hollow wooden mast w/ addition of structural foam?

[farmerRob]

hi all, long time lurker, first post. I've recently acquired a new to me project, an Allegra 24 hull and deck kit. I've done a good bit of research and reading, (skenes , gougeon bros, riggers apprentice, and many others) all in order to help me determine some best practices and rig decisions and other design and fabrication decisions in order to proceed. I'm planning on building a wooden box mast and I have the basic dimensions worked out for a fir spar . One thing I'm currently looking at however is whether the addition of foam to the hollow box spar would be worth considering from a structural perspective. I have access to some free fomular 250 EPS foam which is pretty sturdy stuff as foams go and my thought was that epoxying the foam inside would stiffen up the mast considerably without adding much weight and help to spread loads. I was also trying to determine if by adding foam I could reduce the thickness of the wood slightly. I've seen in other forums not to add fiberglass to the inside of the mast as it will bend / compress prior to the wood and therefore not lend anything to the structure and perhaps it's the same answer with the foam? Perhaps the answer is in increasing the dimensions of the mast and adding the foam and reducing the skin thickness (i know you can do that with just the wood). Anyways, I'm kind of at a loss as whether to keep looking into this or not and figured I would reach out to the group here. I can't seem to find anyone else using a similar technique. Any thoughts on if this would be worth while or just a waste of time and resources? Here's a pic of what I'm thinking. Thanks!!

 

[rxcomposite]

The core acts in shear as the mast bends and amount of bending is dependent on load applied and the length. To theoretically reduce lengths, bulkheads are placed in intervals or fiberglass uni wound at 0 degrees inside to prevent the walls from collapsing. Fiberglass uni increases hoop strength and prevent the walls from collapsing into each other.

Now to answer you question.

Will adding foam core adds stiffness? Definitely using the principle above.
Will increasing foam core thickness reduce skin thickness? Definitely yes as increasing thickness increases moment of inertia and reduces stress on skin.

Which begs the question. Do you need it? If the mast was designed for its use, adding any of the above will increase weight, reducing any benefits you thought would improve it.

 

[farmerRob]

Thanks rx, I think I realized this but just had to get others opinions and thoughts. It’s definitely not needed for sure. I have a tendency to lean towards overbuilding and I know every design decision has trade offs and is in a sense a compromise. What about in terms of safety factor for a given size mast for negligible increase in weight? Or thoughts on negative implications? Thoughts on whether foam help displace water or would it be ripe to trap water up against the wood. I know hollow wooden spars are straight forward and reliable and maybe it’s best not to mess with that but I can’t help thinking if small additions/changes or new materials can help improve upon older techniques.

 

[rxcomposite]

Safety factor is standard 3X load at yield for composites. It is higher in wood due to inconsistensy of material. With metals, Around 2 (depends) because material manufacture have been consitent and can sometimes be verified by a mill certificate. With stringent testings, designers go for 1.75 to 1.8. So it really depends upon the designer and his expertise.

With some foam, it absorbs water so water absorbency is also a factor. I choose close cell foam but when you cut it, some cells gets opened. It is best to coat it with resin before encapsulating it. Same with hollow wood. You have to treat the inside with paint or resin to resist condensation forming inside. Just good practice by craftsman.

Mast are usually made by craftsman in the trade. With published DIY mast, the safety factor is usually designed high. That is why they complain it is too heavy.

It boils down to good engineering analysis and expertise. If somebody has been doing it for years, better trust him. Otherwise, you have to make one and test it to destruction to prove that your second one is better.

Nothing beats metal in terms of stiffness to weight ratio. Carbon mast maybe but the cost of engineering it may be the price of a car.

 

[rxcomposite]

I can’t help thinking if small additions/changes or new materials can help improve upon older techniques

The best I have seen are Water pressure forming and autoclaved cured carbon epoxy filament wound mast.

To design, I would choose carbon filament but it outweighs the cost VS my confidence. E glass no matter if oven cured has just too much flexibility. And weight.

Wood, tightly wound with biax is an improvement from an engineering point of view. Winding 0 degree continuous filament Uni inside is almost an impossibility.

 

[rxcomposite]

Wood is basically a uni with short longitudinal fibers. The forces on the mast are vertical shear and torsion and this is what it should be,

 

[rxcomposite]

I am keyboard happy this moment.

What gives you the best bang for the buck?

Glue is the weakest material. Use birdsmouth method to increase glue surface area. Tapered mast of course.

Insert 0 degree uni INSIDE at the point where the boom is attached. Cut it into 2 parts with 1" overlap and insert into position. About 2 1/2 to 3X the width of the boom. Use inflatable bladder to force it into the inner wall of the mast. Cure under pressure

Apply a layer of 0/90 glass on the outside, same width as as the uni. Same location.

Apply a light biax all along the length of the mast. Wrap peel ply, perf film, bleeder cloth and bag. Cure under vacuum.

These are the points where design is usually compromised. Overbuilt yes, but can take more load and eliminate weak points.