Don't give Jake such a hard time Wouter, the problem with your spreadsheet is that you could calculate that a chocolate boom can be made as good as a carbon one, Yes it would have a large diameter and small wall, just like your aluminium one would.
Exactly !
So you need to look at each individual case in detail to determine what material to use. That is my main point. It is 180 % opposite of making everything from carbon laminate because it is so "good".
Also my alu boom doesn't have a LARGE diameter, it just has a LARGER diameter. There is a difference here. A chocolate boom would have a diameter nearing 1 mtr, that IS an impractical large diameter.
Clearly the larger diameter of alu is still so small that it is not an argument for not using it as a boom.
When you go on to say "a wallthickness of 0.1 mm is impossible to achieve" I think that also makes your post pretty meaningless.
An alu wall thickness of 0.1 mm (0.0001 mtr) may be impossible to extrude but a wall of 1 mm (0.001 mtr) isn't. Therefor my alu boom example still stands as that one uses a 0.001 mtr = 1 mm wall.
This is high school stuff guys, please don't waste our time by making such rookie mistakes.
In general terms most engineering materials except carbon have the same stiffness/weight ratio.
There are other materials like wood, plastics and even metals like copper that have noticeably different ratio's.
Carbon is the worst material for lazy engineers, because it is not homogeneous and can have different properties in different directions. It is a very hard material to design with. But can be amazing when used correctly.
It is really not hard at all to design with carbon. Because it is so light, you can just pile up more and more matts on eachother (and under different directions) till the bloody things holds up under the loads. Without getting hit back by heavyness or a large volume. That is why it is so favoured by homebuilders. They hardly do any real designing.
It may be a hard material to design WELL with but apparently even aluminium is that for most engineers. This has nothing to do with the material but rather with "laziness"
As far as the efficiency of materials goes, you make a good point about efficiency of materials that are volume constrained, but most of the engineering on a cat is weight constrained.
That is not the full picture. If I'm volume constrained then I can still make an alu stiffness equivalent to carbon, I just can't make it the same overall weight. So we are back again at my initial statement. Carbon is preferable over alu when an engineer is limited BOTH in volume AND weight. If either one is not limited then it is not immediately clear which material is to be prefered. It will then dependent on other considerations like cost, availability, and stability of the component (wall thickness)
The only volume constrained case on a cat I can think of is where you are deigning for least drag,
Correct. When it comes down to alu or carbon laminate the difference is volume are often so small that the increase in drag is negligiable. How much more drag will a 59 mm boom have over a 40 mm boom ?
and then it is only really applicable with situations where the material is purely in tension. Like Dolphin striker support and stays.
Wrong again.
Situations with ONLY pure tension favour carbon a little more as here you can't play with factors like diameters to make both components the same in stiffness AND weight. This because both stiffness and weight are influenced to the same degree by a changing length measure. In bending, torsion or buckling this is not the case and here stiffness changes faster with varying length measures then weight. This disportionality is the reason why it is possible to design a tube with equal stiffness and weight when using different materials. Think about this.
The reason why carbon is not used in stays and dolphinstrikers is because it requires too much volume to get sufficient stiffness and strength and carbon is more sensitive to abuse and degradation and often this is compounded by being more expensive as well.
Stainless steel dolphinstrikers can be 3 mm thin (while being 25 to 40 mm wide). An equivalent carbon strap needs to be at least about 2.5 times as thick or 7.5 mm thick. It is alot easier to smash a 3 mm strap through a wave top then a 7.5 mm thick plate. Additionally when a stainless strap is hit very hard it will bend but maintain the beam in shape. Carbon will probably splinter or fracture and fail, bringing down the mast.
Stays is somewhat different. Stainless steel 1x19 cables have the same stiffness per area as carbon laminate. Dyform stainless steel cables have a better stiffness per area then carbon laminate. Again stainless will survive much more abuse then carbon which is important in stays. But most important of all, 1x19 steel cable costs 10 times as little as an equivalent carbon cable. Really the only downside to stainless steel cables are that they weight a little more. But if that is a problem then make your stays out of dyneema or similar fibres. Such fibres will shame both stainless and carbon but with the drawback of being much less abuse resistant then stainless. This last property is really holding back the use of fibres for stays.
Mind you the use of fibres of trap wires is an old trick by now. For nearly 10 years now a portion of the Dutch cat sailors are hanging of 3 mm dyneema or 2 mm D12 lines, with great succes. Much lighter then either stainless 2 mm 1x19 or carbon and significantly cheaper then both as well !
Currently there seems to be a drive to build cat hulls out of kevlar. It is lighter then carbon and it is also cheaper. The superior stiffness of carbon fibres is not really used in the hulls because the crossectional area of the hulls is already so large (think again about our alu/carbon example) that the minimal wallthickness needed allows other material to relatively easily arrive at sufficient overall stiffness. In the case of hulls this favours kevlar and seriously limits the drawbacks of using S-glass.
I guess my mainpoint I'm driving at is that cat sailors would be wise to understand these principles. It will allow them to not get scammed into paying alot of money for boats with fashionable materials that aren't really any better in what they are supposed to do.
Example : If you have the option to have your hulls build in S-glass, kevlar or carbon with the additional cost going up from glass to carbon then don't consider carbon, get Kevlar. You'll get the same quality in the hulls for less cost. If kevlar is not an option then seriously think about how much a little extra stiffness is worth in relation to paying significantly more.
Thanks for reading through my lengthy reply.
Wouter
