Relating speed and engine horsepower of a ship to water resistance

In summary, the force of water resistance opposing the John F. Kennedy's motion at its top speed of 35 knots, with its engines at full power and 70% of that power being used to push the ship through the water, is equal to 208,880,000*35/746 = 9,808,128.6 N.
  • #1
wowser
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Homework Statement


The aircraft carrier John F. Kennedy has mass 7.4*10^7kg. When its engines are developing their full power of 280000 hp, the John F. Kennedy travels at its top speed of 35 knots. If 70% of the power output of the engines is applied to pushing the ship through the water, what is the magnitude of the force of water resistance that opposes the carrier's motion at this speed?



Homework Equations


Kinetic energy: .5mv2
Work-energy theorem: Work=.5mv22-.5mv12
Power = Work/Time
Speed = meters/second
Work=Newtons*meters
1 hp= 746 Watts

The Attempt at a Solution


Convert horesepower to watts: 280000*746=208880000
I then tried to muliply this by the speed (forgetting it was speed rather than time) to find the work (forgetting that this wasn't equal to the force of the ship pushing against the water), then said that was the answer due to Newton's third law. Obviously, it didn't work. I am at a loss.
 
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  • #2
Try this:

Power = Work/time = (Force*distance)/time = Force*velocity
 

FAQ: Relating speed and engine horsepower of a ship to water resistance

1. How does the speed of a ship affect water resistance?

The speed of a ship directly affects the amount of water resistance it experiences. As the ship moves faster, it creates a larger bow wave and pushes more water out of the way, resulting in increased resistance. This means that the faster a ship moves, the more power is needed from the engine to overcome this resistance and maintain its speed.

2. Does the engine horsepower of a ship impact its speed?

Yes, the engine horsepower of a ship is directly related to its speed. Generally, the more horsepower an engine has, the faster the ship can move. However, other factors such as the design and weight of the ship, as well as external conditions like wind and waves, can also affect its speed.

3. Can a ship's speed and engine horsepower be used to calculate water resistance?

Yes, the speed and engine horsepower of a ship can be used to calculate its water resistance. This is because the amount of horsepower needed to overcome water resistance is directly proportional to the speed of the ship. By knowing the speed and engine horsepower, scientists can use mathematical equations to determine the amount of water resistance the ship is experiencing.

4. How does water resistance impact a ship's fuel efficiency?

Water resistance has a significant impact on a ship's fuel efficiency. As mentioned before, the faster a ship moves, the more power is needed from the engine to overcome resistance. This means that ships with higher speeds and more powerful engines will require more fuel to maintain their speed, resulting in lower fuel efficiency.

5. Can changes in a ship's weight affect its water resistance?

Yes, changes in a ship's weight can affect its water resistance. A heavier ship will experience more water resistance due to its increased mass, requiring more power from the engine to maintain its speed. Conversely, a lighter ship will experience less water resistance and require less engine power to maintain its speed. This is why ships must be carefully designed and loaded to optimize their weight and minimize water resistance.

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