See full version: Fixing Boat Propeller Problems
In that case, the rule of thumb is that increasing pitch two inches decreases rpm 400 to 500, while decreasing pitch two inches has the opposite effect. That's pretty much true for all outboards. Inboard applications, with myriad gear reductions and engine rpm, are a little trickier to compute. In general, whatever percentage of rpm you want to increase or decrease, make the same corresponding percentage adjustment in the pitch up or down.
This is pretty easy to identify. Look for damaged and bent blades and chunks of metal missing. The design of most propellers these days is very sophisticated, so damage makes your boat slower and less fuel-efficient. Smooth operation is also compromised, which could damage expensive engine parts such as seals, motor mounts, bearings and struts, and drive shafts. If it looks damaged, it is, and should be repaired or replaced immediately.
If not fixed, a cracked blade will break off and the prop becomes scrap.
Look for repair numbers stamped on the prop. They're easily distinguishable from original part numbers. On aluminum props, you may see black "pot metal" filler or porosity from a welding repair, areas that are prone to failure. A repaired prop isn't a bad thing but it's something you should notice so you're not disillusioned when something that looks good goes bad.
Example of pitting from corrosion that has damaged this prop beyond repair. here
There are several versions of this. One is the "spray can recondition." You've probably seen this if you've looked at used boats. All that shiny paint looks great, so the prop must have been just repaired or might even be brand new. Look more closely and you'll see edges where the old paint ends and new paint begins. That's a sure sign that somebody's trying to hide something. You might see little bends or nicks in the blade tip edge or scratches in the surface. A good prop shop won't leave that behind. Plan on fixing or replacing that prop before you run the boat.
In a nutshell, it means that your prop shaft is turning, but your propeller is not.
To know if you have spun a prop or not, you can try marking a line on the back of the prop hub. Follow these steps below to see if your prop is slipping.
Warning: Make sure you do not over-torque the lock nut onto the propeller shaft. Doing so will result in either stripping the nut or the threads on the shaft.
Your boat’s propeller connects the engine to the water, just as tires connect your car to the road. Unlike tires, your propeller is not going to wear with use but it is prone to damage, which can range from a few nicks in the blade edges to catastrophic damage from a severe bottom strike. The soft propeller hub can also be damaged in a bottom strike, or just from age. Even minor damage can impact boat performance, so it’s important to take good care of your propeller.
If prop blades are bent or otherwise damaged, the prop should either be replaced or repaired. Obviously with those bent blades the prop is not going to function properly and boat performance will suffer, but the damaged prop is also likely now unbalanced, which can cause vibration at high speed which can lead to more problems.
A prop shop can usually replace a spun rubber hub. The plastic hub inserts used on many new props are simple to replace, and you can carry a spare and replace the plastic insert yourself on the water. The rubber hubs will also deteriorate over time and may need to be replaced even if you don’t strike bottom.
One thing is for sure; the knocking sound will not go away on its own. Being able to narrow down possible causes will help in eliminating the knock. more
Before anyone could figure out what was causing it, the owner loaded up his boat and left. Once I got home, I tried to look up what might have been causing the problem, and I found two possible reasons for a knocking noise coming from an outboard engine.
Another possible reason for a knocking sound could be because of missing teeth on both the drive bevel gear and the prop shaft bevel gear. As the gears turn, the two gears would align with one another where the teeth are missing, which could cause the two to slip and make a repetitive knocking noise.