We may earn a referral commission from some links on this page. It doesn't change your price or our recommendations.
Finding one
Proportional vs on/off bow thrusters: what you gain, what it costs
Most tunnel thrusters are on/off. Push the joystick and the motor runs flat out, one way or the other. A 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. puts a controller between the battery and the motor, so the joystick sets how hard it pushes.
This page shows what that controller changes at the helm and at the battery, and what it adds to the price, pair by pair. Then you can decide whether your boat needs one.
Where this comes from. Sleipner’s SEP80 to SEP150 and E100 installation manuals, read 20 September 2026. VETUS’s BOW PRO user manual. Max Power’s ECO page and FAQ, and Sleipner’s catalog and FAQ, as the brand reviews read them. Lewmar’s and Quick’s manuals were searched for the word and found without it. Every price is a dealer’s on the date it was read. Every pair is the same maker, the same tunnel and the nearest thrust the maker sells, within 15 percent.
The short answer
Proportional control does not add thrust. It turns the joystick into a throttle, so a small push gives a gentle nudge.
Sleipner rates its 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. on/off motors for 2 to 3 minutes at full power. At half power or less, Sleipner says its proportional units give close to continuous run time. On the brushless ones the limit is the battery’s charge.
On/off thrust falls with the volts. Sleipner’s SEP100 loses 14 percent of its thrust for a 1.5 V drop. A brushless unit holds its output up as the battery sags, within limits.
Across the 16 matched pairs on this site, proportional adds $698 to $2,700. On a small unit that can more than double the price.
Owners who dock slowly and need a long push, owners docking alone, and anyone on a lithium bank. If a docking is a few seconds of full power, on/off does the job for less.
What the joystick does on each
The two kinds share the same tunnel, much the same thrust and the same hole in the hull. Four things change: how it feels at the helm, how long it runs, how it copes with a tired battery, and what it costs.
On/off. The joystick closes a 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., and the motor gets the whole battery. Sleipner’s SEP manual puts the result in one sentence: “The actual voltage at the motor while running the thruster determines the motor RPM and thrust.”
So the push you get is whatever the bank and the cable deliver that day. Your only control is how long you hold the stick.
Proportional. Electronics sit in between, and the makers do it two ways.
Same motor, new controller. Sleipner’s SEP series keeps the brushed motor and adds a Proportional Power Controller (PPC). It meters the current to match the stick’s position.
New motor. Sleipner’s E-series eVision, VETUS’s BOW PRO and Max Power’s ECO change to a A motor with no carbon brushes: electronics switch the current instead. Nothing wears inside the motor, it runs cooler, and it costs more. motor with an 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., which sets the speed directly.
Either way the stick becomes a throttle. A quarter push nudges the bow in a tight fairway. The whole travel is there when the wind is on her. The sections below cover what follows from that: run time, how thrust follows voltage, and the bill.
Run time
Brushed motors are rated in minutes. At full power the motor heats up until a heat switch inside it trips. On a Sleipner SE that takes 2 to 3 minutes.
The SEP carries the same rating at full power, because it is the same motor. Its manual says: “S2 motor at 2 - 3 min. Duty cycle at 20°c (Ambient temperature)”
An S2 rating is how long the motor may run at a stretch from cold before it must stop and cool. So that line means 2 to 3 minutes at full power.
What the controller adds is running at half power. Sleipner’s catalog says a PRO unit at half power or less gives “close to continuous run time, only limited by your available power supply”.
Brushless motors are rated another way. The E100 manual prints no minutes at all: “Continuous at 22°C ambient, typically limited by battery capacity”. It gives the reason as well: “eVision thrusters have high efficiency and can therefore offer long run times.”
VETUS rates the BOW PRO “S2 - 10 min.”, which is 10 minutes at a stretch. Its user manual says what really decides it: “The runtime is mainly limited by the capacity of the installed batteries.”
Max Power’s ECO page puts it plainly: “Unlimited runtime (until the battery runs out)”
The limit moves from the motor to the battery. With a brushless unit, the motor’s heat stops mattering and the battery’s charge takes over. The run-time page has every maker’s figure side by side, and what each kind of limit means at the dock.
An ordinary docking uses seconds of either. The run limit matters on the day the wind keeps you working in a fairway and the on/off unit’s heat switch trips with the job half done.
Voltage, thrust and the battery
On/off thrust falls with the volts. Sleipner prints both figures. The SEP100 makes 256 lb at 12 V and 220 lb at 10.5 V. Its FAQ says 10.5 V is what a running thruster actually sees.
That is 14 percent of the thrust gone for 1.5 V.
Brushless thrust holds up. The E100 is the brushless unit in the same tunnel, and it prints one figure: 220 lb at 21 V. The inverter holds the motor’s output up as the battery sags.
VETUS says the same of the BOW PRO in its user manual: “The thrust delivered is, within certain limits, not dependent on the battery voltage.”
That holds only within limits. On a 12 V system the BOW PRO controller starts reducing power at 10.5 V. It stops the thruster at 10 V.
Lithium batteries
Battery chemistry splits the same way. Sleipner’s SEP and E manuals carry one block for both: “All Sleipner proportional thrusters are compatible with lithium batteries.”
They add a warning with it: “An under-rated battery management system can suddenly disconnect the load which may lead to dangerous situations.” A lithium bank also wants a controller on firmware 1.033 or later.
Sleipner’s FAQ gives two reasons an on/off unit is the awkward one on lithium. The first is the battery’s own protection circuit, the BMS, which can shut the battery off when it sees a fault. In Sleipner’s words, “thrusters draw a lot of currents quickly, and the high amp draw may lead the BMS system to think that there is an issue”.
The second is the voltage. A “Lithium or Li-ion battery may maintain a significantly higher voltage and higher current under load than a lead-acid, gel or AGM battery”. With nothing between the battery and the motor, the “propeller will rotate significantly faster” than the unit was rated for.
Max Power is blunter still. Its ECO series: “ECO PROportional series are designed to operate with Li-ion or LiFePO4 batteries but can also operate without a problem with lead-acid, AGM, gel, or similar kind of batteries as well.”
Its standard series: “Standard on/off DC thrusters are designed to operate with lead-acid, AGM, gel, or similar kind of batteries. Li-ion or LiFePO4 batteries will cause rapid deterioration of the Max Power equipment (electric motor, relay, control box, transmission, etc) if they are not specified correctly.”
The battery page has every maker’s position. The battery calculator works out the bank each model asks for.
What the controller costs, model by model
Several makers sell both kinds at the same dealer, so you can read the price of the controller straight off. Each of the 16 pairs below is one maker and one tunnel diameter, with the proportional unit at the nearest thrust to the on/off unit’s. Sleipner’s pairs match within a pound. VETUS’s sit within 9 percent, because its BOW PRO sizes do not line up with its standard ones.
The Sleipner E-series rows are matched to the SE’s lower-voltage rating, which is how Sleipner rates both.
Show the 16 pairs, model by model
| Maker | On/off | Proportional | Thrust | Difference | Kind |
|---|---|---|---|---|---|
| Sleipner | SE40/125S, $1,845 | SEP40/125S, $4,545 | On/off 105 lb. Proportional 106 lb | $2,700, 146 percent | brushed motor, PPC controller |
| Sleipner | SE60/185S, $2,295 | SEP60/185S, $4,995 | On/off 161 lb. Proportional 161 lb | $2,700, 118 percent | brushed motor, PPC controller |
| Sleipner | SE80/185T, $3,285 | SEP80/185T, $5,895 | On/off 212 lb. Proportional 212 lb | $2,610, 79 percent | brushed motor, PPC controller |
| Sleipner | SE100/185T, $4,455 | SEP100/185T, $6,975 | On/off 256 lb. Proportional 256 lb | $2,520, 57 percent | brushed motor, PPC controller |
| Sleipner | SE120/215T-24V, $4,905 | SEP120/215T-24V, $6,291 | On/off 306 lb. Proportional 306 lb | $1,386, 28 percent | brushed motor, PPC controller |
| Sleipner | SE130/250T, $5,670 | SEP130/250T, $7,290 | On/off 352 lb. Proportional 352 lb | $1,620, 29 percent | brushed motor, PPC controller |
| Sleipner | SE210/250TC, $8,325 | SEP210/250TC, $9,023 | On/off 550 lb. Proportional 550 lb | $698, 8 percent | brushed motor, PPC controller |
| Sleipner | SE100/185T, $4,455 | E100/185T, $6,885 | On/off 220 lb at 10.5V (256 at 12V). Proportional 220 lb at 21 V | $2,430, 55 percent | brushless, eVision |
| Sleipner | SE130/250T, $5,670 | E130/250T, $7,650 | On/off 286 lb at 10.5V (352 at 12V). Proportional 287 lb at 21 V | $1,980, 35 percent | brushless, eVision |
| Sleipner | SE250/300TC, $8,820 | E250/300TC, $11,025 | On/off 551 lb at 21V (661 at 24V). Proportional 551 lb at 21 V | $2,205, 25 percent | brushless, eVision |
| VETUS | Bow 45, $1,927 | Bow Pro 421, $3,258 | On/off 99 lb. Proportional 93 lb | $1,331, 69 percent | brushless, BOW PRO |
| VETUS | Bow 60, $2,243 | Bow Pro 651, $3,782 | On/off 132 lb. Proportional 143 lb | $1,539, 69 percent | brushless, BOW PRO |
| VETUS | Bow 75, $2,717 | Bow Pro Boosted 76, $4,252 | On/off 165 lb. Proportional 168 lb | $1,535, 56 percent | brushless, BOW PRO |
| VETUS | Bow 95, $3,901 | Bow Pro Boosted 90, $5,528 | On/off 210 lb. Proportional 198 lb | $1,626, 42 percent | brushless, BOW PRO |
| VETUS | Bow 160, $5,814 | Bow Pro Boosted 150, $7,018 | On/off 352 lb. Proportional 331 lb | $1,204, 21 percent | brushless, BOW PRO |
| VETUS | Bow 285, $9,287 | Bow Pro Boosted 285, $10,500 | On/off 628 lb. Proportional 628 lb | $1,213, 13 percent | brushless, BOW PRO |
In dollars, the premium does not grow with the unit. In every family the biggest pair pays less for the controller than the smallest does:
- Sleipner’s PPC: $698 to $2,700 across 7 pairs.
- VETUS’s BOW PRO: $1,204 to $1,626 across 6.
- Sleipner’s eVision: $1,980 to $2,430 across 3.
In percent, it falls as the unit grows. Sleipner’s premium is 146 percent on the SE40/125S and 8 percent on the SE210/250TC. VETUS’s is 69 percent on the Bow 45 and 13 percent on the Bow 285.
The controller is much the same part at every size. So on a small boat it can more than double what the unit costs.
Max Power’s 5 ECO rows are missing from the table because nobody prices them. The maker publishes no price, and neither does the dealer the review priced from.
There is one owner’s figure, for what it is worth. AvalonGB put a Sleipner SEP170 at about $1,000 more than a VETUS BOW160 in street price (Grand Banks 46 Classic, Jan 2018, Trawler Forum). That is one owner comparing two brands in one thread.
The installation is the same job either way. The cost page has the labor.
Who makes which
| Maker | On/off | Proportional | Motor |
|---|---|---|---|
| Sleipner | SE series | SEP series (a brushed motor behind a Proportional Power Controller) and the E-series eVision (brushless) | Brushed on SE and SEP. Brushless on the E-series |
| VETUS | BOW series | BOW PRO (2018) and BOW PRO Boosted, which adds a DC-to-DC charger | Brushed on BOW. Brushless on BOW PRO |
| Max Power | CT series | ECO PROportional series, 24 V and 48 V | Brushed on CT. The ECO 90/110/130 installation manual says a “hair pin” brushless AC motor |
| Lewmar | TT Gen 2 | None in the Gen 2 manual read | Brushed |
| Quick | BTQ series | None in the BTQ manual read | Brushed |
Sleipner is the one maker selling the controller two ways. The SEP puts it on the brushed motor. The E-series changes the motor instead.
VETUS went straight to brushless with the BOW PRO in 2018.
Retractables come both ways too, and the retractables page names them. The Sleipner review and the VETUS review have each range in full.
Who it is for
Buy proportional if your boat docks slowly and needs the push for longer than a burst. That covers:
- a long approach in a crosswind
- a fairway that takes several corrections
- an owner alone on deck who wants a nudge while the lines go on
- a lithium 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.
The brushless units have two more things going for them. They are quieter, which the Sleipner review reports in the maker’s own words. They also have no brushes to check or replace, a job the maintenance page lists for every brushed motor.
Buy on/off for the price, and because a docking is seconds of full power anyway.
The push is the same either way. The size calculator sizes both kinds to the same figure. The thruster finder, our free tool that matches a thruster to your boat, lists both in one table. Choose on run time, battery and price, using the sections above.
Sources: Sleipner’s SEP80 to SEP150 manual and E100 eVision manual, its 2026 catalog and FAQ; VETUS’s BOW PRO user manual; Max Power’s ECO 90 page and FAQ; the owner’s figure from a Trawler Forum thread (Jan 2018).