fluid dynamics - Is this a case of the "venturi effect"? What's behind the "Venturi effect" actually?

Some dinghy sailboats have a hole in the bottom of their deck used to automatically bail out water after a capsize.

bailer of the laser sailoat

The mechanism only works if the boat is moving. The faster it sails, the faster water is drained out of the cockpit, by suction.

Then. Is this an example of the "venturi effect"?

If so, then: why? According to wikipedia

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe

(as the fluid gains speed).

Obviously,

• there is no constricted section of pipe on the sea behind the boat.


• the water is not moving, it could be perfectly still. It is the boat what moves.


• the pressure of the "stopped" sea water can't be lower than that of air on the cockpit, but it happens that the hole continues to suction air while the boat is moving.

Then it is not clear at all (for me) that this occurs by the "venturi effect".

Couldn't it be instead by the suction effect like when you have your hand flat on a table and then you separate the hand from the table very very quickly? (What's the name of this effect?).

Thanks

Answer

This is a classic example, often used in fluid dynamics classes, of Bernoulli's principle. This is the principle which underlies the Venturi effect: increasing the flow speed leads to a drop in pressure. The governing equation for an flow of an incompressible fluid such as water is

$$\frac{V^2}{2} + gh + \frac{P}{\rho} = \mathrm{constant}$$

To answer your points one-by-one:

• The constriction in this case is between the wedge of the autobailer and the water further away from the hull of the boat: when the boat passes over a given 'parcel' of water, the wedge applies a force forward and downward (perpendicular to the plane of the wedge). The fact that the water is incompressible means that to pass the wedge, the flow relative to the boat must speed up.


• It doesn't matter whether it's the boat or the water that's moving; only the relative velocity between the boat and the water is important in this case.


• The pressure of the static sea water at some distance from the boat can't be lower than the air pressure, but that in the thin layer which is accelerated by Bernoulli's principle can be the same as the air pressure, meaning that the suction will continue until there's no water left in the cockpit.

Let's look at the situation with the aid of a diagram:

You can see how, from the boat's point of view, the water far from the hull has a velocity $-V_\mathrm{boat}$. The dashed lines represent streamlines; lines along which a given 'parcel' of water moves. As the flow of water past the boat is constricted by the wedge, notice how these lines get 'squished' together. Water cannot be compressed (i.e. $\rho$ in the above equation cannot change), so to get the same amount of water past the wedge in the same time, the water must speed up ($V$ must increase) and hence the pressure must drop ($P$ must decrease).