Village Science

Outboard Motor Lower Unit


A 15
B 3
D 1, 3



Those of us who have piloted Alaska’s waters with an outboard have learned the importance of having a good lower unit. All the power of the motor is sent to the lower unit. Great forces work through the lower unit. When it works well, all goes well. When it is damaged, everything comes to a halt. Some people have jet units to drive their boats, but they have cost, weight and maintenance problems that keep them out of the reach of most village people. The simple outboard with a lower unit will be around for many years to come.

(While we call them outboard motors, they are really engines. Electric motors are motors, and piston driven engines are different. However, her ewe will ue the commonly used term outboard motor in order to be understood.)

Lower units are expensive to repair or replace. For those reasons alone, it is important to understand them.


The engine turns the drive shaft, but the drive shaft isn’t spinning in the same direction as the prop shaft.

How is the direction of the shaft’s rotation changed? The pinion gear on the end of the drive shaft drives both the forward and reverse gears at the same time. All three gears turn together.

gearPinion Gears

The pinion gear is smaller, with less teeth. It turns 21/2 times for every revolution of the forward and reverse gears. The prop and prop shaft are therefore turning slower, but with more power (torque) than the engine. The engine has a mechanical advantage of 2.5 to 1 over the prop.


gearHow can the forward and reverse gears turn at the same time? Inside the forward and reverse gears is the prop shaft which has four large “dogs”. When the shift lever is moved, the dogs connect with either the forward or reverse gears. Only one of them can engage the drive shaft at a time. The dogs are strong. If the forward and reverse gears were always engaging and disengaging with the pinion gear, they would break off in a moment.

Even though the dogs are thick and made of tough metal, they can be chipped if shifted when the engine is running fast. When this happens, pieces of metal flow through the lower unit grease, causing other chips and bearing wear. Avoid chipping the shift dogs by slowing the engine to the lowest rpm before shifting.



Taking a lower unit apart isn’t hard. Describing the process is hard. With the prop off, look around the prop shaft and find a ring that is screwed into the housing. Often there is a locktab to keep it from unscrewing by itself. With the locktab removed, the ring can be unscrewed. See a local expert if you need help in this. With the ring removed, turn the lower unit with the prop shaft down. Holding a hand over the prop shaft so parts don’t fall on the floor, rap the whole lower unit on a bench or block of wood, hitting on the skeg. Inertia quickly drives the gears, seals, and other parts into the waiting hand. Science in action!


Without lower unit grease, friction would destroy the gears and bearings in the lower unit within minutes. If the grease is too thick, it can’t get into the small places and prevent the metal from wearing. If the grease is too thin, it doesn’t protect the metal parts from grinding each other. Lower unit grease is specifically designed to work well even if there is a little water in the lower unit.

An oldtimer had no lower unit grease, so he put wheel bearing grease in his lower unit. All the gears and bearings spoiled within two hours.

It is good to change lower unit grease often, as the little metal chips that break off continue to cause great damage. Fresh grease prevents unnecessary wear.

The transmission in a helicopter has a magnet to pick up small pieces of metal that float about before they damage the unit. Newer outboards have a similar magnet on the drain plug to pick up chips and filings. Many snowmachine chain cases have a magnet on the end of the dipstick to pick up loose particles that would cause excessive wear.

Changing Grease

There are two holes through which the grease is injected into the lower unit. Drain the old grease out of the bottom hole. Fill the lower unit from the bottom until the grease comes out the top hole. If it is filled from the top after inserting the bottom screw, there are many air bubbles. It never would fill completely.



The prop shaft and drive shaft are in constant motion. Seals keep the water out and the grease in.

The seals must be snug against the moving shafts. Good, healthy, soft seals help to keep the grease in and water out. Since water is not a lubricant, friction in a greaseless environment will destroy the lower unit in minutes.


There are two shafts in a lower unit.

  • The drive shaft
  • The prop shaft

The drive shaft runs from the bottom of the crankshaft to the lower unit.

The prop shafts bend when impacted with the bottom is great enough. Once the shaft is bent, the shaft and prop vibrate because the prop is not turning in a perfect circle. If this continues, seals wear out from the vibration and water enters the lower unit. The bearings wear excessively from vibration. Roll the prop shaft shaft and if it wobbles, it is bent.

If vibration is allowed to continue, replacing the bent shaft will still allow some vibration because the bearings will be worn.


If the lower unit is not deep enough in the water the prop “catches air”, or more accurately, “cavitates”. 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 14.7 psi, the pressure of the atmosphere pushes air from the surface of the water to the prop. 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. It is called the “anti-cavitation plate.” Without it the motor catches air, or cavitates, greatly frustrating the pilot.

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.

finInterestingly enough, this fin is made of zinc. When outboards are in salt water, there is a great chance of a chemical reaction, like a battery, “eating” one metal on the motor. The fin is made of a zinc that sacrifices itself. It is “eaten” before the other more valuable motor parts. 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.





back side


propsProps 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 13.

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.

one revolutionA 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

The load determines the engine’s speed.

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 2,000.

Pistons are now made from aluminum because aluminum is light. Modern engines turn 5,000 to 12,000 rpm.

If the prop doesn’t have enough pitch, the engine will turn too fast.

If the prop is has too much pitch, the engine will turn too slow.

Balancing the prop to the load and controlling the engine’s speed is critical for the life of the engine.

problemA 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.

solutionNew Solution

Manufacturers tried different innovations and came up with the slip prop. A look at the end view of a prop shows the blades, the hub, an artificial rubber ring, and the inside bushing that has splines and slips over the prop shaft. When the prop hits the bottom, the prop shaft can still spin in the middle while the blades slip on the artificial rubber ring.


The slip prop supposedly saves the gears from breaking and the prop from being severely damaged. However, when the prop is new, the ring is stiff, and too much stress is communicated to the gears. When the prop is old, the rubber slips too easily, even when the motor is accelerating. It would be nice to have a way to adjust the tension on the rubber ring to fit the boating conditions, but manufacturers haven’t gotten that far yet.

Once the prop has hit bottom several times, it bends and chips. If the prop is out of balance, it will vibrate, damaging the bearings and seals. It is possible to shape the prop again by tapping it with a hammer on a solid surface. It can also be filed, however, it is important to file the front of the prop, giving a flat surface in the back to push against the water.


One time I bought a brass prop. I thought it might be better. Wrong! Brass props are okay in salt water, but their mass is so great small chips result in severe vibration. The inertia of a heavy brass prop is much greater than a light aluminum one. The vibration was so great from the brass prop, it snapped my prop shaft in a very short time.

Many people are using stainless steel props, as they are tougher, and last longer. However, the owner of a prop shop recently told me that aluminum props are cheaper in the long run. Aluminum easily deforms. Stainless steel is much tougher, but the damage is transmitted to the gears, which are far more expensive and difficult to change. It is better to change aluminum props than lower unit gears.


Most outboard motor problems that we have faced in our part of Alaska are lower unit problems. We travel shallow rocky rivers and lower units give out long before the upper units wear. Understanding them and treating them carefully helps lengthen their life, saving money and long trips poling, paddling, or drifting home.



  1. Find a complete lower unit. Identify the parts, prop shaft, drive shaft, anti-cavitation plate, skeg, water intake, and engine exhaust.
  2. Look at the motors in the village, new ones and old ones. In what ways are they similar? In what ways are they different? Trace the changes in outboards through time. Ask oldtimers about inboard engines. How were they better? How are outboards better?
  3. Compare the lower units made today and those of years ago. Ask the oldtimers about the advantages of the shear pin type lower units. Is there one in a cache somewhere? Why did the outboard manufacturers change from shear pins to slip props?
  4. What kind of metal do you think the gears are made of? Try to file them. Are they hard or soft? Try to file the drive and prop shafts. Are they hard or soft?
  5. Feel the seals. Are they soft? Are they worn? What holds the seals tight against the shaft?
  6. Find the intake for the water pump. Why do you think the holes aren’t bigger?
  7. On a complete lower unit, turn the prop as if the boat were going forward. One side of the prop has low pressure coming from the top; the other side has low pressure coming from the bottom. Identify each. If the prop were to cavitate, which side will it cavitate on?
  8. Look at a prop shaft with the gears attached. Explain to someone else how the motor shifts from forward to neutral to reverse. Try to draw this so someone else can understand by your picture.
  9. Change grease in a lower unit. Make sure the bolts are tight once you are done. Did you see the new grease pushing some old grease out of the upper hole? Did bubbles come out too? Are you confident that the lower unit is full of grease? Was there any water in the lower unit when you first drained it?
  10. Stir a clean magnet in the grease that has just been drained from a lower unit. Are there any metal chips? (Cover the magnet with thin plastic wrap before doing this to facilitate cleaning.) Rub some of this grease between your fingers. Rub some new grease between your fingers. Can you feel a difference in friction? In thickness (ability to keep metal parts from touching each other)?
  11. Tap an old drive shaft with another piece of metal. Does it ring, indicating high carbon steel? How was it attached to the end of the crankshaft so it wouldn’t spin? Ask someone what these are called.
  12. Roll an old prop shaft on the table. Look closely. Does it wobble, indicating that it is bent? What do you think happened to the seals if the shaft was bent?
  13. Try paddling a boat with the motor up, out of the water, and then lower the motor. Paddle again. Note the resistance of the lower unit. Can you now see why design and size are so important? Imagine the resistance at high speeds.
  14. Imagine that the prop has just hit a big rock. What parts absorb the stress and shock?
  15. Where does the exhaust leave the engine? Why doesn’t it exhaust into the air?
  16. Check five to ten props in the village. What is the average pitch? Are these mostly working boats or speed boats? Do you see any relationship between the prop diameter and the horsepower?
  17. Does anyone in your location have a jet boat? Talk with them about the advantages and disadvantages of jet units. Why don’t more people have a jet unit?
Student Response

Student Response

  1. What gear is at the end of the driveshaft that turns both the forward and reverse gears?
  2. Why is lower unit grease important?
  3. Should you fill the lower unit with grease from the bottom or the top hole?
  4. What do seals do?
  5. Why are the thin shims important in a lower unit? Explain or draw.
  6. What are the two shafts that turn in a lower unit?
  7. Draw a picture of cavitation.
  8. What does the skeg do?
  9. What does the trim tab do that is behind the prop?
  10. A prop is marked 11 x 13. What do each of these two numbers mean?
  11. One prop is marked 13 x 13 the other is 11 x 13. Which is the speed prop? Which is the work prop?
  12. Why is it important to balance the load and the rpm?
  13. Describe or draw a prop that has a shear pin.
  14. Describe or draw a slip prop from the rear view.
  15. What happens when a prop gets out of balance?


  1. Consider that an outboard usually turns 5,500 rpm. How many revolutions per second is this? Can you even imagine something moving up and down that fast? How many times a minute can you clap your hands? How many times faster is a piston? (Time and count yourself for a minute.)
  2. Find some old forward, pinion, and reverse gears in the village. Count the teeth on each. How many are there on the pinion gear? On the forward gear? If the forward gear turns one time, how many times has the pinion gear turned?
  3. Using the information you discovered from the above question, if the engine is turning 5,000 rpm, how many rpm is the prop turning?
  4. The pinion gear in my motor turns 21/2 times for every turn of the forward gear. The engine is turning 5,000 rpm. How fast in rpm is the prop turning?  
  5. Find the cost of a new outboard motor. Find the cost of a new lower unit for the same motor. What percentage of the cost of the whole motor is the lower unit? (A 30-horsepower Mariner costs $3,200 and a whole new lower unit costs $1,356. Use this example if you can’t find your own figures.)
  6. Pat has to buy parts for his lower unit. Pinion gear $32, forward gear $45, seals $5.75, reverse gear $45, new prop shaft $37, new prop $127, and grease $3.75. How much did it cost him to hit the rock?
  7. A new stainless steel prop is $250. An aluminum prop is only $105. The stainless prop lasts two times longer than the aluminum. Which is more economical?
  8. A speed prop supposedly goes forward 13” for every revolution, but in reality it only goes 6.5”. What percentage of efficiency is this?
  9. Frank can get a second hand lower unit for $675 or a new one for $1,172. The used one will last two seasons and the new one will last four. Which is more economical?
  10. Aluminum props used to cost $15 each in 1972. Now they are $120. What percent increase does this represent?

Questions or comments?
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