Why don't inboard motor boats have gearboxes?

Question

I mean, cars have gears and gearboxes to increase speed by utilising the revolutions made by the motor and transfering that to the wheels. But boats just seem to have a single forward and reverse gear. Wouldn't the boat move faster if, like a car, it progressed through a range of gears?

Answer 1

The propeller slips threw the water & gradually bits to increase speed an the water acts as a gear selection>There has been some that tried gear selection trans but not many>??

Answer 2

A gearbox would add weight and isnt really necessary for propper function. It would only make the system more complex and expensive.

Answer 3

The lower unit on the outdrive is the transmission, however it does not work the same as an automotive transmission. The pitch of the prop gives you your speed or power variences.

Answer 4

The performance boats have a prop that changes pitch to achieve a similar result. Once the prop is at maximum rpm it begines to increase pitch and move the boat faster without sacrificing rpm . Similar to a variable speed clutch in a snowmachine. On your average pleasure boat it`s limited top speed is mostly because of hull design, and most can be overpowered with standard direct drive

Answer 5

ZF makes a two speed marine gear for boats, and larger boats and ships have controlable pitch props, essentially the same as having multiple gear ratios.

Answer 6

Some boats do have automatic transmissions, with multiple forward gears. I bet servicing them can be a chore, since the filter is in the pan, under the transmission. These are mostly custom speed boats with car engines (big V-8s, some times supercharged :))

Most boat owners have enough problems with out needing transmission problems to worry about.

Answer 7

Holy Cow, no one has a clue...

Here's the actual truth, not just a guess.

Actually, spinning the prop faster is not the solution to more speed in the water.  As speed increased, the hydrodynamics around the prop change requiring a different pitch and blade design.

The same holds true when the prop is spun at higher rpms.  The prop shape would need to be dynamic to compensate for the changing hydrodynamics as the rpms increase.