A tunnel thruster is the simplest and most common choice when the bow has enough depth below the waterline. Retractable thrusters drop out of the hull only when needed, so they suit sailboats, fast boats and shallow bows, but cost more and take more interior space. External stern kits bolt to the transom and add a stern thruster without cutting the bottom.
| Factor | Tunnel | Retractable / external |
|---|---|---|
| Where it sits | Tube through the hull, usually in the bow | Retractable: drops from a hull opening; external: tunnel kit on the transom |
| Drag underway | Some, from tunnel openings; reduced with recesses or deflectors | Retractable: flush hatch when stowed; external: hangs off the transom |
| Depth needed | Tunnel centre at least 1x diameter below waterline (Sleipner) | Retractable drops below the hull, so it works on shallow bows |
| Interior space | Motor above the tunnel | Retractable needs a tall housing (Max Power Compact Retract: 555 mm) |
| Hull work | Cut, glass in tube, round ends | Retractable: flanged or bonded opening; external: transom cut and kit bonding |
| Moving parts | Prop and gear leg only | Retractable adds a deploy mechanism |
| Typical use | Most powerboats and many sailboats with enough bow depth | Sailboats, fast boats, shallow bows; external for stern thrusters |
| Relative cost | Lowest | Retractable highest; external stern kits moderate |
If the bow has the depth for a properly positioned tunnel, a tunnel thruster is the best value and the easiest to service. Choose a retractable when the hull cannot take a tunnel deep enough, or when underway drag matters, as on many sailboats. Use an external transom kit when you want a stern thruster without cutting through the bottom.
Three ways to get sideways force into the water, three very different jobs. Which one fits your boat comes down to hull shape, how fast the boat goes, how much room you have inside, and whether the thruster is going at the bow or the stern. Here is how they compare from the installer's side.
Tunnel thrusters
The classic: a fiberglass, aluminum or steel tube laminated or welded through the hull, with the motor sitting on top and the prop inside the tube. It is the simplest design, the most common, and usually the least expensive to install and maintain. Most of the brands we install, from Sleipner SE to Vetus BOW to Lewmar Gen 2, are tunnel units.
The constraint is depth. Sleipner's guide wants the tunnel centre at least one tunnel diameter below the waterline, as far forward as possible, with rounded ends and a tube no longer than about four times its diameter. A fine-entry bow with little depth forward forces compromises: a tunnel too close to the surface sucks air, and one moved aft to find depth loses leverage.
The other cost is drag. On fast boats and sailboats, the back face of the tunnel can catch water. Sleipner's fixes are a recess aft of the opening or a small deflector ahead of it, and its rule of thumb is that you should not see the back face of the tunnel when looking aft from in front of the boat.
Retractable thrusters
A retractable thruster lives inside the hull with a hatch flush to the bottom. When you switch it on, it lowers a short tunnel and prop below the hull; when you are done, it folds back up. No tunnel drag underway, and the prop sits below the hull rather than in it, which helps on shallow bows.
Examples that show the range:
- Sleipner SRL low-profile units, such as the SRL80 for boats roughly 10 to 15 m with 80 to 96 kg thrust.
- Sleipner SRVP vertical proportional units, such as the SRVP170 PRO rated for 49 to 72 ft boats.
- Max Power Compact Retract for 30 to 52 ft yachts: 70 kgf at 12V, 85 at 24V and 100 at 48V, 40 kg, bolted onto a flange rather than laminated directly into the hull, with a 185 by 241 mm hull opening.
The trade-offs are cost, complexity and interior height. The Max Power Compact Retract stands 555 mm tall when stowed, which has to fit somewhere under a berth or locker. There is a deploy mechanism and position sensing to maintain, and Sleipner recommends an annual inspection of its retractable units. See our retractable installation page for what the job involves.
External thrusters (transom kits)
External thrusters are mostly a stern story. A stern tunnel kit is a short tunnel that bonds and bolts to the outside of the transom, with the motor mounted inside. It adds a stern thruster without cutting through the bottom.
- Sleipner 90124I: 125 mm stern tunnel kit for SE30, SE40, SEP30 and SEP40.
- Vetus STERN110P to STERN400P: GRP stern tunnels, with SDKIT extensions that push the tunnel deeper when the transom opening sits close to the waterline.
- Lewmar 589310: stern tunnel kit for the 185TT.
Sleipner's stern notes are practical: keep the thruster wash clear of sterndrives and trim tabs, and do not let the tunnel disturb water flow under the hull. External kits hang in the water, so they collect growth and add a little drag. More on stern installs on our stern thruster page.
Install scope compared
A tunnel install is mostly fiberglass work: cut, fit the tube, laminate inside and out, round the ends, then mount the drive and motor and run the electrical system. On a typical boat that is a 2 to 3 day job, outlined in our installation steps guide.
A retractable install cuts a larger, shaped opening and mounts a housing that has to be watertight and strong enough for the unit's weight and loads. Max Power's Compact Retract bolts onto a flange rather than being laminated directly into the hull, while other designs bond a housing in. Either way it needs interior height, a clear path for the mechanism, and the same battery and cable work as a tunnel thruster.
An external stern kit is usually the smallest hull job of the three: a transom opening for the drive, the tunnel kit bonded and fastened to the outside, and the motor inside. The electrical work is the same, and at the stern the battery often ends up close to the thruster anyway, which helps.
Electrical and battery notes for all three
Whatever the mounting, the motor needs voltage. Our standard is a dedicated AGM battery near each thruster with its own fuse, switch and charger, so the bow or stern unit never depends on a long cable run back to the house bank.
How to choose
- Bow with good depth forward, cruising speeds: tunnel.
- Sailboat, fast boat or shallow bow: look hard at a retractable, or a tunnel with a recess or deflector if depth allows.
- Adding stern control: a transom kit if the stern has room, or a tunnel if the hull shape suits.
- Very limited interior height: a low-profile retractable like the Sleipner SRL or Max Power Compact Retract, or a tunnel if the bow allows.
Thrust sizing works the same for all three; start with our sizing guide.
The bottom line
Tunnel when the hull allows it, retractable when it does not or when drag matters, and an external kit for adding a stern thruster with the least hull surgery. If you are not sure which camp your bow falls into, send us photos of the bow profile and the forward compartment and we will tell you what fits.
Questions people ask
Do tunnel thrusters slow a boat down?
They can add some drag, mostly from the back face of the tunnel opening. Sleipner describes recessing the hull aft of the tunnel or adding a small deflector in front, and says that standing in front of the boat looking aft you should not be able to see the back face of the tunnel.
Why choose a retractable thruster?
A retractable unit lowers its prop below the hull only when needed, so it avoids tunnel drag and works on bows too shallow for a properly submerged tunnel. Sleipner's SRL low-profile models and Max Power's Compact Retract are built for boats where interior height is limited.
Can I add a stern thruster without a tunnel through the hull?
Yes. Sleipner, Vetus and Lewmar sell stern tunnel kits that mount externally on the transom, and Vetus offers extension kits to push the tunnel deeper when the transom opening is close to the waterline.
Are retractable thrusters less reliable?
They add a deploy mechanism and position sensing, so there is more to maintain than a fixed tunnel unit. In return, the unit is tucked up inside the hull when stowed instead of sitting in a tunnel in the flow, and inspection points are listed in the maker's service schedule.
- Sleipner SE80-SE150 installation guide (tunnel position and drag)
- Sleipner SRVP170 PRO retract thruster
- Sleipner DC thruster spec sheet (SRL, SRV)
- Sleipner FAQ (retractable inspection)
- Max Power Compact Retract
- Vetus stern thruster tunnels
- Sleipner 90124I stern tunnel kit (Eshopmarine)
- Lewmar stern tunnel kit 589310 (iMarine)
Manufacturers update their lines. Confirm current model numbers with us or the maker before you buy.