Is A Prop Your Performance Problem?

Who among us hasn’t worried about performance? Even if boaters don’t need to run with the stallions anymore, it's nice to know their steeds still have what it takes. Every now and then many find themselves checking their speed and rpm and wondering if their vessels are still performing like they did a few seasons ago. Also, when buying a new or used boat, they want to make sure that she performs well right from the get-go with the number of people and the load they plan to carry. In either case, if some performance dysfunction is detected, they need to get to the bottom of it quickly. Ironically, when certain prop experts are called with this confidential problem (we call it PD), rather than ask about the prop, they will usually ask other questions about the boat’s general health and condition.

Quite often, a patient’s performance problems have nothing to do with their prop and to get to the bottom of it they must examine some other factors–

Factor #1: The obvious: Is the engine tuned up? Does it have the proper compression and is it turning up to its stated max rpm at the dock?

Factor #2: A boat's hull design may limit the overall performance of the boat. Even a highly efficient, handmade propeller will not overcompensate for an inefficient hull design. Boaters should not assume that because a boat is built by a famous brand name that it has a good hull for their intended purpose, or even an efficient hull. While competition and the increased knowledge of boat owners over the last decade or so has forced some builders into re-tooling old designs, there are still many inefficient hulls on the market. Don’t blame the prop in all cases.

Factor #3: Is the hull clean? Boat hulls foul faster than many may think, and very fast, indeed, in saltwater. Basically, in saltwater locations in the summer – even in the north – hull surfaces need to be wiped down every week or two to keep them absolutely clean. BoatTEST.com has found that dirty hulls are the #1 factor in disappointing performance in an otherwise well-set-up boat.

Factor #4: The angle at which the prop attacks the water will affect performance in all cases. For example, the down angle in a conventional inboard boat will always be inefficient. This was made dramatically apparent by the introduction of pod drives, which are generally 30% more efficient than conventional inboard propulsion for a variety of reasons, but the down angle is the most important. Reducing the down angle can increase the efficiency of the prop – something a boater can’t do with an inboard after the boat has been bought, but can be done with an outboard or stern drive.

In outboard-powered boats, transom height, angle, and engine trim angle all affect propeller performance. If any one of these is not optimum a boater cannot expect optimum performance from the propeller. Likewise, sterndrive lower units must be placed correctly on the transom by the builder and the operator must dial in the proper trim.

Factor #5: Comparing a boat’s performance with test numbers presented on BoatTEST.com may leave a boater finding a boat’s performance wanting. We pride ourselves in getting “everyday use” performance numbers, so ours should be close. But there are other factors at play: atmospheric pressure, altitude, temperature, and humidity all affect engine performance, which directly affects propeller performance. If a boat’s performance is not up to what is shown on BoatTEST.com, look at where we tested the boat and the conditions. Expectations could be unrealistic given the geography.

Factor #6: Salt water is more buoyant than fresh water; this may cause some hulls to run faster than identical freshwater setups. Looking at the results of tests done in flat conditions in saltwater, they may seem unattainable, all the more so after checking to see how many people were aboard and how much fuel the boat carried, as well as the atmospheric conditions and wave height. This could be the reason for the difference.

Factor #7: Water conditions can play a big part in boat performance and propeller efficiency. Short, steep seas will slow a boat greatly and strain the engine and make the prop inefficient. If they’re boating in an area that always has a jarring chop, then boaters need to talk to a prop expert about maximizing a prop’s efficiency.

Factor #8: The boat's load and the position of the load can significantly affect performance. This is the #1 factor in lack of performance in planing boats of nearly all size ranges. More weight adds wetted surface. Weight plus wetted surface make the boat harder to get on plane and harder to stay there. The deeper in the water a boat rides, even though it is planing, the more water the engines and props must push out of the way, which will spike fuel consumption and drain speed.

Look at where the boat’s weight is located. If too much is in the bow, or the stern, there is not much a prop can do to overcome a boat that is significantly out of trim. Again, speak to a prop expert and find out exactly what he might be able to do to help. We have all seen some stern-heavy boats plowing along, and that is usually because of a combination of factors including several of the above, plus an engine that is too small to make up for the other sins.

Factor #9: Are the props clean, smooth, and undamaged? While there is some debate as to whether or not a “satin” finish (slightly rough) or a “polished” finish is best, we’re not going to slice the issue that thin and few boaters need to. Even a small nick will rob a boat of performance. Also, make sure one of the blades is not bent. If in doubt, send the prop to an expert.

Factor #10: Now, after checking all of the above, it is time to check the prop’s geometry. Engine RPM at WOT must be within the recommended operating range with the rated horsepower RPM as the target number (see the specification in the Operator's Manual). Increasing or decreasing propeller pitch directly affects engine load throughout its RPM range.

*A high load (high pitch and/or diameter) on an engine will result in lower engine RPM throughout its entire rpm range for a given throttle setting.

*A light load (low pitch and/or diameter) on the engine will result in higher engine RPM throughout its entire rpm range for a given throttle setting.

Now, and only now, is it time to consult a prop expert. These people are busy and they will appreciate the fact that a client has already gone through the above checklist before they put their stethoscope on the lower unit.

The “X” Factor: If a boater has gotten to this point and still hasn’t found the performance problem, chances are they know what it is and didn’t want to admit it – could it be that the boat is woefully underpowered? It could have been all right new, but over the years it has filled up with heavy gear.

We know that it is hard to believe that a famous brand name would underpower its boats, but it has been known to happen.

Consult with a knowledgeable boat dealer, surveyor, or a prop expert and get their opinion – they may be able to help with a satisfactory fix.