# Why a ship can't pass through its own wave?

This wikipedia article is written like the perfect answer to your question:

https://en.wikipedia.org/wiki/Hull_speed

Question 1: When the wavelength of the bow wave is equal to the length of the ship, the bow wave interferes constructively with the stern wave, causing taller waves in the wake and thus more energy radiated in the waves and more drag.

Question 2: This effect is not a hard "speed limit" at all. In fact, compared with the sound barrier at Mach 1 in aerodynamics, it is hardly noticeable. The drag continues to increase above the hull speed, with only a modest bump in the trend for most common boat hull shapes.

The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.

1) Why this phenomenon occurs when the wavelength of the wave and the length of the ship become equal?

Because you are trying to add more "inertia" than the "external field" (mostly Gravity) can absorb. This leads to the effect explained by the comments of John Forkosh; to ship starts also to try to "get over with gravity". Adding further inertia would lift the ship and as this lifting energy is mostly dissipated by wave creation, it just causes more losses. In ideal theoretical case this "lifting" would only cause some potential energy to be "lost" until it's recovered when the ship stops. But as in the reality the ship starts an infinite climb, the losses are extreme; up to 5-10 times more than in optimum.

But this Lifiting explanation is just a simplification. (As my posts are commonly deleted here, if I don't simplify.. ) As this aspect also works with airplanes and submarines! So what actually happens, is that the ship starts to transport also the surrounding fluid. These fluids adds the total transported mass of the vessel, and it's flows adds losses. The bulbous bow is an example how by adding more on the hull, the drag can be reduced up to 15%. The main reason why this "bulb" is good, is just the flow stability it provides when the ship is running on max speed. (Froude =1)

2) Why a ship can't pass through its own wave?

The only problem is the lack of engine power. As said above, it need's 5-10 time more power to lift up the boat to planing than is needed to just cruise in Froude number 1 speed. Ie. Emma Maersk has an 81 MW propulsion power. So do the math, and find out that it would need a 400-800 MW power source to get higher speed. This is about the amount of typical nuclear Powerplant. So it's possible, it's just not realistic.

One such fast freight ship was even once build; GTS Finnjet. It had the needed engine power, but it actually used this full power only 10 years, before it was already completely impossible to accept it's 16 000 litres/hour full power fuel consumption. Even air-plain's does better with travel-time-freight-ton-fuel-used comparison. Note; This ship was ordered just before the 1973 oil crisis.

Please note that when you get the ship up on planing, adding further velocity doesn't add too much of losses, and doesn't need much more power either. From here you can find resistance/weight vs. Speed/length ratios.

The hydrofoil ships are working with this principle. Their drag is acceptable small, as they really get out of water on high speeds.