It seems as these arguments tend to forget that form drag consists of two components, and wave drag is one of them.
That is one of the reasons why I don't the identifier "form-drag".
I'm very strict in the wording of definitions if it is ambiquous then I consider it useless.
Even submerged submarines and airplanes at lower speeds are subjected to form drag even when no waves are produced. Hence, increasing cross-sectional area increases form drag even when wave drag is (almost) constant.
All hulls have aerodynamic drag as well and we haven't included that one yet either. Gareth mentioned also excluding induced drag (because of the sideways slipping). We can think of a bunch of other factors not included, this still doesn't mean that they are large enough compared to wave-making drag and wetted surface drag to really factor in in the overall picture. Any scientist will have to simplify his models at some level, in this thread we implicetly decided to only look at the wave-making drag and wetted surface drag factors as they are the largest ones.
But still my original statement applies even for these simplified models. If the wetted surface drag decreases by a larger amount then the wave-making drag increase then the overall drag of our (simplified) hull will decrease.
The question we are trying to find out in this thread is whether this is found to happen in the hull length and weight ranges that are associated with beach catamarans. I'm on record stating that I think that for some lightweight craft this may be the case.
I have been trying to interpret your arguments that the shorter 16-foot hull should produce less drag than a 19-20 foot boat (that this thread was about).
I did not really state it that way. I said that I thought that a really lightweight (105 kg) 20 footer may be better off with shorter hulls like for example 5.7 mtr long hull. I later refered to the F16 design to show how a smaller boat with less sail area can still run with the larger F18's by having been made both lighter and shorter. It does so over a wide range of conditions. This was achieved, in my opinion, by both reducing wave making drag and wetted surface drag by equal percentages, in this shortening the hull was a way to achieve this.
I cannot agree that introducing the observation that sailboats almost always tend to be at maximum class length, or mentioning 'planing' or 'semi-planing' (either defined as transition between modes or as part of the boat being dynamically lifted), can be brushed away as simplistic absolutes: "always" or "never".
I'm not sure whether I did that but I do know that I brush away descriptions like "semi-planing" because they are not well defined and so allow everybody to read something else into it. As such these "definitions" are useless and can even be confusing.
Rather to me it seems that absolete denial of the possibility that beach cats are subjected to some dynamic lift, or that 'semi-planing mode' does not exist, classifies as absolutes.
I did not deny that. And I argued that some hull shapes will not have a planing mode no matter how fast they travel. Therefor the definition of semi-planing is increasingly useless as it implicetly assumes that all hulls plane at some speed. Submarines are great counter examples, so two are ships with fully submerged floats where only the skirts penetrate the watersurface.
I am not at all an expert in boat design. Furthermore, the texts that I read tell me that several different mathematical models/algorithms are being employed to model different phenomena of hydrodynamics and boat design.
Correct.
We have to live wih disagreements and uncertainties of what is really correct until somebody comes along and brings us hard empirical data.
True
Stein
