The overwhelming many engine-driven craft use propellers to transform the power of the engine to useful work. There are many alternatives which might have advantages for specific applications, but propeller systems are great all-rounders which can be reasonably cheap, simple, reliable, efficient and easy to utilize.
They suffer, however, in one potentially significant drawback, which can be a large slow turning propeller is often extremely effective when compared to a small fast-spinning one. Even on small cruiser, in which the maximum size of propeller is usually limited by hull shape, the optimum shaft speed is often within the order of 1000 rpm – merely a half or even a quarter on the speed from the engine. Running the engine more slowly isn't the response, because you'd just lose the majority of the power you've bought by buying it initially. What exactly you need is a thing that will lessen the shaft speed but increase its torque (turning effort).
It is also very beneficial every single child reverse the direction of rotation, to produce astern power to stop the boat or pass go backwards, or to fit counter-rotating propellers using a twin-screw boat. Again, you'll find alternatives including variable-pitch propellers whose blades swivel around the hub, scoop-like deflectors, or even boat engines that could be stopped and restarted inside other way, but by far the most popular arrangement is really a reversing marine gearbox.
Reversing marine gearboxes 
Two gearwheels, whose teeth mesh together design one turns, the other has got to turn likewise. Small gear has 9 teeth, therefore it can be turning at 1000 rpm, its teeth are moving at 9000 teeth each and every minute. The bigger wheel is twice the dimensions, and contains 18 teeth, so although its teeth also needs to be moving at 9000 teeth for each minute, that means only 500 rpm. Note, too, that in case small wheel is turning clockwise, the larger wheel must be turning anticlockwise.
Imagine that you're employing a spanner a foot long to show small wheel. If you apply an attempt of 10 lb towards end from the spanner, you're applying a torque of 10 lb/ft (10 lb for a radius of a single ft). Kit-wheel is a lot smaller – suppose it's got a highly effective radius of just one in. Actually its teeth should be pushing for the teeth in the other wheel with a force comparable to 120 lb (120 lb at a radius of a single/12 ft = 120 x 1/12 = 10 lb/ft). The more expensive wheel comes with a effective radius of 2 in, so a force of 1201b to its teeth corresponds into a torque of 20 lb/ft (120 x 2/12 = 20 lb/ft).
Basically, simply by using a 9-tooth wheel they are driving an 18-tooth wheel, we've halved the pace doubled the torque, and reversed the direction of rotation.
Simple gearboxes
Real marine gearboxes look more complex, but count on exactly this principle. In fact, the total amount saved between the simple gear train and also the Volvo MS2 is the MS2 uses coneshaped bevel gears, to ensure that however the input shaft is horizontal, the central driven shaft is vertical.
The motive for this is the fact it possesses a simple strategy for achieving a choice of ahead or astern gear.
The pinion and ring gear about the input shaft turns two slightly larger bevel gears that spin freely around the vertical shaft. You are driven because of the the top of input gear as well as the other through the bottom of computer, in order that they rotate in opposite directions, but you are identical size as one another so they really turn in the same speed.
Between two gears is often a sliding clutch assembly, shaped like two shallow cones mounted base-to-base around the shaft. Ridges called splines within the shaft and matching grooves inside clutch be sure that it may easily slide along, but can't turn without turning the shaft at the same time. Moving kit lever slides the clutch up or down to ensure among the cones engages right matching hollow with the spinning gears, which locks that gear about the shaft.
At the end of the shaft, a somewhat small pinion and ring gear engages that has a bigger gear on the output shaft, to present a horizontal output, at the much lower speed of rotation.
Layshaft gearboxes
Even though MS2 and derivatives are normal on Volvo engines around about 200 hp, it's somewhat unusual in using a vertical shaft. A more widespread arrangement is the layshaft gearbox.
In this case, a small gear about the input shaft turns a greater gear about the layshaft. Such as gears around the vertical shaft on the MS2, it is free to gyrate the layshaft, but could be locked through to it by a clutch. In the event the input shaft is turning clockwise along with the clutch is engaged, the layshaft turns anticlockwise and more slowly. At the opposite end in the layshaft an inferior gear meshes having a large gear for the output shaft, driving the output shaft clockwise sometimes more slowly.
Astern gear is achieved with a second layshaft. Its very like the primary, with two gearwheels, one of which has a clutch which could get out free-spinning or lock it to the shaft, even though the other is actually in mesh using the output shaft. The true secret aspect of your second layshaft is the fact it is not driven directly by the input shaft, but
because of the first layshaft. Because of this it's turning in the opposite direction – therefore one clutch is released plus the other engaged, the direction on the output shaft are going to be reversed.