Offshore Optimising
October/November 1996
Carbon masts are a widely discussed but seldom seen commodity in the Australian racing fleet. Several smaller production classes are now using them as standard while few larger yachts have taken the plunge. The basic superiority of the concept is indisputable but what keeps people away is cost. This will inevitably drop as production techniques allow for a range of sections to exist similar to the current situation in alloy spar making. However for the moment most yacht masts are one - offs with the resultant non-amortizable costs of tooling and design. It is also quite difficult to set up a competent business in making carbon spars as there are many issues to be resolved and most of them require some elements of suck it and see regardless of the range of technical experience the company may or may not have at its disposal. Suffice it to say that any carbon mast maker that does not have a bin full of rejects in its early days is not really trying.
What is it that makes a carbon mast superior to aluminium? Four terms need to be understood and I would like the engineers in the audience to suspend judgement at this stage as I am trying to impart some useful (as opposed to absolute) knowledge here. These factors and their approximate definitions are as follows :
Material density : easy , the mass per unit volume, i.e., grams per square cm .
The modulus of elasticity (E) : technically the rate of change of stress with respect to strain. However the critical factor here is that a higher value generally means a stiffer material. A material with the same strength properties in all (isotropic) has one value for E while a composite material (non - isotropic like a tree) will have different values depending on the axis relative to fibre orientation. This is very important in carbon mast design since typical mast design only deals with compression. E is usually constant for a class of materials such a 6000 series aluminium or steel.
The yield strength : the stress a which a material will deform permanently. While E may be constant the yield value can vary enormously for a given class of material. Metals change mainly due to temper and alloying but a 6000 series alloy commonly used for mast extrusions can have a yield varying from 76 to 300 megapascals (MPA). All alloy masts are not created equal! Yield is usually the lesser of the compressive or tensile yield values.
The moment of inertia (I) : this is a bit abstract. Let's suffice it to say that it is not a factor of material but shape. As a value in itself it has long been the factor that differentiates one aluminium mast extrusion from another since, if the material is the same , a larger moment implies a stronger section. If you introduce a variety of materials for mast making (timber, steel, E glass , etc. ) the I value is no longer enough and the value of comparison become E x I which takes into account material and shape. A comparable alloy vs carbon mast would have similar values of E x I if they were to carry similar loads.
Now let us deal with the two material main contenders and see how they stack up in such a way as to make carbon the material of choice. For this example I will use 6061 T6 aluminium which is the material of choice in the industry to represent the aluminium side of the argument. For carbon, well, it's not so simple as there is an enormous range of materials and fibre orientations to choose from and this is where the skill of engineer and manufacturer comes into play. Not only does the choice of materials make a difference the way they are processed impacts on the strength of the final product. However, so that we don't grind to a halt, I will nominate the most popular stacking sequence with a standard modulus carbon and a processing technique of 150 degrees and 3 atmospheres of curing pressure. This produces the following comparison :
MATERIAL DENSITY YEILD E
6061 T6 2.58 g/cm 276 Mpa 69,000 N/mm2
Carbon laminate 1.4 g/cm3 752 Mpa 94,000 N/mm2 (longitudinal)
You can quickly see that in every respect the carbon laminate is superior. It is 54% less dense which means that even if you replicate the physical properties of an extruded mast in every dimension then the mast should be both stronger and half as light. Perhaps if anything the danger is making the mast too stiff. Too good to be true? Well you are partially correct. This is because the strength of the laminate is not as great in other directions and there are a lot more factors that need to be considered such as sheer strength and torsional rigidity. Another term worth keeping in mind is process engineering. This is where the calculated values are turned into reality. Is the pressure and the heat evenly applied? Is the material and stacking sequence accurately implemented? Have the calculated laminate properties been physically tested in advance? Have materials moved in the mould and thus made for variations in the wall thickness? And so on and hence the full rubbish bin.
If you have come this far you must be serious so here a few standard questions you should be armed with when discussing your future mast with Oz Spars :
1. Is the E x I value the same or near the ideal aluminium mast for my boat? If too high it will be too stiff (or heavy) and if too low the opposite. You want values for 0 and 90 degree orientations.
2. Can I see your calculations for E x I for each panel?
3. Can I see all cut-outs to confirm that wall thickness in each area is as calculated? Spreader cut-outs and halyard exits are particularly useful.
4. Can I see the reinforcement schedule for the rigging attachments? The compressive strength of the laminate times bearing area of the fitting should be equal to the breaking strength of the rigging.
5. The proposed laminate should have validated values for strength in at least the two primary directions. Confirm that these values meet or exceed the values for your equivalent alloy mast. A high compressive strength can still leave you with a mast like a wet noodle if the torsional strength has not been attended to.
Finally, don't look at the beautiful little samples in the salesman's hand. It is not too hard to make a small object in carbon. The same quality in a 15 meter length is what you want to see. Good luck!