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Ships resistance

The resistance a ship can be divided into 5 main reasons.

Frictional losses
Pressure resistance
Wavemaking resistance
Resistance due to drift

Resistance due to friction.

This is the resistance due to friction of the water along the hull.
It is depending on the roughness of the hull, the area of the submerged hull, the type of boundary layer, and of course the ships speed.
It is the resistance created by the water the hull surface is speeding up close to the surface.
It is the resistance that creates the boundary layer.

For most people it is easy to imagine how the wall roughness is influencing this.
The rougher the hull surface the more water is dragged with it.

Also the influence of the area size of the hull is comprehensible for most people.
The more are the more water is dragged with it.

The type of boundary is a little harder to comprehend.
In general there are two types of boundary layers: the laminar and the turbulent.
The laminar boundary layer is a nice smooth boundary layer.
Very close to the hull its speed is 0 and the farther away from the hull the higher the speed becomes, until it reaches the ship speed.
This laminar flow has a low frictional loss, but is not easy to maintain when it gets thicker.
Even the smallest surface irregularities might disturb is, resulting in it becoming Turbulent.
A Turbulent boundary layers has a higher friction, and is in general not smooth layered, but is more turbulent, as its name already makes clear.
A turbulent layer has compared to a laminar boundary layer a smaller part in which the speed is very low.

Typical a boundary layers is the first meter laminar, and more aft becomes turbulent.
The rougher the hull and the higher the speed the easier is becomes turbulent.

The thickness of this boundary layer is exaggerated for this length of plate.
For a nice clean hull and very low speeds the boundary layer can be almost fully laminar.
In heavy weather, planing and with a dirty hull there is virtually no laminar part.
Of Course the thickness of the boundary layer grows when it comes more to the stern, more water is slowed in that time.

The frictional resistance of a sailing boat can be minimized by having a nice clean hull, and moving around slowly to not disturb the laminar boundary layer, especially not in light weather.

There are some other options to reduce the frictional loss, but in my humble opinion they are too complicated to use on a sailboat.

  • Suction of the boundary layer, by drilling small holes in the hull where the boundary layer is being sucked through, resulting in virtually no boundary layer and no frictional loss.
  • Energizing the boundary layers, through some slots directed aft water is being pumped at high speed reenergizing the boundary layer.
  • It is logical to combine these, since the sucked up water needs to be dumped somewhere.
    This technique uses small holes, that can be blocked easily.
  • Moving the wall, If the hull is moved around like a conveyor belt the shell has no speed through the water, and therefore no frictional loss.
  • Accelerate the molecules by electricity, like JLN lab
  • Where the boundary layer would be having a liquid (or air) that has a lower viscosity. Catching air between the hull and the water like a Hoovercraft is an example of that. Other examples is to leak some less viscous liquids from your foredeck, or heat the hull.
  • Directing the turbulence with small scratches. A sharkskin uses this principle.
    This has been used for rowing boats, I believe manufactured by 3M, but was forbidden by the International Rowing association.

  • pressure resistance.

    If the flow around an object is nicely streaming and is after the object more or lees undisturbed this means no energy has been put in disturbing.
    Is the flow around the object breaking loose and creating a lot of disturbance after the object the pressure resistance is high.
    This pressure resistance is often also called shape resistance, since it is a lot depending on the shape of the object.
    This resistance is depending on the streamlines around the object, If you now the streamlines the pressure resistance can be calculated.
    Please take a look at the picture below, It shows that the triangle has a much higher pressure resistance than the circle.
    For the circle the streamlines after the object are undisturbed, while for the triangle the streamlines have shifted.
    An other way to look at this is that for the triangle the force C which is directed partly forward is missing.

    The path of the streamline is also depending on the boundary layer. If the circle was rough and would have a thick boundary layer the flow could look like this:

    It is proven in real life that a turbulent boundary layer due too its thin layer with virtually no speed prevents flow separation.
    This means a turbulent boundary layer results in a lower pressure resistance.

    This Explains why some professional speedskaters have some irregular stitching at strange places. The same is done with a Golf ball. The dints result in a turbulent boundary layer.

    This are nice tricks, but unfortunately not applicable to a ships hull.
    The boundary layer is after a meter or so anyway turbulent.
    The hull should be as smooth as possible.

    Interesting to notice is that for some ships and most cars the pressure resistance would be lower if they would go in reverse.
    This can be explained by the manual design of most cars and ships, and feeling says a triangle with its point first gives less resistance as a triangle with a flat end.
    The aft design -where the "pressure recovery" takes place- is in general of more importance than the front end, but who looks aft?

    wave making resistance.

    A ship makes waves when it sails. In this wave energy is stored. This energy is the wavemaking resistance.
    A wave is produced by the curvature of a ship.
    Around a ship changing the direction of the water results not only in a pressure difference, but also in a change of waterlevel.
    The more and the sharper the water is changed in direction, the bigger the waves.
    Positive pressure results in a higher level, negative pressure results in a lower level.
    Typical high pressure is built at the bow, where the water is deflected from the hull. For now I name this "bow hill"

    A low pressure is built where the water is directed away from the hull like where the greatest width of the ship is.
    For now I name the resulting lower level "shoulder valley"
    At the stern the water is being deflected inwards, resulting in a higher pressure, for now I call this "stern hill"
    Water has the tendency to level out, else we would use dragliners instead of locks etcetera.
    This means that the hill will fall down, and even will fall further down creating a valley.
    The valley will rise up again to a hill, and so it keeps repeating.
    In the time the hill has become a valley and a hill again you have sailed a distance.
    If this distance you have sailed in the time a hill becomes a valley is half the ships length the bow hill has becomes a valley where the ship already creates a valley, thus giving more waves.
    Also the valley from midships has become a hill at the stern, thus amplifying the stern hill. Producing more waves means a higher wave making resistance.
    This specific situation where the wave pattern is amplifying each other is called hull speed.
    Your wave pattern will look like this:

    It is a clear peak in the resistance curve for many ships.
    (The hull speed can be calculated in km/hr by the formula 4.5*Squareroot(length), in which length is in meters.
    A ship of 6m has a hull speed of 11 km/hr.)

    Beside amplification of the waves there is also the possibility to make them cancel out eachother.
    If one travels half the ships length in the time the bow hill becomes a hill again it cancels out the shoulder valley.
    Then one makes little waves, and the wavemaking resistance is low.
    If one exceeds the hull speed this is in general called Planing.
    It comes for many ships with a steep drop in resistance, because the wave pattern is no longer amplifying.
    And low wavemaking means low wavemaking resistance.
    In planing conditions one is more or less sailing on the bow wave and one is therefore a little higher as at hull speed, where one is sailing in its shoulder valley.
    This Rise of the boat must nut be confused with the lift a waterski produces.
    A waterski works because the aft end of the ski is directed down, thus deflecting the water down, and the ski up.
    This waterski effect is needed for proper planing, else the ship will suck itself down, like a tugboat that has big engines, but is directing the water up at its stern, and thus is sucking the boat down.

    That is why some small motorboats have "Planing flaps" below the waterline.
    This isto prevent being sucked down at planing speeds.

    A catamaran, a race row boat have low curvature in the length direction, This means all waves will be low compared to an ordinary wide hull.
    The wavemaking resistance becomes relative low, and the peak at hull speed is more hard to notice.
    Hull speed is not limiting them as much as on a normal ship.

    Some ships have less pronounced curvature when heeling, what results in less wavemaking resitance.

    Resistance by drift.

    Especially when close hauled the boat is drifting, making leeward.
    This because the sailing force is more directed to the side as forward.
    This result in the boat not going straight through the water but under an angle.

    If one is making much leeward way a high drift resistance occurs.
    This is comparable with the resistance when using a lot of rudder angle.
    This because the force directed to the side is coming a little into the direction one sails.
    Note the difference between the direction one sail and the direction where the bow is pointed to.
    In other words, one needs a lot of force to pull the boat sideways though the water.

    To reduce this resistance one needs a big, well shaped, deep, keel to minimize the sideways slip. A relive deep keel does the job better as a shallow keel, just as with the sails.
    A deep keel has the disadvantage that the point where the sideforce also becomes deep, resulting in more heeling.
    Often this is compensated because the weigh of the keel is also placed lower.
    Ther is not much one can do to tune the keel, the only thing is to keep it clean, to prevent flow separation. The hull sometimes also helps preventing drift.
    Principle of driftresistance stays the same.

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