Slip Fit Method Carbon Mast

I have been toying with the idea of building a carbon mast using pre-fab oval tubes of the same general dimensions as the original aluminum spar. Now I finally have my own boat to work with, a 1981 Catalina 22, so I can contemplate 'actual' rather than theorectical dimensions.

Obviously this is for for cruising use, not racing and not for resale. This is just an idea at this point and I am not hell bent on doing it, fully understanding that the cost could be greater than just buying a new alumnium mast. So why? Because its an idea and I'd like to at least investigate the possibility and maybe it will work, maybe it won't. Please forgive me if I get some of the terminology wrong.

I searched for other threads on this topic and came up with a few. There were and a few bits of useful info but particularly this link; http://www.aes.net.nz/comment19.html

So for the sanity check here is as much info as I have at this point.
- Mast is 25 ft overall, cross section roughly 2-3/4" x 4", .125 wall (1/8")
- The article link above suggests the composite wall thickness should equal the original aluminum spar. Is that accurate?
- I've looked at several suppliers of Carbon Tube. Oval sections are available in near identical profiles. There are 'stock' sizes and most suppliers will custom make whatever you want but obviously that will come at a greater cost. There are U.S.A. based companies and a bunch overseas... If you have been following the news about the Alibaba.com IPO then you would understand why I would consider and overseas supplier... Its worth a look.
- Sections lengths are most commonly 8 foot, but there are 10 and 12 foots mentioned (again, custom sizes). For the sake of simplicity and shipping lets consider 8 foot sections.
- The method of joining would be slip fit with expoxy. So besides 8 foot sections used to make the mast body a few 12 inch sections (maybe 8 inch) that would be the 'nesting size' inner diameter of the main sections... proab somewhere between .0007 to .010 gap for epoxy.
- The slip fit unions would not be exactly at each 8 foot interval, I would plan them at key strength areas, gooseneck and spreaders comes to mind. Besides, only 3 sections at 8 feet gives me 24 feet, still a foot short of my 25 foot requirement... you get the idea.
- Unless the oval profile of the tube exactly matches the orginal spar the masthead will not fit. Possibly a big deal for some, but I am a well equipped welder and somewhat well equipped metal fabricator so I can work it out. I also have a friend with a 3D printer that owes me a favor so its possible I could have him design and print a plastic masthead so I can create a mold for a new one in composite.
- Surface mount mast track and bus cars...
- The linked article above mentions the orientation of fibers in the layers of lay-up. With pre-fab sections having that specific lay-up probably won't be possible unless you go custom and then it becomes cost prohibitive. So unless a specific lay-up is specified by a particular supplier lets assume a twill (+/- 90) fabric on inner and outer layer with UD layers in the middle... which 'should' be a various axial and hoop plies. Does that sound right?
- When slip fit and epoxied would it still be a good idea to do a layer on the outside? One possibility is to plan the lay-up with thinner than .125 wall, do the slip unions, and then do a continuous outer sleeve.
- My spreaders are simple tubes, probably the easiest part. I can retain the orginal aluminum spreader mounts or make new ones in composite. Same with much of the other hardware. All of the alloy parts will be bedded/isolated properly. That is an issue with the orginal mast, maintenance was not done in the last 4 years resulting in bi-metal corrosion between stainless fasteners and the aluminum spar.

Well, that about covers it... Any comments or suggestions would be greatly appreciated!

 

a few thoughts: I see no points in putting a layer on the outside, it would be difficult to get a good bond on cured parts, if you sand it to rough it up you could be cutting the surface fibers, actually weakening the assembly. It would just add weight and cost and not make it stronger. you should choose the size of the tubes to be strong enough without the extra layers on the outside. The epoxied joints in the slip fit should be more than strong enough if you get a good bond.

I actually like the idea, where did yo find slip fit oval sections carbon tubes? how much are they?

 

By searching for 'Oval Carbon Tube' I came up with several shops that offer a true oval or oval 'D' rectangle profile, but few that publish pricing for that, they offer a custom quotes mostly... I am still looking. There is published prices for 'stock' profiles, almost always round. Here is one example of one U.S.A. supplier offering both round tubes and oval tubes with prices as well as the 'inner ferrule tubing' for slip fit connections;
http://www.carbonfibertubeshop.com/large tubing.html
http://www.carbonfibertubeshop.com/filament wound tubing.html

It might be possible for me to go with a round tube but I'd rather stick with the original spar profile for many reasons. The above link could be helpful to someone considering a slip-fit carbon mast of smaller dimensions for a racng dinghy or similar. Looking at those prices you can see why I am willing to consider an asian supplier as long as the shipping isn't murder.

 

You need a 45 degree layer over uni or protruded to take and bending or twisting loads because all of the strength of uni is along the fibers.

 

Will this be a free standing or stayed version of the Catalina stick?

 

Stayed... Not trying to change the overall design in any way. Thats why I'd like to make sure I match the original mast cross section as close as possible. If I can find the right tube supplier at a reasonable price, and the overall strength is there I would share the info on the Catalina owners forum if they might want to try the same thing. I am also considering synthetic line rather than the wire rope standing rigging, I have already experienced 'Meathooks' when taking the rig down to transport the boat. I know its more expensive but I think if I shop smart it could be reasonable.

 

On second thought, the Un-stayed Rig would be impressive... It would need to be a strong mast obviously and have to be hull stepped rather than deck stepped. Those modifications would require a commitment to that rig permanently, but they are possible. It would probably be a bit much for a Catalina 22 but dang, the lack of stays would make topside much less claustrophobic and mast stepping could be greatly simplified (or complicated considering there are no longer any stays to support manual mast stepping, except the forestay for roller furling).
Interesting concept to ponder...

 

If you're not going to take advantage of a carbon spar, what's the point of a stick that costs 5 times as much?

 

You lost me PAR... So you are saying if I don't go un-stayed on a Carbon spar then I'm not taking full advantage of the material technology?

As far as price goes I do understand that purchasing a new carbon Spar from Hall or Southern Spars or the like would cost 5 times as much (or more) as a new AL spar, but the point of this 'slip-fit' method is to at least investigate the possibility that pre-fab 8 ft tubes could be assembled into a mast that would still exceed performance the characteristics of the original but at lower cost than a continuous spar from a commercial builder. I'm not saying at all that I'm sure it will work, but I'm taking a look. The only thing I can say for sure from a logistics stand-point is that it will be cheaper to ship a small bundle of 8ft tubes across the US than it would be a 25ft single spar. At least that is a start.

 

Simply put, carbon has a few advantages, least of which is it's cost. A replacement DM-6 extrusion is about $750 at full retail, if ordered directly from Dwyer. I can get them for less than $500. I'm not sure what a carbon version of the same section might be, but tubes wouldn't offer the same modulus and bending attributes, as the DW-6 section. Used DW-6 (probably one of the most popular sections) are all over the place, so even more savings, plus no new attachment issues.



Once the carbon stick is up, what do you get for the cost? The boat will have a quicker roll moment, likely on the jerky side, but I keep coming back to justification per cost, on this type of boat. I can see the rational on a Windmill or other boat, where a direct performance improvement could be had, but a Catalina 22?