Hull length and 'hull speed'

Could somebody please kindly explain to me in words (not equations) what exactly it is in hull length that makes a boat faster ?
Would it be because the bow wave has a greater distance to travel before reaching the stern and hence attaining 'hull speed' ?

But then again, what has the stern wave got to do with anything, if a boat begins to reach a speed where the bow wave is pushed up to the boats driving-force limit, and would need extra power to plane over it ... what's this got to do with a stern wave and hull speed ?

Trying to get my head around the Froude number and hull speed ...

 

Wave making resistance, generated by water line length and hull shape, eventual cause the drag to be sufficiently high, that the boat can not escape it's generated wave train without an exponential increase in propulsive power.

 

Hard to explain without equations, but here goes:

Think of hull speed being the speed where the bow and stern waves are at the ends of the waterline length. On longer boats the waves are farther apart. Waves that are farther apart travel faster than waves that are closer together, so a longer boat with it's longer wave separation has a higher hull speed.

Froude number is a means of expressing speed relative to length. Lets say two vessels of different lengths are traveling at the same Froude number. The longer boat is traveling faster than the shorter boat. Froude numbers are helpful to naval architects because some (not all) resistance components are related to the Froude number. That is how model test results are used to predict resistance on a full size ship.

A Froude number of 0.40 is considered "hull speed". If two boats are identical in every respect except length, the longer boat will be faster with the same power. They will travel at about the same Froude number with the same power, but since Froude number is relative to length, the longer boat is faster.

 

A boat creates a number of different waves. However...the ones relating to hull speed are the bow wave and the stern wave and the separation between the two. The faster waves travel the greater the separation between them. The bow creates a wave as the boat moves through the water. At low speeds this wave has a trough behind it that crests only part ways aft along the hull. Once the boat reaches "hull speed" the wave trough runs the entire length of the hull and a bit aft before it crests again. Here you see the stern of the boat squat because it is down on the face of the stern wave...now a following wave and it is trying to climb the bow wave. This is the maximum speed that he boat can travel in Displacement mode... or it's Hull Speed. If the shape is right and the power is sufficient...it can then start to semi-plane or start riding the crest of the bow wave as it moves further aft under the hull. If still more power and optimum shape is there...you get up to full plane. Hull speed occurs between 1.2 and 1.5 times the square root of the waterline length...or the distance between the crests of the waves. The reason there is a variable there is because some hulls are more efficient and create smaller waves...so the crests aren't as big and the trough isn't as deep so they can cheat a bit and go a little faster. These hulls are generally very slim and carefully shaped. A fat tub of a hull with lots of rocker and displacement will generally not make it to near what the slim hull will...even with the same waterline length. A good indication of a hull's ability to extend it's reach in the displacement mode is the run aft or buttocks lines. If there is an almost straight line from the lowest point of the hull aft to the transom and the transom is somewhat wide then the boat has more support aft and can hold itself up a little further into the trough than one that has a narrow stern with buttocks lines that curve a lot. Hulls that slice through the water rather than push it aside and down or are very shallow and ride mostly on the water can also extend their hull speed to the higher ranges of 1.5 or 1.6 the waterline length. These are general descriptions and there is much room for interpretation but I hope this gives an idea of the theory.

 

At exact hull speed the boat sits in the wave it created itself and rides in its trough.
Below hull speed it's own wave/s lifts part of the boat.
Above hull speed the wave is longer than the boat and it has to constantly climb its own bow wave. Here is a pic I found on the net:
http://www.northcoastjournal.com/media/issues/041008/SCI-hull-speed-graphic.jpg

 

GRAVITY
The physics of a wave in deep water is similar to a swing. Both work on the restoring force due to gravity. If you pull a swing back and let it go it will oscillate. At the mid position the swing will have velocity but there is no force. As it swings past the mid point the restoring force increases while the velocity reduces.

FREQUENCY
The frequency of oscillation is controlled solely by the length of rope or chain supporting the swing.

If you disturb water, waves emanate from the source of the disturbance. The water particles move in a circular motion under the influence of gravity. The frequency of oscillation is solely a function of the length between wave crests; not the height of the wave.

PHASE VELOCITY
The speed of propagation of the wave energy, or phase velocity, is solely a function of gravity and the wavelength.

So hull speed is the phase velocity of the wave train that matches the waterline length of the hull. The velocity of propagation of the wave energy is the same as the boat. The boat is riding the wave it creates. Going faster means the boat is leaving the wave energy behind so has to put considerable more energy into the water to create the disturbance unless it is a slender hull. With planing the boat gets dynamic lift and the water disturbance diminishes so the energy going into wave making reduces.

Rick W

 

This was also covered in another thread. See post #81 on this page:

http://www.boatdesign.net/forums/boat-design/center-flotation-calculation-implications-30857-6.html

Eric