Why don’t boats have transmissions? (like cars) I will school you

Many people know that boats lack gears as that of cars. But why don’t boats have transmissions like cars or other vehicles? Wouldn’t that make them be able to go faster? Well, you are in the right place to know all the answers, and you can do that by continuing the article.

Boat engines don’t have transmissions because they add a lot of undesired complexity for a minimal speed gain. Usually, the propeller doesn’t turn at speed equal to its engine; if it does, it cavitates & reduces its life span. Adding gears increases speed, which increases cavitation, so no gears.

Even if the propeller rotates at its engine speed, it cavitates. And if we add transmissions to increase the speed, it cavities more, so no gears in boats. But, the boat engines have reduction gears that turn the propeller at a slow speed than the engine speed to prevent cavitation on the propeller at engine speeds.

NOTE – Check the effects of having transmissions in boats below to know more about the propeller cavitation.

The “propeller” is like wheels for a boat. A car’s speed depends on how fast the wheels rotate. Similarly, a boat’s speed also depends on how fast the propeller rotates. If the propeller rotates fast enough, you can go fastly; otherwise, you can’t go fastly. But, if the propeller rotates fast, it begins to cavitate.

That’s why boats don’t have transmissions like cars and other vehicles. Some high-performance boats do have gears, but the normal boats don’t have gears.

Why don’t boats have transmissions? The problems of having transmissions in boats

Wait, there are effects of having transmissions in boats? Increasing the boat speed (rotating the propeller fastly) will create more problems than benefits? We will now see the effects of having the transmissions in boats.

It causes cavitation on the propeller

Cavitation When the propeller rotates fast, it will create a low-pressure area on the propeller blades, which boils the water, thereby forming steam bubbles on its surface. And when they pop after some time, it creates a tiny explosion on the propeller’s surface, which will damage the propeller over time (1 year or more).

Now, you might be wondering how the water boils at the low temperatures inside the water. Usually, water boils at 100 ℃, 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.

Fastly rotating the boat propeller 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 or more) if those bubbles pop on the propeller blade’s surface.

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

Increases water friction

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, the harder it pushes back (friction). If you push the water fast with a boat, they resist and push it back.

As you spin the propeller faster and faster, it gets harder and harder to turn—also, the drag of the boat against the water 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. If you try to increase boat speed, it increases water friction and needs more power (gas) to push the water out of its way. Even if you increase the speed, the resulting losses are more for the smallest increase in speed.

That small increase in boat speed is canceling with an increase in water friction, fuel consumption, and also putting more pressure on the engine, resulting in us paying extra money for a minimal increase in speed or performance.

Adds undesired complexity for a small benefit

Having transmissions on the boat engine will cost more. Requires us 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.

Unlike car engines, we can’t simply hold a boat in the middle of the lake and repair it if anything happens. If the boat has an inboard engine, the boat needs to get out of the water to inspect it. Boats require a lot of maintenance; even unused boats need to be maintained well.

Many people are OK without transmissions on a boat due to all those added complexities for a boat engine. And one more thing to note here is different gear levels 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, transmissions 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 as often (quickly) as cars do.

Boats have reduction gears (they are not what you think)

I am not talking about the high-performance boats which have gears. Every boat has reduction gears, but they are not the same as transmissions in cars for increasing the speed. The propeller rotates at the same speed as the propeller shaft because it’s fixed to the propeller shaft.

And the engine drive shaft is connected to the propeller shaft. The propeller rotates slower than the main engine due to the reduction gears between the engine drive shaft and propeller shaft. It makes the engine drive shaft rotate at one speed (higher speeds), different from the propeller (lower speeds).

They are intentionally decreasing the speed of the propeller rotation to reduce cavitation on the propeller blades. If the propeller rotates at the engine drive shaft speed, it cavitates. Increases further if transmissions are used (source).

Even at normal speeds (without transmissions), it’s causing cavitation on the propeller. 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 more). And the overall speed gain is minimal.

Variable pitch propeller (substitute for transmissions)

A variable-pitch propeller is a sort of alternative for gears. By changing the pitch of the propeller, you can go at different speeds. (Pitch means: Suppose if the propeller’s pitch is 20 inches, then for one complete rotation (360 degrees), the propeller will move the boat 20 inches in the forward direction).

Unlike a normal propeller, each blade on the variable pitch propeller can rotate to a certain degree. You can rotate each blade of the variable pitch propeller. Rotating the blades alone will cut the water at different angles, increasing the pitch for each complete rotation, making the boat move faster.

Many people will refer to variable pitch propellers as alternatives for gears, but it’s not. Those propellers are expensive than normal propellers (even normal propellers are expensive in general). Check this small video on how the blades will rotate for a variable pitch propeller to visualize the above procedure.

First, starting at a short pitch and then increasing the pitch in a step-by-step order will act similar to gears. A small pitch propeller will give less thrust, and a big pitch propeller will give more thrust (push) for the boat. You can find these variable pitch propellers mostly on ships, not usually in small boats.

Do ships have transmissions?

Now, you know that most small (recreational) boats don’t have transmissions like cars or other vehicles, but what about ships? Do ships have transmissions like cars?

Ships don’t have transmissions like cars or other vehicles; some of them will have variable pitch propellers to adjust the propeller’s pitch, increasing the speed a little. Other than that, ships are similar to small boats; they rely on propellers that work efficiently at operating speeds.

As mentioned earlier, “cavitation” the same problems occur if the propeller rotates fast with the help of transmissions (gears). Still, they have reduction gears, and the propeller doesn’t operate at the engine drive shaft speed. The same effects apply here too.

The key takeaways from the post

Boats do not have transmissions like cars because adding transmissions to a boat engine will cause undesired complexity, creating more problems such as cavitation, increasing the boat’s drag & fuel consumption, etc., for a small increase in speed. That’s why boats don’t have gears like cars do.

For a small increase in performance (speed), we need to spend more money, and it’s creating more problems such as cavitation, energy loss, increasing complexity, etc. Maintenance would be more and difficult for a boat with transmissions.

And one more thing to note here is boats mostly move at constant speeds (frequently, 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 obstacles.


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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