Why do boats don’t have gears? All you need to know

Gears are essential for cars or any other vehicles because they will increase the speed of cars and or other vehicles. Not only the speed, but it also uses power and energy effectively. Then you might be wondering why can’t we use gears in boats. Let’s see why do boats don’t have gears or transmissions first.

One of the essential reasons why boats don’t have gears is that the boat’s performance without gears vs. with gears isn’t increasing much, and it requires more power and money. Instead of yielding a good performance, gears are giving unneeded complexity. So, that’s why boats don’t have gears.

That’s one of the essential reasons why boats don’t have gears. And there are some effects of running the propeller at high speeds such as cavitation on a propeller, unnecessary energy loss, increase in fuel economy, etc. With all these still, we are not getting the required output (increase in speed).

The high-performance boats have gears. But for most normal boats, the gears add a lot of complexity and, most of all cost. Meaning, it would only add unneeded complexity for no gain, and if you have transmissions or gears for a boat, then for every transmission or gear change you are making is a source of energy loss.

Now, we see how the increase in speed will affect the propeller, and then we will see some of the effects of having gears on a boat.

Some of the effects of spinning the boat propeller faster (increasing the boat speed)

1. Causes cavitation on the propeller. Cavitation is another issue with increasing the propeller speed. When the propeller rotates quickly or fastly, it will create a low-pressure area on the propeller blades, which boils the water and forms steam bubbles on the propeller’s surface. And they will pop after some time, and while poping, it creates a tiny explosion, which will damage the propeller over time (1 year).

Now, you might be wondering how the water boils at the normal temperature inside the water. Water boils at 100 degrees celsius, and if you decrease the pressure, the water starts boiling at lower temperatures. With a significant decrease in pressure, you can boil the water, even at room temperature.

Rotating the boat propeller quickly or fastly creates a low-pressure area on the propeller blades, thereby boiling the water and forming steam bubbles on the blade edges, which will damage the propeller blades over time (1 year) if the bubbles pop on the propeller blades.

However, there are super cavitation propellers, which will reduce the cavitation effect completely (mostly) by making the bubbles to pop after leaving the propeller edges at the end. But, these propellers are used for military purposes and high-performance racing boats as well as model racing boats, not for normal boats (generally) (source).

2. Increases water friction. As you spin the propeller faster and faster, it gets harder and harder to turn—also, the drag of the water against the hull increases, requiring more effort to push the boat in the water. So, a boat in the water is like a car that is going up a very steep hill. It needs to gear down to maintain speed or accelerate. With a boat, it works out that you only need one or two gears.

Water behaves very differently from the air. Water doesn’t like to be pushed out of the way by any object (boat), and the harder you push the water out of its way by the boat, the harder it pushes back (commonly known as water friction, which will be more while pushing the water out of its way by a boat in the water).

If you try to increase in boat speed, it increases water friction as well. Meaning, to move the boat faster in the water, it needs more power (gas) to push the water out of its way. The increase in boat speed is canceling, with an increase in water friction and fuel consumption, resulting in us paying extra money for a minimal increase in speed or performance.

Some of the effects of having gears on a boat

  1. It costs more. Having gear on the outboard motor or I/O will cost more. We need to pay so much for a minimal increase in speed, and the maintenance of the gears is another big work to do, and it should be done regularly to reduce any damages in the future.
  2. Performance isn’t increasing much. There is no dramatic increase in the performance of the boat without gear vs. with gear. If there is no huge performance increase, then using a boat with gears is not much effective.
  3. Causes cavitation on propeller blades. Cavitation can damage the propeller. If you rotate the propeller fastly with gears, then it will create more cavitation, which damages the propeller furthermore in the long term usage.
  4. It is increases complexity. Let’s not talk about high-performance boats because the majority of the people won’t use them. But for most normal boats, the gears add a lot of complexity, and maintenance would increase for a boat with gears, creating a lot of problems for the users.

Many people are okay with having no gears on a boat due to all those added complexities for an outboard or I/O motor. And one more thing to note here is different gears are used to go at different speeds, but boats mostly go at the same pace all the time.

Boats no need to stop and go like a car in traffic or on the road. For cars, gears are essential because they need to change the RPM every time (such as in traffic or if someone comes in the middle). But boats are not like that; they no need to change the RPM often as cars do.

Alternatives for boat gears

Okay, first of all, there is no alternative for gears. A variable-pitch propeller is a kind of alternative for gears. By changing the pitch of the propeller, you can go at different speeds. (Pitch means: Suppose if the pitch of the propeller is 20 inches, then for one complete rotation (360 degrees) of the propeller will move the boat 20 inches in the forward direction).

But here the pitch is not that; it is different. A variable-pitch propeller will have rotation kind of thing for each blade, meaning you can rotate the individual blades of the propeller. By rotating the blades alone will cut the water at different angles, making the boat to move faster.

Many people will refer to variable pitch propellers as alternatives for gears, but it’s not. Check this small video on how the blades will rotate for a variable pitch propeller.

First, starting at a low pitch and then increasing the pitch in a step by step order will act like gears. A small pitch propeller will give less thrust, and a big pitch propeller will provide more thrust (push) for the boat. You can find these variable pitch propeller on the ships, not on small boats (generally).

Boats have gears (not forward gears, reduction gears)

I am not talking about the high-performance boats which have gears. I am talking about normal boats. Every boat has gears, but they are not the same as gears in cars for increasing the speed. Boats have reduction gears. Before going into reduction gears, let’s see the basics first.

The propeller shaft drives the propeller, and the engine drive shaft drives the propeller shaft. The propeller rotates at the same speed as the propeller shaft, because it’s fixed to the propeller shaft. The propeller shaft turns slower than the engine drive shaft due to reduction gears between the engine drive shaft and propeller shaft (source).

That is the reason why the engine rotates at one speed (higher speeds), and the propeller rotates at different speeds (lower speeds). They are intentionally decreasing the speed of the propeller rotation to reduce cavitation on the propeller blades.

The essential takeaway from this is (Boats have gears) even at lower rotation speeds of the propeller (without gears), it is creating cavitation. Again if you increase the speed of the propeller by installing gears, it would furthermore increase cavitation on the propeller blades, which damages the propeller over time (1 year or less).

Bottom line

Boats don’t have gears because they will create a lot of unnecessary complexity for a boat’s engine, and many people don’t want that. And the performance of the boat without gears vs. with gears isn’t increasing much. That small or minimal increase in boat performance is coming at higher costs.

So, boats don’t have gears. For that small increase in performance, we need to spend more money, and it’s creating many problems such as cavitation, energy loss, increasing complexity, etc. Maintenance would be more and difficult for a boat with gears.

And one more thing to note is mostly boats move at constant speeds (meaning, often there won’t be any significant decrease or increase in the speeds, like cars). Different gears are used to go at different speeds, but the boat won’t go like that; they go at a constant speed (mostly). So, boats don’t have gears due to all those problems.


My name is Mahidhar, and I am passionate about boating. Every day I learn some new things about boats and share them here on the site.

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