The F31 is a powerful platform and we're excited to see what we can do to make the old girl more competitive again against the younger crowd!
First item to 'evolve' is the rudder blade.
As the 31 developed from the uber-successful 28 it also inherited the rudder blade. If you look at the 28's blade, it's rather short, fat and blunt. With little area behind the axis where it rotates, it feels rather 'wooden' although the width of the rudder gives it go through when the boat is sailing at slow speed.
But the major 'problem' with it is the area ahead of the axis line (a line extended from the middle of the gudgeon pins down to the tip). If this area is to little you end up with a blade that is hinged much like a door. This gives it a heavy, lethargic feel on the helm.
A balanced blade will have as much as 13% of the rudder area ahead of the axis. This gives the helm a much lighter precise feel.
Most of the Corsairs come with what could be called a "trailing rudder" i.e. one that is not balanced as discussed above. The real problem with this is, as you load the boat up i.e. sheet the main on hard and start powering up, the helm goes into heavy weather helm (loads of pull on the tiller). Once we balance the blade properly this load disappears and then we can start it increase mast rake.
By increasing the mast rake, we will improve windward pointing angles as well as off wind boat speed so it's an altogether better deal. Increasing the forestay length allows the mast to lay back at more of an angle. This shifts the Centre of Effort of the sail further back and transfers some of the lateral load away from the dagger board to the rudder - kind of like load sharing!
In the photo at left, Black is the original size of the 28 blade. We've added on a 6" tip as well as 1.5 inches to the leading edge.
Here's what we started with. The red line is the line taken through the pivot point of the gudgeon pins. With such a small projected area ahead of the pivot, the helm is heavy. This is the same as sailing with a trailing rudder and there is no way you can increase the mast rake as it would just translate into more weather helm (pull) on the tiller.
The new C970 Sport has an altogether new blade which is deeper, narrower and balances much better right out of the box! But that's the benefit of 20 years in the production cycle. We could have purchased a new 970 blade but since we already had a blade it seemed more fun to remake it!
So after some manipulating, here's what we have done.
1. Red line is the pivot axis.
2. Blue line is the new leading edge.
3. The black edge between the red and blue lines, is the old leading edge that had a half inch shaved of to make it a straight edge.
4. Half an inch was shaved off the trailing egde so we have effectively shifted the balance point further forward.
This is just how blunt the leading edge was! It might be "forgiving" but it sure is thick. By projecting the new leading edge ahead of this we can make a better edge with a finer entry.
Finer entry = less resistance although not as forgiving. But in the true sense of the word, we are only dealing with 'slow' sailing speeds and not some airliner!