Am I just slow or...

I've been reading up on the Sears-Haack body (reasons don't matter)http://en.wikipedia.org/wiki/Sears–Haack_body but it suddenly struck me that the issue that boats have in transitioning from displacement mode to planning mode seems to have similarities with the wave drag that the Sears-Haack body experiences.

Have I just taken one too many hits of Aspertame and my brain is full of jellied doughnut holes, or am I just slow coming to this realization that others already get?

 

I am sick to death of "planning" hull threads. I am "planning" to put a bounty on offenders !

 

I don't even know the difference between a dispersion hull and a planning one

 

Well there isn't to the extent that all hulls are subject to the same overall forces, just like all aircraft are subject to the same forces. And yet there are aircraft labeled "super sonic" and others labled "sub-sonic"

And there is a difference between a 6 Meter, which will never ever climb out of the hole it digs in the water because by the time its going fast enough the negative lift the hull generates will make it create the next Mohole - and say an 18' skiff

 

Now back to the original question on the Sears-Haack body and its applicability to hull design.

Or does that term just intimidate you folks too much?

 

If you had read the background work, you would know that Sears-Haack bodies do not apply to non-compressible fluids or surface interfaces (i.e. they are totally submerged in the fluid). They are theoritical contrivance based upon the work of Havelock extended to compressable flows. For surface hydrodynamics, the applicable body for wave making resistance is the Wigley shape and/or Inui's, Mitchell's, and Daube's work. If you want the planing transition, see Savitsky's work.

 

Baltic, it is a very complicated problem mathematically. There is a mathematical similarity between near-free-surface wave solutions and compressible flows in the supersonic range. There are some comments on this towards the end of DCockey's wide ranging thread on Hull Asymmetry.

This may help - http://wiredspace.wits.ac.za/bitstream/handle/10539/2102/Research Report.pdf?sequence=1

 

Thanks for that!!

 

The analogy between surface waves and two dimensional compressible flows only applies when the free surface wave lengths are long compared to the water depth. For boats this means the boat length is long compared to the water depth. Otherwise there are major differences in the physics of free surface waves and compressible flow.

Waves in compressible flow are non-dispersive; the wave speed is independent of the wave length. Free surface waves are dispersive; the wave speed is dependent on the wave length as well as the water depth. If the wave length is large compared to the water depth the dependence of wave speed on wave length is small. This causes fundamental differences in the behavior of free surface waves and compressible flow waves.

As Phil mentioned, this topic was previously discussed at the end of the http://www.boatdesign.net/forums/hy...ull-asymmetry-minimum-wave-drag-38260-13.html thread starting with post #193.