Why don't boats have transmissions more similar to cars that would allow for multiple forward gears?

Question

It seems that boats would be able to go a lot faster if they had something similar to an automatic transmission that cars have. By limiting a boat to one forward gear it seems as if a lot of the engines potential is wasted. If your car only had one forward gear you would never be out of first and really only be able to do a maximum of like 40 miles per hour.

Answer 1

Some ski race boats and offshore race boats use modified automotive auto transmissions. They do this so they can get monster surface piercing props on the plane.

Surface piercing props are designed to have half the prop (approx) out of the water at speed, so at low speed when the entire prop is submerged, the driver can select a lower gear wich will allow the engine rpm to get to a level where it can make power enough to acheive planing speed.

There is an advantage on recreational boats to have variable ratio "gearing" but not nearly as much as on a land based vehicle. Water has some "slip" so works like the torque converter on an auto transmission. This allows the engine to not stall when foreward gear is selected. The drag of the water running under the hull is a "constant" load, like going up a long, steep hill in your car. If all you did in your car was drive up an infinate hill with a constant payload, you could get away with a single speed gearbox with torque converter. Of course there would be advantages with a shifting box but not as much as in normal driving with ups and downs, stop start driving etc.

There are a few variable pitch propellers available for this reason.

Basically, there are advantages of more "gears" on a boat but not nearly as much as in a car. Then you have to factor in cost, weight, reliability and compactness v's performance gain.

Answer 2

Well actually some do as an option. Yanmar has a 2 speed marine gear that I have seen installed on boats as small as 35ft. The biggest problem with having multiple gears is weight and size. It has to be packaged for small craft. For many years ships have had controllable pitch propellers. The pitch is changed to match desired speed range. Someone above mentioned airplanes. Prop driven airplanes have had controllable pitch props for years also. So instead of having a transmission as you describe the pitch of a prop can be changed to give more thrust. A car can have a transmission because once it gets rolling the engine can handle heavier loads. On a boat, its always got to overcome the resistance of water. Kind of like its always going uphill.

Answer 3

I don't know much about smaller boats but many larger ships that need "gearing" actually use variable pitch propellers. You can probably look up lots of info on them on the internet. This is important for large diesel engines that have smaller RPM ranges then smaller engines engines do. For a small boat I would think that the large RPM range of a gas engine would make gearing not needed unless you are trying to get a planing hull to go really fast. Also I believe that boats that are designed to really fly are set up with a prop with a pitch designed for the top end. Kind of like starting your car down the road in 5th gear. Makes them slower to get up and plane but I think people with boats like that spend most of the time with the throttle all the way down anyway so they probably don't care. Once again don't know much about small boats, just my 2 cents.

Answer 4

In this case the difference is in friction.
The car has to overcome two main types of it: tires vs. the road and body vs. wind.
The boat needs to overcome the friction of the hull vs. water.

Transmission is needed to vary the power supplied from the engine to the weels. With cars, on lower gears we speed up the car to the cruising speed (more power to the wheels), and at the cruising speed the friction is still considerably low, but the car mass already gained enough inertia. To maintain that inertia (and speed), the car does not need as much power, just enough to overcome the friction.

On the other hand, the boats have to overcome lots of friction. So when the boat is moving slowly or fast, it still gets lots of resistance from the water (friction). So to maintain the higher speed of the boat, the motor needs to supply much more power compared to cars.

There are other factors come into play, such as displacement speeds and planing speeds when the friction is lower compared to semidisplacement speeds; as well as slippage, cavitation, etc. But for our simplified example, the basic principle holds true -- the power needed to overcome the water resistance is increasing linearly (almost) with the speed of the boat.

Hope this helps.

Answer 5

No,It aint that way, Boats have no wheels and can't get traction ! The prop can go extremely fast and no faster even with some kind of transmission,! All it would do is slow the prop down! This is hard to explain but I did the best I could!

Answer 6

Marine engines are designed to run in a specific RPM range ..
The prop along with transmission ratio determines this as does weight and length of boat.
If you run over the range you will damage engine as well as under range you will lug engine and damage it.

Answer 7

u can't have gears on a boat because if u did the prop would get destroyed, you will have all the power of the engine w/ one gear on a bigger boat because of the rpm and the setting.

Answer 8

stern drive inboard engines have a variety of stern drives and gear ratio take the mercury engine line you can choose from different ratios with there regular drives with the 1:62 or 1:81 ratio or you can choose from the bravo 1, 2, or even 3 depending on the size of the engine or its application of use so there are different options you can choose from to get your optimal power from your engine for your use

Answer 9

props can only spin so fast before cavitation starts to have an effect on the amount of push the prop generates..

CAVITATION...props work like wings of a plane..

there is a low & high pressure side to each prop because the water has to travel further over the curved surface on one side..
when water is at low pressure it boils at a lower teperature..
(on top of a mountain water can boil at 60degrees F)

so the water on the low pressure side of the prop boils as the speed increases.....

this slows the boat down because the prop wont push against the air coming out of the boiling water as much as it does against normal water...

Answer 10

a proper is pitched to achieve maximum torque and speed at various rpm's. turning a prop faster than it's designed rpm will gain you nothing also a transmission robs power. think about it if it was more efficient prop style air planes would have transmissions too