Originally Posted by David M
Why would they want to increase the wetted surface area per amount of volume with a hard chine as opposed to a parabolic cross section?
The major factors in hydrodynamic resistance are:
1.) Friction due to surface area
2.) Wavemaking resistance
To keep this simple I will attempt to explain my understanding of what happens in basic terms.
Friction as a function of speed grows roughly with the square of velocity.
Once we get into the higher speed range (or Froude number) the wavemaking resistance comes into play. (this is akin to the "hull speed" of a mono, but we can't use the usual 1.34 x SQRT Lwl here)
Wavemaking resistance has 2 main components, but simply put, the wavemaking resistance grows roughly with the cube of the speed.
So the amas are optimised to keep wavemaking resistance down, and this is best achieved with this kind of shape. Long, slim and with an immersed transom that "cheats" the water
into thinking the hull
The use of the hard chine to help bring down wavemaking and lower drag is seen more often these days, but I haven't seen published data on how efficient this really is, and the guys who know aren't going to tell us,as they have paid alot of money
in tank tests and advanced CFD (computational Fluid dynamics) tests.
The theory is that they create a seperation zone to the highly turbulent flow, (that creates drag) by helping air get mixed with the water
, and thereby lowering the viscosity. This happens best when the hull has vertical movement, like in a chop.
Some of the big French tris use a flared chine to push the waves away from the hull sides to bring down the secondary drag this creates as well. See the picture showing the hull shape on Idec, the current
singlehanded round the world record
holder with Francois Joyon at the helm