12 March 2019
Getting into specifics, as commented early in the piece, I’m 95kgs as is Chris (though Chris is a whole lot more fit), so it’s pretty logical that the initial work has been done on the big rig.
The photos below are the Mk2 rig, the Flame rig was the Mk4 and the C8 is a Mk6 rig and in both cases the boat is being sailed by Tom Burton. In the first photo, the shot is taken in the sound of Castle Rock, Middle Harbour and the other sailor in the Laser is Gerard West.
In the black/grey/white photo, the green line is horizontal (off the shore line), the blue line is parallel to the green, call it a waterline. The red line is up under the gunwales aft, the boat is being sailed at 5.25° heel, pretty flat. The 2 yellow lines are dropping from the CoG of the respective masts. (Click on photos to enlarge.)
Again, I stress the Mk6 C8 mast is heavier, but it’s also straighter. And I would expect the alloy mast to bend more lower down.
Some people have no idea what I am talking about re the Flame Rig, so I have included that also, it’s Dec 2015 and a Mk4 rig.
Just want to touch on a few points.
TGi – Temperature Gradient index. I’m not an industrial chemist or a structural engineer, so I have been told this is a TGi thing, but don’t quote me on it.
I was born in NZ but lived all my life in Australia, and Australia is hot, much hotter than Europe and most of the US. But nothing prepares you for the tropics. And a lot of the people we are targeting live in Asia not far from the Equator.
We had a stark lesson not so long ago, we had supplied 20 odd masts for a single-hander to a sailing club in Singapore, and Singapore is less than 1° from the equator. The kids came in for lunch, dropped the booms off (Pin head rigs) and got out of the sun and had lunch. Came out after lunch, hook the sails up went sailing but something was wrong. Cut a long story short, all 6 masts had got so hot that they had re-set, bent. Took a week but as always is the case, we heard about it. What we found out was someone in Singapore plugged the topmast “to make them float”. What transpired was the core temperature of the mast rose to well in excess of 150C, well above the masts’ TGi.
It’s just one of those things, all these masts are made as part of a std production process, the tapes, the resins, all of that is designed for specific temperatures and if you want to go hotter, then the prices skyrocket.
To overcome this we leave the mast open, so it can behave like a chimney, with the hot air going up the inside, meaning that in Arabia, the Caribbean, Singapore, Brunei all of these places the mast will remain “within its TGi”. You will never see this in Europe, most of the USA, in fact most places outside the tropics, but if you want a world-wide boat with a FRP mast you had better have a plan.
Vang loads One of the big differentiators is vang loads. In std Laser, be it Radial, 4.7 or Std rig, first thing you do when you get on board is strap on the vang as hard as you can, and once you have done that you pull it some more.
With a Square-head rig, it is very easy to over vang the sail. First thing is the downhaul becomes your primary camber control. We have even gone so far as to couple the Downhaul and the Outhaul together to make one Camber control.
This opens up the vang as a twist control, it de-burdens it as a camber control, and that in turn allows a lighter crew to ease the vang and opening the leech to de-power the top of the sail. It also allows a heavier crew to firm up the vang, close the leech and power up the whole rig.
The boom is also a lot more substantial, so you simply don’t have to “vang it down”.
Mast bend One of the reasons we use a Square head is that the de-powering process is spread across 3 “design tools”. The bend of the mast is one, and it’s very evident in the shot of Gerard and Tom, the mast bend of a Square-head rig is about 1.5-1.7% max, whereas the mast bend of a Pin-head or quasi Pin-head rig is 5-6%. Added to that, the FRP mast has a smaller diameter. And this is a x² law, so for every extra 1% in bend, and extra diameter, the elongation of the fibres is exponentially more. “elongation of the fibres” = mast longevity, as in the less they move the longer the mast will last, and unlike alloy, provided you stay under a %, the elastic limit of the carbon which is about 0.9%, simply won’t fatigue.
1st and 2nd batten, I call it Euler Crippling load, I know Steve Clark calls it Buckling load, maybe it’s the difference between Aviation speak and Boat speak, but he is the same guy, 17th Century mathematician. The top 2 battens on a square head also play a pivotal role in the depowering process, they can be set to trigger the “stand up” or “lay down” mode, and that in turn can be set by the downhaul tension.
Sail cloth, the choice of sail cloth and the rocking of the panels is also very important. Again, stay within the elastic limits of the Mylar, rock the panels correctly and couple all that with the top 2 battens and the elasticity of the sail-cloth can be dialled in to get the mainsail to do the right thing at the right moment.
Full length battens, while on the subject of battens, we did try ½ battens, and we would have saved a bit more weight but the whole performance of the rig becomes compromised.
It’s a very small price to pay, about 400gms, for a far more complete package, that will last longer and behave consistently all the time.
Start line, people say you can’t start well with full length battens but just go look at any AC race, or closer to home, 49er and 29er racing, Int Moth Racing. I know in the 49er class they have developed a technique to “crab to windward”. It’s just a different skill set and the C5 kids will get it in the blink of an eye.
Performance, Theory vs Empirical. Tom and Gerard are highly skilled sailors who have worked very hard to get the very best out of the existing Std Rig. The day the shot was taken, by me from Michael Blackburn’s rib, exactly what you would expect to happen happened, it was close but he who sailed the std rig won out, certainly upwind, not by much and for anyone to have stepped into a new set-up and expect instantaneous speed advantage is wishful thinking. This was the first time they had seen the rigs. Downwind, it was not so clear and the new rig had as many wins as it had losses.
This was the Mk2 rig, we are now at a Mk6 with the Flame Rig (Mk4) being the break through.
The simple fact of the Mk2 rig is the mast weighs 4.4kgs complete (it’s now a little heavier being check-in-able) whereas the std mast is 10.4kgs. The centre of gravity of the Alloy mast is 3m (approx.) above WL, the Carbon rig is 2.5m. That sum is pretty easy, 10.4 x 3m = 31.2kgs/m² inertia, whereas the Carbon rig is (say 5kgs with the heavier battens) 5 x 2.5 = 12.5kgs/m².
What this tells us is that if (when) a gust hits you and the rig starts rotating to leeward, it is going to take less than ½ the effort to stop that rotation.
The other key factor is what I call counter-RM [righting moment]. Let’s say Tom weighs 80kgs, a laser is 1.39m beam, Tom’s feet are ½ way, maybe, so 0.7m, his backside is say 300mm to windward of the gunwale and the CoB [Centre of Buoyancy] at 6° heel is 100mm to leeward, so he has 1.1m Arm.
Therefore, he is generating 80 x 1.1 = 88kgs/m torque (don’t ask me to go into SI’s).
The rig is hanging out to leeward of the boat and if you do that maths, looking at the photos you will see that a std rig, 3m up, is 475mm to leeward.
You can again can now do another sum, which is 10.4kgs x .475 = 5 kgs/m negative or counter RM. The Carbon rig 5 x .356 = 1.7kgs/m counter RM.
So, some assumptions:
- I am assuming all other things are being equal, the weight of the sail is roughly the same (but I have bounced the C-rig weight up because of the battens).
- That they are going to hang to leeward about the same. The alloy rig being a straight extruded tube will bend most as it comes out of the deck, at the max load point, whereas the FRP tube, is 4.5mm thick as it come out of the deck so it’s less likely to “kink”, as in, bend excessively at deck level.
- This is a Mk2 C8 rig, and it has evolved dramatically over the last 4 years (how time flies when your having fun).
- The weight difference between the 4.7 mast and the C5 is greater, but the kink to leeward will be less pronounced with the C5.
The simple reduction in weight, the dramatic reduction in inertia and the reduction in Counter RM, given the history of carbon rig conversions means that with time, the Carbon rig boat will get to max speed earlier, stay at max speed longer and/or point higher due to lower drag.