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Wednesday, December 12, 2012

Exploring the Envelope

Not much time behind the desk recently, and for the coming weeks.
After the planning and design phase and a smooth build period, we are now exploring the performance profile of the new A Cat.

Here is our first in-depth discussion of the project outside this blog:

There is a bewildering array of variables to grapple with.
And this is just one of the configurations we plan to test.

Our approach is to quantify as many as is practical and gradually develop a tuning manual to allow customers to dial in fast settings out of the box as a starting point for individualised tuning.

We will post more detail but the short summary is as follows:
The skipper controls trim through fore/aft weight placement.
Foil AoA is controlled by a single line system and rudder foil angle is set 'on the beach'.
The interaction of these three variables is fertile ground for getting the best performance out of the boat.

In addition there are the usual tuning variables such as mast rake, rudder toe-in angle as well as sail trim settings.
Simple enough to explore with a good plan!


  1. The downwind moderate vid looks like it could use more positive AOA on the front foil? I was thinking the same thing about the upwind vid though it did seem to be pitching very little with the bow piercing nicely. I suppose one has to start with a conservative setting offwind to avoid liftoff.

    What sort of gasket are you using at the hull exit underwater for the main foils? Thanks.

  2. Hi Karl,

    That vid shows our first time in those conditions: moderate breeze and a messy sea state.
    The rudder foils are set to the minimum increment on the positive side of neutral.
    It is the first step in a methodical programme where we will explore different settings in a structured way to validate the predictions made by the optimisation programme.

    So in the vid the rudders are actually lifting the sterns up ever so slightly. Ofcourse if the boat pitches down the AoA on the rudder foils goes through neutral and then starts pulling down to restore a level attitude.

    Note where the skipper is standing. He is easily a metre further forward than where he would be comfortable on any other A Cat. And the boat is nowhere near its limit.

    Foil lift can be controlled by rudder foil angle but also (and more efficiently) by increasing toe-in angle. Takeoff speed is in fact determined by toe-in angle and that is a variable we intend to explore. Basically we must keep everything the same and change one thing at a time so that we can compare the effects to our simulations. Once that is done we will supply the boat with a simple manual showing optimum base settings for each condition in a simple form.

    No gasket under the hull, only a foil bearing and a case through the hull to the deck.
    The case is shaped to minimise its volume while allowing the foil to pass through.

  3. Thanks for the thorough reply Dario! Very interesting stuff. I thought the rudder might be lifting. The bows being immersed in small waves must reduce pitching which has to be very important with a very tall rig like an A-cat rig.

    It looks like bearing off with the boat heeled will lift the stern and decrease lift on the mainfoil, which sounds like a good combination in moderation - in Moths one has to reduce heel before steering aggressively into the tack, otherwise the stern lifts too much and the boat plunges down. But there is less heel here so probably not as important.

    Having two boards to push against each other with a toe setting does create quite a lot of combinations! I'm sure it will be fun to sort through it all. In some conditions offwind I'm sure you want both toed in, but in others the windward one less positive (or negative?).

    The most interesting aspect of it all from my point of view is the autoregulation of vertical lift through board shape, rather than mechanical adjustment. I suppose one could hook a wand to the toe setting... =:-)

    Final question (for now): are you using an asymmetric foil section on the mainfoils?

  4. There are some interesting dynamics at play with heel and pitch variations.
    Definitely the boat is faster when sailed flat so all the foils work as intended.

    Martin has applied some fiendishly clever detail design to satisfy the brief of keeping the boat simple to sail.
    Toe-in is not adjusted on the water. It is set by replacing the central plate in each set of foil bearings.
    However, since foil rake can easily be adjusted on the fly, raking the windward foil fully back when going upwind has the same effect as reducing toe-in angle. The windward foil then stops working against the leeward one and starts pulling down and to windward. It is like lighting the afterburners. Boatspeed stays the same and the boat just climbs away. You can see it at work in some of the vids:

    Foil section is symmetrical but there is a subtle twist in the chord plane.

  5. That explains the symmetrical section.

    I did notice that difference in windward performance on the vid; did not know what to attribute it to though. Looks like the extra drag pays for itself with more power; the boat did not look to be losing much speed to the conventional boat at all, despite pointing higher. The foil areas and spans must be fairly specific for that to pay off in a particular application; clever indeed.