replacing timber beams with FG

What purpose does thy Kevlar serve?

 

Each fiber has different strengths. Using a combination of 2 or 3 fibers would use the strengths of one to offset the weaknesses of the other(s). Fiberglass still has strengths, we just overlook those because the carbon and kevlar fibers are significantly better in most areas.

Of course the problem would be the cost.

Use epoxy and bond the fibers together. And that is where most of your expense would be.

I would also wrap in the three different directions. 0 degrees, 45 degrees, and 90. With each of the fibers used. This also offsets weaknesses with strengths.

Start with three 2x6's, hollowed some in the middle (reduce weight), epoxied together, and then rounded down to about 5". Wrap up to the right diameter.

 

2 2x6's for the outside of the beam, and 2 2x2's between the 2x6's. One top, one bottom.

You might get by without rounding the inside hole any - just use the square inside?

You would end up with about 1" of wood at the corners of your square hole.

Use KNOT free wood! Do NOT go cheap at all.

 

Unless you fully understand what you are doing mixing reinforcement materials is recipe for disaster. For instance, if you use uniaxial carbon fiber tapes, and you don't use enough, the tapes will take all of the load and will break under the applied load. This occurs because the carbon fiber has a higher modulus and it takes all the load before the other fibers do. You need to design the layup in materials and direction of the fibers to control the stress and make sure the system is sufficiently strong and has robust structural margins.

The use of hollowed out wood is also pretty much a waste of weight. The reason is the modulus of the fiber reinforced outer skin is much higher and the core is doing little more than providing a shear tie between the sides, and if you hollow it out, there is no shear tie so it is doing very little structurally, where a foam core will do the same thing for a lot less weight.

Replacing wood with a composite structure is not just a matter of looking up the material properties and plugging in one piece for another, or wrapping a cloth layer over a piece of wood. Do it right and you can save a good bit of weight, make the system stronger, and have something that won't rot. Do it wrong and you will have something that could snap and cost you your life.

You really need to get some help from an engineer who has composite experience and can help you actually design this structure or you are asking for trouble.

 

16 feet long, 5-1/2 inch diameter Pine poles seem weak for this application.
There are 5 of them, whereas most cats seem to have only 2 or 3 beams.
Generally you would want to overbuild them. The current ones rotted?
Do they need to keep the same shape? Aluminum tubing is commonly used,
maybe easier to crossover the strength of 1/4in thick al tubing to fiberglass?

 

Raf, as soon as you change something you are making an engineering decision, if you don't have the background or experience you are here trying to fill in the gaps(pun intended). The best thing to do is to rebuild as original, the tubes you wish to make are they lashed? bolted? bolted with rubber? all this makes a difference in the way the fastenings interact... sometimes using a tube complicates things. You could approach CST in Kurnell for an idea on engineering & manufacturing costs... they do this for a living, the cost might be surprising either way but their manufacture of composite tubes is pretty slick & could give some guidance.
Best thing is to put it back as designed using good quality materials, that has worked for the vessel until now, call Barrenjoey timbers or Swadlings if near Sydney for pricing on clear Douglas Fir or close to it & scrarf & laminate to dimension with epoxy if required, other cities might have other reliable supply.
Jeff.

 

This vessel seems to want to invoke the spirit of ancient Polynesian war canoes, using hi-tech marine ply.

16 ft Radiata Pine is about as 'Polynesian' as fibreglass, but very weak structurally.

I would get James Wharram to design proper marine ply box beams, as I am sure these planned beams need to be at least clear Oregon or similar. No way would I use Radiata pine, even IF you could get it in 16ft lengths, which you cant.

Depending on your location, there may be some sawmillers who can provide a more structural timber - that you can laminate. Australian or Western Red Cedar would be a possibility. Fibreglass isnt really in keeping with the 'spirit' of the craft, and YellowJ and others have pointed out, the engineering needs specialist input.

To repeat, get back to James, and let him provide an alternative - I am sure he will be happy to help.

 

I don't think you really know what you're talking about.


.

 

Thank you.

Now that we have heard from me, what would thee suggest?

 

Its not a job for glass at all (except for epoxy gluing and covering of course ). A quality timber solution will be cheaper, easier and better performing.

Aluminium would be the next solution to consider.

How many small cats were ever built with glass beams ?

 

It was my way of thinking.
Aluminium and fibreglass seem to have similar Tension/Compression properties and thought that rapping a PVC pipe with 8mm thick fibreglass would give a similar if not stronger beam. Because this type of F.B. beam would be way lighter then a timber one, is possible to make'em a bit bigger in DIA and still be lighter then those in timber.
The flection is the greatest concern here as beams are lashed and free to rotate to some extent so, tork force would be negligible. As for compression, end to end of beam, how much would that be? No more then the weight of the boat for each beam when the boat is falling on his side. Here my guideline is a comparison with FG masts. they take tones. as for compression under the must and under the lashes, of course there would be reinforcing of extra layers.
thank you for your input.

 

I have no valid suggestion, that's why I didn't offer one. On such a vital component on such a large, sea going craft, an informed, engineered opinion seems better suited than a wikiHow type guess.

 

Yes, we call it the connective structure..... it's what keeps a catamaran being a catamaran.
Jeff

 

I thought you might reveal wiki as your source of knowledge.

regards.

 

You can replace the members, but you are best to go back to the designer and ask the designer to update his design.

First, it would be easiest for him. He knows what he intended the members to provide as far as support.

Second, it would be good for all owners of this design in the future.

Third, he may have already thought of this and have a solution on hand.

You can do this on your own, the information about the strengths of the different fibers are easily found on the web, I have studied them and seen the contrasts of each material.

I didn't save those pages, or I would post them for you.

Just remember, the point where you are attaching the mast is your weakest point. All of the other attachment points have space to spread the forces from one member to the other. I.e., from the cross-member to the hull.