stiff from the beginning. What do you mean by "the rotating wing masts are build with an optimized shape to start with and are very stiff." ? My mast is not round or square piece of stick either. It is a wing mast providing a lot of lift.
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