Is Your Prop Up to Spec?
When did you last spend quality time with your propeller? I'll bet it was a long time ago, if ever—not many of us obsess over our props like we do our engines and electronics. But when it comes to performance, your propeller is arguably the most important thing on your boat. A prop that's out of spec, whether from damage, wear, shoddy repair, or simply poor manufacturing, can waste power, reduce top speed, increase fuel burn, and produce excessive sound and vibration. If your boat shows any of these symptoms, spend a bit of time and cash on a propeller tune-up; the fuel you save over a couple of seasons will pay for it.
Step one is to find a good propeller shop, one with modern equipment that can analyze your prop, blade by blade. A typical high-tech propeller-analysis system—two of the best are Hale MRI and Prop Scan—employs a precision measuring device connected to a computer. The prop is mounted on a special platform that allows the technician to rotate it by hand. As the prop turns, the tip of the measuring device moves across each blade at various radii, transmitting measurements, accurate to within thousandths of an inch, to the computer. Proprietary software crunches the numbers to determine the characteristics of each blade at the different radii—pitch, camber, rake, and so forth. The computer then creates an easy-to-interpret display showing any measurements out of whack. Scanning typically takes a few minutes.
Both Hale MRI and Prop Scan analyze props based on the ISO standard for propellers: Standard 484/2 lists four levels of manufacturing accuracy, from Class III (widest per-blade tolerance vs. listed specs, i.e. pitch of each blade within +/-4 percent and mean pitch within +/-3 percent) through Classes II and I to Class S, described as "Very High Accuracy." Class S props measure within +/-0.75 percent per blade vs. specified pitch, and +/-0.5 percent mean pitch. (Most props used on civilian vessels are Class II or III.) A skilled technician using either Hale MRI or Prop Scan can tune your propeller to tolerances even narrower than Class S.
Why is this important? In the perfect propeller world, each blade has the same pitch from root to tip. If one blade—or even a section—has less pitch than the others, it won't pull its weight; conversely, an overpitched blade will do more than its share, resulting in excess blade loading and possibly cavitation; cavitation creates vibration and corrosion on the forward surface of the blade, especially near the tips. It's as if the four white horses harnessed to Ben Hur's chariot all ran at different speeds: The fastest horse would do more work, while the slowest one would be pulled along by the others, and the bad guy, whose black horses were all working equally, would win the race.
To find a high-tech propeller shop in your area, check Prop Scan's Web site, Google "Hale MRI," or refer to the Michigan Wheel Web site and click on "Inspection Equipment." (Michigan Wheel is a distributor for Hale MRI.) No matter what the system, there's no extra charge for high-tech propeller analysis, but shops using it can do a better job bringing your prop up to spec.
Ask the Experts: Randy Dinter
The Problem: Last summer PMY editor-in-chief Capt. Richard Thiel noticed that his diesel engine was low on coolant, but he couldn't find any leaks. He did notice, however, a light, oily sheen on the water near his exhaust and wondered if he had a blown head gasket or cracked block. For advice Thiel turned to Randy Dinter, shop foreman at Brewer's Yacht Haven Marina in Stamford, Connecticut.
The Solution: Since there was no coolant under the engine, the problem wasn't a leaking hose or connection. The lube-oil level wasn't rising, and the oil wasn't getting lighter in color, which would have indicated water or coolant getting into the oil, maybe through a blown gasket. The oily sheen at the stern was the clue: "The coolant must be coming out with the raw water, and the only place coolant and raw water mix is in the heat exchanger," Dinter says. "The engine is older, so we can assume vibration or galvanic corrosion have caught up with the heat exchanger."
Thiel's engine is more than 20 years old, so the heat exchanger owes him nothing. However, Dinter advises, a boat owner can head off trouble by having his yard pull the core out of the heat exchanger every five or six years ("ten years, max"), clean it (Brewer's dips the cores in acid), and inspect it for cracks or pink metal, the sign of galvanic corrosion. According to Dinter, you should remove the exchanger's end caps every couple of years to see what's inside. "It costs very little, only a new gasket," he says.
Dinter also suggests you change your antifreeze every two years for "the green stuff" and every five years for the red. Antifreeze will get acidic if left in the system too long and damage the soft metal in the heat exchanger. He says you should replace the zincs at least annually.
Other factors come into play, too, and not always ones that are obvious. For example, flexible engine mounts wear out, and the increased vibration can cause problems, in the heat exchanger and elsewhere. "Maybe you have to change the mounts every five or six years," advises Dinter. You might not notice this because it happens so slowly: Dinter once repaired a boat where the mounts were so far gone, the prop shaft was taking the weight of the engine; the owner saw nothing, until the shaft broke.
Fortunately, Thiel got off easy this time: For the 2008 season, his diesel should be running nice and cool, thanks to that new heat exchanger. Now, what about those old engine mounts...?
This article originally appeared in the January 2008 issue of Power & Motoryacht magazine.