Boats and cars help us to cross oceans and borders. That doesn’t mean that they are fast enough. Both of them lack in speed. But, if it is cars vs. boats, then which one will go faster?
Boats are slower than cars because water is denser than air, and the boat needs more power to push the water out in its way (more friction). At the same time, cars are just moving through the air by pushing them away (less friction). So, boats are slower than cars.
The main reason behind the slowness of a boat is friction (water friction). When you keep the boat in water, it displaces the same amount of water space in the water, which is equivalent to the weight of the boat. Meaning, if your boat weighs 1000 pounds and you kept your boat in water, the boat displaces the same space in the water, which is equivalent to 1000 pounds.
Whenever you are boating in water, the boat is displacing (pushing the water away) the boat’s weight of water, every time. So, it requires more power to displace (push the water away) the water on its way. Since the water is denser, the boat requires more energy to push the water out of its way. Water friction is making a boat to travel slowly.
On the other hand, the car is not pushing anything which is denser than water on its way. The car has to overcome its frictional components in the drivetrain or driveline, friction between the tire and the surface (road), and the air. And the car is moving (driving) against a solid surface (road), unlike the boat is moving against a fluid surface (water).
The boat has to overcome more dense medium (water) on its way, as well as air. So, it takes more power to move on its way. On top of its (boats) own weight, it also must move an almost equal weight of water out of the way. Meaning the boat is moving twice its weight of water out of the way. The boat can travel faster, but not on the water.
An example of water friction is running in water vs. running on the road. If you run in water (say 2 feet depth water), you will feel the drag (you can’t run fastly in water). In the same way, the boat can’t go fastly in water due to the drag caused by the water. It is because the water is denser than air.
Similarly, if you run on the road, you will not feel any drag (you can run faster on the road than in water). In the same way, the car can go fastly on the road. It is because the air is less dense than air. Air is around 800 times less dense than water.
Check my article on Essential factors that affect the boat speed (don’t worry, it will open in new tab) to know more about how the boat hull design affects the boat speed.
Factors affecting the speed of the boat against a car
The main things that affect the boat speed are friction (water friction), boat design, and performance, its conversion from horsepower to thrust.
- Friction (boat vs. car)
- Design (boat vs. car)
- Performace (boat vs. car)
These are the main things that affect boat speed. Check my article on Why boat speed is affected in shallow waters? (don’t worry, it will open in new tab) to know more about what affects the boat speed in shallow waters in detail.
1. Friction (car vs. boat)
The car has friction between the car tire and the surface (road), which is solid-solid. At the same time, the boat has friction between the boat hull and the water, which is solid-liquid.
For a car, the friction is between the tire and the road. For a car, friction is more when it is stable, and the friction reduces if you start moving the car. Meaning while the car is moving, it has less friction, which makes it move fast on the road.
For a boat, the friction is between the water and the boat hull (solid). Friction is more when the boat is moving fast, and it is less when the boat is moving slowly. Meaning while the boat is moving fastly in the water, it has more friction, which makes it move slowly in water.
And another friction is the air, which also slows down the speed of a car and a boat. Which is also known as drag (Drag is a force acting opposite to the motion of any object moving, in our case air). Next, we will see how the boat and car speeds are affected by design.
2. Design (boat vs. car)
Friction can be reduced with design. If the design of a car or boat is so smooth and streamlined, then it reduces the friction and helps the boat or car to go fastly. A flat design at the front of a car or boat increases the friction (for both car and boat), which slows down the boat or car.
If the boat is more streamlined, the less water needs to be pushed aside by the boat. Similarly, for a car, if the car is more streamlined, then the air will slide over the car quickly. For both cars and boats, the streamlined design will increase speed.
Example of a car: Take an example of a sports car, a sports car is designed long and smooth – streamlined. The air can slide over it quickly. But if the front design of the car is flat, such as trucks or big vans, then there is much more friction, and it slows down the car. Then the air can’t slide over it quickly; instead, the air pushes against the front of the car. Then the car won’t be able to go fast enough.
Example of a boat: Take an example of a spirit of Australia (fastest boat in the world), it has a smooth, long hull that will be perfect for sliding the water quickly. But if the front design of the boat is flat, such as barge boats, then there is much more friction, and it slows down the boat. Then the water can’t slide over it quickly; instead, the water pushes against the front of the boat. Then the boat won’t be able to go fast enough.
Overall, the design affects both the car and boat (if it has a good streamlined design, then they will move faster). But for a boat, the design doesn’t affect much because the boat is already moving in a more dense medium. At the same time, the car is moving in the less dense medium. (source)
Even if we change the design of the boat and the car (more streamlined), still, the boat can’t move as fastly as a car on the road because the boat is moving in a more dense medium (water) than a car (air).
3. Performance (boat vs. car)
If we keep the same horsepower engine in a car and a boat, still, cars will move faster than a boat because there is a huge drag in the water than in the air medium. For example, if we keep a 400 hp engine in boat and car, then the car will go around 150 mph (miles per hour) or more. And a boat with the same weight might go at 50 mph. It is because of the drag created in the water (source).
The massive difference between the mph is due to friction and propeller output.
Friction is a known thing: as I discussed above, water is denser than air. So there is more friction in the water while boating than driving a car (cars have only air friction and friction between tire and road). Boat needs to push twice its water on its way. At the same time, cars need to push air out of the way.
Propeller output: For a car, it takes only a few horsepowers to get a good speed on the road (because less air friction and less friction between the tire and road). For a boat, it is not like that, as I said earlier, the boat has to move almost twice its weight in the water, and it needs more power as well, and it consumes more fuel or gas as well.
Boat propellers try to increase the speed; they do a great job. But there is less productivity (efficiency) between the engine in an outboard motor block and the propeller. At the same time, there is more productivity (efficiency) between the car engine and the wheels.
And another problem is cars have gears (4 or 5 gears). Meaning you can increase the speed if you want. But, for boats, it is not like that because it has only one gear. Even if you increase the rpm of a propeller, the speed of the boat doesn’t improve much because the water will slide from the propeller more quickly.
Cars are faster than boats because the car has to push the air out of its way (less friction). Whereas for a boat, it has to push the water out of its way, which is denser than air (more friction). And boats are pushing twice its weight in the water, so the boats move slowly than cars.
Even if the car and the boat have the same horsepower engine and the same weight, the boat won’t go faster than a car. It is because of the water friction.