|Katana in red, Octave in grey|
Armed with higher resolution tools, a more linear pressure recovery could be engineered reliably. The newly resolved pressure recovery rate is achieved through straighter diagonals from the mid section to the transom, combined with aft sections that are closer to semi-circular.
A very simplified overview might go something like this:
On the one hand we want to bring the flow back together (from max beam/draught to a point on the centreline/waterline near the transom) to
1) Make the wake as small as possible – smoothly refill the hole in the water made by the boat and
2) Get as much ‘push’ as we can from the water pressure on the aft surfaces of the boat – since the surfaces are angled inward, the normal pressure that acts at 90 degrees to the surfaces has a component pushing the boat forward. This component would in an ideal world be the same as that pushing back on the forward parts of the hull, but in reality is less due to energy lost through viscosity in the boundary layer.
On the other hand we want to maximise volume in the stern to
1) Get as much support as possible from the stern wave,
2) Damp pitching,
3) Avoid flow separation and
4) Maximise power.
All the while we want to keep wetted area to a minimum…
So you can see how nailing down more exact values makes our design choices much clearer!
In most conditions this particular change as implemented on Katana is near neutral. It trades the power and support of firm aft sections for reduced drag.
But in specific conditions (namely low to medium speeds, very high speeds, and in waves) our updated analyses show a small but measurable gain.
The new aft treatment has the advantage of less wetted surface area, which is a bonus at low speeds. At higher speeds the risk of laminar separation is reduced.
The forward volume distribution has been revised with a less aggressive rocker profile but more angular sections in the forefoot.
In fact the new stern treatment achieves a similar effect to the characteristic soft chine/tumblehome of Octave but does away with some associated minor penalties.
However, since the sections are more rounded, the actual distance along the hull surface between the two separation lines is only marginally greater than before.
To tip the scales, the principal advantage of the new stern shape is enhanced pitch damping. Marbleheads are inherently susceptible to speed sapping pitching due to their deep bulb, tall rigs, fine ends and (obviously) their small size relative to common wind generated waves.
Major values such as waterline beam, midsection area and prismatic coefficient have not changed.
Management of the flow has been refined whilst still achieving a 1.5% reduction in wetted surface area and an increase in power to carry sail, especially downwind.