Why don't boats have gears like cars?

Nostrodamus

Nostrodamus

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Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?
 
They do, they are called sails. An engine is an alternative source of power and as a boat only travels relatively slowly, like a car in first gear, the engine only needs forward and reverse.
 
Nostrodamus said:
Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?
Click to expand...

Some tugboats used to have 2 speed gearboxes to optimise steaming and towing requirements but the controllable pitch propeller is a more more flexible solution.
 
Boat speed is limited by the waterline length of the hull, unless you have a planing hull, so the engine doesn't need a gear box just enough power to hold hull speed. Cars are not limited to speed hence the gearbox.
 
A car has wheels and keeps rolling on its own for quite some time after you take your foot off the gas (and clutch). That's because the wheels have very little resistance and the air it has to push out of the way isn't too thick either (except in London, which is why cars go so slowly there). So with a car, you need some torque to get going, but once you've picked up speed, you don't need that much to keep going. And since cars cover a very wide range of velocities, from crawling through cities to speeding along the Autobahn, you do have to have a choice between high torque and high RPM (at the wheels, not the engine) to go fast. Try and see how fast your car can go in first gear, or how well you can drive away from a stop in fourth gear. And then cars also go up and down hills, which boats mostly don't (apart from the odd wave).

A boat floats, which is pretty efficient just like wheels, but it has to push a lot of water out of the way to keep going. Water is quite a bit thicker than even London air, so much harder to push out of the way. When you put your gear lever in neutral, it won't keep going nearly as far as a car with the same inertia would go on land. So it needs a lot of torque all the time just to keep going and the speed is limited by the water resistance, except for crazy fast hydroplaning boats which are closer to airplanes than boats anyways (and tend to use airplane engines). So there isn't much reason to have multiple gears on a boat, apart from forward and reverse, which most do have.

That's my theory anyways :)
 
Basically, a boat can cover its complete operational range of speed without a need for a gearbox. A car, to travel between (say) 5 mph and 105 mph needs a gearbox to enable the car to travel at those speeds. A boat needs to travel between 0mph and ??? 30 mph (large cargo ship), which can be covered just by the rev range of the engine
 
Even a fixed pitch propeller works fairly well over a wide range of boat speeds.
The drive is via a viscous fluid that allows the prop to have a varying speed relative to boat speed.
So gears are not as necessary as when driving via wheels which don't slip on the ground.

Hence old-worlde car auto boxes had fluid torque converters to have the advantage boats have and could get away with only 2 or 3 gears to cover a wide speed range.
 
Nostrodamus said:
Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?
Click to expand...

Fixed pitch propellers are only efficient at absorbing 100% of engine power at rated speed. Once again the simple rule crops up that propellers move boats engine merely turn them.

ZF introduced a range of two speed transmissions for powerboats in order to address the issues of highly loaded planing boats getting over the hump and excessive boat speeds at idle. However it has to be remembered that in high gear propeller could not be sized to absorb any greater than 100% engine load. Concept fell on its face with the introduction of electronically controlled high torque diesel engines and ZF have quietly dumped the concept.

Take a look at any diesel engine power curve, and the propeller law curve, you will see that at all speeds other than rated speed engine has far greater potential power than power propeller demands.
 
Yes, one gear is enough to cover the needed speed range, but having more gears would save fuel at lower speed.
The prop is usually designed so that the engine can just reach the max power RPM. At this operating point you will utilize the rated maximum power of the engine. If the max speed is for example 8 knots, and you slow down to 6 kn, then the required torque to push the hull through the water will be much less. As a result the engine will output a torque which is much below the maximum available torque at that RPM, which means non-optimum fuel economy. (= consumed diesel/produced mechanical work) Having the possibility to switch to a bigger gear would reduce the RPM further and thus increase the torque taken from the engine to be closer to the max torque at that RPM. (the prop torque would remain the same)
Please google “Brake Specific Fuel Consumption Maps” to see how the fuel economy depends on the RPM and the torque.
 
Nostrodamus said:
Would gears make them more efficient and use less fuel depending on sea state?
Click to expand...

While not using gearboxes with different ratios as in a car there are various devices that change the pitch of the propeller to siut different circumstances. These include variable pitch propellers where the revs are kept constant and the pitch is changed manually, infinitely variable pitch as in the Bruntons Autoprop which adjust the pitch to suit the load and speed, and "overdrive" feathering props such as the Gori which can engage a different pitch on demand.

All of these devices aim to overcome the limitations of a fixed pitch prop as explained by Latestarter. The Bruntons in particular claims to be more efficient in motorsailing which is where a fixed prop is most inefficient, and adjusting the pitch to cope with adverse conditions. This makes it a popular choice for those who cruise long distances under a mixture of sail and motor.
 
Nostrodamus said:
Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?
Click to expand...
Do you change gear when going between 0 and 10 MPH in a car?
 
An old friend of mine fitted a Reliant Robin engine complete with gearbox in a converted ships life boat he used for fishing.

He used third gear to go ahead which was fine but going astern needed lots of revs to achieve not much propulsion. Didn't seem any advantage using the other gears.
 
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