Okay so i am currently designing a 98" model of a supermaxi. The mast of this beast is going to be 145" long/high, and hopefully made of carbon. None of the suppliers have a carbon mast this long (for obvious reasons), so i am wondering, if i splice a few carbon tubes together, will that be strong enough? Or does carbon fiber not like to be cut and glued together, and will it break? I would rather not have a 12 foot mast come down on the boat, if it can be avoided.
There will probably be someone more expert to comment on this, but I’ll take a shot at it. Carbon fiber is probably as good a material as you can get for this purpose, with outstanding strength to weight, and also just plain outstanding strength. It’s a little sensitive to nicks, and can fail easily if you should happen to ding it. Even so, carbon fiber tubing commonly has small holes drilled in it for attachment points. I feel better about drilling such holes when my plan includes using epoxy to refill them, although epoxy does not have the modulus of carbon fiber, so there is still some question about how much support the epoxy would provide. Much will depend on how you design the mast, which includes what diameters and wall thicknesses you use, as well as how you support it. Carbon fiber tubing splices well by epoxying carbon fiber connections whose OD just fits the ID of the tubing it is connecting. Tapered masts are often made by epoxying OD inside ID in that same manner, something like the extendale antennae on most of our transmitters. The epoxy gets a better bite if you dress the areas with fine sandpaper first.
Take a look at sailboard masts if you can use a round one. If you are trying to retain a somewhat scale appearance - consider using some nice (but expensive) straight-grained Sitka Spruce (try Aircraft Spruce parts supplier).
Route a trailing edge groove, then shape to desired profile. Finally cover with a carbon-fiber “sock” (or tube). After cure, use a thin diamond wheel on a Dremel tool or similar to just cut open the carbon over the groove to allow sail to be threaded. Even with a wood core and carbon layer - I would think the use of fuctional and working diamond wires and spreaders might be needed.
If you use the sailboard mast idea, the sail will probably have to be a sleeved sail. If I proceed with my RXY (ROXY) yacht, I think I am going to try the wood method since building new ones are far less expensive that buying a custom carbon layup - and wood masts “have been known” to be used for many years before the birth of carbon - albeit a bit heavier too. Alternatively - but not sure of profile size) a Hobie 14 mast that has been damaged could possibly be cut and used. I have a feeling the wall thickness might be too much.
I agree with dick wood covered cabon would be a better idea.
depending on you wood working skills, you might be able to scale this down.
I’ve made a couple of CF spars for my A-class, and SAILSetc told me what to do… (1) At joins, use an internal joining piece of cf tube, length around 10 x dia of spar, and epoxy it in. (2) If you need holes, reinforce the area by first wrapping some cf tow around the spar, and epoxying in place. Amount of reinforcement approx 1 dia above and below the intended hole. You know to roughen any spar surface where you’ll epoxy, otherwise it won’t key. (3) If the end of a spar, eg the mast heel, is to take a pure compression load, no problem. But if there is any possibility of forces in other directions, the end must be reinforced otherwise the tube will split. Reinforcement is as for 2, wrap some cf tow and epoxy around the end to a height of 2 dia. (4) SAILSetc sell (expensive, though!) grooved cf tubes. My A-class mast is made of two joined lengths using method 1 above, one length 15.4 mm dia, the top length 12.7 mm. The SAILSetc spars come in lengths up to 2400 mm.
I was involved in helping a friend build a plywood/carbon wing mast (32 foot) to use on a NACRA 6.0 Worrell 1000 boat. We built using solid Sitka Spruce, and 3 mm. (1/8" marine plywood). The only difference is that instead of an internal sail track - he bought and added an external aluminum sail track to the trailing edge. Like Dan - one could probably scale this down to size.
The leading edge had the insets cut first, then it was shaped. The ply is glued to leading adge. After cure, it is bent, glued and clamped to the trailing edge. If you elect to use internal shear web - glue to one side panel at same time you glue to leading edge. The angle of the leading edge is what determines the thickness of the mast. Requires a lot of clamps depending on length. Use the PVC tubing clamps idea - inexpensive and can always use more clamps.
Just another idea if wood working tools and experience is your interest. Surprising light weight for it’s profile. You can make up sections about 4 inches in height to try various leading edge angles to get desired wing profile. I suppose balsa could be used, but you would need to add “hard points” for mounting of tangs for diamonds and shrouds.
If you build from shorter ply sections - stagger the joints so they don’t fall across from one another.
We built our in basement - had to get it out via windows. Forgot about getting the mast out of the house after completion.
why don’t you try buyin some carbon fibre tube from another supplier (in other industries) & just cut the sail track in it (as long as you have a angle grinder with a diamond blade on it) or maybe use carbon fibre mast track off a dinghie (i14’s use carbon fibre sail tracks, in aus they are about 80 bucks for 3m or somethin like that iirc). i will either make my own cf mast or buy a cf tube & cut a sail track in it (with a little help from my work mates)
I think if you cut a slot the length of the carbon tube, you will wind up with a tube that has virtually no torsional strength. The full round, or foil shape being solid is what gives the mast (tube) it’s strength. As the mast bends, one side goes into compression while the opposite side tries to lengthen to match the other side. As long as the tube is not split this will resist bending to the point of breaking. On the other hand, if the mast is vertical with a vertical trailing edge slot, as the mast bends to the side, one surface will be in compression and bow outwards, while the other side cannot help support the bend (or stretch) and the mast will fail. This is why on a mast, you need a central shear web support to help reduce mast bend, and you need some kind of trailing edge web to prevent the mast track from opening.
For a real short mast, a slot full length might not fail, but when you get up to 12 feet in length (4 meters approx.) you need something to keep the training edge slot from getting wider when the mast bends.
Simply take a drinking (soda) straw and slice it lengthwise with Xacto blade. Holding each end, begin to bend the plastic straw and watch what happens to the slice that once was very narrow and fairly close together. It will take very little pressure to “kink” the straw. Assume “kink” in plastic straw equals catastrophic failure in a carbon tube.
Back to the drawing board!