Yamaha Outboard Sizes: Horsepower, Blocks and Shaft Length

There are 32 outboards in Yamaha's Build a Boat lineup, from a 9.9 hp up to a 350 hp. Size also means shaft length, such as 25 or 30 inches. The shaft has to match your boat's transom just as the horsepower has to match the hull.

Horsepower families that share an engine block

The horsepower numbers look like a smooth ladder. In fact, several ratings are built on the same engine block. Yamaha's horsepower guide points out which ratings share a block, and the real jumps in engine size come where the block changes. Here's our summary of those examples:

Ratings	Engine block
F50, F60, F70	1.0L inline-four
F75, F90, F115	1.8L inline-four
F150, F200	2.8L inline-four
F225, F250, F300	4.2L V6

Take the F70 and F75. They sound like neighbors, but the F70 shares the small 1.0L block with the F50 and F60, while the F75 sits on the 1.8L block used by the F90 and F115. That five-horsepower step puts a larger-displacement engine on your transom. When you compare two ratings, check the block first. If both ratings share a block, you're choosing between ratings built on one engine block.

Does a bigger rating on the same block burn more fuel?

At cruise, not much. The F225, F250 and F300, which share the 4.2L V6, get nearly identical fuel economy at cruising speed when fitted with the same props. The extra horsepower only shows up at high rpm and wide-open throttle. That includes the holeshot, when the boat climbs from a standstill onto plane.

Say you're torn between an F225 and an F250, both on the 4.2L V6. The F250 costs you little extra at the fuel dock on a normal run. You get it back when you need to climb on plane with a full load, and in more top end. If you head offshore for halibut or albacore with full tanks, ice, gear and a crew, the bigger rating earns its place. If you mostly make short, light runs around the bay, the smaller rating will cruise the same, and you won't miss the top end.

Shaft length: the size that has to fit your transom

The transom is the flat back of the boat where the outboard bolts on. Its height sets the shaft length you need. With a shaft that's too short, the prop sits high and can suck air and lose its bite in a turn or a following sea. With one that's too long, the lower unit hangs deep and drags, which costs speed and fuel.

Two engines listed on our site show how much this varies (call us to confirm what's currently available):

A pre-owned 2017 Yamaha LF225NCA is a 225 hp four-stroke with a 30-inch shaft and left-hand rotation, sold AS-IS. Left-hand rotation matters on many twin-engine rigs. There, the two engines spin in opposite directions, which balances prop torque so the boat doesn't pull to one side.
The 2026 Yamaha F90JB jet outboard has a 25-inch shaft, which our listing calls extra long. It weighs about 354 lbs.

To check your own boat, measure straight down from the top of the transom to the bottom of the hull at the centerline. Engine weight belongs in the same check. Compare the engine's weight with what your hull's builder says the transom can carry. A heavier engine changes how the boat sits at rest and how the transom handles the load.

Jet outboards: rated horsepower vs. power at the pump

In the shallow stretches of the Coos, Millicoma and Coquille, a spinning prop can be risky. A jet outboard pulls water through a pump and pushes it out the back, so there's no prop and a much shallower lower unit than a prop motor's gearcase, though the intake still sits under the hull. That pump costs power. The number on the cowling is the powerhead's rating, and less than that reaches the water.

The 2026 F90JB is a good example. It's built on the same 1.8L inline-four as the prop-driven F75, F90 and F115. Its powerhead is rated at 90 hp, and it delivers about 65 hp at the jet. So when you size a jet rig, think in terms of output at the pump. A 90 hp jet pushes the boat about the way a 65 hp prop motor would. If a prop engine would feel just right on your river boat, you need a jet with a higher rating to get the same push.

Getting the right size on your boat

Start with where you fish. A river drift boat, a bay boat, and an offshore rig that crosses the bar each want different power, and the hull you pick sets the shaft length. If your hull has a capacity plate showing a maximum horsepower rating, don't go past it; if there's no plate, check with the builder before you buy power. When we put a boat package together, we match the hull and power to the water you actually fish. We can also check transom height and rigging before an engine goes on.

If you're keeping your hull and replacing the engine, our post on what drives Yamaha outboard repower cost on the Oregon Coast covers what moves the price. The engine you pick there comes down to the same things: the block, the shaft length and, for river boats, jet versus prop. To have your transom measured and your current rigging checked before you choose a new engine, bring the boat to our boat service and repair department in Coos Bay. A tight fit between transom and shaft keeps the prop biting cleanly on a choppy bay run.