Converting an Outboard from Prop to Jet: Hull Fit, Mounting Height, Power Loss and Cost

Be honest about where you boat. A jet shines on rocky, shallow, clear rivers. If you mostly run lakes or deep water and only touch bottom now and then, a prop with a jackplate, a shallow-water anchor or simply learning the river may serve you better. Jets also don't troll well, because they are designed to run on plane; most river anglers who troll use a separate electric trolling motor.

A jet needs a steady supply of solid, air-free water flowing into the intake. That depends on the hull more than anything else.

• Best: a flat bottom or a modified V with a small deadrise. Bass Pro singles out welded aluminum jon boats with about 6 to 10 degrees of deadrise as ideal.
• Poor: a true deep-V. Honda's field service manager told Boating magazine that jets work on flat bottoms or boats with a little V, but not on a true deep-V.
• Watch for: a center keel, strakes or a big dent right ahead of the motor, all of which can feed air to the intake.

Yamaha markets its larger jets for some deck boats and V-hulls, so makers don't all say the same thing. If your boat is anything other than a flat or mild-V aluminum hull, ask a dealer who rigs jets before you buy a kit.

Because a jet delivers less push, weight matters more than with a prop. Add up the boat, motor, fuel, battery, gear and the people you normally carry. A heavy boat that barely planes with a prop may never plane properly with a jet. Planing quickly matters, because a jet running slowly off plane pulls gravel up into the pump. Bass Pro also notes hull thickness: a heavier-gauge welded hull is less likely to open up on a rock strike, but it also adds weight.

Before anything changes, record wide-open-throttle rpm, top speed by GPS and how long it takes to plane with a typical load. These numbers help the dealer pick the right impeller and let you judge the result honestly afterward.

Jet pumps are not universal. The kit has to match your motor's make, model, year, shaft length, driveshaft and shift linkage. Write down the motor's model and serial tags (see the engine serial number guide) before you shop. Ask the kit maker or dealer about:

• Shaft length: the jet must match your motor's existing shaft length. You can't fit a short-shaft jet to a long-shaft motor to avoid raising it.
• Shift system: the reverse gate is worked by your shift cable or a shift rod. Motors with digital (drive-by-wire) shift may not be compatible with aftermarket jets.
• Cooling water pump: most kits reuse the motor's own water pump, but some models need a different pump housing.

This is the part owners most often underestimate. A jet pump hangs lower than a gearcase's water intake needs to be, so the whole motor has to go up on the transom until the front edge of the intake is roughly level with the bottom of the hull, as your kit instructions describe. That usually means a riser block or a jackplate. Two things follow:

• If the motor moves aft on a jackplate, the hull bottom may need a short extension plate so the intake still gets clean water.
• The transom has to carry the motor at a new height. Check that the transom is solid and that the mounting bolts land in sound, backed structure (see the transom stringer rot check guide).

The exact height is set during setup and fine-tuned on the water. Leave that to the installer, using the kit maker's figures.

Price both paths before you decide:

• Converting: the kit, a riser or jackplate, shift parts, dealer labor and possibly transom work. Your existing motor must be in good health; a tired powerhead is a poor place to spend jet money.
• Buying a factory jet: Mercury, Yamaha, Honda and others sell jet outboards ready to bolt on. In 2018 Boating listed Honda's 40 jet at $11,360. A factory jet comes with the right pump, setup data and, often, warranty that covers the pump; Honda's covered both powerhead and pump.

When you compare, read the factory spec sheets for the details a kit won't change on your old motor. Mercury, for example, lists a stainless impeller on its 25 to 40 hp jets (it says stainless stands up better to abrasive debris) and a charging output of 17 to 18 amps on those models. Yamaha lists which transom height each of its jets fits: 25-inch transoms for its 60 to 150 hp jets and 20-inch for the 40. Check that against your boat before you price anything.

Ask the dealer how a conversion affects your engine's warranty before you do it on a newer motor.

Store the removed gearcase clean, drained or filled per the service manual, and protected from freezing. Some owners switch between prop and jet with the seasons; others sell the motor later as a prop outboard. Either way, a kept gearcase is worth money.

A good installer will:

• mount the motor at the starting height in the kit instructions
• connect and adjust the shift linkage so the gate locks fully in forward and works through neutral and reverse
• set the impeller clearance (see the jet impeller shimming guide)
• grease the pump bearing
• sea trial the boat and adjust height until it runs cleanly without sucking air

Practice in open water. You need some throttle to steer, and docking takes a different touch. Practice your stopping: shift to neutral or reverse with the gate, not by just letting off the throttle. Get used to how the boat slides in turns, especially on a flat bottom.

After the first few trips, check:

• Rpm jumps and the boat slows, especially in turns or chop: the pump is getting air. The motor may be too high, or the hull ahead of the intake is feeding bubbles.
• Lots of spray up the transom and extra drag: the motor may be too low.
• Wide-open rpm well below the kit maker's range: the gate may not be locking fully in forward, or the impeller doesn't suit the load.

Take these notes back to the installer. Small height changes make a big difference.

Once converted, your maintenance changes. There is no gearcase oil, but the pump bearing needs frequent grease, the grate and impeller need regular checks, and the impeller clearance grows as gravel wears it. Follow the jet outboard care guide and your kit's manual. Outlaw Eagle, a river jet boat builder, lists greasing the bearings every 10 to 20 hours, more in harsh conditions; follow your own kit's schedule.