Questions about surface tension

I couldn’t find a thread that most closely relates to this so:

Is a given wetted surface area draggier on the surface of the water than fully under it? And if so:

Does the way a hard surface interacts with the water surface affect drag/area, I’m thinking planing vs displacement. If a board/boat skims across water it’s dragging on the water surface, but a boat cutting through the water is more changing the shape of the water surface.

But then, how is that different from being underwater, except that the water touching the hull used to be the water’s surface(kind of)?

 

I don't quite follow the question. If it is on the surface it is not wet from what I visualize. Do you mean fully submerged, like a submarine, vs a boat? In that case the drag would be different because of wave making among others. The difference between displacement and planing modes is the dynamic pressure. The same displacement boat will have less wetted surface when planing. Are you comparing same displacement or same wetted surface?

 

Yes, it is. If you have ever watched a big beach ball in a pool, it slides around but it doesn't roll. There is a difference between the force peeling the water off a wetted boundary and the force of a running attachment. When stationary, the forces is the same, but when the body is being peeled out the water, the force is greater, and when it is being immersed, the force is less. So beach balls drag a boundary layer around with them but don't roll over the surface.

 

The "drag" per unit of area of a planing surface is more than when in displacement mode. Consider that the surface area that is supporting the planing boat is smaller than the surface area of a displacement boat. The pressure per unit of area is much higher on the planing boat but there is less area exposed to that pressure. As usual in boats and physics, there are a lot of variables to deal with.

 

Displacement, Drag

+ Friction (wetted surface)
+ To displace, to move water away
+ The Horizontal aft/back component of the hydroStatic vector

Planing, Drag

+ Friction (wetted surface)
+ The Horizontal aft/back component of the hydroDynamic (Lift force) vector (which in its calculation already includes a hydroStatic part, see Savitsky 1964)

---

This is the reason why Surfing ...

... with big Waves ... heavy sailboat (If the sailboat enjoy a proper hull attitude) is faster than the light one, because in the end (Fn > 0.8) the bulk of the resistance is Friction ...

... and the bulk of the forward Force (in Newtons) = Mass in kilos x Gravity (9.8) x Sine of the Angle of the Slope of the Wave

"Why does my boat Surf so fast"

Why does my boat surf so fast? https://www.boatdesign.net/threads/why-does-my-boat-surf-so-fast.50195/

Because The powerful Force of The Earth loves Big Mass and Heavy Displacement

And because the sailboat adopts a good and proper Hull Attitude for Surfing

 

Andrew Claughton (1984) at the Southampton Wave Tank was amazed at how good old, heavy sailboats Surfed big Waves.

The powerful Force of the Earth, and a correct order of High and Low pressures, Lateral and Vertical, in relation to the Center of Gravity of the sailboat.

Pure physics

 

In our Wave tank, which I don't know if you know it: the Atlantic, we have repeated Andrew's experiments.

And as a guinea pig we have used an old retired sailor: Roman Sanchez.

Who with an old boat reached a tip of 20 knots in glorious Surfing

 

Román Sánchez Morata

Román Sánchez Morata is known to be a very precise and meticulous navigator (and writer)

He averaged a speed of 4.378 knots which fits his length (19.19 feet LWL) exactly (SLR = 1)

In big waves he sailed at 5 knots and the top speed of the Surf was 12-14 knots.

With Force 7 the boat sailed at 6 knots of speed and the peak Surf speeds were 14-16 knots.

And at the end of a tremendous Surf "wild planing" in his words) ... Román measured (with GPS) a peak speed of 20.3 knots.

The number is the least important thing.

The big issue was and is the total control: the good hydroDynamic behavior.

(The documentary film could not be carried out in this aspect of the seventh art. But in the scientific aspect it was a magnificent experiment).

Stability and Control

 

"Main Part of Michell"

https://www.researchgate.net/figure...blue-passes-above-the-residual_fig1_341840100

 

Drag (Fn > 0.4) can be reconstructed as the sum of Wet Surface Friction + "Main Part" of Michell 1898 + Savistky 1964, modified (2025) in the case of a sailboat

When the boat is planing, Michell 1898 leaves the stage and only Friction and Savistky (1964, 2025) remain.

But the Big Question is Stability

Stable Balance on the Three Axes (Pitch, Roll, Yaw)

 

Motor Boat

< 0.40 Fn: Displacement
> 0.40 < 0.80 Fn: Semi-Displacement
> 1 Fn: Planing

Sail Boat

< 0.35 Fn: Displacement
> 0.35 < 0.50 Fn: forced Displacement
> 0.50 < 0.80 Fn: semi-Planing
> 0.80 Fn: free Planing

 

Well

I expressed it badly above.

The point is this: the boat moves through the water and piles up water at the bow, making a mountain of water at the bow.

If

If the boat overcomes the bow wave ....

Then

Then what remains is Friction and Geometry: the orientation of the hydroStatic and hydroDynamic vectors due to high and low pressures.

 

And the geometry of the Wave prevails

In the end (Fn > 0.80) the bulk is the Geometry

 

(Although in the end the documentary film was not made, Román Sánchez Morata thanks the team of the project, the old man and the Sea)

 

(Southampton, 1984)

(Bottom line: the hydroDynamics of a small sailboat in a real world with big ocean Waves is a question that only interests a tiny minority of eccentrics)