I generally accept what comes out of Godzilla.  Although if it has no rocker I will play with the displacement design speed to get some.  In my view it is preferred to have a nose that will generate dynamic lift even if the boat is pitched bow down. So some rocker at least in the last 15 to 20% of the hull either end.  Usual range is foot of the stem just below water level with light boat to foot of the stem no deeper than half the draft.

The 18m proa has round sections and no rocker.  It turns a bit too easily in a shunt.  The fastest way to shunt is to turn it into the wind to slow it down.  Turn rudders for the next tack; head off downwind till doing 3kts then start to crank on the mainsheet and come on the wind as speed gathers.  Typically there is a tiny loop in the track where a shunt has been executed so ground is lost.  If the boat is below 3kts and the rig sheeted in, the Aerorig tends to turn the boat under the sail and is then pointed into the wind and stuck in irons.  The only way out is to shunt back downwind and try again.  

I know of one proa where the longitudinal section forms a circular arc on the keel with the stems at loaded water level.  The owner/builder is considering adding keels either end in the hope of improving tracking.

The 18m proa is impossible to tack to sail the lee hull to windward without pulling it through the wind with a motor.  So it is quite different to a tacking catamaran.  It is very easy to take from the wrong way around to the right way around as it has strong lee helm when sailed the wrong way around.  I expect the HPs with twin mast would be possible to tack without motor and also have less lost ground in a shunt just by working out the sheeting sequence and timing for the sails.  

So there is not much point comparing rocker in a proa hull with a catamaran hull.  Shunting is different to tacking and probably easier to execute once you understand what is happening.  

The only experience I have on large sailing cats is to do with pedal drives.  Have had some experience with getting a beach cat without jib to eventually change tack but  essentially through shunting rather than smooth tack.  The biggest cat I have sailed seriously but briefly was a Tornado and it had no trouble tacking; stem quite deep but higher than mid-section; nominally dry transom.

The ability for a catamaran to tack will depend on its balance and overall efficiency, which includes rig and appendages as much as the hulls.  For light displacement, high performance boat the hulls will not have much rocker.  As displacement to length goes up and design speed comes down there may be more rocker.

I know of one cruising cat sailor who is happy to motor through a tack.  I suspect he is not alone.

Rick

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