Spun Prop Hub - How to Tell, Quick Marker Test and Repair Options

Mercury and Boating Magazine describe the same pattern:

• When you open the throttle, the RPM shoots up but the boat barely accelerates, almost as if there's no prop on the engine.
• The boat is slow to get on plane, or won't plane at all.
• At idle and low speed it may still push the boat. Boating Magazine calls this a sort of "get home" mode.
• It may only slip above a certain RPM, so it can feel like a power problem or ventilation.
• A "clunk" when you shift into gear is not one of the signs by itself. Mercury explains that shift clunk is the normal sound of free play in the drivetrain being taken up and isn't damaging; rubber-cushioned hub kits are sold to quiet it. A clunk that suddenly gets much louder, or a prop that rattles at low speed, is still worth a look while the prop is off.

• Engine down on power: the RPM stays low and won't reach its normal range. A spun hub does the opposite: RPM goes higher than normal for the speed you're going. See the loss-of-power guide.
• Ventilation or cavitation: the prop grabs air, usually in tight turns, with the engine trimmed too high, mounted too high, or with a damaged prop. It usually recovers when you trim down or straighten out. See the prop-ventilation-cavitation guide.
• Trim problem: Mercury notes that if the drive or outboard won't trim all the way down, the boat may struggle to get on plane. Check that trim works normally (see the trim-tilt-fluid guide).
• Something wrapped on the prop: a rope, weeds or a plastic bag around the blades or hub can make the prop slip and vibrate much like a spun hub. Look before you start pulling parts.
• Worn blade edges: Yamaha points out that grit, sand and silt slowly wear away a prop's edges, so a prop that looks fine at a glance can still lose grip and performance. Compare it with a new prop if you can.

If the hub slips when you try to plane but still pushes at idle or slow speed, Mercury suggests you may be able to limp home without changing the prop. Keep the RPM low and steady, avoid sudden throttle, and head to the nearest safe dock or ramp. If you carry a spare prop and the water is calm, you can swap it (steps 5 to 9), but only with the engine secured as described in the safety box.

With the boat out of the water, the key out and the lanyard off, shift into forward gear. Then try to turn the prop by hand (wearing gloves). In gear, the prop should resist strongly, since you're trying to turn the whole engine. If the prop turns easily while the shaft stays still, the hub has let go. A hub that only slips under heavy load may still pass this check, so use the marker test next.

Return the shift to neutral when you're done.

This is Boating Magazine's simple way to confirm a spun hub:

1. With the engine secured, draw a straight line across the end of the hub insert and the propeller barrel at the back of the prop with a paint marker, grease pencil or lipstick. You can also scratch a light line with a small file so it can't wash off.
2. Run the boat as you normally would, including the throttle range where it slips.
3. Secure the engine again and look at the line (pull the prop if you can't see it clearly).

If the line has broken into two pieces that no longer line up, the insert has turned inside the barrel: the hub has spun.

With the engine secured (key out, lanyard off, battery switch off):

• Wedge the block of wood between a blade and the anti-ventilation plate.
• Straighten and remove the cotter pin, or bend back the tabs of the tab washer, if your prop nut has one.
• Loosen the prop nut with the correct wrench.
• Slide off the nut, washers, any spacer or adapter, the prop and the thrust hub or washer behind it.

Photograph or lay out each part in order. Hub kits and mounting hardware go back in only one correct order.

• Rubber hub: look for the insert sitting crooked or turned in the barrel, rubber that is melted, crumbled or torn away from the barrel, or gaps around the sleeve.
• Replaceable hub: check the sleeve for cracks, melting or rounded drive splines, and the insert for wear.
• Heat damage (melted, glazed or burned material): suspect a cooling or engine problem, as Mercury warns. Check the telltale stream and water pump before running the boat hard again (see the engine-overheating and water-pump-impeller guides).
• Blades: look for bends, cracks or missing chunks.
• Prop shaft: look for fishing line wound around the shaft where it enters the gearcase, which Boating Magazine warns can damage the seal and let water in or gear oil out. Check the splines and threads for damage. After a hard strike, have a shop check that the shaft isn't bent.

• Pressed rubber hub: take the prop to a prop shop to have the old hub pressed out and a new one pressed in. Boating Magazine puts a typical rehub at roughly $50 to $100, depending on the engine and the shop. It can be done at home only with the proper press tools.
• Replaceable hub kit: buy the correct kit for your prop and engine, and swap it yourself following the kit's instructions. Mercury says its Flo-Torq SSR kit can be installed by owners in most Mercury props.
• Damaged blades as well: a prop shop can often repair and rehub at the same time. If the repair costs close to a new prop, consider a new one with a replaceable hub so the next spun hub is a quick fix.
• A spun hub with signs of heat: fix the cooling or engine problem first, or the new hub will fail too.

Clean the shaft and coat it with the lubricant your owner's manual specifies. Mercury's pages differ on how much: one recommends a liberal coat to keep the prop from seizing on the shaft, while its newer prop-inspection quick tip asks for a light, even coat on the splines and around the thrust washer and warns against overdoing it. Cover every spline and contact surface without packing grease in; your manual has the final word. Reinstall the thrust hub, prop, hub kit parts, spacer, washers and nut in the exact order you removed them. Tighten the nut to the torque in your owner's manual, then fit a new cotter pin or bend the tab washer as required. Spin the prop by hand (shift in neutral) to make sure it turns smoothly and doesn't rub. Mercury also recommends checking that the prop is still tight after 20 hours of operation and never running with a loose prop.

Remove the block of wood, check the shift is in neutral, and launch. Accelerate gradually and watch the tachometer. The boat should get on plane normally, and the RPM at full throttle should be back in the range your owner's manual gives. If the RPM still flares, recheck the installation and look at ventilation, trim and engine power.

Mercury recommends carrying a spare prop and, for props with a replaceable hub, a spare hub kit, plus the tools and fasteners to fit them. Mercury's FAQ also suggests a floating prop wrench as part of the spare-parts kit, so a dropped wrench doesn't end your day.