Reinforcing a windsurfer mast

I am using a windsurfer mast on a foil-stabilised sailing canoe (Hinged Bruce foiler - Amateur Yacht Research Society https://www.ayrs.org/phpbb/viewtopic.php?t=3876&p=4423#p4423). I was advised that a windsurfer mast is not strong enough for the purpose, so bought myself enough tubular carbon sleeving to add three layers of unidirectional (1in to 2.75in UNI Carbon Sleeving (sold by the running ft) (#15016) - Soller Composites, LLC https://sollercompositesllc.com/product/1in-uni-carbon-sleeving/) and one layer of biaxial carbon (2in 3K Carbon Light Sleeves (sold by the running ft) - Soller Composites, LLC https://sollercompositesllc.com/product/2in-3k-carbon-light-sleeves-sold-by-the-running-ft-copy/).

I had assumed I could slide those sleeves onto the mast dry, then wet them out all at once before compressing everything with heat shrink. Now it occurs to me that this might be a bit optimistic. I am not sure I have enough spare carbon for a test lamination. Any advice?

Also, the normal procedure is to wet out everything, put the heat shrink on, then shrink it from the middle to the ends. But if the wetting out of 4.6 m (15'4") of mast takes long enough for the resin to kick off, the heat shrink may not have opportunity to compress the laminate. Assuming I can wet out four layers at once, would it be a realistic option to start at one end, wet out a bit of laminate, shrink the tubing, then go on to the next bit?

If I have to wet out one or two layers at a time, I think I will need some slow hardener, wet out the whole length of one or two layers, slide on the next layer, and then slide on the sleeve.

 

Try UV curing Epoxy. Most Surfboard shops sell it.
That way you can do multiple layers without it curing too soon.

 

Buy the slowest hardener the manufacturer has, or switch the epoxy brand. For example here you can buy a GL approved combination with 210 minutes pot life, that doesn't require post-curing at elevated temperatures. 3.5 hours of working time at 20°C should be enough for your lamination.
Resins | Epoxy-resin | R&G Faserverbundwerkstoffe GmbH - Composite Technology https://r-g.de/en/list/Resins/Epoxy-resin/&suchbegriff_5=above-120-Min&ButtonName=05

To put the sleeves over the mast you need a plastic tube, slightly larger then the mast, ideally full size, but at least one meter long. You can make it yourself out of some stiff PET plastic, flare one end, or you can use a PVC pipe.
The mast gets extensions out of wood, about 10cm, so you can securely fix the sleeve against sliding (ties, staples, etc.)
The sleeve goes onto the tube, bunched up, then you slide the tube over the mast. Tie the sleeve of, then slide the tube along and the sleeve will unroll onto the mast. Apply epoxy, then repeat with the next sleeve, making sure the fibres remain mostly straight. This means you move the tube a few centimeters holding on to the sleeve so that it unrolls under tension, then you squeeze the sleeve onto the previous layer into the wet epoxy, repeat. It's easier if you have three hands, a helper is welcome.

 

Would I need a UV lamp, or could I just leave the mast in (Norwegian) sunlight for some time, turning it every few hours for even exposure?

I am really glad I asked, that is a better idea than anything I thought of. Thanks.

 

Either. The instructions vary with the brand.

 

I am not sure how you calculated the mast is not strong enough. However, is the problem the strength or the stiffness?

 

Strength is not an issue but bending (stiffness) is. It is mainly defined by the EI. EI is defined by the cross sectional area of your mast and the ratio of uni to biax ratio plus the glass to resin ratio.

 

Flexibility of an unstayed windsurfer mast is an important element.
More carbon fibers will certainly make the mast stronger, but also more brittle.

 

I may be tackling exactly the same job in due course - is it the 3 layers of unidirectional first and the biaxial on top?
Also I'm not familiar with 'heat shrink' - at least not at this scale - do you have a link you could share please?

Many thanks

 

Robert’s initial post stated that he was advised that a windsurfer mast was not strong enough for his application, but was there data to back up that advice?
Windsurfer masts come in a wide variety of specs, a stronger one is probably available.
Years ago when windsurfing in big waves I found that more flexibility was preferred over extreme stiffness, not so much for performance reasons as survivability of the rig.
Stiff high carbon sticks were much more likely to break in the inevitable wipeouts.
Hopefully Robert’s rig won’t be subjected to sloshing about in tons of whirling whitewater, but I suspect that his mast may be fine as it is.

 

"Strong" means two types. Tensile/compressive strength due to bending (stiffness) controlled by the elastic modulus and shear (twisting) controlled by the shear modulus.

Uni is the dominant fiber in the flexing and biax is for shear. Biax is always the last laminate on the outside.

It is important that the laminate should have the same or greater modulus (tensile/compressive/shear) as the original laminate otherwise you will just be adding weight with little to no improvement in the overall properties.

 

Strength and stiffness are two different properties. Shear is a type of stress that acts coplanar to the cross section of the body. You are mixing concepts and definitions.

 

No calculations. I was discussing my planned design and was told of a home-built dinghy that used a windsurfer mast and broke it just from bouncing around in a bit of swell. I forgot to ask how much bury that mast had. I have 45 cm for a 460 cm mast. But also, depending on which tack the boat is on, it can have several times the righting moment of a dinghy of similar weight, so strength is the potential issue. Maximal righting moment is about 4000 Nm or about 3000 ftlbs.

I currently use a smallish lug sail, and may replace it by a junk sail, neither of which is very sensitive to the bending characteristics of the mast, so stiffness is not the issue.

That is what I intend to do, but I am not an engineer and have nothing but a hunch to back up that choice. I assume rxcomposites is better informed.

You can find both tubular sleeves and large enough heat shrink here: Tubular Sleeving - Soller Composites, LLC https://sollercompositesllc.com/tubular-sleeving-2/

 

The mere fact that it broke is already an indication that it was built to high modulus (brittle). Carbon fiber perhaps.

Heat shrinking might not work. You need a lot of pressure to squeeze the excess resin out. That is what makes it high modulus. Minimum process is filament winding in tension and excess resin squeegeed out.

Please post a picture of the mast that broke. Preparably with dimensions and showing the profile.

 

Heat shrinking is the most elaborate method accessible to me. I could put a vacuum bag over the heat shrink to get a bit more pressure. I do have access to a vacuum pump. If that doesn't do the job, I should look to buy a different mast instead. I do have room for a slightly larger diameter, so I could see what the dimensions of a Europe dinghy mast are.

I don't have a picture, it was not my boat.