Modification of standard Nacra 5.5SL downhaul to a 16:1 downhaul.
I won't go into detailed benefits of this mod. But using a 16:1 downhaul can give you a flatter sail. A flatter sail is important to have when the wind is blowing hard. It depowers the sail and will help keep the boat flatter and can help keep you from flipping over.
This is a 8:1/2:1 cascading system, which gives you a 16:1 overall downhaul.
I have seen many different ways this can be done. This is just one possible solution.
The photos below can explain thing a lot better then I can, so I will keep my descriptions brief.
For the 2:1 portion I run a line from the bottom of the mast through a block that attaches to a hook that goes through a grommeted hole on the main sail, down to a double block for the 8:1 system. I have a roller furling jib so do not use the old hardware on the mast for the jib halyard downhaul. So I used an existing cleat one one side, and a tie off on the other side. If you are adding hardware, I would recommend the cleat on both sides. You can then ease this line way out for hooking up the hooks, then pull it down some once on, which will give you more travel on the rest of the system.
I have exit cleats on both sides of the mast. I am not sure is this was originally standard.
The bottom 8:1 line runs from exit cleat, up to double block on 2:1 portion, down to old cheek block on side at bottom of mast, back up to double block, down to newly added cheek block at center of mast groove, then on to the other side which is set up the same way.
The hardest part installing this was figuring out how to attach the cheek block in the mast groove. What I ended up doing was cutting a piece of scrap aluminum so it would fit inside the groove. I filed the sides down at an angle so it press evenly up against inside of groove. The sizing needs to be right so you leave space inside for the main halyard. I then drilled holes in this and bolted the block to the aluminum. Heads are inside and bolts and nut stick out so it won't interfere with the halyard.
I sail solo a lot and thought this was a good modification I wanted for safety reasons. Pulling this tight in higher winds depowers the sail, and can help keep you flatter on the water.
When I take the sail down I just unhook the sail, then put the hooks in the mast grove opening were the sail is fed in, and put it tight. I can then leave the whole setup in place.
The 16:1 is legal for racing, but may void your warranty.
From the class web site: "5.6c Downhaul ... Nacra 5.5sl, Uni, 5.8, 5.8na and 6.0na may not exceed 16 to 1. All purchases above 8:1 may void manufacturers warranty.