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Boat Builder

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Which type of boat hull is the most efficient for transporting heavy equipment? What about going fast in calm waters? Which type of boat hull do you choose if you need to make a long trip in rough seas? Your challenge is to find out which types of boats are the best suited for these different situations. Are you ready? Then it’s time to hit the high seas!

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What is a displacement hull?

A displacement hull has a rounded shape and pushes the water around the hull when it moves. Displacement hulls are typically limited to lower speeds because of this. However, they tend to be very stable in rough surface conditions and can carry heavy loads. An example of a displacement hull is a round hull. Displacement hulls are very efficient at low speeds, and as such they typically don't need powerful motors.

What is a planing hull?

A planing hull is a type of hull that is designed to allow the boat to plane. Planing is when the boat rides on top of the water instead of moving through the water like a displacement hull. Planing can only be achieved by special hulls at high speeds; thus, planing boats usually have a flat hull and a powerful motor.

What is a semi-planing hull?

A semi-planing hull is a combination of a displacement hull and a planing hull. At low speeds it can act like a displacement hull, but it can also plane (though not as well as a pure planing hull) at high speeds. It isn't quite as stable in rough seas as a pure displacement hull because it has a shallower shape, but can go much faster than a displacement hull. An example of a semi-planing hull is a shallow vee hull. This type of hull is shaped like a gentle V with the deepest part at the center of the boat.

What factors impact the resistance of a hull?

As a ship moves through calm water, the ship experiences a force acting opposite to its direction of motion. This force is the water’s resistance to the motion of the ship, which is referred to as “total hull resistance” (RT). A ship’s calm water resistance is a function of many factors, including friction and viscous effects of water acting on the hull, the energy required to create and maintain bow and stern waves, and air resistance. These factors are all influenced by the ship speed, hull form (draft, beam, length, wetted surface area), and water temperature.

How does speed affect a hull's resistance?

Total hull resistance increases as speed increases as shown in the figure below. Note that the resistance curve is not linear, but increases more steeply at higher speeds. The humps in the graph are due to resistance due to waves generated by the ship.

							
								Line graph of total resistance versus ship speed. Resistance increases slowly at first, has a small peak, then increases sharply

What is viscous resistance?

Water is not an ideal fluid, and therefore an object moving through water will experience resistance. The figure below shows a hull submerged in a real fluid with viscosity. Fluid particles cling to the body, resulting in the formation of a “boundary layer,” where the flow rapidly changes speed, from zero speed at the side of the body, to the free-stream speed. Two forms of resistance happen as a result of viscosity: Friction Resistance and Viscous Pressure Resistance. Friction arises from the shear stresses in the fluid and acts tangential to the body. Viscous pressure resistance acts normal to the body.

							
								Diagram showing flow of water around a ship's hull

What is friction resistance?

Viscosity is a temperature dependent property of a fluid that describes its resistance to flow. Maple syrup is said to be a very viscous liquid; the fluid particles in syrup being very resistant to flow between adjacent particles and to other bodies. On the other hand, alcohol has a low viscosity with little interaction between particles. <line-brake> <line-brake> As a ship moves through the water, the friction of the water acting over the entire wetted surface of the hull causes a net force opposing the ship’s motion. This frictional resistance is a function of the hull’s wetted surface area, surface roughness, and water viscosity. Although water has low viscosity, water produces a significant friction force opposing ship motion.

What is viscous pressure resistance?

In the forward portion of the hull pressure forces act normal to the surface; however, in the aft portion of the hull the boundary layer reduces the forward acting component of pressure. This reduction in the forward acting component results in a net resistance force due to pressure acting on the hull. This increase in resistance due to pressure is called “viscous pressure drag” or “form drag”, and is sometimes also referred to as the normal component of viscous resistance.

As you might expect, the shape of a ship’s hull can influence the magnitude of viscous pressure drag. Ships that are short in length with wide beams (a low length to beam ratio) will have greater form drag than those with a larger length to beam ratio. Also, ships that are fuller near the bow (e.g. bulk oil tanker) will have greater form drag than ships with fine bows.

The viscous resistance of a ship is: Rv =Cv 1/2 (ρV2S) where: Cv = coefficient of viscous resistance ρ = water density (lb-s2/ft4) V = velocity (ft/s) S = wetted surface area of the underwater hull (ft2)

Ships are often designed to carry a certain amount of payload (weight and volume) at a given speed. Therefore, the means of reducing Viscous Resistance for a design is to reduce the coefficient of viscous resistance or to reduce the surface area for a given volume. A sphere has the smallest wetted surface area per unit volume, but it would be expected to create a lot of waves at the surface.

Increasing the length of a ship, and reducing beam for a given speed tend to reduce the viscous resistance coefficient; however this increases wetted surface area. Thus, the design of a ship is a trade-off between a sphere (minimal wetted area) and a toothpick (minimum viscous coefficient), with suitable concerns for stability and seakeeping added in.

What is wave resistance?

Wave theory states that the energy in a wave is proportional to the square of the wave height. Since the energy in a wave depends on the square of the wave height, any increase in wave height requires a subsequent increase in energy required to create the wave and an increase in wave making resistance. Thus, if wave height doubles, a four-fold increase in energy required to create the wave occurs. Therefore, as ship speed increases and wave height increases, wave making resistance becomes dominant.

In the design phase of a ship there are two things that can be done to reduce the effects of wave making, and therefore improve the performance of the ship: • Increasing length of the ship increases the speed at which the length of the wave system generated by the ship is equal to ship length and therefore reduces the impact of wave making resistance. • Bulbous Bows. Bulbous bows are one attempt to reduce the wave making resistance of surface ships by reducing the size of the bow wave system. The idea behind a bulbous bow is to create a second bow wave that interferes destructively with the bow divergent wave, resulting in little to no wave at the bow.

What is air resistance?

Air resistance is the resistance caused by the flow of air over the ship with no wind present. This component of resistance is affected by the shape of the ship above the waterline, the area of the ship exposed to the air, and the ship’s speed through the water. Ships with low hulls and small “sail area” or projected area above the waterline will naturally have less air resistance than ships with high hulls and large amounts of sail area. Resistance due to air is typically 4-8% of the total ship resistance, but may be as much as 10% in high sided ships.

What is antifouling paint?

Antifouling paint is a special type of paint that kills barnacles and other organisms that would otherwise grow on the hull. If organisms grow on the hull, the hull could be damaged and would be subjected to much more drag, therefore decreasing the efficiency of the boat by up to 15%. Modern antifouling paints also often protect against corrosion on the hull and improve the flow of water past the hull, and therefore increasing the efficiency of the boat.

One type of antifouling paint is made from copper (I) oxide (cuprous oxide). Other paints include biocides that limit the growth of algae, barnacles, and other marine organisms on boat hulls. Some bottom paints are designed to fall off as they move through water. As the paint falls off, any organisms attached to the hull also are removed from the hull. Other types of antifouling paints include those infused with Teflon or other slippery silicon finishes.

What are problems associated with antifouling paints?

Copper-based antifouling paints have been reported to show an adverse effect on marine life. High concentrations of copper have been found the waters of harbors and other high-traffic boating areas. Antifouling paint particles can be eaten by zooplankton or other marine species and move through in the food chain. Organisms at the secondary and tertiary consumer levels often exhibit high levels of copper in their tissues as a result of bioaccumulation.

What is the difference between an outboard motor and an inboard motor?

An outboard motor is attached to the rear of the boat. The propeller is located on the bottom of the motor. When the boat turns, the entire motor turns to push the boat in that direction. Because of this, no rudder is needed. Inboard motors, on the other hand, are housed inside the hull, and the propeller extends to the rear of the boat. A rudder is used to control the direction that the boat moves in. Inboard engines typically last longer than outboard engines, because they are sheltered from the elements.

What is effective horsepower?

“Effective Horsepower” is the power required to move the ship’s hull at a given speed in the absence of propeller action. It is equal to the product of the resistance of a ship and the speed of the ship. The largest losses in the system are the thermodynamic and mechanical losses in the engines, which cause the loss of roughly 60% of the fuel energy before it becomes rotational power at the output of the engine. These losses include: exhaust = 30%, engine cooling = 27%, and internal friction = 4%. Additionally, the propeller accounts for 15% energy loss resulting in a net thrusting energy of only 24% of the original 100% energy input into the engine.

What are trim tabs?

Trim tabs can be attached to the bottom of outboard motors to help adjust the angle at which the boat moves through the water. Trim tabs can be adjusted depending on the speed of the boat and the surface conditions. They help keep the front end of the boat lower and helps level the boat in the water, which both makes it easier for the helmsman to see and control the boat.
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