Alan C: I have not checked the hull shapes you refer to regarding validation. Assuming they follow multihull "practice" I'd guess they have a transverse shape that is more ore less semioval or U-shape. The validation seems ok as long as the shape is not too different from the generic one.
I fully agree with you. As precised in the post #16 :
Regarding the model and the remark that has been made concerning the result I produced, I have the feeling that the coefficients used in the polynomial expressions must be corrected, as the contribution of the different parameters to the various drags are highly depend on the hull shape on which they are applied. This is because these coefficients has been made by mathematical regression of towing tank measurements on hulls, whose shape's parameters were bound to a particular domain. When the model is used on other shapes, these regression are no more applicable.
Initially made for IRC monohulls boats in 1997, I have updated the coefficient of the model to take into account more modern monohull shapes, including chines and more flat bottoms. I have also determined a set of coefficients to be used for the determination of the drag of slender hulls, like catamaran and trimaran. These coefficients have been obtained by regression on CFD and tank test performed on sailing and motor multihulls, from 56' to 140'. These hull shapes are quite different from those of kayaks. This may be the reason why my model would be inadequate for drag prediction of kayak shapes. It lacks some adaptation.
How far are those calculations from the reality ? According to the study I gave in the post #4, the order of magnitude is, at least, correct.
Are these calculations able to distinguish between section's influence ? My opinion is that, unless we do not use more sophisticated methods of calculations, we cannot know.
The missile example given earlier is not representative for this case; if you look at the drag coefficient a zero AoA it is quite high for both shapes. In fact the body is truncated at a full transverse section, which means that the dominant drag factor comes from the transom pressure loss. All other losses present are minor in comparison. In addition, the missile experiment was carried out at Mach 0,8 and the body was "fully submerged", which is a different condition from a low speed body semisubmerged and depending on gravity waves.
There is quite a gap between the kayak shape floating on the water and the missile shape flying in the air. The problem that I (we) have is to find publications of experiments or calculations representative of our case. I doubt that kayak's designers would spend a sufficient amount of budget in CFD nor in tank testing in recognize facilities, even at olympic level. The same goes for me. I have taken the opportunity to apply the analytical model that I have on kayak's hull shapes, with a faint faith that someone could have any better quality datas. However, in the scope of this discussion, I have no intention to use my CFD code, without any fundings.
That said, I may recall that the transom drag of a fully submerged body or the transom drag of a floating body both depend on the aspect-ratio of the transom, which is rather the same for the missile of round section and the missile of the square section, depending on the definition that we give of the aspect ratio itself. So can we get, with this study, an intuition of the influence of the shape of the section on the body drag. If, in both cases, the transom drag are almost the same, then the body drags should be also almost the same. If we assume that the transom drag of a square shape is greater than the transom drag of a circle shape, due to vortices generated at the wedges of square shape, then the body drag of the square shaped missile should be less than the body drag of the circular shape missile.
In addition, when you look at racing multihulls history, a clear "squarization" of floats tendencies is to be observed. To my knowlegde, there is two main reasons for this : First, CFD simulations & tank tests performed on racing boats ( again, bigger project, bigger budget, bigger studies ) have shown that the hydrodynamic drag of chined hull is, at worth, equal to the one of rounded hull. At best, chined hull could have less drag than rounded hull. Second, hard chines could be used as natural stiffeners, adding subdivisions to more flat surfaces, where it is easier to place core materials.
Allow me to share with you pieces of this study, that has been realized at the ECN ( Ecole Central Nantes, Fr), whose hydrodynamic laboratory is recognize and well known, and ordered in 2007, by MULTIPLAST, a boatyard famous for their racing multihulls. At this time, the Gilles Ollier Design Team, where I was employed, was working on a new generation of hull shape (in parma). -drawings have been stretched to make difficult any use of this study-. We were testing U shapes versus elliptic shapes. CFD has been performed on bare hull.
i. Presentation
ii. Typical output
iii. Drag computation
In the figure down below, "Cx" is the drag coefficient. The pink curve correspond to the Q6 hull drag, with squared sections. The grey curve correspond to the drag of our reference hull, namely ORANGEII, round the world record holder in 2005, in 50 days, 16 hours, 20 minutes.
As you can see, the drag coefficients are almost the same, with a very little advantage to Q6. The wave drag generated by Q6 with U sections is lower than the wave drag generated by the reference, with rounded sections.
So, in the absence of more representative studies, I find quite conservative to conclude that slender bodies with square section or rounded sections have almost the same drag coefficient. Regarding the manoeuvring capabilities of light and small monohulls, and from my experience of windsurfing, both in waves and on flat water, I can say that, indeed, chined sections are more appropriate at high Froude, allowing a better heading in straight line, and that rounded sections are far more easy to jibe or tack in medium waves.
Nevertheless, kayak shapes are also clearly out of my league, and I would be very pleased to have a better knowledge of these wonderfull crafts. Do you have any contradictory experience or study to share ?
