petterg said:
Thanks for all the answers. I feel like I've learned more than I was asking.
This also made me wonder on more thing:
Assumed a symetric vertical foil moving horizontally with 0 angle of attack. Will it create more drag if it is half submerged than if it is fully submerged?
How will a plot of drag vs height above/partly submerged/under the surface look? As airplanes has less drag when flying high, I assume the drag in the plot will be minimum high up in the air, then drag increases as the height comes down towards sea level. When it touches the water surface the drag increases significantly. As the foil is more submerged the drag increases. But as the top of foil gets below the surface I don't know what happens. Does the drag jump down and then increase with further increased depth?
For a partially submerged foil, you will basically have three regions you will care about: the submerge portion dominated by viscous and form drag in water, the exposed part dominated by viscous and form drag in air, and the surface region dominated by wave drag. The surface region producing wave drag will be the biggest driver of drag in this scenario (assuming the foil isn't extremely long), followed by the underwater portion, and then the exposed portion.
So, depending on the length of the foil, there will be an absolute minimum in drag if the whole thing is in air, then an increase as you dip it into the water, and a maximum right before it becomes fully submerged (i.e. the entire surface is generating underwater drag with a small sliver that is still above to generate wave drag). Then it drops off again as it becomes fully submerged.
This is why the above and below-water portions of the keel of a ship are typically dramatically different shapes. The design principles for minimizing drag in each region are different.
Lnewqban said:
You will mainly have underwater skin friction plus turbulence at the surface.
No. "Turbulence" is not a form a drag. Turbulence is a property of the fluctuating nature of a shear flow. Its effects can lead to more (or sometimes less) drag, but it does not directly produce drag.
Underwater and above, the foil is subject to skin friction drag and form drag (which is largely driven by separation in this case). An increase in turbulence in the boundary layers will increase skin friction drag but tend to decrease form drag due to its tendency to delay separation. In a situation like this, I'd imagine separation is minimal and the increase in skin friction drag would be the dominant factor.
At the surface, wave drag is dominant. Sure there might be some turbulence, but the drag created by pushing water out of the way in the form of waves, especially when they become nonlinear and "break," is by far the dominant mechanism for drag in that region. This is one reason some modern ships are starting to use wave-piercing hulls.