Bow thruster terms: plain-English definitions

The short answer

Steering and handling

These are the words behind how a boat turns. They explain why a thruster is a steering control, what a lever and a couple do, and why the rudder goes quiet at docking speed.

couple

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.

moment

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.

lb·ft

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.

lever

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.

pivot point

The spot the boat turns about when she swings. With the boat moving ahead it is usually about a third of the way back from the bow. In reverse, it moves further back.

prop walk

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.

rudder

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.

rudder authority

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.

steerage way

The slowest speed at which the boat still answers the wheel. Below it you are not steering, you are drifting in a chosen direction.

windage

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.

single screw

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.

twin screw

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.

sterndrive

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.

spring line

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.

on plane

A hull running fast enough to climb on top of the water instead of pushing through it. Anything hanging below the hull adds drag and can be damaged at that speed.

hull speed

The fastest a boat that pushes through the water can go before it starts fighting its own bow wave. The waterline length sets it: about 1.34 times the square root of the waterline length in feet, in knots.

Beaufort scale

The wind scale sailors use, from 0 (calm) to 12 (hurricane). Force 4 is 11 to 16 knots, force 5 is 17 to 21, and force 6 is 22 to 27.

The boat

These are the parts of a hull our guides name when they tell you where a thruster can go.

LOA

Length overall: the whole boat from the tip of the bow to the back end. It is not the waterline length, and not the number in the model name.

displacement hull

A hull that pushes through the water rather than climbing on top of it. Most sailboats and the heaviest trawlers have one, and they carry a lot of weight for their length.

semi-displacement hull

A hull that can be driven a little faster than a pure displacement boat of its length without climbing onto the surface. Most trawlers, and most motor yachts with an open upper helm, are one.

planing hull

A hull built to climb up and skim on top of the water at speed instead of pushing through it. Most powerboats under about 40 ft have one.

freeboard

How far the deck sits above the water. More freeboard means more of the boat is exposed to the wind.

topsides

The sides of the hull above the water.

transom

The flat surface across the very back of the boat.

forefoot

The part of the hull just behind the bow, where the front of the boat curves down toward the keel. It is the shallowest part of the underwater hull, so a tunnel there has little water above it.

keel

The lowest part of the hull, running along the middle from bow to stern.

fin keel

A short, deep keel bolted under the middle of a sailboat, with the rudder separate behind it. The boat turns quickly and drifts sideways easily at low speed.

chine

The angle where the side of the hull meets the bottom. A hard chine is a sharp corner, and a hull with a rounded bottom has none.

deadrise

The angle of the hull bottom from flat, seen from the front. A deep V has a lot of it and needs more water under the bow before a tunnel will fit.

amidships

The middle of the boat, halfway between bow and stern.

athwartships

Across the boat, from one side to the other.

lazarette

The storage locker at the very back of the boat, under the aft deck or cockpit. On many boats it is already full of steering gear, tanks and hoses. That is why a stern thruster often has nowhere to go.

flybridge

The open upper helm on top of a motor yacht or trawler. It adds a lot of height for the wind to push on.

swim platform

The shelf across the back of the boat at water level, for boarding and swimming. A stern thruster can hang underneath it.

tritoon

A pontoon boat with three logs instead of two.

windlass

The electric winch at the bow that raises the anchor. It has its own heavy cable and often its own battery. That is why a bow thruster is sometimes wired alongside it.

GRP

Glass-reinforced plastic: what European makers call fiberglass. Most production hulls are built from it, so "GRP work" means cutting into the hull itself and glassing it back up.

cored

A deck or hull built as a sandwich: two skins of fiberglass with balsa or foam between them. Bolts through it must be sealed, or water gets into the core and rots it.

through-hull

A fitting that passes through the hull below the waterline, sealed with a wide lip on each side. Cutting one means the boat comes out of the water.

seacock

A valve on a fitting through the hull that shuts the sea out. Good practice, and some insurers, say to close every one of them when you leave the boat.

backing plate

A plate of metal or plywood on the inside of the hull or deck, under the bolts. It spreads the load over a wide area, so the bolts do not pull through the fiberglass.

through-bolt

A bolt that passes right through the hull or deck with a nut on the inside, instead of a screw that only bites into the surface.

stringer

A long rib bonded inside the hull, running from bow to stern, that stiffens the bottom. Cutting through one weakens the boat, so a tunnel is kept clear of them.

bulkhead

A wall inside the boat, running across it from side to side.

forepeak

The space right in the bow, inside the hull, usually a locker for sails, fenders or the anchor chain.

full keel

A keel that runs most of the length of the hull, with the rudder hung off its back edge. The boat holds a straight course well and turns slowly.

trim tabs

Two flat plates hinged along the bottom edge of the transom, one each side, that tilt down to lift the stern and level the boat at speed.

gelcoat

The smooth, colored outer skin of a fiberglass hull: a layer of resin over the glass. It keeps water out of the laminate under it, which is why ground areas are coated again.

layup

The layers of glass cloth and resin built up one over another to make or repair a fiberglass hull.

chamfer

An edge cut at a slant instead of left square or rounded.

The thruster

Here are the kinds of thruster, the parts inside one, the ratings printed on the box and the pieces that wear.

bow thruster

A small propeller at the front of the boat that pushes the bow sideways, so you can steer the front end at docking speed, when the rudder does almost nothing. The engines still move the boat; the thruster steers her bow while they do.

tunnel

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.

retractable

A thruster that lives inside the hull and drops down through a hatch when you press the button, then pulls back up. It leaves no permanent hole in the water flow.

external unit

A thruster that bolts to the outside of the hull, up near the bow or on the back of the boat. Its propeller hangs in the water below it. Nothing is cut through the hull, so there is no tunnel.

hydraulic thruster

A thruster whose propeller is turned by oil under pressure instead of an electric motor. A pump on the engine or a power pack supplies the oil. The thruster needs no battery bank at the bow and can run as long as the pump does.

proportional thruster

A thruster with a controller between the battery and the motor, so the joystick sets how hard it pushes rather than just on or off. The motor and the mounting are the same. Only the control differs.

gearleg

The underwater part of a thruster: the housing in the tunnel that holds the gears and the propeller. The motor sits above it, inside the boat.

tailpiece

The gearbox at the bottom of a tunnel thruster, in the water. It turns the motor’s shaft through a right angle to drive the propeller. Some makers call it the gearleg.

shear pin

A small, deliberately weak pin between the shaft and the propeller. If the propeller jams on a rope, the pin breaks instead of the gearbox, and it costs a few dollars to replace.

coupling

The joint between the motor’s shaft and the gearbox below it. On many thrusters it is the part built to give way if the propeller jams, so the gears survive.

anode

A block of zinc or aluminum bolted to the thruster that corrodes on purpose, so the seawater eats it instead of the gearbox. It is replaced when it is about half gone, usually once a year.

galvanic corrosion

Metal eaten away because two different metals sit in seawater connected to each other, or because stray current leaks into the water. It is the damage an anode exists to absorb. Most warranties exclude it.

brushes

Small carbon blocks inside a motor that press on the spinning part and carry the current into it. They wear down, they shed dust, and they are the usual reason an old thruster motor stops.

brushless

A motor with no carbon brushes: electronics switch the current instead. Nothing wears inside the motor, it runs cooler, and it costs more.

inverter

Electronics between the battery and the motor. They turn the battery’s direct current into the alternating current a motor without brushes runs on, and they hold the motor’s output steady as the battery voltage drops.

cavitation

Bubbles forming on a propeller blade when it is working too hard or running too near the surface. The propeller spins and the push disappears.

fouling

Weed, slime and barnacles growing on anything that sits in the water. Inside a thruster tunnel it quietly costs thrust, and on a retractable it can stop the unit coming down.

haul out

Lifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself.

duty cycle

How long a motor may run before it must rest, as the maker rates it. "2 to 3 minutes" means that long at full power, then a pause to cool down.

S2

A motor rating from the electrical standards: the motor can run at full power for the stated number of minutes from cold, then must rest. "S2 - 4 min." means four minutes.

kgf

Kilogram-force, the metric thrust rating most European makers publish. One kgf is about 2.2 pounds of push, so a 50 kgf unit is roughly 110 pounds.

lbf

Pounds-force: a pound of push, written as a thrust rating. It is the same figure most people write as lb.

nominal

The figure a maker prints as the normal running value, as opposed to a peak at start-up or a minimum.

actuator

A small electric motor and screw that pushes a part into position. On a retractable thruster it is what lowers the unit out of the hull and raises it again.

limit switch

A small switch that tells the controller a moving part has reached the end of its travel. It is how a retractable thruster knows it is fully down or fully up. A dirty or failed one stops the unit from running.

ignition protected

Built so it cannot make a spark that would ignite gasoline fumes. Required for anything electrical in the same compartment as a gasoline engine or tank.

fairing

Smoothing a repair or a fitting into the shape of the hull with filler, so the water flows past it without a step. On a thruster it is the work that rounds the tunnel mouth into the hull.

antifouling

The paint on the underwater hull that stops weed and barnacles growing. It goes on the gear leg and the propeller too, and never on the anode.

lay days

The days a boat sits out of the water in the yard, billed by the foot per day on top of the lift itself.

stray current

Electricity leaking into the water from a fault or a poor connection on a boat, yours or a neighbor’s at the dock. It eats metal under the water.

isolation

Keeping two metal parts from touching or being joined by a wire, so no current can pass between them. On a metal boat the thruster motor is kept isolated from the hull.

The electrics

This group covers the battery, the cable and the switches. Most thruster trouble starts here.

CCA

Cold cranking amps: how much current a battery can deliver in a short, hard burst, which is the rating starter batteries carry. A thruster draws like a starter, so the makers size its battery this way rather than by capacity.

DIN

One of two test standards for a battery’s cold cranking amps, the German one. It gives a lower number than the American test for the same battery. Check which standard a label uses before comparing figures.

SAE

One of two test standards for a battery’s cold cranking amps, the American one. It is the figure on most battery labels sold in the US. It gives a higher number than the German test for the same battery.

DIN CCA per amp

The minimum battery the maker asks for, divided by the current the motor draws. A ratio near one means the maker wants a battery that can crank roughly what the thruster pulls.

amp-hours

How much a battery can supply slowly, over hours: ten amps for ten hours is a hundred amp-hours. It is the right measure for lights and a fridge. It is the wrong one for a thruster, which wants a lot of current for a few seconds.

BMS

Battery management system: the electronics inside a lithium battery that protect it by cutting the connection when something looks wrong. A thruster’s sudden heavy draw can look wrong to it.

AGM

Absorbed glass mat: a sealed lead-acid battery with the acid held in fiberglass matting. It gives off no gas in normal use and holds its voltage well under a heavy draw. That is why thruster makers name it.

starting battery

A battery built to deliver a large current for a few seconds, the kind that starts an engine. Many thin plates suit a thruster’s short, hard burst.

deep cycle

A battery built to be drained slowly and recharged many times, the kind that runs the lights and the fridge. Its thick plates last through many cycles, but they give up current slowly, where a thruster asks for a lot of it at once.

house bank

The batteries that run the boat’s lights, fridge and electronics, as distinct from the engine’s starting battery. They are built for a slow, steady draw.

voltage sag

The drop in a battery’s voltage while a heavy load is drawing from it. The battery recovers when the load stops, but the motor only ever sees the sagged figure.

solenoid

An electrical switch worked by a small current that closes a large one. In a thruster it connects the battery to the motor when you press the joystick, and it is a common part to fail. Solenoid, relay and contactor are three names for this one part.

contactor

A heavy-duty electrical switch that carries the full motor current, closed by a small signal from the control panel. It is the part between the joystick and the motor, and it can corrode or stick. Contactor, relay and solenoid are three names for this one part.

relay

A small electrical switch that a low current closes so a bigger current can flow. A thruster panel works the motor through one, and a relay that sticks closed keeps the motor running. Relay, solenoid and contactor are three names for this one part.

main switch

The heavy switch, usually near the batteries, that disconnects the thruster from its battery entirely. It is the only switch that stops a motor that has stuck on.

AWG

American wire gauge, the size scale for electrical cable. The number falls as the cable gets thicker: 2 AWG is thicker than 6, and 1/0, 2/0, 3/0 and 4/0 are thicker still.

ABYC

The American Boat and Yacht Council, the body that writes the standards US boats are built and wired to. A surveyor or an insurer who asks whether wiring is to standard means ABYC’s.

ABYC E-11

The American Boat and Yacht Council’s standard for electrical systems on boats, the one US surveyors and insurers check wiring against. Its tables set the most current each cable size may carry and the most voltage a run may lose.

DC-to-DC charger

A charger that takes the boat’s 12 volt supply and puts out a different voltage. It lets a 24 or 48 volt thruster bank charge from a 12 volt boat.

series-parallel switch

A switch that connects two 12 volt batteries end to end to make 24 volts for the thruster. It then connects them side by side again, so the boat’s 12 volt charger can charge them.

voltage drop

The volts lost in the cable between the battery and the motor. A long or thin cable loses more, and the motor only ever sees what is left.

ampacity

The most current a cable can carry all day without overheating, as the standards table it. It depends on the size of the copper and how hot the space it runs through is.

ANL fuse

A flat, bolt-down fuse made for heavy loads like a thruster or a windlass. It is built to carry a big overload for a while and open only on a short circuit.

bus bar

A metal bar with several studs that joins many heavy cables at one point, so each does not have to reach the battery itself.

lug

The metal end crimped onto a heavy cable so it can be bolted to a terminal.

crimp

Squeezing a metal end onto a cable with a tool made for the size, so the copper and the end become one solid piece. It is how heavy marine cable is terminated, never soldered.

tinned cable

Cable whose copper strands are coated in tin, so they do not corrode in salt air. Most marine battery cable is tinned.

multimeter

A hand-held meter that reads volts and resistance. It is the tool behind the makers’ isolation tests.

in parallel

Batteries joined side by side, positive to positive and negative to negative. The voltage stays the same and the cranking amps add together.

in series

Batteries joined end to end, the positive of one to the negative of the next. The voltages add, so two 12 volt batteries make a 24 volt bank, and the cranking figure does not add.