Description
Two Engines, One Horsepower Rating
Before anything else: Mercury sells two different 40 HP FourStroke outboards at the same time, and they are not variants of each other.
The three-cylinder runs 747 cc through a 2.00:1 standard gearcase and weighs 204 lb. It accelerates hard, reaching around 30 mph on a suitable hull and covering 0 to 20 mph in roughly five seconds. It is the choice for lighter boats where you want responsiveness and where 56 lb of transom weight is important.
The four-cylinder runs 995 cc through a 2.33:1 Command Thrust gearcase and weighs 260 lb. Command Thrust means a gearcase that sits deeper in the water and swings a larger propeller with more blade area. It does not make the boat faster at the top end. It makes a heavy boat come onto the plane and stay there at lower speeds, which is a different problem and a real one if you are pushing a loaded pontoon or a heavy aluminum hull.
Neither is better. They answer different questions. If your boat is light and you want to get up and go, take the three-cylinder. For heavier boats that struggle to plane, the four-cylinder is the better choice. The rest of this page describes the three-cylinder.
The 747cc Three-Cylinder in Use
The engine uses a six-valve single overhead cam layout with electronic fuel injection, operating between 5500 and 6000 rpm. Three cylinders means the inherent balance advantage discussed on the 25 and 30 HP pages carries through here: 120-degree firing intervals cancel the rocking couple that gives twins their character, so vibration through the transom and the tiller stays low even at sustained throttle.
At 204 lb this engine is at the boundary of what a transom notices. On a 16- to 18-foot aluminum hull, it sits without changing the trim. On something lighter, the 56 lb saving over the four-cylinder version genuinely affects how the boat rides, especially with weight forward.
EFI provides turnkey starting in any weather, instant throttle response, and protection against poor-quality fuel, which is more important in some markets than others. The engine is SmartCraft compatible, meaning it will talk to Mercury’s digital gauges and give you real-time fuel burn, engine hours, trim position, and fault codes rather than a needle and a guess.
Gearcase, Steering, and What to Specify
The standard gear case runs. 2.00:1. The gear shift is forward, neutral, and reverse. Exhaust discharges through the propeller hub. Cooling is thermostatically controlled.
Steering is available as a tiller or remote. At 40 HP, the choice is genuinely a toss-up. A tiller keeps the layout simple and puts you where you fish. Remote steering from a console is more comfortable over distance and necessary if you carry passengers forward. Mercury also offers a Big Tiller kit on some configurations, which is a heavier-duty tiller built for engines in this power range rather than a scaled-up small-engine part.
Specify the option at order. Converting between tiller and remote afterwards means a control box, cables, a steering system, and labor, and it costs considerably more than getting it right the first time.
Starting is an electric turnkey. A high-output alternator supports fishing electronics and battery charging.
What It Will Move
Aluminum fishing boats and console boats from roughly 16 to 18 feet, fiberglass runabouts, larger RIBs, and pontoons on the lighter end. As a repower, it directly replaces aging 40- to 50-hp two-stroke engines, with substantially better fuel economy and noise.
If your boat is a heavy pontoon, a loaded workboat, or anything you currently struggle to get on a plane, the four-cylinder Command Thrust version is the correct engine, and we can quote it separately.
Every Mercury 40 HP outboard we ship is new, factory sealed, and pre-delivery inspected before crating. Compare across the Potent Power range and the Mercury outboard motors hub. For hull matching, see our pontoon boats and rigid-hull inflatables. Cross-brand, see the Yamaha midrange series. If you are unsure which 40 HP build suits your boat, request a freight and spec quote with your hull details.
Mercury 40 HP FourStroke EFI (3-Cylinder): Full Factory Specifications
Confirm your stock configuration before publishing. Four-cylinder figures are shown in the comparison column.
| Feature / Specification | 3-Cylinder (this page) | 4-Cylinder (alternative) |
|---|---|---|
| Horsepower | 40 HP / 30 kW | 40 HP / 30 kW |
| Engine type | Inline 3-cylinder | Inline 4-cylinder |
| Displacement | 747 cc | 995 cc / 60.8 CID |
| Bore and stroke | See supplier data. | 65 x 75 mm / 2.56 x 2.95 in |
| Valves | 6-valve SOHC | 8-valve SOHC |
| Full-throttle RPM range | 5500 to 6000 rpm | 5500 to 6000 rpm |
| Fuel system | Electronic fuel injection (EFI) | Electronic fuel injection (EFI) |
| Gearcase ratio | 2.00:1 | 2.33:1 |
| Gearcase type | Standard | Command Thrust |
| Dry weight | 204 lb / 93 kg | 260 lb / 118 kg |
| Steering | Tiller or remote | Big Tiller or remote |
| Recommended fuel | 87 octane, up to 10% ethanol | 87 octane, up to 10% ethanol |
| Recommended oil | Mercury FourStroke 25W40 Marine | Mercury FourStroke 25W40 Marine |
| Ignition | Digital inductive | ECM 07 Digital Inductive |
| Cooling system | Water-cooled with thermostat | Water-cooled with thermostat |
| Alternator | High-output | 18 amp / 226 W |
| Starting | Electric (turn-key) | Electric (turn-key) |
| Gear shift | Forward, Neutral, Reverse | Forward, Neutral, Reverse |
| Trim system | Power trim and tilt | Gas assist, power tilt, power trim |
| Shallow water trim range | See supplier data. | 20 degrees |
| Exhaust system | Through the propeller | Through the propeller |
| SmartCraft compatible | Yes. | Yes. |
| Engine protection | SmartCraft Engine Guardian | SmartCraft Engine Guardian |
| CARB star rating | 3 stars | 3 stars |
| Color | Phantom Black | Phantom Black |
| Condition | New, factory sealed, PDI completed | New, factory sealed, PDI completed |
Frequently Asked Questions
Why are there two different Mercury 40 HP outboards?
Mercury builds a 747cc three-cylinder 40 HP and a 995cc four-cylinder 40 HP simultaneously. They are different engines, not variants. The three-cylinder weighs 204 lb with a 2.00:1 standard gearcase and accelerates harder. The four-cylinder weighs 260 lb with a 2.33:1 Command Thrust gearcase that plants heavy boats on the plane. Choose by hull weight, not by horsepower.
What is a Command Thrust gearcase?
Command Thrust is Mercury’s heavy-duty gearcase that sits deeper in the water and swings a larger propeller with roughly 20 percent more blade surface through a lower gear ratio. It does not raise top speed. It lifts heavy boats onto the plane more easily and holds them there at lower engine speeds. Specify it if your boat is a loaded pontoon, a heavy aluminum hull, or a workboat.
How fast is the Mercury 40 HP three-cylinder?
On a suitable hull, the 747cc three-cylinder 40 HP reaches around 30 mph and accelerates from 0 to 20 mph in roughly five seconds. Actual figures depend on hull weight, load, and propeller pitch. The three-cylinder is the quicker of the two 40 HP builds because it carries 56 lb less weight through a higher 2.00:1 gear ratio.
Should I order a tiller or remote steering for the Mercury 40 HP?
Order a tiller if your boat is an open layout and you operate from the stern, since it keeps the installation simple and puts you where you fish. Order remote if you have a console, carry passengers forward, or run distance. Converting afterwards requires a control box, cables, and a steering system, so decide before ordering. A heavy-duty Big Tiller kit is available on some configurations.
Is the Mercury 40 HP SmartCraft compatible?
Yes. The engine communicates with Mercury SmartCraft digital gauges, giving real-time fuel burn, engine hours, trim position, and diagnostic fault codes rather than analogue readings. Engine protection runs through SmartCraft Engine Guardian, which reduces power automatically before damage occurs rather than simply warning you.
