What this engine does - and what it does not
Everything below is from Yamaha's own European technical guide, linked at the foot of this page. What the engine will not do is set out just as plainly as what it will.
Included
- ✓A proper reverse gearF-N-R, where the 2.5 is forward and neutral only. If you come alongside single-handed or berth somewhere tidal, that matters more than the horsepower.
- ✓Still a one-person lift at 27 to 28 kgThe same weight as the 4 and the 6, because it is the same engine. You gain output without gaining weight.
- ✓Shallow water drive, three positionsRaises the leg for creeks, slipways and beaching without losing drive altogether.
- ✓Five trim positionsMore adjustment than anything above it in the range, which have four.
- ✓Charging available as an optionYamaha lists a 6 A alternator with rectifier regulator as an option on this model. Ask us before you order if you want it fitted.
- ✓Four-stroke, so no mixingNeat unleaded, RON 90, and oil in the sump. 0.6 L of engine oil and very little to service.
Not supported
- ✗The middle of three identical enginesThe 4, the 5 and the 6 share one 139 cm3 block - same bore, stroke, compression, gearbox and weight. Only the state of tune differs. Stepping up or down the trio changes output, not the machine.
- ✗No charging as standardThe 6 A alternator is an option, not fitted. If you want charging built in without asking, the 8 hp is the smallest with it as standard.
- ✗Manual start and tiller onlyPull start and a tiller handle. No electric-start or remote-control version of this model.
- ✗One carburettor, not injectionOnly the 25 hp in this range is fuel injected. In practice that means leaving good fuel in it over winter and using the choke on a cold morning.
What you get
The same engine as the 4 and 6
Identical 139 cm3 block, 62.0 x 46.0 mm bore and stroke, 8.9:1 compression and 27 kg. What changes across the trio is tuning: 2.9, 3.7 and 4.4 kW.
Reverse gear
A real F-N-R gearbox, which is the practical difference between this and the 2.5.
Shallow water drive
Three positions to lift the leg clear in thin water, then straight back down.
27 kg, still portable
No heavier than the 4 hp. One person can carry it down a pontoon and lift it onto a transom.
Reading a Yamaha model code
Yamaha's codes are on every listing, invoice and dealer site, and nothing on the engine explains them. Take F9.9JMHS - once you can read it, the whole range makes sense in about ten seconds.
F9.9 - four-stroke, 9.9 hp
F is the four-stroke family, and the number is the horsepower at the propeller shaft. Every engine in this range is a four-stroke, so every code starts with F.
J - design revision
A letter marking which generation of that model it is. It moves on when Yamaha revises the engine, so it is how you tell a current unit from a superseded one: F9.9J is the current 9.9.
MH - manual start, tiller handle
M is manual pull start, H is a tiller handle. An E in place of the M is electric start, and a code without the H is built for remote controls rather than a tiller.
S - short shaft
S is the short shaft, L the long. On this range the two are exactly 127 mm apart - five inches - which is where the 15 in and 20 in descriptions come from.
How much engine do you actually need?
Horsepower is the number everyone looks at and rarely the one that decides it. Weight, shaft length and whether the engine has a reverse gear matter more on a small boat.
2.5 to 6 hp
A tender, a small dinghy, a yacht's shore boat. Light enough for one person to carry down a pontoon and lift onto a transom. The 4, 5 and 6 are the same 139 cc engine at three states of tune.
8 and 9.9 hp
A bigger tender, a small day boat, or a yacht that wants to push against a foul tide. Both are the same 212 cc twin - the 9.9 is the same engine tuned harder.
15 and 25 hp
Enough to get a small boat planing rather than pushing water. This is transom-bracket and two-people territory, not something you lift off at the end of the day.
Measure the transom
Short shaft suits a transom around 15 in, long shaft around 20 in. Yamaha's own figures put S and L exactly 127 mm apart on every model in this range. Too short and the propeller ventilates in a chop; too long and you drag the leg.
F5
Every figure below comes from Yamaha's own European technical guide for the F5A.
| Yamaha model code | F5AMH |
|---|---|
| Engine type | OHV, 1 cylinder |
| Displacement | 139 cm3 |
| Bore x stroke | 62.0 x 46.0 mm |
| Compression ratio | 8.9 :1 |
| Fuel induction | 1 carburettor |
| Maximum output at the propeller shaft | 3.7 kW (5 ps) |
| Full throttle operating range | 4500 to 5500 r/min |
| Idling speed | 1450 to 1550 r/min |
| Lubrication | Wet sump |
| Ignition | CDI |
| Alternator | 6 A with rectifier regulator, optional |
| Gear shift | F-N-R |
| Gear ratio | 2.08 (27/13) |
| Shallow water drive | 3 positions |
| Trim | 5 positions |
| Transom height | S 435 mm, L 562 mm |
| Dry weight | 27, 28 kg |
| Engine oil capacity | 0.6 L |
| Gear oil capacity | 0.100 L |
| Recommended fuel | Unleaded regular, RON 90 |
Fitting it
- Match the shaft length to the transom before anything else - S for around 15 in, L for around 20 in.
- Check the boat's maximum power plate. That figure is the ceiling, whatever the transom will physically carry.
- Unleaded regular, RON 90. Fuel goes in neat - the oil lives in the sump.
- Rinse through with fresh water after salt, and stow the engine upright so the leg drains.
- On a small tender, fit a safety lanyard and use it. It is the one piece of kit that only matters once.
Every Yamaha outboard we stock
Live prices and the figures Yamaha publishes in their own European technical guide. Every column links to that guide.
| Model | F2.5 2.5 hp | F4 4 hp | F5 5 hp | F6 6 hp | F8 8 hp | F9.9 9.9 hp | F15 15 hp | F25 25 hp |
|---|---|---|---|---|---|---|---|---|
| You are viewing | ||||||||
| Model code | F2.5BMH | F4BMH | F5AMH | F6CMH | F8FMH | F9.9JMH | F15CMH | F25GMH |
| Engine | OHV, 1 cylinder | OHV, 1 cylinder | OHV, 1 cylinder | OHV, 1 cylinder | SOHC, in-line 2 | SOHC, in-line 2 | SOHC, in-line 2 | SOHC, in-line 2 |
| Displacement | 72 cm3 | 139 cm3 | 139 cm3 | 139 cm3 | 212 cm3 | 212 cm3 | 362 cm3 | 432 cm3 |
| Max output | 1.8 kW (2.5 ps) | 2.9 kW (4 ps) | 3.7 kW (5 ps) | 4.4 kW (6 ps) | 5.9 kW (8 ps) | 7.3 kW (9.9 ps) | 11.0 kW (15 ps) | 18.4 kW (25 ps) |
| Full throttle range | 5250-5750 r/min | 4000-5000 r/min | 4500-5500 r/min | 4500-5500 r/min | 5000-6000 r/min | 5000-6000 r/min | 5000-6000 r/min | 5000-6000 r/min |
| Compression | 8.8 :1 | 8.9 :1 | 8.9 :1 | 8.9 :1 | 9.4 :1 | 9.4 :1 | 9.3 :1 | 9.1 :1 |
| Ignition | TCI | CDI | CDI | CDI | CDI | CDI | CDI | TCI |
| Alternator | None | 6 A optional | 6 A optional | 6 A optional | 6 A | None on MH | None on MH | 16 A |
| Gear shift | F-N only | F-N-R | F-N-R | F-N-R | F-N-R | F-N-R | F-N-R | F-N-R |
| Gear ratio | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) | 2.08 (27/13) |
| Shallow water drive | None | 3 positions | 3 positions | 3 positions | Manual, 2 positions | Manual, 2 positions | Manual, 2 positions | Manual, 2 positions |
| Trim | 4 positions | 5 positions | 5 positions | 5 positions | 4 positions | 4 positions | 4 positions | Manual, 4 positions |
| Transom height | S 433 mm / L 560 mm | S 435 mm / L 562 mm | S 435 mm / L 562 mm | S 435 mm / L 562 mm | S 431 mm / L 558 mm | S 431 mm / L 558 mm | S 438 mm / L 565 mm | S 424 mm / L 551 mm |
| Dry weight | 17, 18 kg | 27, 28 kg | 27, 28 kg | 27, 28 kg | 40-43 kg | 40-45 kg | 51-61 kg | 56-70 kg |
| Engine oil | 0.25 L | 0.6 L | 0.6 L | 0.6 L | 0.8 L | 0.8 L | 1.8 L with filter change | 1.1 L with filter change |
| Gear oil | 0.075 L | 0.100 L | 0.100 L | 0.100 L | 0.150 L | 0.150 L | 0.250 L | 0.370 L |
| Price | £558.33 £670.83 | £862.50 £1,016.67 | £925 £1,091.67 | £1,026.67 £1,216.67 | £1,604.17 £1,891.67 | £1,812.50 £2,354.17 | £1,933.33 £2,275 | £2,967.50 £3,491.67 |
| Yamaha specification | ||||||||
| View product | F2.5 | F4 | You are here | F6 | F8 | F9.9 | F15 | F25 |
Choosing an outboard
Three questions settle it, and only one of them is about horsepower.
What is the boat rated for?
Most boats carry a plate stating a maximum engine power and often a maximum weight on the transom. That is the ceiling. Fitting more than the plate allows affects handling and, on an inflatable, can lift the bow in a way that is genuinely dangerous.
If there is no plate, tell us the make, length and construction and we will work it out with you.
Short shaft or long?
Measure from the top of the transom straight down to the bottom of the hull. Around 15 in wants a short shaft, around 20 in a long one. Get it wrong and the engine either ventilates when the boat pitches or drags its leg through the water all day.
This is the single most common ordering mistake, and it is not something you can adjust out afterwards.
How often will you lift it?
An engine that lives on the transom can be as heavy as the boat allows. One that gets carried down a pontoon, lifted into a car boot or stowed in a locker has a practical limit well below the legal one - 17 kg is a different proposition from 45 kg every weekend.
Be honest about this. It is the reason people end up not using a perfectly good engine.
What you will also want
You may also need
Yamaha outboard FAQs
What is the difference between the 4, the 5 and the 6?
Will it charge a battery?
Which shaft length do I need?
Will it fit my boat?
What fuel does it take?
It is a four-stroke - do I mix oil with the fuel?
Can I use it in salt water?
How long will it take to arrive?
Not sure which one?
Tell us the boat, the transom height and how often you will be lifting the engine, and we will tell you which Yamaha to buy - including when the smaller one is the right answer.
Talk to the chandleryDelivery & Returns
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