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 stopped working. This page shows why, explains why backing down is worse again, and walks through the fixes, starting with the ones that cost nothing.
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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 exactly as hard as before. Slowing from 6 knots to 2 hands it about 9 times the advantage over your rudder. That is why the bow blows off.
- Backing down is worse. On a single-screw inboard with the rudder behind the propeller, 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 turning moment.
Your rudder has already quit
You throttle back for the fairway. Somewhere in the last few boat lengths she stops answering the wheel. The bow goes where the wind wants it. Back down and it gets worse. Most people decide they are bad at docking, when what they are is out of rudder.
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. That force rises and falls with the square of the speed.
At 6 knots you have all of your How much steering the rudder is giving you at a given moment. It falls with the square of boat speed, so at two knots you have roughly a tenth of what you had at six.. At 3 knots you have 25% of it. Slow down and the steering falls off a cliff.
| Boat speed | Steering you have left | Wind's advantage over it |
|---|---|---|
| 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× |
At the speed you enter a slip, you have almost nothing left. The wheel still turns, but the boat has stopped answering it. Below The slowest speed at which the boat still answers the wheel. Below it you are not steering, you are drifting in a chosen direction. you are drifting in a chosen direction.
Why the bow blows off
Wind force is set by the wind. It does not care how fast you are going. The wind 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., meaning the topsides and the house. That push stays exactly where it was while your rudder falls off that cliff.
Both are square laws, and they run in opposite directions. So halve your speed and the wind has 4 times the advantage over your rudder. Slowing from 6 knots to 2 hands the wind about 9 times the advantage it had at the top of the fairway.
The breeze did not get up when you slowed down. It felt that way because that is the moment your steering gave out under it. The wind itself never changed. So the fix is steering that works without boat speed.
Astern is worse, and prop walk is why
Everything above is about going forward. Backing down is a different problem, and a worse one.
Take a One engine driving one propeller. It steers with a rudder, so it loses steering at low speed, and in reverse the propeller kicks the stern one way only. boat with the rudder behind the propeller. Going ahead, the prop blows water straight over the blade. That is why a burst of power in gear steers her even when she is barely moving. Coast in with the engine in neutral and that steering is gone.
Put her astern and the flow over the rudder stops. The propeller now pulls water from around the hull and pushes it forward, away from the rudder. The blade sits in slack, disturbed water. It does very little however far you turn the wheel.
What turns the boat 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 propeller pushes the boat along the shaft. It also walks the stern sideways, and going astern that sideways push is at its strongest.
Which way it walks depends on which way your propeller turns, and it goes one way only. You get the direction your propeller gives you.
If your boat will not back straight, this is almost always why. It is not a fault in the boat or in you. Prop walk is fixed for your boat, so find out which way yours goes and plan to back into the turn it already wants. Fighting it with the wheel does not work, because the rudder is doing almost nothing astern.
This does not apply to every boat. Outboards and An engine inside the boat driving a steerable leg out through the back of the boat. It steers like an outboard with the motor inboard. It is common on express cruisers, and it leaves no room for a stern tunnel. swivel the whole drive, so they point the thrust and back down accurately. Prop walk is a minor effect on them, and reverse steering is good. Two engines driving two propellers, set apart from each other. Running one forward and one in reverse turns the boat on the spot without any rudder at all. That is why twins need less help at the stern. steer on the throttles, one engine ahead and one astern, and barely need the rudder at all.
This section is about single-screw inboards with a rudder behind the propeller. That covers most trawlers, most displacement cruisers, most sailboats under power, and every tournament ski and wake boat.
What fixes it, in the order to try
Work down this list in order. The first three cost nothing, which is why they come first.
- Keep steerage way for as long as you can. Every knot you carry is worth more than the one below it, because the force goes as the square. Arriving with a knot in hand is the difference between 25% of your steering and 11%. The table above shows the whole curve.
- 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 into one side of a fairway and badly into the other is telling you which way its propeller turns. Astern is worse explains why.
- 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 and costs nothing. It has been fixing this for four hundred years.
- Then, if the problem is still there, 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 a thruster sits beats how hard it pushes
A thruster is a rudder that works at zero speed. A rudder steers by making a sideways force at the very back of the boat, the furthest point from the pivot. That only works with water moving past it. A thruster makes the same kind of force without needing the flow. Fit one at each end and you have a rudder at both ends.
So where it sits matters nearly as much as how hard it pushes. Turning 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. is thrust multiplied by 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., the distance from the point the boat pivots about.
A A tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Fitting one is mostly a labor job rather than a hardware one. has to go where there is enough hull to take a tube. On a 40-footer that is about 6 feet back from the bow. An external unit can sit almost at the bow itself, or right on the The flat surface across the very back of the boat..
| Where it sits | Lever arm | Thrust needed |
|---|---|---|
| Tunnel, 6 ft back from the bow | 14 ft | 150 lb |
| External unit 1 ft back from the bow, where an external unit sits | 19 ft | 111 lb |
| External unit on the transom | 20 ft | 105 lb |
Moving from the tunnel's position to the transom cuts the thrust needed by about 30%. That is why a modest external unit does more than its spec sheet suggests.
Position does most of the work, but it does not close the gap on its own. A 90 lb unit at the bow produces 1,710 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. against the tunnel's 2,100. That is 81% of it. On the transom it is 86%.
A unit at each end is where it becomes decisive: 3,510 lb·ft, or 167% of the tunnel. 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 whole length of the boat between them.
What none of this will do
Two units at opposite ends of the boat give you far more steering than one. They will not pin your boat to a dock in a strong crosswind or a running current. Nothing at this end of the market will.
What they will do is turn her, in either direction, the instant you touch the control, with the stern line still in your hand. They turn her more sharply than the tunnel most yards would quote you.
If you cannot get her nose round without a go-around, or you get uncomfortably close to the boat alongside, that is a steering problem. It can be solved without spending five figures. Before any of that, look at your spring lines. A line ashore is cheaper than any thruster.
How much do you need?
You need enough to keep steering the bow in the breeze you dock in, at the speed you dock at. Your windage decides that far more than your boat's length does. Windage is how much wind your The sides of the hull above the water. catch. Sleipner says so in its own words: “Thruster power is mainly determined by a boat’s design and not by its weight.” VETUS’s own sizing method multiplies the wind pressure by the windage area, though the range it publishes is a chart by length.
Length on its own is in neither. Two boats of one length can carry very different amounts of house, and a chart by length cannot tell them apart.
This is also where a lot of money gets wasted, for the same square-law reason this whole page runs on. Wind load rises with the square of wind speed too. Sizing for thirty knots instead of fifteen asks for about four times the thrust.
The calculator works your number out from what kind of boat she is and how long, at a 15 knot breeze. It also shows you what those last few knots are adding.
Do not read this as an argument for buying small. The regret that comes up in owner threads is a thruster that cannot move the bow in a real breeze. The point is narrower. The fix people reach for is a bigger single unit in the same place, and that is the expensive one. A unit at the other end of the boat buys more turning per dollar, because it works on a longer lever.
No thruster fixes being pinned alongside in thirty knots. That call gets made before you come in.
- Run the size calculator for your boat. Tell it what kind of boat she is and how long, and it sizes a thruster to keep you in control in a 15 knot breeze.
- See which types can sit at the ends, taken from the installation manuals.
- Or try the thruster finder, our free tool that matches a thruster to your boat. Answer seven to nine questions and it lists the units that fit, with what each costs and what you give up.
- Check a quote you are holding, and ask what wind it was sized for.
What this means when you dock
If the bow blows off as you slow for the slip, your rudder has quit under the wind. Carry a knot more for longer and get a line ashore sooner. If that is not enough, a thruster steers the bow when the rudder cannot.
If she won't back straight, that is prop walk. Find out which way yours walks and set up every approach to back into the turn it wants.
If you have outboards, a sterndrive or twin screws, the astern problem mostly does not apply to you. The drive points the thrust, and twins steer on throttle.
If you need a go-around to get her nose round, that is a steering problem. A unit at each end gives you 167% of the tunnel's turning moment on the 40 ft example above.