Propeller tips

While bad fuel and a dead battery remain the primary reasons a modern outboard might leave you in the lurch, another area that deserves your frequent mechanical attention is the propshaft. The propshaft connects the pinion bearings of the lower gearcase with the propeller. It’s the one part of the outboard that makes the passage from the cozy, lube-drenched confines of the mechanical womb out into the harsh, unfriendly environment of air and water. The two propshaft seals act as a membrane around the shaft to keep water from entering the gearcase. If these seals fail, it will cost you big bucks. Here’s a simple propshaft inspection routine you can follow to help avoid seal failure.

Let’s assume your boat is on a trailer or at least out of the water. Start by shifting the motor into neutral, removing the key from the ignition and pulling the kill switch to prevent any chance of accidental starting of the motor why you are working around the prop.

If you watch the end of the shaft, and sort of line it up with an object behind it, you should be able to see if there’s any wobble when you spin the prop, If it’s wobbling, the shaft is bent and that needs to be fixed.

Boating with a bent shaft is like driving with a wheel out of balance - it will put a lot of stress on those propshaft seals and on the bearings that support the shaft. I watched Jansen use a dial gauge clamped to an outboard skeg to check the run out on a suspect shaft. He said run out of 0.007-inch is acceptable by Mercury service specs, and that you can notice a shaft with 0.015-inch with your eye.

Next give the prop a quick inspection. See if the blades are in the same plane when you spin it, and check for bent blades or bad dings in the blades. A bent prop won’t perform well, and will also spin out of balance, again putting undo stress on the shaft seals and bearings. You might feel this through the wheel or tiller, although hydraulic steering can mask this vibration. A good prop shop can repair minor blade damage.

Now you’ll want to remove the propeller. You’ll need a socket (a 1 1/16th-inch nut is the most common size on mid-size to V6 motors) and a screwdriver to do this job. Use the screwdriver to bend up the tabs on the lock washer that fits under the prop nut. We used a Mercury 125 for our example in these photos. Some other brands have a slightly different style lock nut, and some older motors use a cotter pin that you’ll need to pull out with a pliers

Place a piece of wood — Jansen has a nice pine 4×4 — between a prop blade and the anti-ventilation plate to keep the prop from turning when you loosen the nut. Spin off the nut and lock washer and then slide the prop off the shaft. On our Merc, the composite Flo-Torque prop hub slides out of the prop first. On other brands splines in the prop hub mate to the shaft, and the prop will just pull off. Unless it’s stuck on there due to corrosion. In which case you might need to smack it with a rubber mallet a few times. Behind the prop is the thrust washer. Slide this off next and pay close attention to which way it should go back on. The prop shaft is tapered, and the inside of the washer is tapered to match so it should only slide all the way down the shaft when it’s on correctly. Jansen always wipes off the thrust washer and inspects it for wear.

“If it looks like the washer has been spinning under the prop, it may be because the prop nut was not adequately tightened,” said Jansen. “If the thrust washer wears down, it can allow the hub of the prop to run on the gear case. The main thing is to remember to put the washer back on. I’ve serviced three motors already this year with no thrust washer because the owner forgot to replace it.”

Your main goal here is to keep the prop shaft well-lubed so that the propeller does not corrode itself permanently to the shaft. Jansen likes a Merc product called Quicksilver 101 Lubricant, and he is liberal in its application to the shaft. Grease that thing up, drop on the thrust washer and prop, and using your wood scrap to hold the prop in place, tighten the nut. The torque spec for a Merc 2.5-liter outboard is 55 ft. lbs., which is pretty tight if you don’t have a torque wrench handy. If the lock-washer tabs don’t line up with the slots on the hub, tighten the nut a little more - NEVER loosen the nut to make the lock tabs fit. Jansen and the Merc service manual suggest retightening the nut after you’ve run the outboard once and thrust has seated all the parts.

While that prop is off, you’ve also got a chance to inspect the outer seal, which is right behind the thrust washer. This area is always going to be greasy from the prop shaft lube, but if you see anything that looks like 90-weight gear lube, a seal could be leaking. Other signs of trouble might be a little dribble of lube on the ground below the gear case when the boat is parked for awhile, or any signs of oil in the water around the motor. Of course if the prop shaft is bent, it could be causing the seals to leak. However, the most-common cause of propshaft seal failure is fishing line that gets wrapped around the shaft.

excert from Jansens diary

When describing a prop you'll always hear two numbers:
something like 13 x 17 .
The first number is always the outside diameter of the propeller in inches (outside edge to outside edge measured across the center).
The second number is always the Pitch in inches of the blades.

What this means is:

If the propeller was in a large vat of soft butter:
One revolution of prop would move it that far forward in inches through the butter.
Of course this would be in an ideal situation with no slip factor.
Everyone in the prop business seems to have a different formula for figuring prop slippage and boat speed to motor RPM ratio.
Most of them are sort of close but all of them are only theoretical rules of thumb.
A good prop shop will usually let you buy and try a prop for suitability with the option of exchanging it for a different Dia. X Pitch Prop.

Most of today's props have a cup at the trailing edge of the propeller blade.
This curved edge on the propeller lets it get a better "grab and go" thereby reducing slippage.

You'll also hear about double-cupping which is about the same deal:
improved for running in a trimmed out position at or near the water surface.

Air from the water surface or exhaust exiting from the exhaust outlet being drawn into the prop blades causes Ventilation.
When this situation occurs, boat speed is lost and engine RPM climbs rapidly(seems to rev up or breaks traction).
This is often the result of trimming the engine too far up or out, sharp turns, and outboards mounted too high.

Cavitation; often confused with ventilation, is a result of water boiling because of extreme reduction of pressure on the surface of the propeller blade.
Propellers often cavitate during normal operation, but extreme cavitation can cause a cavitation burn on the prop's surface which looks like little pits on the blade.
Cavitation is caused by incorrect prop selection, bent or distorted blades, bent prop shaft, and the list goes on.

The propeller should be checked regularly to be sure all the blades are in good condition.
If any of the blades become bent or nicked, this condition will set up vibrations in the drive unit and the motor.

Remove and inspect the propeller.
Use a file to trim nicks and burrs.
TAKE CARE not to remove any more material than is absolutely necessary.

For a complete check, take the propeller to your marine dealer where the proper equipment and knowledgeable mechanics are available to perform a proper check of pitch, rake, and diameter at modest cost.

Inspect the propeller shaft to be sure it is still true and not bent.