I have just compared quite many masts in ORCi certificates for 34-38' C/R-boats with P about 14 m and RM 120-170 kgm/deg. All are 9/10 - 19/20 fractional two spreader rigs without runners. Boats like Salona 37/38, First 35, X-37 etc.
The aluminium mast rig weights are 150-180 kg and carbon 92-180 kg. Most carbon rigs are 120-160 kg with a very few under 120 kg and some over 160 kg.
Can you explain where does the huge span in carbon rigs come from?
Not for those particular masts, but some of the reasons are:
Stock alloy masts cover a range of boats for each size. The mast might only just fit in the lower end of the range, so the mast is excessive.
Larger dimensions usually mean thinner walls and less rigging.
Some manufacturers/engineers vary the laminate along the length of the mast (alloy masts are the same wall thickness all the way up). Varying the laminate is harder to build, but uses less material.
Build methods vary enormously. For example, the cheapest mast will be filament wound over metal mandrels, an automated process using low cost carbon tow. Most manufacturers cannot vary the laminate or the shape of these sections (we can) so they are heavy. The lightest method is to build in one piece in a female mould. This requires expensive carbon, skilled laminators and a lot of practice, especially if a sheer web is included.
Interestingly one of the heaviest carbon and lightest mast is found from boats that have done very well in ORC Worlds:
http://data.orc.org/public/WPub.dll/CC/32035.pdf
http://data.orc.org/public/WPub.dll/CC/32032.pdf
It's hard to imagine that this purpose built boat would have chosen a bad quality carbon mast. Maybe they wanted a very stiff one?
Seems like the likeliest answer. Perhaps it enabled less rigging/windage/weight as well. Or maybe the alloy one broke or was under specced.
Are all carbon masts about 100% carbon or are some using a mix of glass and carbon? What about HM carbon and laminating methods?
Most are all carbon. We have built unstayed masts for cruisers with glass off axis material to save cost at a small weight increase.
I have a boat in that range with a quite typical Selden C211 profile. The rig weight is 172 kg including Furlex 207 profile. The profile should be 5.34 kg/m and the mast is 17 m thus it should be about 91 kg. The rods should be about 20 kg + turnbuckles and fittings. I couldn't find weight for the Furlex profile nor for the spreaders. So changing to carbon should give much lower section weight and also lower spreader weight. Also changing the aluminium forestay profile would save something.
All these would help. Biggest will be engineering the mast and laminate precisely for your boat. Carbon or textile rigging will also help. All are expensive though, so unless you are wealthy or serious about racing (clean bottom, practised crew, no junk on the boat, etc), it may not be worth the money.
Other comments:
Prepregs are not inherently lighter. They only become so if they are cured at high temperature and pressure. Wet laminates in the same circumstances are equally strong, stiff and light.
An example of the above:
We have just built a 15m wing mast for a 9m catamaran using high modulus carbon and resin infusion. The required sideways stiffness was calculated as 4 x 10^6 mm^4 with variations along the length. The nearest alloy mast was 5.45 x 10^6 mm^4 and was 20% fatter (ie less efficient). The next alloy size down was 2.7 x 10^6 mm^4.
The alloy section weighs 123 kgs. The standard modulus tube 58 kgs. The high modulus tube weighed 41 kgs. This is a huge saving for a lightweight racing cat. We also custom designed and built the fittings and bonded them on, which saved further weight. The mast was infused in one piece including the sheer web. I have attached pics of the set up pre infusion and an offcut.
This mast is not directly transferable to your boat, but does give an idea of the weight and windage savings that can be made.
