Splicing 2 Carbon Masts together

[Jim Cate]

Aluminum masts are usually joined at the spreaders, or above. I don't know why that is, as I'd imagine the bottom of the mast would have less need to flex. Perhaps it's cosmetic.

I think the reason many aluminium masts have a join in them is to get the lengths of individual bits short enough to meet shipping requirements, and this means the join is near the middle in many cases.

Jim

[Sparx]

Great discussion.

GRIT, I like your idea of using a section of mast to make an inner plug. If the mast sections are not tapered I would do this.

Thickness of the inner plug can be calculated using the same approach. Recognize the stiffness of a hollow cylinder varies with the fourth power of diameter. So an inner plug will have to be very thick to be equal in stiffness to the original section. This is why I recommend the outer over sleeve. It is can deliver the strength with fewer layers.

Regarding the track, if the track is internal to the mast sections then the approach is laminate up to but not over the track. I don't think a gap in the external over sleeve would be a problem as long as the over sleeve layers are well adhered. External track would of course be better. The inner plug also helps here.

I suggested a 6 foot over sleeve as a way of spreading the load over a large area, minimizing the opportunity for stress risers causing a problem. I did not use any calculation to arrive at 6 feet.

I don't see a problem scarfing the mast sections as long as the layers that fill the scarf are well adhered and at least as strong as the material removed. The over sleeve can provide some atonement for minor defects here.

Think carefully about infusion over a solid substrate, unless you are confident in being able to get every layer resin saturated. Carbon does not wet out as easily as fibreglass. This is going to take careful planning. When infusing foam core we use foam that has holes in a grid or kerf lines to make sure resin gets to the innermost layers. It might be worth building a mock up section to confirm good results.


GRIT's point about wrinkles is valid. That uni carbon just loves to move around. My suggestion of an outer layer of biaxial is for aesthetics. The biaxial can be pulled tight to a smooth finish. I really like winding peel ply tightly around the build up to pull everything together nicely.

[GRIT]

Jim. That's my understanding too. I just wondered if there's a reason the shorter piece is always on the top part of the mast (assuming unequal lengths).

Sparx. I misread your original post, and thought you were advocating an outer sleeve of biax, without uni. My apologies. Though the thickness, due to track issues, still make me question the idea of an outer sleeve.


Aluminum masts are simply sleeved with a similar thickness insert. Could an insert be built, of the same or slightly thicker carbon, then just have a small 20:1 scarf on the outside? That's only 100mm, which is pretty simple to do, and could be all biax, since the strength will come from the insert.

Cheers, and thanks for your input.

Paul.

[Chotu]

Brilliant discussion!
.
Thank you all.

I will definitely practice my carbon hand layups before attempting this. There are still a few things I can make and I'll just make them out of carbon. Chainplates is one I still need to do, but I'll start with some stairs or covers for integral tanks. Maybe a new countertop.

[GRIT]

Chris has a point too Chotu. Do you know the mast moments?

Paul.

[GRIT]

You could certainly infuse the inserts. That would be easy to avoid wrinkles, and you'd be nearly completely void free.

Paul.

[Chotu]

Originally Posted by chris95040

Wouldn’t two smaller masts likely be of insufficient dimensions in cross-section for use as lengths of a longer mast?

Missed this one. Sorry.

I've got a lot of things to figure out here. Whew!

That's a very good question. We're looking at taking a 56ft mast and giving it an extra 9 feet from an identical mast.

Possibly the dream I have of using a tabernacle to make life easier with this boat could allow me to just use the 56ft mast or at least only do a smaller little extension.

I need some data on these masts to have a more productive conversation.

Grit:. Did you just volunteer to help?? Ha ha ha.

[Chotu]

Originally Posted by GRIT

You could certainly infuse the inserts. That would be easy to avoid wrinkles, and you'd be nearly completely void free.

Paul.

This is where my head was going.

Proper epoxy to fiber ratio, zero voids.

Seems like infusion is the more fail safe way to go depending if we are talking plug or sleeve.

I really need to get the mast dimensions though.

[GRIT]

Yes, I did.

Though my time will be limited, as you know.

Cheers.
Paul.

[chris95040]

Originally Posted by Chotu

I need some data on these masts to have a more productive conversation.

At least with an aluminum mast, the design is pretty straightforward: You just need to identify the required stiffness fore/aft and side to side (Iyy and Ixx). This number is based on details about your standing rigging placement, spreader placement, your sailplan, and the righting moment of your boat. A competent rigger can do it in a matter of hours.

From there you shop for a section that meets that requirement.

I’m guessing carbon fiber adds more to that process but I think your step #1 is to work with a rigging expert to identify how strong this mast needs to be. Surprisingly this is as much a function of your boat (righting moment, chainplate location, etc) as it is a function of mast height.

Maybe carbon is so stiff the design process is completely different... With aluminum, by the time you are stiff enough for the unsupported spans to not buckle you are typically way overbuilt for the straight compressive load. I imagine carbon might be a different story!

[Boatguy30]

The real issue here is the specified moment of inertia for the boat vs. the moments of those 55' mast. An extreme example would be making a 65' mast with a bunch of used laser mast. Could easily be done but would never be strong enough regardless of how strong rhe splices.

I've worked at a major carbon spar builder and been a bit surprised at how crudely they were spliced together. These were very large sailing yacht masts and no doubt had much excess material anyway. So overall, it's no biggie IF you're starting with adequate sections which I somehow doubt you are.

[JDanielBryant]

I have sectioned carbon windsurfer masts, which undergo tremendous strains. The solution used here is an insert. For a round, tapered mast this is simply an aluminum extrusion (most likely 6061-T6) that is bonded to one end with the upper section sliding over it. For a more complicated cross-section, an extrusion is out of the question as it would require a custom die to extrude it. I would suggest building one up an insert on top of an undersized foam core (hand shaped maybe 1/2" smaller in cross-section and say 24" long). Lay it up with either carbon or glass cloth and epoxy to build up to the inner diameter of the mast junction. Sand/grind it down until you get a tight fit and then affix it either permanently with Dow 5200 our use a softer adhesive if you think you you want to disassemble it someday.

[chris95040]

Originally Posted by Boatguy30

An extreme example would be making a 65' mast with a bunch of used laser mast. Could easily be done but would never be strong enough regardless of how strong rhe splices.

Exactly.

[stormalong]

Originally Posted by JDanielBryant

I have sectioned carbon windsurfer masts, which undergo tremendous strains. The solution used here is an insert. For a round, tapered mast this is simply an aluminum extrusion (most likely 6061-T6) that is bonded to one end with the upper section sliding over it. For a more complicated cross-section, an extrusion is out of the question as it would require a custom die to extrude it. I would suggest building one up an insert on top of an undersized foam core (hand shaped maybe 1/2" smaller in cross-section and say 24" long). Lay it up with either carbon or glass cloth and epoxy to build up to the inner diameter of the mast junction. Sand/grind it down until you get a tight fit and then affix it either permanently with Dow 5200 our use a softer adhesive if you think you you want to disassemble it someday.

I hope you thoroughly rinse that mast after use. You will get electrolysis between the carbon and aluminum. Carbon is electrically conductive. Not a good idea for a large mast.

[BrianFahey]

I am NOT an expert in any of these methods; only a practical guy that enjoys infusing a little common sense into problems. So; this possibility could not even apply. I do agree that compression is a very important load to consider and I believe the side loads closer to the bottom of the mast to be less critical (even though the stays transmit that load to the base- it's probably already thicker at the base and would more tolerate the load. Consider a butt joint. Consider spacing that joint enough so that a flange is formed by the following idea, wetted carbon fiber pushed out from the inside and pushed in from the outside giving the joint itself a 100% 'seat'. Consider that; to control the density and the evenness of the 6' inner 'splice' section, the wetted, vertically aligned carbon fiber be 'pressed against an inflated bladder. Maybe do it dry and allow a vacuum to draw in the resin/epoxy before inflating the bladder. To prevent the pressure from splitting the mast ends put the outer patch carbon fiber on with a jubilee patch of sheet metal held fast with a slew of those huge screw clamps (giant radiator hose clamps). The pressure (via the hose running up the bottom to the bladder) would be fighting the band clamps holding the sheet metal, holding the horizontally aligned and wetted fibers, and the finish on both inside and outside sides would be smooth. If you wanted a 40:1 taper build it and clamp it in-place under the sheet metal form and just remove it later. The sheet metal would overlap, and so it would show a seam that would only be the height of the gage of the metal. It could be baked or ultraviolet-assisted if the bladder was made to be clear and have a center cavity or make it multi cylinders made of clear bladders that are all pressurized. As for the tracks- remove them entirely and re-install them with a spacer the same thickness as the Jubilee patch. Unconventional, but I'm curious to see what REAL engineers think. Good luck. SV Ellipsis beached and destroyed at Elbow Cay in the Bahamas [as in; I'll live my dream through other people now ;-( ]