Decide
When a propeller accelerates water aft, what pushes the boat ahead?
The lesson in plain words
- Action and reaction
- A propeller accelerates water aft, towards the back of the boat.
- The water exerts an equal force in the opposite direction, pushing the boat ahead, forwards. This push is thrust.
- It is an example of action and reaction: pushing water one way pushes the boat the other way.
- Thrust and resistance
- Resistance is the force opposing your boat’s movement.
- It comes mainly from water friction, wave-making and swirling water, with wind also contributing.
- If thrust exceeds resistance, you speed up.
- At a steady speed through the water, thrust equals resistance.
- More thrust can give a higher steady speed, but resistance also rises as you go faster.
- Astern thrust
- Astern means backwards. Reversing a fixed-pitch propeller’s rotation makes it push water forwards, giving astern thrust.
- Most such propellers work better ahead than astern.
- Astern thrust is often about half to two thirds of ahead thrust, though the amount depends on the propeller and installation.
- Do not expect equal stopping power from the same engine setting.
- Carrying way
- In neutral, the engine no longer drives the propeller.
- It does not cancel your boat’s motion.
- The boat carries her way: she keeps moving until resistance or opposing thrust slows her.
- While you are still moving ahead, astern thrust first slows you; only after stopping can you move astern.
- Allow stopping room and begin slowing early.
- Learn your boat’s response in clear water.
Key data
- Steady speed
- Thrust = resistance
- Typical astern thrust
- About half to two thirds of ahead thrust; installation-dependent
- Neutral
- Removes engine drive, not the boat’s movement
Key points
- A propeller accelerates water aft; the reaction pushes your boat ahead.
- At a steady speed through the water, thrust equals resistance.
- Astern thrust is generally weaker than ahead thrust.
- In neutral your boat carries her way. Allow room and time to stop.
Check what you have read
Your boat holds a steady speed through the water. How do thrust and resistance compare?
True or false?
Decide each statement before you see the answer.
In neutral your boat carries her way. Allow room and time to stop.
Astern thrust is generally weaker than ahead thrust.
Selecting astern makes the boat move backwards immediately.
Neutral stops the boat.
At a steady speed through the water, thrust equals resistance.
The same engine setting gives equal thrust ahead and astern.
Practice case & exam keys
Situation
You select neutral while moving towards a landing place. The engine idles, but the boat keeps moving ahead. Why, and what should you allow for?
Show the answer
Neutral removes engine drive, not your forward motion. Your boat is carrying her way. Astern thrust can slow her, but it is usually weaker than ahead thrust and needs distance to work. Allow stopping room and start slowing earlier on your next approach.
Exam keys
- Thrust is the reaction to water accelerated in the opposite direction.
- Steady speed through the water: thrust equals resistance.
- Astern thrust is usually weaker; neutral does not remove way. Stopping takes distance.