Here's how to diagnose and remedy common problems.
With all due respect to the captain, the propeller drives the boat. Its condition determines how fast you go, how much fuel you burn, how long your engine will last, how smoothly your boat rides, how quickly you pull a wakeboarder out of the water, and perhaps even if you'll get back to the dock at the end of the day.
Very few marinas or boat dealers fix propellers themselves; it’s simply too complicated. Your marina sends your prop to a prop shop or you can visit one yourself. Make sure that shop is reputable, preferably a member of the National Marine Propeller Association, the only organization in the world that provides training and certification for the propeller-repair industry. Gone are the days when props could be repaired with an anvil, a welding machine, and a coat of paint. Today's props require trained technicians to make them run right. While prop makers today deliver accurately sized props, there is still much that can be done to fine-tune them for a particular application. So, take a peek at your propeller. Here are some of the things to look for.
Flat Tore Up
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.
Looks Good, Works Badly
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.
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.
Often, especially with inboards, it's difficult to see a big bend that starts at the root of the blade where it meets the hub. All blades look the same and there's no obvious damage but one or two may be completely out of "index," which is the spacing between blades, or "track," the relative height of each point. On a four-blade prop, for example, each point on each blade should be at a 90-degree angle and at the same relative height as the same point on a neighboring blade. It's hard to see this, which is a good reason to have your props checked every time you haul your boat or when at your dealer for regular service. Most prop shops do inspections for little or no charge.
Little Crack? Big Bad Is Next
Ignoring tiny nicks in the propeller edge is a big deal. Those nicks can create stress risers, areas that are more prone to crack as the propeller blade works through the water. You can smooth out those areas yourself with a file, but be careful not to grind too much. You'll alter the blade geometry and kill efficiency. Best to let a professional handle that.
Many stainless-steel props are heat treated to improve strength, a process that can make them brittle and susceptible to cracks. Carefully inspect stainless props and look for hairline cracks, especially in the leading and trailing edges. Most props so afflicted can be repaired, but if you don't fix it, you'll end up losing the blade, damage that cannot be repaired. Prevention costs much less than the cure.
Worn Won't Work
All the blades look the same, so what's the problem? Often, they're all worn evenly. Look for blades that aren't naturally rounded at the tip or are worn to a point. Feel the edges, but be careful. Wear makes them sharp. On the other hand, the tips may be particularly blunt on the leading edge, where the prop makes first contact. Paint worn evenly is also a clue.
If you know the design diameter of your propeller, you can eyeball its actual dimension by measuring from the center of the hub to the tip of one blade (the radius) and multiply by two to get the approximate diameter. Diameter and blade area are two of the most critical aspects of propeller design. Missing material causes higher rpm and diminished efficiency, potentially damaging the engine at worst, and slowing the boat and using more fuel at best. A prop shop can build up diameter and blade area to original specs. If too much is gone, especially on aluminum props, it's often best to buy a new one.
A metal propeller soaking in saltwater (or any other electrolyte) can suffer a form of corrosion that appears to "eat" away the metal, leaving it pitted where some of the alloy has left the metal structure.
What is really happening is that a cathodic (more noble) metal is taking the less noble (anodic) metal in. To see where a metal stands in the order of cathodic to anodic elements, check the galvanic series chart, which lists metals in order of their nobility with the least noble at one end and the most noble at the other. With a bronze alloy prop, which is an alloy that can contain copper, lead, tin, zinc and other elements, less noble metal can be stolen (leached out) from the alloy and taken to another metal. The vehicle for this transfer is the "electrolyte" and, in the case of a boat, that is saltwater or brackish water.
Another factor affecting this process is the bonding system of the boat or the anodic protections. If this activity is not controlled by a bonding system and/or the use of sacrificial anodes, you'll see pitting all over the blades. Bronze propellers can develop a pink tint that is a symptom of the "theft" of anodic metal in the bronze alloy and, when you rap a blade, instead of a resounding, bell-like "ding," you'll hear a dull "thud." Unchecked, the process can leave prop blades of any metal alloy looking like Swiss cheese.
In that case, your propeller is dead. Especially on a bronze or aluminum propeller, where the missing material likely can't be adequately restored by welding, because the base metal is too corrupted. (Nibral and stainless-steel propellers may be salvageable.) Critical areas, now brittle and soft, eventually crack and fail. The only option is to replace the propeller. Carefully check other underwater gear like shafts, struts, rudders, and sterndrives and get to the root of the problem.
Every marine engine has a recommended operating rpm range, and the correct propeller turns the engine within that range. A typical outboard, for example, might be rated at 5,000 to 6,000 rpm and the proper propeller turns within that range at wide-open throttle (WOT). (Check your owner’s manual for the actual WOT rpm range for your engine.)
All of the visual symptoms identified so far can cause the rpm to be too high or low. Worn blades, for example, cause high rpm and prop repair solves many rpm problems, but it's not uncommon for boats to have improperly sized props in good condition, especially as boats get heavier, motors get weaker over time, and boat owners fiddle with diameter and pitch to resolve perceived or real performance 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.
A good prop shop can adjust the pitch on most props up or down about two inches depending on the material. More than that usually requires a new propeller. Diameter can also be adjusted to influence rpm, but outboard props are typically designed with specific diameters that work well with each pitch, and maximized diameter is almost always critical. Best to work with pitch first and adjust diameter only as a last resort. The good news is that most of these problems are visible to the naked eye. A simple inspection makes your drive train last longer and gives you one less thing to worry about as you enjoy the pleasures of boating.
A surveyor can develop a preliminary opinion on propeller problems by devising a simple "eyeball" test method. Here a clamp attaches to a metal rule (a section of bar stock or a length of metal rod works, too) to the leading edge of the rudder. The prop is rotated by hand and each blade tip is observed as it passes the straight edge. Blade anomalies are revealed by variations in blade tip clearance as the prop is turned. It’s only a rough field test but, short of a high-tech evaluation, it sets a basic reference point for spotting problems that may require the prop be sent to a shop. A super-simple test to detect obvious blade discrepancies consists of firmly and steadily holding a ruler or even a screwdriver (tip down) vertically above the prop and against the hull bottom while turning the prop.
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