Dinghy development: Bendy carbon masts

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Bendy carbon masts

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=12023
Printed Date: 08 Jul 25 at 4:37pm
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Topic: Bendy carbon masts

Posted By: NHRC
Subject: Bendy carbon masts
Date Posted: 31 May 15 at 10:05am

Has anyone found that their ageing carbon mast gets tired and loses its design characteristics, stiffness in the right places while flexing in others.

On my boat, Merlin Rocket 3450 the rig tension is applied to the shrouds, which ineffect pulls on the forestay to give me a tight luff on my jib.

At the moment when rig tension is applied it is compressing the mast and not tensioning the forestay. Is was varified as well by having a coach in a rib following us upwind in 15-20 kts yesterday.

There are other factors that I am considering that could have cause and effect on rectifying the situation. But I am interested first of all in people's thoughts on whether they have noticed softening in the properties of their rigs over time.

Might get my mast ultrasounded to look for issues.

Replies:

Posted By: Blackie
Date Posted: 31 May 15 at 11:03am

I'm pretty sure carbon masts lose some stiffness. It maybe just a very small amount and probably shows itself by the mast being less crisp feeling in it's responsiveness.
But with a Merlin of that age (wooden Chipstow perhaps?) I would think it's more likely to be the boat flexing and allowing rig tension to vary in turn allowing the mast to bend more. ie the shrouds and spreaders, which should be keeping the mast in the shape you want it are allowing the mast to bend as the rig tension drops when you hit a wave or get a gust etc.
Or it maybe as simple as not having enough lowers on when you apply the kicker in windier conditions...

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B14 - 764 Admiral Trailers

Posted By: NHRC
Date Posted: 31 May 15 at 11:30am

The boat is surprisingly stiff, I pulled across the beam from the shroud points to 350kgs without any creaks or changes in shape.

Chepstow mast Ron hall hull.

The lowers are a possibility... More interested in people's reflections and experiences on ageing carbon masts and their longevity.

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www.thenottinghillriggingco.com

Posted By: iiiiitick
Date Posted: 31 May 15 at 12:25pm

I only know of unstayed carbon masts on Byte C2. My friend is sailing regularly with a nine year old mast including winning two championships. He is on his third boat but has retained the original mast. He is happy with the mast characteristics and sees no reason to change.

Posted By: NHRC
Date Posted: 31 May 15 at 12:35pm

Cheers iiiiitick...!

That is good to know.

I don't think that there is any scientific or engineering evidence that carbon tubes should develope a softness like a metal, without there being unseen cracks, de-lamination, splits or voids causing softness.

Please can more people post stories of what they have experienced with carbon spars, masts booms skinny poles and bowsprits regarding them becoming more bendy, losing eir stiff characteristics with age and heavy use...

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www.thenottinghillriggingco.com

Posted By: Oatsandbeans
Date Posted: 31 May 15 at 2:44pm

Yes, the stiffness of a carbon composite will drop as it is fatigue cycled. If you are into this sort of thing here is a link.

http://web.ornl.gov/~webworks/cpr/v823/rpt/106099.pdf

The graph on page 57 shows how the strain ( equivivilant to deflection on a mast) changes if it is cycled at 1/2 its ultimate strength. After 10,000 cycles ithe strain goes from 0.8 to 1.0 % which doesnt sound much but is a 20% increase

Posted By: NHRC
Date Posted: 31 May 15 at 2:53pm

Thanks for the link there O&B I'll have a read later... I have been trying to find some research on fatigue on line but was struggling.

Not sure how scientifically fatigue works on a composite as opposed to a metal.

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www.thenottinghillriggingco.com

Posted By: JimC
Date Posted: 31 May 15 at 3:16pm

Interesting... I haven't studied the paper properly but I note that it says that (para 4.1) in the composite they used "fatigue is unlikely to be a problem when cyclic stresses coincide with the fiber orientation". Of course in a mast nearly all the fibres are oriented the same way and so are all the stresses.

Merlins are intrinsically not the best engineered of boats, it might be worth taking measurements to see if the hull is distorting. The trickiest, because the foredeck is in the way, but possibly most useful is across the hull midway between bow and mast. Between the shrouds is good as well of course.

Posted By: Woodburner
Date Posted: 31 May 15 at 7:54pm

They do vary their curve especially if left under tension, even when new, we had a bunch of brand new hi quality 100% carbon masts at a dealer conference, they were rigged and left under tension for three or four days, when we derigged they were all bent.

We tend to bend them the opposite way, which as advice, isn't much use to dinghyists with bolt rope tracks, so the best bet always ensure the rig tension is off, don't over tension in hot weather, (the event we were at was in Greece mid summer).

There is also a lot of variation in the tolerance of masts from differing manufacturers, and the type of lay up and weather the tube is autoclaved, or not, braided tubes also suffer more than the vertical then cross wound layups. There was a store in Hood river with a mast bend test machine and they used to keep notes, even of the same mast from the same brand there were often quite stark variations in stiffness even though the manufacturer claimed less than 5%.

In those days there were two or three mast builders in town and this pre dated the explosion of carbon tube from the former USSR factories that happened in the mid to late nineties before the Italians geared up.

So anyway enough waffle, the moral of the tale, you pays you're money and you takes your choice, but they need looking after and UV aint their friend either.

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https://www.ease-distribution.com/moses" rel="nofollow - Foil fun

Posted By: NHRC
Date Posted: 31 May 15 at 11:10pm

Originally posted by JimC

Interesting... I haven't studied the paper properly but I note that it says that (para 4.1) in the composite they used "fatigue is unlikely to be a problem when cyclic stresses coincide with the fiber orientation". Of course in a mast nearly all the fibres are oriented the same way and so are all the stresses.

Merlins are intrinsically not the best engineered of boats, it might be worth taking measurements to see if the hull is distorting. The trickiest, because the foredeck is in the way, but possibly most useful is across the hull midway between bow and mast. Between the shrouds is good as well of course.

I seem tobe lucky. If anything my hull is over engineered.

I have tested it with a load cell to far more than the loads it will need to withstand and it is fine.

The issue is either with the carbon tube or the rigging. There are things I know I can try with the rigging, hence asking for people's experience with carbon spars suffering fatigue over time.

Have you ever had any experience with that?

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www.thenottinghillriggingco.com

Posted By: alstorer
Date Posted: 01 Jun 15 at 3:38pm

Epoxy resins can degrade and soften with weather and cyclic loads. Even without weather, carbon composite can most certainly undergo fatigue failure. Graeme is right- paying more (autoclave cure means more expensive resin in most cases) gets you better more reliable performance*

*most of the time. Pretty much. Sort of. For a value of "better".

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Al

Posted By: NHRC
Date Posted: 01 Jun 15 at 9:21pm

No doubt money buys quality...

Thanks for your posts guys...

Just wondering if any skiffies have noticed their spars booms sprits loosing their stiffness with time???

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www.thenottinghillriggingco.com