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Finding one
Retractable bow thrusters: what they cut, and who they suit
A 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. is the thruster people get pointed at when they do not want 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. through the bow. This page shows what four makers’ manuals ask of the hull, how to use one, and what it costs. Then it tells you which boats it suits.
Where this comes from. The figures are read from four makers’ own installation manuals, on 19 September 2026. They are Lewmar’s RT manual, Quick’s BTR series manual, Sleipner’s SR80 and SR100 guide and VETUS’s retractable thruster instructions. Each is linked where it is quoted. The prices are this site’s own, dated. Where the page does its own arithmetic it says so. The one figure nobody publishes, the hours a yard takes, is labeled as the estimate it is.
The short answer
A retractable swaps a tunnel’s two side openings for one hatch in the bottom. In a 185 mm size that hatch is about 3 times one tunnel mouth, and about 1.5 times both.
A watertight compartment sits above the hatch and carries the unit. Lewmar wants a naval architect to design it.
Quick says to open and close its unit at no more than 4 knots, and to go astern with it down at no more than 2 knots.
That holds in every length band this site prices. Only 18 of the 44 units listed here carry a published US price, and the yard hours are our estimate.
Sailboats, fast planing boats, and hulls with nowhere to put a tube far enough forward. The thruster finder, our free tool that matches a thruster to your boat, blocks it on some hulls, listed below.
What a retractable is
It is a tunnel unit on a lift. The propeller and its short tube live in a housing inside the hull. They drop through a hatch in the bottom when you press the button, and come back up flush.
Mechanically it is a tunnel thruster. It has the same motor, the same 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., the same propeller in the same short tube, and the same battery and cable.
The depth rules are the same too. Quick’s BTR wants at least three quarters of a tunnel diameter below the waterline, which is what it asks of its tunnel units. Sleipner’s minimum depth for the deployed SR80 and SR100 is 185 mm, the tube’s own diameter. Its tunnel guides ask for one diameter as well.
The difference is that the tube is not glassed into the hull. It sits in a housing above a hatch, and an 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. lowers it into the water and raises it again.
That gives you two things:
A closed hull with the unit up. There are no tunnel mouths in the water flow, which is why sailboats and fast boats get them.
Depth. The propeller works well down on a hull that has nowhere to put a tunnel far enough forward. The unit drops below the hull line instead of needing depth inside it.
It does not give you a smaller hole. The thruster finder describes a retractable as a hatch cut into the bottom of the boat. The no-hole page puts it in the same group as the tunnel. The units that leave the hull whole are a separate group there.
How big the opening is, against a tunnel
Two makers publish the size of the opening. VETUS gives a template for each tunnel size. Lewmar gives a minimum height, width and depth for each model.
Both cut the hull at 45 degrees and reuse the sawn-out piece as the hatch. The angle is what makes the hatch close flush.
| Unit | Tunnel size | Hatch opening | Hatch area | One tunnel mouth | Hatch, in tunnel mouths | Hatch, against both mouths |
|---|---|---|---|---|---|---|
| VETUS STE, 150 mm tunnel | 150 mm | 224 by 285 mm | 638 cm² | 177 cm² | 3.6 times | 1.8 times |
| VETUS STE, 185 mm tunnel | 185 mm | 254 by 322 mm | 818 cm² | 269 cm² | 3 times | 1.5 times |
| Lewmar RT140 | 140 mm | 220 by 265 mm | 583 cm² | 154 cm² | 3.8 times | 1.9 times |
| Lewmar RT185 | 185 mm | 235 by 330 mm | 776 cm² | 269 cm² | 2.9 times | 1.4 times |
| Lewmar RT250 | 250 mm | 325 by 460 mm | 1,495 cm² | 491 cm² | 3 times | 1.5 times |
Take a retractable in a 185 mm tube, the size most of these units use. Its opening is about 3 times the cross-section of one tunnel mouth. That is about 1.5 times the two mouths a tunnel cuts.
So you trade two holes in the sides for one in the bottom, and the one is bigger.
Lewmar’s cut grows with the model. It is 220 by 265 mm for the RT140, and 325 by 460 mm for the RT250.
Hull thickness matters here in a way it does not for a tunnel:
- Quick’s BTR fits a hull up to 35 mm thick in the 185 mm size, and 50 mm in the 250.
- VETUS wants a bevel cut on the inside of any hull thicker than 15 mm. That lets the connection between the tube and the motor swing out far enough. Its housing must not sit more than 15 mm beyond the opening.
Where it goes
A retractable wants the same two things a tunnel does: forward and deep. The placement page has the arithmetic behind both.
Lewmar wants the RT as far forward as possible. With the unit down, the centerline of the hub has to be at least 1.5 times the propeller diameter below the waterline, and clear of the hull.
Quick uses its tunnel rule, at least three quarters of a tunnel diameter. It adds one of its own: the hatch must sit clear of the wave the bow throws, going ahead or astern.
Sleipner puts the SR on the keel centerline at the bow, as far forward as the 185 mm minimum depth allows. It lets a stern unit sit off the centerline to clear the propellers, trim tabs and rudders.
It also needs more room inside than a tunnel does, because the whole thruster rises into the boat. Sleipner asks for 100 mm of clear space all round, for molding the flange and for service later.
Lewmar asks for room for the hoses, the cables and access, kept dry and ideally ventilated. If the space is used for stowage, it wants a cover or a barrier. Nothing may jam the lift or reach the stop switches.
The compartment and the hatch
This is the part of the job a tunnel does not have.
Lewmar’s manual asks for a compartment built inside the hull, with an opening in the bottom. The top of that watertight compartment is the seat the base plate bolts to. It carries the weight of the unit, the thrust, and the forces under way.
Lewmar wants the compartment designed by a qualified naval architect. Quick refers any beams or supporting structure near its unit to the builder, a naval architect or a specialist.
Quick sells its counter flange separately, in stainless steel, aluminum alloy or GRP. It is laminated into a fiberglass hull, or welded into a metal one.
On a 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. hull, Sleipner has the inner laminate and the core cut back at least 100 mm around the flange to reach the outer skin. Then it is reinforced.
The hatch
The hatch comes out of the hull itself. Lewmar cuts at 45 degrees on all four sides and usually reuses the piece as the closing plate. It sits on a landing molded into the cut-out, so it seats in place when the unit is up.
That landing does two jobs. It stops the sea flooding the housing at pressure, and it stops the leg taking the load when the boat pounds.
VETUS saws at the same angle. It says the angle is what lets the sawn piece close the opening fully.
Sleipner cuts 45 degrees on three sides and 30 on the face toward the bow. Water hitting the closed hatch is taken by the hull. Its hatch always opens toward the bow.
Quick keeps 3 to 5 mm of clearance around the hatch, most carefully on the hinge side, with the inner walls at 45 degrees.
Get the cutting angles wrong and the hatch jams as it opens. That is Sleipner’s warning. VETUS ends its instructions with one the other manuals leave unsaid: check for leaks the moment the boat goes back in the water.
Using it: the speed limits
A tunnel has no moving part in the hull. A retractable has a hatch and a lift, and the manuals put limits on both.
Quick’s limits are the most explicit:
- Open and close the unit at no more than 4 knots.
- Go astern with it down at no more than 2 knots.
- Do not run with it open, and do not enable it above 4 knots.
Its fault table says why. The control box flashes once when the unit has tried three times to raise against too much friction. The causes it lists are a foreign body in the mechanism, the boat sailing at high speed, or a mechanical problem with the unit or its hatch.
VETUS’s unit retracts on its own after 15 minutes unused, so nobody leaves it down by mistake. Lewmar’s RT deploys when the control panel is turned on, which is a reason to keep the panel off under way.
Sleipner’s SR guide and VETUS’s manual give no speed figure at all. Sleipner’s answer to water pressure on the closed hatch is its shape. The hatch opens toward the bow, with the shallower cut on that face.
The moving part is also what fails. The thruster finder says so on its card. The Sleipner review carries one owner’s forum account of factory-fitted SRV100 retractables whose couplings sheared repeatedly on a new boat.
Marine growth is the other risk, and it bites differently here. A tunnel with growth in it loses thrust. A retractable with growth on the hatch may not close.
What it costs
This site lists 44 retractables from 5 makers. They run from 150 to 386 mm of tunnel and 124 to 1,433 lb of thrust. Where the makers publish a length at all, they place them on boats between 26 and 151 ft.
Only 18 of them carry a published US price. The other 26 are sold through dealers who quote, which is the usual arrangement for Sleipner’s and VETUS’s retractables.
| Unit | Thrust | Tunnel size | Price |
|---|---|---|---|
| Max Power Compact Retract (3 versions) | 220 lb and up | 185 mm | $7,370 |
| Max Power VIP 150 (3 versions) | 254 lb and up | 185 mm | $8,598 to $8,698 |
| Lewmar RT185 MX Gen 2 (2 versions) | 214 lb and up | 185 mm | $8,687 to $9,034 |
| Sleipner Side-Power SRVP80 PRO (2 versions) | 212 lb and up | 185 mm | $10,510 |
| Sleipner Side-Power SRV80 (2 versions) | 212 lb and up | 185 mm | $11,000 |
| Sleipner Side-Power SRV100 (2 versions) | 256 lb and up | 185 mm | $11,750 |
| Sleipner Side-Power SR80 (2 versions) | 212 lb and up | 185 mm | $11,850 |
| Quick BTR 185 (6 versions) | 231 lb and up | 185 mm | $12,436 to $17,991 |
| Sleipner Side-Power SRVP100 PRO (2 versions) | 256 lb and up | 185 mm | $13,070 |
| Sleipner Side-Power ERV100 eVision (2 versions) | 220 lb and up | 185 mm | $14,500 |
| Sleipner Side-Power SRP80 PRO (2 versions) | 212 lb and up | 185 mm | $14,700 |
| Sleipner Side-Power ERL100 eVision (2 versions) | 220 lb and up | 185 mm | $18,050 |
| Sleipner Side-Power SRV130 (2 versions) | 353 lb and up | 250 mm | $22,730 |
| Quick BTR 250 (3 versions) | 529 lb and up | 250 mm | $23,840 to $26,476 |
| Sleipner Side-Power ERV170 eVision (2 versions) | 375 lb and up | 250 mm | $26,455 |
| Sleipner Side-Power ERV210 eVision (2 versions) | 463 lb and up | 250 mm | $28,545 |
| Lewmar 250VRTT Vertical Retractable | 353 lb | 250 mm | $29,417 |
| Sleipner Side-Power ERL170 eVision (2 versions) | 375 lb and up | 250 mm | $29,500 |
For the installed price, the cost page puts the retractable beside the tunnel in each length band. It builds the figures from those units and this site’s labor figures.
The retractable column starts at 30–35 ft, because no priced retractable is sized for a smaller boat. In every band it fills, its cheapest unit costs more than the cheapest tunnel.
| Boat | Retractable, installed | Tunnel, installed | Yard hours, retractable | Yard hours, tunnel |
|---|---|---|---|---|
| Under 25 ft | No priced unit for this length | $8,700 to $10,600 | — | 32 |
| 25–30 ft | No priced unit for this length | $9,000 to $11,100 | — | 32 |
| 30–35 ft | $15,402 to $30,467 | $9,500 to $12,500 | 40 | 32 |
| 35–40 ft | $15,412 to $34,307 | $10,100 to $14,200 | 50 | 40 |
| 40–50 ft | $15,502 to $48,312 | $11,800 to $19,700 | 80 | 64 |
| 50–55 ft | $24,800 to $41,300 | $16,700 to $20,300 | 88 | 70 |
| 55–60 ft | $25,700 to $42,000 | $17,400 to $20,800 | 88 | 70 |
| 60 ft and up | $27,500 to $44,000 | $17,800 to $22,500 | 88 | 70 |
The hours are the weak spot in these figures. Nobody, maker or distributor or yard, publishes installation hours for a retractable. Here is what the trade does say:
- Fisheries Supply says the installation is significantly more expensive than a tunnel’s.
- Imtra calls it the hardest install, and says few yards have done one.
Neither of those is a number we can price from. So the retractable is priced at the tunnel’s hours for that boat length plus 25 percent. That 25 percent is our own figure, and every cost figure for the style carries that basis.
Got a quote for one? Check it against the quote-checker, with the hours basis above in mind.
Who it is for
A retractable is for getting a tunnel’s thrust and depth on a hull that stays closed under way. That fits three kinds of boat:
A sailboat. The thruster finder moves the retractable up for one. A tunnel mouth is a permanent hole in the water flow, and a sailor pays for that in speed. Fisheries Supply, Max Power and 48 North all give that as the reason the style exists.
A fast planing boat. This site lists no 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. bow unit for one. That leaves a retractable and a tunnel as the two bow routes, and the retractable is the one that closes the hull at speed.
A hull with nowhere to put a tube far enough forward. The retractable’s propeller works below the hull line instead of needing depth inside it.
It is not for every hull. The thruster finder blocks the style on a pontoon, a ski or wake boat and a bass boat, for the reasons it gives on the page:
- On a pontoon: a retractable drops down inside the hull, and a pontoon has no hull depth to drop into.
- On a ski or wake boat: a retractable needs hull depth to drop into, and these planing hulls have almost none forward.
- On a bass boat: a retractable needs hull depth to drop into, and these planing hulls have almost none forward.
Avoiding a tunnel because of the hole itself? A retractable will not help. The no-hole page compares the units that bolt on above the waterline against the tunnel band.
Sources: Lewmar’s RT manual (a dealer’s copy), Quick’s BTR series manual, Sleipner’s SR80 and SR100 guide and VETUS’s retractable thruster instructions, all read 19 September 2026. The opening table is our arithmetic on the makers’ published sizes. The prices are this site’s own, read 12 October 2026 to 15 October 2026. The installed bands and the hours basis are the cost page’s. That page marks the retractable’s hours as an estimate.
What this means when you dock
If yours is a Quick BTR, slow to 4 knots or less before you lower it. Keep astern to 2 knots while it is down, and raise it before you speed up again.
If yours is a Lewmar RT, the unit goes down when you turn the control panel on. Keep the panel off once you are under way.
If you forget to raise a VETUS, it retracts on its own after 15 minutes unused.
If the unit will not come back up, Quick’s fault list starts with a foreign body in the mechanism. Growth on the hatch can also stop it closing.