Spy photos

However, I'm in complete agreement with the fact that a foil designed to work with a winglet from the beginning, should be more efficiant that one that has had a winglet retro fitted.

You have to overcome inertial to gain momentum

The greater the inertia to overcome, the greater the momentum

BTW don't confuse momentum and speed

Think of it as 2 cars of vastly different weights traveling at the same speed.  Turn the engines off and let them roll.  The heavier one will roll further (frictionless system)

I guessed it may be for extra momemtum, but didn't want to appear silly!  Still, you need greater lift and hence end up with more drag.  Presumably the added momemtum more than makes up for this?

I knew someone would ask that.... It's not easy to explain, and for the sake of my typing I'll try to keep it short so sorry if it appears simple.

A glider will fly a certain distance for a height, this is measured as height = 1, so using the ASH-25 (its a type, think 18ft skiff feel free to Google, sadly it's not the -30mi) that'll be about 60:1. So, for one mile of height, it'll cover 60 miles forward. That's all you need to know if you are just cruising about. However, if you are racing them, you also care about the speed at which it does this. Normally the best glide ratio is found at about 56knts, but that's too slow to race around a course, so you'd want to fly between thermals at something like 75-95knts, depending on conditions. At that speed the glide ratio drops to something like 40:1 - 20:1. This is called a 'speed polar'.

Now, if the wings are stuffed full of water (and we are talking significant amounts, up to 200kg), the speed at which you can fly for any ratio changes. the 60:1 will be achieved at something like 68knts, the 20:1 at something like 110knts. This may not seem a great advantage, but it means for a given amount of height, you travel the same distance, but get there nearly 20% faster. or, you can cover the same distance at the same speed but keep the extra altitude.

The pay off is that the glider will not cimb quite as well in a thermal, however, it pays off to climb slightly slower and cruise faster as long as you are in strong conditions.

 

And that's keeping it short.

Remember though that the fastest way to steer the boat is to only use small rudder movements. The rudder should effectively follow the direction of the boat which is steered by changing the centre of effort of the rig and foils using heel. The small yawing angle required to steer the boat should not increase the drag sufficiently to penalise the tip devices.
The real benefit for using them on the moth is that designed correctly the stalling speed of the rudder horizontal foil will be reduced, meaning that Mike has better pitch control of his boat at low speeds, as he transitions to or from foiling mode.