Laser C-Rig continued
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Laser C-Rig continued

Laser C-Rig continued

7 March 2019

99% of design is from top down.   I’m big (95kgs) and when being asked to design for 45-60kgs your first reaction is to scale down.

But I am also lucky in that we tried to do that, with Takao WRT the 29er and we had learnt that it did not work.   The 29er is not a scaled down 49er, rather it is its own unique design and that’s possibly why 29ers are so much faster than the 49er at the extreme.

So, when we started to get bogged down with the C5, Takao suggested that we forget about “Flame rigs” and go right back to first principals.     We did an analysis on the Std Laser Rig, the Radial Rig and the 4.7 rig.    We knew the 4.7 was great in a breeze which is why it’s so successful in Europe, but it’s poor in light air, and with that in mind, we looked at 100s of photos of rigs and also of people sailing Lasers.   We also had a raft of data from Tom Burton from a program called SailEQ which is another spin off from this whole program.

We used the std rig as a base line, and we determined that a well-sailed Laser was sailed at 6° heel, as said before this is mostly to keep the skipper’s butt out of the water.  We also determined that the “lead” so the distance that the CoE of the sail is in front of the CLR was about 130mm.    In a boat sailed flat, this would lead to lee-helm but because a Laser is sailed at best, at 6° that results in the well known weather-helm.

What we found with the 4.7 rig was this “lead” was upwards of 300mm so in conditions of not so much wind where the skipper is sitting upright and the boat is less than 6° heel, and without mainsheet tension pulling the CoE aft, you do get a lot of, in most cases lee-helm and that has always felt awkward.

“Lead” is when the CoE is in front of the CLR
“Trail” is when the CoE is behind the CLR

CoE = Centre of Effort, the point on the sail where all the forces are resolved

CLR = Centre of Lateral Resistance, the point (in the case of a skiff) in the centreboard where the forces are resolved.

Lasers have Lead because when the boat is heeled it wants to round up, like a yacht, and a 49er has Trail because when it’s sailed flat there is no round up.  In a perfect world you carry about 10% of the side load on the rudder, it’s about 28% of the combined area so you load it up about ½ (kgs/m²) of the centreboard, hence wanting a little weather helm.

In a breeze, the kids in Europe are good, they strap their sails down to the deck, pulling the CoE aft and they sail often at heels far greater than 6° because it feels good.    Pretty obvious why.

The next thing we found when we actually measured the area, a 4.7 is not 4.7m² it’s actually a little under 5.1m².  In fact, we found all the areas across all 3 rigs are understated.    It all depends on how you measure the area, and to be honest, the area is just a number, what’s important is that you use the same system across the whole design spectrum.

Next bit gets interesting, in that we already knew that if you reduced weight aloft, as in going to a mast like a carbon rig, you have to increase sail area or you will skew the rig to lighter weights.   With the 49er we tacked on 170mm of mast height and approx. 1.5m² in area on a total area of 20.5m², but with the 49er we also wanted to increase crew weight, and to date, that’s exactly what’s happened.

We also had the Flame rig data and to a lesser extent information from the 29erC rig.

We knew we had to up the area, approx. 10%, so say 0.5m² was about the target, but we decided to hold that down to 0.4m² because the target was smaller Asians and it also allowed us to reduce the mast height so even though there was more area, the Arm (distance between CoE & CLR vertically) actually reduced.

To be honest, I don’t know where we ended up, because empirical testing always trumps theoretical but my guess is we are at 5.4 – 5.5m²

By this stage, we were committed to a square head, but the big issue with a square head is how big should the length of the top batten be.     If it’s too big/long then you have to use excessive downhaul tension to make it reactive.   Too short, and it may as well be a pin-head rig and you lose all the advantages.

The big advantage of a square-head over a pin-head is the mast bend is ½ !    Long term that leads to near indefinite mast life.    It also reduces problems with hoisting the main,  and having overly hooked leaches in light airs.

Ian MacDiarmid and I worked on this along with mast stiffness and position of the bend so that we could end up with reasonable downhaul tensions which allows us a wide range of crew weights and without excessive vang loads.   We reduced foot lengths, but the big breakthrough was being able to “splice” the mast at deck level.  This allowed us to drop the whole mast further aft, it allowed us to use a stiffer topmast and it got the Lead down into the 120-130mm in all wind ranges, so the helm felt good.   Later Takao made some unilateral decisions to reduce the “splice” to reduce the helm when sailed at 6° and Clive Watts (CST) was able to deliver us a “kinked” mast at deck level and that reduced weight and cost.

In December this year Takao and I played with vang lengths, which allows us control over lower mast camber.   Sure, we threw away some booms that had become Swiss cheese and the lower mast of the now C8, it’s quite hard to believe it’s still standing but we had something really special.

Late Feb this year I happened to be in Longchi, China, working on the 29erC rigs and I watched 2 smallish Chinese girls struggling with a 4.7 rig.   Everything Takao had talked about was there in front of my eyes.

Above is a work sheet which was the initial drawing for the proposed C5 rig, dated July 2016 (click on image to enlarge).

The Lead on the laser std is 136.83 but would obviously be less than that with the main sheeted hard down.    4.7 is 295.84, more than double.   What’s important when you do this is to remain consistent, so these may, in fact, not be the “actual” numbers, but they are relative.

We have the splice at 7.5° in this, we ended up less after Takao opted for less weather helm, we also ended up a bit bigger than this shows, with longer cord lengths.     I need to re-do those measurements.

Julian Bethwaite

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