Semi-displacement boats don't generate lift??

So I have been reading a book written in the early 1960's called "Naval Architecture of Planing Hulls" by Lindsay Lord. In this book it is often stated that surfaces that don't redirect water flow do not produce any lift. (Read the screenshotted texts below.)

I am not an expert in naval architecture, but I can't see any reason why Lord's statements would not be true. But still there are many motorboat designs, even from well respected designers (Grand Banks for example) that include such surfaces and many of them seem to perform decently. Many of these designs are what most people would call a "semi-didplacement" or "semi-planing" hulls. Below is a picture of an Elling E6, a 60ft semi displacement yacht that reaches 22 knots WOT with single 900hp Volvo. Note how the bottom slopes up towards the stern.

The statement that in order for a vessel to significantly exceed its hull speed, its hull must generate great hydrodynamic lift, seems to be generally agreed to be true. But is it really? If semi displacement hull's flat, upsloping buttocks do not generate lift, how does it exceed hull speed without extreme trim angle?

Could it be, that the hull of such boat exceeds hull speed not because of the lift produced, but because of the flat and wide after body resisting immersion (caused by wave making resistance) effectively enough for the hull to ride bow wave rather than sinking between the bow and stern waves.

What do you think?

 

Short answer...No. First of all, there is no "lift"...it is only a mental concept that separates out some portion of drag in the direction you want it to go.

Short answer...Of course; but not the way you think. Secondly, there is also no such thing as "hull speed". A vessel will go as fast a power allows. The concept of hull speed is an archaic holdover from early theorists best left behind in the hubris of history.

Finally, I have always felt that often the different sides in a hydrodynamic discussion are more about how an individual internalizes the concept rather than a difference in fact. Ask 3 Naval Architects to state the differences between displacement, semi-planing, and planing hulls and you will get 5 or more answers.

All that said, all floating bodies are supported in the water by the dot-normal integral over the wetted surface of the fluid pressure, both static and dynamic. This interaction between the pressures and the shape produces both the buoyant force and the drag. Whether a vessel "lifts" or "squats" is just a physical manifestation of the interaction between the hydrodynamic pressures, the shape, and the vessels mass in the vertical plane. Generally a vessel is said to "plane" when the majority of the vessels weight is supported by dynamic pressure, it has nothing to do with the shape of the wake except as it relates to pressures on the wetted surface. So specifically, the shape and speed of the hull influences the dynamic pressure which then forms a dynamic force over the hull surface which retards the hull and causes it either to "lift", to "sink", or to stay neutral based on the static waterline. However, the integral of the pressures over the hull are always identical to the weight of the vessel.

Now the shape of the hull for a given weight that causes the minimum drag at speed while meeting all other items in the SoR..... that's the rub.

 

Notice what the stern is doing, and its depth in the water. And don’t fall for the idea that Robbie’s weight is being lifted much by the sail- try hanging from a lat bar on a weight machine & put your feet on a weight scale-

 

Yeah.... and that brings up something that gets into the quaggy scrum of the "ancient interface". Rather than just look at the hull and the water which we can do with a CVN (which can go well beyond "hull speed" BTW), lightweight high-speed vessels need to be thought of as being fully submerged at the boundary between two fluids of different densities. So for that 3-point hydroplane it is just as important that the aerodynamics hold the hull down on the water as it is that the water lifts the hull up.

 

fun mind game- ~ D/L of the windsurfer hull as shown in the pic ~ 300, and the wave drag is probably off the charts- not bad for a narrow squash tail with a relatively small sail…. Except for wetted surface (bow out) what’s changed? Stability?