How does outboard lower unit work

If vibration is allowed to continue, replacing the bent shaft will still allow some vibration because the bearings will be worn. The prop spins in the water, not giving much thrust at all. If the lower unit is too deep in the water, it will hit the bottom too easily and will present more drag than is necessary. The height of the back of a boat is critical. A quarter of an inch can make a difference.

One time while traveling in the ice I took seven hours when another boat made the trip in three hours. My motor was cavitating when chunks of ice were trapped between the boat and lower unit; his was not. Why does this happen? Behind the moving prop is a high pressure area created by the prop pushing water backwards. In front of the prop there is a low pressure area as the water is being drawn away by the prop.

If the pressure in front of the prop gets below The prop spins in a pocket of air, and lose its thrust. The flat plate above the prop is designed to keep the air from doing just that.

Even with the anti-cavitation plate, cavitation occurs when seaweed, floating leaves, and grass stick on the front of the lower unit, breaking the smooth flow of water. The water pressure in front of the prop is drastically reduced. The air is then driven by atmospheric pressure around the anti-cavitation plate, causing the prop to spin in a pocket of air.

Housing Shape The lower unit has to be big enough to hold the gears, and small enough to present the least resistance possible. If the gears are too small, they break easily. If they are too big, pushing the oversized lower unit through the water takes energy away from the forward motion of the boat.

The skeg on an outboard protects the prop from impact. If it were shorter, it would expose the prop to obstacles in the water. If it were longer, it would hit the bottom. If it were thinner, it would easily break off.

If it were thicker, it would cause too much drag. The angle of the skeg deflects the lower unit from rocks and logs. Trim Tab The prop spins in one direction causing the motor pull to one side, making steering tiring. On most motors, particularly the big ones, there is a little fin that hangs down behind the prop to counteract that pulling while the boat is in motion.

By turning the fin to one side or the other, the twisting of the motor is offset. Interestingly enough, this fin is made of zinc. The fin is made of a zinc that sacrifices itself. It might need to be replaced at some time if the motor is in salt water, but it is cheap and easy to replace.

This prop is too damaged. It will vibrate. Props come in different sizes and pitches. The size of the motor determines the diameter of the prop. The size and load on the boat determine the pitch. On each prop there are two numbers. For example: 10 x 10 or 10 x The first number is the diameter of the prop. A smaller motor will usually have a smaller diameter prop. The second number tells how far the prop would move forward in one revolution if there were no slippage in the water.

This is the pitch. A 10 x 10 prop is a work prop. It will go forward ten inches in one revolution. It will push a heavy load. However, the engine turns too fast if there is no load in the boat.

A 10 x 13 prop is a speed prop. It will go forward thirteen inches in one revolution. It will cause a light boat to go fast, but it will work the engine too hard if there is a big load. Most pilots have extra props to account for the different load conditions under which the boat might operate. Balancing the Load and Prop. If the load is too large, the rpm is too low and there is great stress on the motor.

If the load is too light, the rpm is too high, and the motor will self-destruct from the inertia of the piston as it goes up and down.

Long ago, pistons were made of steel, but their great mass kept engine rpm under 2, Pistons are now made from aluminum because aluminum is light. Modern engines turn 5, to 12, rpm. If the prop is has too much pitch, the engine will turn too slow. A Problem Overcome. Older motors had shear pins.

When a prop hits bottom or an obstacle, the force of impact is great. Action equals reaction. There is a great action driven by the engine, and a great reaction when the prop hits bottom. If there were no shear pin, the prop would be damaged and the pinion gear might break on impact. Designers put a metal pin through the prop shaft that was weaker than the gears. This pin was the only connection between the prop and the shaft. On impact, the shear pin broke and was inexpensive to change.

However, when it broke in rough or dangerous water, which it often did, there was furious paddling to safety. There are a couple of things pertaining to the outboard cooling system that need regular maintenance, or require attention from time to time. First, lets look at the typical water intake opening, and water pump assembly Figure 1.

Water is drawn into the water pump through the intake opening of the lower unit, where it passes through a cavity that ends up entering the pump itself. At that point, the impeller pressures the water into the long inlet tube leading to the bottom side of the powerhead. Regularly check water inlet grate to make sure nothing is obstructing water flow. The water pump impeller should be replaced every two years, regardless of usage.

An impeller will normally go bad from dry rot attributed to non-usage more so than wearing out from excessive usage.

One thing to remember on an outboard is the warning horn. You will hear a loud constant shrill whistle if your outboard is in an overheat condition. UTI Campus Alerts. Specialized Training Student-Paid vs. Manufacturer-Paid Programs Catalogs. Meet Our Graduates Find out what some of our graduates are doing today in pursuing their successful careers.

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These include: Top outboard powerhead Mid-section Outboard lower unit The top half of the engine contains the outboard powerhead, which is made up of several different components.

Outboard Powerhead The top section of an outboard motor, or the outboard powerhead, is composed of various components that make up a combustion engine. Essentially, the powerhead is made up of the bare bones of the engine, which include the following components: Engine block : The engine block is where the moving components of the engine are located, which include the piston rods, pistons and the crankshaft.

It also contains the cylinders, which is where the pistons are. Crankshaft : Pistons attach to the crankshaft inside of the engine block. As the pistons move up and down, the crank spins around them to generate power. Piston rods connect the piston to the crank, and as the crankshaft spins around, the piston rods move up and down to move the pistons inside of the cylinders.

Together, these components make up the main part of the powerhead. Mid-Section The mid-section of an outboard motor is a metal casing designed to connect the engine to the lower unit. Outboard Lower Unit Similar to the powerhead, the outboard lower unit has a lot of moving part. The outboard lower unit sits under the water and contains various components, including: Shift mechanisms Driveshaft Propshaft Gear set Clutch dog Bearings Seals and shims Shift mechanisms control the rotation of the propeller.

The non-mechanical shift mechanisms include an electric shift, a hydroelectric shift, or a hydro-mechanical shift mechanism The gears in the lower unit transmit rotational force by applying force against the teeth of another gear.

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To learn more, visit our website and request information to get in touch with one of our Admissions Representatives today. Classes start soon. With classes starting every weeks, no need to wait to start your career. Which really is just a toothed gear that meshes into the center of either the forward or reverse gears. Think if you had a donut and a stick that fit perfectly well at the center of that donut. How would you make the donut and the stick turn together without either one slipping?

Easy right? You drill a hole through the side of the donut then through the stick then through to the other side of the donut and then you would insert a smaller dowel into that small hole you just drilled hence coupling the donut and the large stick in its center together. Now that dog clutch must be able to slide a small distance forward and backward along the prop shaft to engage the forward or reverse gears.

This is accomplished through a rod in the center of the propshaft. The prop shaft being hollow for about half its length.