Composite Mast - foam core

Any ideas on the feasibility of building a composite mast with a solid core to simplify construction? I've been considering building a 35' mast with a divinycell or corecell foam core, and then wrapping this core with several layers of unidirectional carbon fibre.

The mast is for a custom boat of my design and i would rather not use an aluminum extrusion while looking for weight savings aloft. I do not have the means of molding a hollow composite mast and the design does not require internal halyards.

Thoughts?

 

Can be done, but I think making 2 female moulds for the front and backside of the mast is cheaper, and construction more simple.

Female moulds can be MDF, coated or teflon film stuck on.

 

It's makes so much more sense to build a mandrel then a mold for this sort of thing. How did you calculate the wall thickness and diameter?

 

That was actually my follow up question, this is still in a conceptual phase so I am yet to do that. What formula would you recommend for calculating wall thickness / diameter? I was also wondering if having the solid core would help reduce the wall thickness, carbon is expensive.

Herman, I already have several sheets of 1" Corcell, which is partly what led me to think of this. I was also thinking it might quicker to do this that to build molds or a mandrel.

 

It is a challenge to keep things straight. This is already a challenge at 4', let alone 35'

Also you would like to bag the laminate, to increase fiber/volume ratio and reduce weight. This is easier done in a female mould than around a mandrel or core, where the fiber will inevitably be crimped around the mandrel, causing wrinkles.

 

Somewhere I saw the idea of using a line stretched very tight to support a tube (mandrel) around which to build the carbon fiber spar (use a come-a-long to tension). I've been thinking to try foam pipe insulation as the core, perhaps with a temporary plastic pipe inside during the construction process. There are carbon fiber and carbon/kevlar sleeves available in a variety of weights. I was thinking of using one of these. My project is a small sail-row-cruising boat. Mast will be about 18 feet tall, though probably in shorter sections, one embeded in the hull, 2 above, fitting together like a kayak paddle.

 

That's a very interesting idea how would you remove the mast from the foam pipe? or would it remain in the mast?

 

You can attempt to stretch a line or cable but it will always have some level of sag in it.

 

Here is a link to a site where the guy home builds a carbon graphite mast. IT is worth a look, seems simple enough. For a large one I would get some help with the design and stress analysis unless you can do it yourself.

http://www.gust.ax/gallery/mast/

 

Plywood is about the worst material you can employ for a mast. It has 1/3 of the fibers running in the wrong direct, which is just adding weight. The whole point of a carbon spar is light weight. What you gain in weight savings with the carbon is shot to hell with the plywood.

If I were to build a foam cored spar, I'd use a pre-woven inner carbon sleeve if possable. I'd make a steel tube mandrel, around which the carbon sleeve would be placed. With this curing, I'd apply the foam, wrapping it around probably in spiral. Lastly would be the outer skin of carbon and/or uni-dia again probably spiraled onto the foam. If I could work fast enough I'd do this all in one shot and bag it.

This would produce a very light, quite strong (depending on laminate schedule and diameter of course) spar. The center of the spar would be free to run halyards, lifts and lighting wires too, once removed from the mandrel (an air chuck and some pulling).

About the mandrel, on a 35' spar it has to be metal. Assuming a free standing spar, the diameter will be such that a pretty damn big hunk of custom welded tubing will serve as the mandrel, even if it's a stayed rig the mandrel will be no small piece of steel. Regardless of what you do, there will be some sag associated with this length of material, so it has to be built on a bench, with a tray that also conforms to the shape of the spar (just to make things easy). Welcome to the wonderful world of composite spar making.

 

There were several masts built by Geoff Cruse in Brisbane using Divinicel and carbon using hand layup in a half female mould split front to back.


While they worked very well I was not convinced that there were any weight savings over a traditional alloy section, though of course the alloy rig did not have that sexy foil shape or pre-bend

These rigs and their variants went on Crowther Supershockwave 40's and Southern Ocean 50's (2 big cats in this pic)


http://www.rawnerveracing.com/index.php

Supershockwave 40 below

 

I once had the same opinion as you Sabahcat, but now have come about on the material. It's significantly lighter in a stayed arrangement and in a free standing environment, it doesn't have a rival.

I've been known to have long discussions about the viability of such an expensive material, but I'm slowly liking on it. The cost can be justified if you look at the big picture and all the rigging you can save in a free standing rig and if a stayed rig, the shapes and weight savings, still make one look hard. The cost is still an issue, but if we ever get out of these damn wars, the prices will drop as free market development has to kick in to keep the industry alive. There's nothing better then not having billion dollar contract locks in place, for 100 units of what ever military spec to keep your production facility honest.

If your budget can tolerate the cost, then designing up a free standing stick will offer long term rewards, particularly if racing.

 

First of all, you may read my article on the state of the art of free-standing masts, which describes some manufacturing methods:

http://sponbergyachtdesign.com/StateoftheArt.htm

Some of the material in there will apply to stayed rig manufacturing as well as free-standing.

You cannot design and engineer the diameter and wall thickness of a mast by any one or two simple formulas. You have to design to the loads at hand, depending on the type of mast that you are designing, and you have to use a whole engineering process to determine the wall thicknesses all along the mast. Regardless of mast type, diameter and wall thickness will vary all along the mast. Then you have to draw up or develop the laminate schedule, the ply-by-ply lay-up of the mast. There are certain techniques to use and procedures to follow. You have to get the mix of fibers correct for the design, that is, the ratio of unidrectional fibers to off-axis fibers, and the orientations of those off-axis fibers, whether they are 90 deg or +/-45 deg, and where they reside in the laminate stack. The mix and locations of all the layers are extremely important. Your laminate mechanical properties (strength and modulus) will vary greatly depending on how you mix the fibers. So this is not a simple process.

Building a foam core with something like Divinycell is actually a very heavy way to make a mast. If you calculate the volume and weight of all that foam, you will see that foam adds a lot of weight. That foam does not have to be there. Better that you go for a hollow section. I have used a technique wherein I build a very lightweight wood form with thin plywood and veneer or very thin aircraft ply, and then lay up the mast over that, keeping the wood in the mast. This, too, is a relatively heavy way to build a mast, but it makes it fairly easy to actually lay it up. You can do it all on a straight, flat, and level table.

I have found that using carbon sleeves is not very good. The sleeves are expensive, and they snag a lot as you try to slip then onto the mandrel. It is easy to ruin the laminate just trying to get them on. Better that you stick with tapes and cloths.

That's my two-cents worth. I hope that helps.

Eric

 

I plan to leave the foam in the mast. For the small boat and mast I'm planning the weight will not be such a big issue, I think. And the foam will provide buoyancy even if the mast could otherwise fill with water - a possible help with self-righting.

 

Reading Eric's post really brings one thing to mind for me:

If you already have the complete rig plan for a boat as part of a design you bought, is there a simple translation that can be done to go from say aluminum to carbon fiber?

I mean, the engineering numbers for aluminum are known. If I know my thickness and my taper and whatever else... couldn't I then take those numbers and translate them to the same physical characteristics, but in carbon?

Just a question... a thought. I'll personally probably stick with aluminum so I have a mast left after a lightning strike.