Low-speed handling
Boat won’t steer at low speed? Why, and how to get it back
If she stops answering the wheel in the last few boat lengths, you are not bad at docking. The rudder has run out of water to work with, and the first fixes cost nothing.
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- Single screw, outboard, sterndrive and twins
- Spring line steps
- Steering left at 2 knots 11% › of what the rudder gives you at 6 knots.
- The wind's edge at 2 knots 9 times › the advantage it had over your rudder at 6 knots.
- Steering astern, single screw Prop walk › The rudder does very little. The stern walks one way only.
- Fixes that cost nothing 3 › Carry speed longer, use your prop walk, get a line ashore.
- Thrust needed on the transom 30% less › than a tunnel 6 ft back from the bow, for the same turn.
- A unit at each end 167% › of one tunnel's turning moment, on a 40 ft boat.
The short answer
- Your rudder has quit. Rudder force falls with the square of your speed. At 2 knots you have about 11% of the steering you had at 6.
- The wind has not changed. It pushes as hard as before, so slowing down hands it about 9 times the edge over your rudder. That is why the bow blows off.
- Backing down is worse. On a single-screw inboard the rudder does very little astern. The sideways kick a propeller gives when you put the boat in reverse. A single propeller pushes the stern one way and only that way. That is why a single-screw boat backs predictably in one direction and badly in the other. turns the boat, and it only goes one way.
- Try the free fixes first. Carry The slowest speed at which the boat still answers the wheel. Below it you are not steering, you are drifting in a chosen direction. for longer, use your prop walk, and get a line ashore earlier.
- A thruster is a rudder that works at zero speed. Where it sits matters nearly as much as how hard it pushes. On the transom it needs about 30% less thrust than a tunnel for the same turn.
Your rudder has already quit
A The blade behind the propeller that steers the boat, and it only works when water is flowing past it. At docking speed it does almost nothing, and in reverse it does nothing at all if it sits behind the propeller. is a foil. The sideways force it makes comes from the water flowing past it, and that force goes with the square of the speed. At 3 knots you have 25% of the steering you had at 6.
At the speed you enter a slip there is almost nothing left. The wheel still turns, but the boat has stopped answering it.
| Boat speed | Steering left | Wind’s edge |
|---|---|---|
| 1 kt | 3% | 36× |
| 1.5 kt | 6% | 16× |
| 2 kt | 11% | 9× |
| 3 kt | 25% | 4× |
| 4 kt | 44% | 2.3× |
| 5 kt | 69% | 1.4× |
| 6 kt | 100% | 1× |
Both columns are against 6 knots of The slowest speed at which the boat still answers the wheel. Below it you are not steering, you are drifting in a chosen direction.. They are ratios, so they hold for any boat.
Why the bow blows off
Wind force does not care how fast you are going. It pushes on your The area of the boat the wind can push on: the hull sides, the cabin, the mast and rigging, or the canopy. A tall, boxy boat has a lot of it, and wind force does not care how slowly you are moving., the topsides and the house, just as hard as before. Your rudder is what gave out. Halve your speed and the wind has 4 times the edge it had.
The breeze did not get up when you slowed down. That is the moment your steering gave out under it. The fix is steering that works without boat speed.
Astern is worse, and prop walk is why
Going ahead, a single-screw boat's prop blows water straight over the rudder. A burst of power in gear steers her even when she is barely moving. Put her astern and that flow stops. The prop pulls water from around the hull and pushes it forward, away from the rudder.
What turns her astern is The sideways kick a propeller gives when you put the boat in reverse. A single propeller pushes the stern one way and only that way. That is why a single-screw boat backs predictably in one direction and badly in the other.. A turning prop walks the stern sideways, hardest going astern. Which way depends on which way your prop turns, and it goes one way only.
The rudder does very little astern. Most trawlers, displacement cruisers, sailboats under power and tournament ski and wake boats.
Reverse steering is good. The whole drive swivels and points the thrust. Prop walk is minor.
Steer on the throttles. One engine ahead and one astern, and the rudder is barely needed.
If your boat will not back straight, this is almost always why. It is not a fault in the boat or in you. Find out which way yours walks and back into the turn it already wants.
What fixes it, in the order to try
- Free Keep steerage way for as long as you can. Every knot you carry is worth more than the one below it. Arriving with a knot in hand is the difference between 25% of your steering and 11%. The table
- Free Use prop walk instead of fighting it. Set the approach up so the stern walks the way you want it to go. A boat that backs well to one side and badly to the other is telling you which way its prop turns. Why it walks
- Free Get a line ashore earlier. A A dock line run forward or aft at an angle rather than straight across. Held against the engine, it pulls one end of the boat in to the dock. It is the cheapest docking aid there is. steers at zero speed. It has been fixing this for four hundred years. How to use a spring line
- If it is still there Then add a steering force that does not need boat speed. That is what a thruster is. Where it sits matters nearly as much as how hard it pushes. Where it sits
How to use a spring line
A spring line runs from a cleat near the middle of the boat to a cleat on the dock. Put the engine in gear against it and the line holds her while the rudder or the drive swings her in. It works with the boat stopped.
- Rig it before you arrive. Cleat one end on the boat’s midships cleat and leave the rest where your crew can pick it up. Hang the fenders on the dock side.
- Get the line ashore. Point the bow past the end of the dock. Your crew steps off with the line and puts it around a dock cleat.
- Go ahead against it. Keep the engine idling in forward and let the line come tight. Turn the wheel away from the dock, and the boat swings in and lies alongside.
- Tie up while she is held. Leave her idling in gear. The engine holds her against the dock while the crew puts on the bow and stern lines and sets the fenders.
Practice it on a calm day at a quiet dock. The same line can take you off the dock again.
The steps follow Boating Magazine. For leaving the dock on a spring line, see trick one in BoatTEST’s five tricks.
Where a thruster sits beats how hard it pushes
A thruster makes the same sideways force a rudder does, without needing water flowing past it. How much it turns the boat is the thrust times the The distance from the point the boat turns about to the place the push is applied. Double the distance and the same push turns the boat twice as hard., its distance from the point she pivots about. That turning force is its Turning effort, measured in pound-feet. It is the force multiplied by how far that force sits from the point the boat turns about. A small push a long way out beats a big push near the middle..
A A tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Installing one is mostly a labor job rather than a hardware one. has to go where there is enough hull to take a tube, about 6 feet back from the bow on a 40-footer. An external unit can sit almost at the bow, or right on the The flat surface across the very back of the boat..
| Where it sits | Lever | Thrust |
|---|---|---|
| Tunnel, 6 ft back | 14 ft | 150 lb |
| External, at the bow | 19 ft | 111 lb |
| External, on the transom | 20 ft | 105 lb |
The external unit at the bow sits 1 ft back from it. Each row makes the tunnel’s 2,100 Pound-feet, the unit of turning effort: one pound of push, one foot away from the point the boat turns about. It is how you compare a thruster at the bow against one at the stern, because pounds of thrust alone say nothing about where the push is applied. of turning moment, with the boat pivoting about her middle.
Position does most of the work, but not all of it. Here is a 90 lb external unit against the 150 lb tunnel:
of the tunnel’s turning moment (1,710 lb·ft).
of it (1,800 lb·ft).
of it (3,510 lb·ft). The two pushes work as a Two equal pushes in opposite directions, one at each end of the boat. A couple spins the boat about her middle without shoving her sideways. The further apart the two pushes sit, the harder the boat spins, which is why moving a thruster further from the middle does more than making it stronger. on the length between them.
How much do you need?
Enough to keep steering the bow in the breeze you dock in. Your The area of the boat the wind can push on: the hull sides, the cabin, the mast and rigging, or the canopy. A tall, boxy boat has a lot of it, and wind force does not care how slowly you are moving. decides that far more than your length. Sleipner says so in its own words: “Thruster power is mainly determined by a boat’s design and not by its weight.”
Wind load goes with the square of wind speed too. Sizing for thirty knots instead of fifteen asks for about four times the thrust. The size calculator works your figure out at a 15 knot breeze.
What none of this will do
No thruster at this end of the market will pin your boat to a dock in a strong crosswind or a running current. What one at each end will do is turn her, either way, the instant you touch the control.
The expensive fix is a bigger single unit in the same place. A unit at the other end buys more turning per dollar, because it works on a longer lever.