Mast building

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skenn_ie

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CB's demise explains why I couldn't resurrect a thread therein.
Can anyone advise me on calculating dimensions for a BIG (100ft keel to head), self supporting timber mast ?
I am thinking thin wall, hollow, Douglas Fir, with inlaid glass/epoxy unidirectional roving. I believe that glass/epoxy has 4x elastic stiffness and over 10x yield strength of Douglas Fir. The mast would be one of 3 for my little ship. I have ideas for a unique (as far as I know) sail too, but that will be tested at about 5% scale before I spend loads of money that I don't have ! I also have a habit of making crazy ideas work.

Thanks in advance
Shane
 
Where to begin? It rather depends on how much you know.

Essentially it is a tapered cantilever problem. You have to determine your maximum loads: sail, self weight, dynamic; factor of safety; how you are going to support it. Work out your moduli .... and then determine the composite construction.

100' above keel is huge. How big is this 'little ship'? It's hardly a project for someone that doesn't have a few bob.
 
It is a massive problem and no one on an internet forum is going to do the engineering for you.

If you really are serious, then have a look on Eric Sponberg's website. He probably knows about as much as anyone on wooden free standing wooden masts.

If you are really, really, really interested, pay him some money and he will answer all your questions.


However if you are just 'thinking' do this experiment on mixing materials of different moduli of elasticity; take three or four same size elastic bands, then make a knotted loop of thin cotton the same size as the unstretched elastic bands. Then use all the elastic bands and cotton loop together to suspend a carrier bag (or the like) from a single hook. Progressively add weight to the bag. Just watch how much (if any!) load is carried by the elastic whilst the cotton is still intact. Watch the shock loading on the elastic bands when the cotton eventually breaks. Engineering highly stressed mixed material cantilever structures aren't going to be done with a 1:20 scale model.
 
Try posting the question on Boat Design.Net.My suggestion would be to try the Multihull section where you will find details on plywood/composite masts.But,I am 'biased'.
Good Luck.
Cheers
 
I would think that if you were going to the trouble of fibre glass on the inside of a hollow wooden mast that a fully carbon fibre mast would be a far better idea.
There are people who make these (in stayed form) in exchange for large amounts of cash and they may be able to design a self supporting mast. good luck olewill
 
I would think that if you were going to the trouble of fibre glass on the inside of a hollow wooden mast that a fully carbon fibre mast would be a far better idea.
There are people who make these (in stayed form) in exchange for large amounts of cash and they may be able to design a self supporting mast. good luck olewill
 
...... inside is not where I would put structural fibre either .... it wouldn't help the less tensile capable douglas fir.

It is an interesting subject for sure ...... and my curiosity as to the nature of the project is aroused, but as others have indicated it is a serious subject with some serious consequences if you get the calcs wrong. Efficient composite structural design is one of the more complicated areas and most definitely an area for the professionals with indemnity insurance.

Tim Bennet is right when he says the 5% model would not achieve much, and certainly should not be relied on as a indication of the capabilities of the full size structure. It would however be interesting to test your basic understanding of the analysis of the structure. In particular I think you would find it difficult to scale the effects of the glass/resin component.
 
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