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Motor Lower Unit
Motor Cooling System
- Find a complete lower unit. Identify the parts, prop shaft,
drive shaft, anti-cavitation plate, skeg, water intake and engine
- 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?
Inboards were slow but powerful. Outboards are easy to steer,
are lighter, quieter, and can be removed to work on them. They
travel much faster than inboards.
- 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?
Shear pins were inconvenient and sometimes dangerous to change,
but the lower units laster for a very long time. The modern lower
units are terribly expensive to change. Sometimes I wish for the
inconvenience of the shear pins.
- 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?
Gears are made of very hard steel.
- Feel the seals. Are they soft? Are they worn? What holds the
seals tight against the shaft?
Many seals have a small spring that holds the seal to the
- Find the intake for the water pump. Why do you think the holes
If the holes were bigger grass and small rocks would enter
and plug the cooling system.
- 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
The prop will only cavitate on the side that is on the downstroke.
- 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.
This will jiggle your brain for a little while, but is worth
- Change grease in a lower unit. Make sure the bolts are tight
once you are done. Did you see the new grease pushing the 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?
- 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)?
- 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 wouldnt spin? Ask someone what
these are called.
It should ring as most of them are good quality high carbon
steel. There are splines on the end that fit into the splines
on the crankshaft.
- 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?
If it was bent, the seals were worn.
- 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.
- Imagine that the prop has just hit a big rock. What parts absorb
the stress and shock?
The blade of the prop transmits the impact to the gears through
the prop shaft. Something has to give between the pressure in
the cylinders and the blade of the prop. Something has to break
- Where does the exhaust leave the engine? Why doesnt it
exhaust into the air?
Much of the exhaust goes out the back of the prop. It is much
quieter this way.
- 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?
Bigger motors usually have bigger diameter props.
- Does anyone in your location have a jet boat? Talk with them
about the advantages and disadvantages of jet units. Why dont
more people have a jet unit?
Jet units are very expensive. They dont carry a big load
well, and they plug with gravel, leaves, and even pieces of plastic
in the water.
- What gear is at the end of the driveshaft that turns both the
forward and reverse gears?
- Why is lower unit grease important?
It lubricates the gears and bearings.
- Should you fill the lower unit with grease from the bottom
or the top hole?
From the bottom hole or there will be too many air bubbles.
- What do seals do?
Keep the grease in and water out.
- Why are the thin shims important in a lower unit. Explain or
They determine how loose or tightly the gears fit together.
- What are the two shafts that turn in a lower unit?
Drive shaft and prop shaft
- Draw a picture of cavitation.
This should show air bubbles going down to the prop.
- What does the skeg do?
Protects the prop from hitting the bottom
- What does the trim tab do that is behind the prop?
It corrects for torque that might cause the engine to want
to turn to one side.
- A prop is marked 11 x 13. What do each of these two numbers
11 = the diameter. 13 = the amount the prop would move forward
if it were auguring through a solid.
- One prop is marked 13 x13 the other is 11x13. Which is the speed
prop? Which is the work prop?
- Why is it important to balance the load and the rpm?
Too much load or too many rpm both put stress on the engine
- Describe or draw a prop that has a shear pin.
- Describe or draw a slip prop from the rear view.
- What happens when a prop gets out of balance?
It vibrates because the forces arent balanced. If this
is allowed to continue, bearings and seals will be damaged.
- 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.)
91.7. Answers will vary
- 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?
Answers will vary
- Using the information you discovered from the above question,
if the engine is turning 5,000 rpm, how many rpm is the prop turning?
Answers will vary
- The pinion gear in my motor turns 2 1/2 times for every turn
of the forward gear. The engine is turning 5,000 rpm. How fast
in rpm is the prop turning?
- 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 cant find your own figures.)
Answers will vary. 42.37%
- 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
- 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?
- A speed prop supposedly goes forward 13 for every revolution,
but in reality it only goes 6.5. What percentage of efficiency
- 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?
The new one is cheaper
- Aluminum props used to cost $15 each in 1972. Now they are $120.
What percent increase does this represent?