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.
Now, you might be wondering how the water boils at the low temperatures inside the water. With a significant decrease in pressure, you can boil the water, even at room temperature. 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.
Water behaves very differently from the air. 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. 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.
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. I am not talking about the high-performance boats which have gears. A second major difference between the two types of transmissions is the fact that the vast majority of marine gears have only one set of gears and therefore one ratio.
ZF makes a two-speed marine gear. One is to engage and disengage the engine from the propeller—in other words, to provide neutral. Another is to provide reverse rotation so that you can back your boat into your slip. These two functions are accomplished by a series of internal clutches, which when moved via the gear lever at the helm, engage different gear sets. Each marine transmission has an internal oil pump that generates hydraulic pressure, which forces these clutches together to provide engagement.
Because the pump generates heat as it pressurizes the oil, every marine transmission also must have an oil cooler, which is typically mounted on the outside of the transmission housing and is identifiable by the water hoses leading into and out of it. When the gear lever is at one position there is no engagement neutral among the gear sets; in another there is engagement between two gear sets rotation in one direction and in yet another there is engagement among three gear sets resulting in rotation in the opposite direction.
Note that strictly speaking there is no such thing as forward and reverse, just clockwise and counterclockwise rotation. In some installations counter-rotation of the propellers is actually accomplished by simply swapping the linkage so that the opposite gear set is engaged when the lever is moved.
The other function of the marine transmission is to set the ratio between engine rpm and propeller rpm. A typical diesel rotates at around rpm at cruise setting.
A propeller turning this fast would be highly inefficient, so it is the job of the gears inside the transmission to slow down the propeller rotation to roughly half that of the engine, which is why this is always referred to as the reduction ratio. Note that this ratio is fixed; the only way to change the ratio between engine and propeller rpm is by changing marine transmissions or changing the gear sets within the transmissions.
The marine transmission performs one other vital function: It is the most convenient location from which to drive an auxiliary device, such as a hydraulic pump. Most marine transmissions have a power take-off point, which if unused, is usually covered by a plate. Remove this plate and you can see the transmission gear, which engages the gear of the auxiliary device.
Pretty exciting stuff, huh? Okay, maybe not, but you should still be impressed. For a comparatively simple device, the marine transmission does a lot of work. Display as a link instead. Clear editor. Upload or insert images from URL. Why Don't Boats Have Gears? Share More sharing options Followers 0. Reply to this topic Start new topic. Recommended Posts.
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