I've always been unhappy with the mainsheet rigging at the clew of the 5.5m² blokart^® sail.

I'm a big guy so I tend to sail with the 5.5m² sail in higher winds than most people; I also use a pretty stiff mast most of the time.
Adding to the problem, I have pretty severe tendonitis in my elbows, so gripping and sheeting-in hard can become painful pretty quickly.

There are two problems I set out to remedy:

1. The mechanical advantage of the stock system actually goes below 1:1 when you sheet all the way in (i.e. you need to pull on the sheet with more force than the sail is pulling up with).
2. You can't sheet the sail down near the end of the pully-whip.

I've been trying to think of a (legal) way to move the block on the sail up and back, while moving the location where the end of the mainsheet terminates forward. Ideally you would like to have the pully-whip block "nest" between the forward block and the attachment point.

What I came up with is to slide a 3/8" (9.5mm) diameter rod into the loop where the original block attaches to the foot of the sail and tie the other end of the rod to the end of the boom. This gives me a way to hang the block and the end of the mainsheet anywhere between the two points.

In my first test, I used an aluminium bar I had lying around. You can see it bends a bit even without any wind-load.
In the "mark 2" version I used something with a larger cross-section to eliminate the bending.
The "mark 3" version is almost identical to the original test, but with a 10mm dia. pultruded carbon fiber rod that I got on E-Bay for $10.

One thing you can't see in the photos and video is how much harder I had to pull on the mainsheet with the original rigging.

To get an idea of the improvement with the new system, I set a reference at the height I could pull the boom down to with the stock system.
Then I measured the force necessary to pull the boom down to that point with both systems (no wind load, just the force from a standard 900/900/900/600/2nd/tip carbon mast).

The stock rigging took about 65-70 lbs.
The protoype system took between 30-33 lbs.

In all fairness, the force in the stock system increases dramatically as you get near the fully sheeted in point (as you would expect given the geometry). Once the boom was out about 12" (30 cm) the sheet force became similar to the protoype's.

Here are a short video sailing with the new sheeting sytem in wind gusting over 15kn:

Here's a very brief video of the system in action. The wind is about 10mph (4.5m/s):
(at about 15 seconds into the video you can see me recover from a sharp hike)

Here's a very brief video of the system in action. The wind is about 10mph (4.5m/s):

Click on the image to switch between the original and protoype rigging.