Unlike owners of high performance and bass boats, houseboat owners frequently don’t pay a lot of attention to their propellers until they have an encounter with an underwater obstruction. After their props get hammered back into shape, they frequently forget about them again until the next time they run afoul of a “skegosaurus.”
But high fuel prices are just one reason houseboat owners might consider paying a little more attention to their wheels. Props that have been on a boat for a long time and even props on newer boats that look fine can reduce performance and lower efficiency which costs extra money at the fuel dock and also puts an added strain on the drive train.
Over time, the size and shape of a prop changes. Prop erosion from normal wear reduces a prop’s diameter and the erosion is quicker and greater on boats that are frequently beached. A boat also undergoes changes over time so what was the right prop for a new boat may no longer be the correct prop as the boat ages.
Newer boats with Electronic Control Modules (ECM) also have a critical need for properly sized and tuned props in order to perform at their peak efficiency. One of the module’s jobs is to make adjustments that protect the engine so when a prop is badly out of specifications, the module will make sure the engine is safe regardless of how that lowers efficiency.
Although propeller discussions can get incredibly technical, houseboaters need to understand a few basics. Props come in a wide variety of shapes, sizes and materials and houseboats with sterndrives generally swing aluminum props but inboard-powered boats use naval bronze or brass propellers.
The crucial factors are the diameter and pitch and those are stamped on every prop with the diameter always being the first number. Larger diameter wheels are usually better for houseboats so correct pitch selection follows from there. The right combination allows the boat to run in the engine manufacturer’s wide open throttle (WOT) revolutions per minute (rpm) range. WOT varies from engine to engine. Some older engines may reach WOT around the 4,000 to 4,400-rpm range while newer boats can peak around 5,000-rpm. At WOT, the engine produces maximum horsepower. If the engine turns at higher rpms, or more commonly, less rpms than the WOT recommendation, there is an overall reduction in power and efficiency at all speeds including the cruising rpm range. For every one-fourth of an inch a prop is short-diametered, for instance, the boat loses about 150-rpm so a boat propped one inch undersize will lose 600 rpms at WOT. Improper propeller pitch adds more rpm drop.
The process of propeller manufacturing, selection and repair has evolved significantly over the last few years due to the use of computerized equipment and sophisticated programs. Prop selection is no longer a trial-and-error-guessing-game but is done with computer programs that analyze data about the boat, engines, transmissions, gear ratios, and the existing WOT reading to determine what the optimum prop should be.
Boat manufacturers are increasingly building boats with higher performance standards so new props have to attain much higher standards than in the past. Highly accurate Computer Numerical Controlled milling machines can produce those tight tolerances. Many prop companies used to measure accuracy with their own standards but most now adhere to tolerances developed by the International Organization for Standardization (ISO). A Class S prop has the tightest tolerances and props are increasingly less precise down to Class 3. Most factory props now meet Class I standards which allow a deviation of three-fourths of one percent and some even reach the higher Class S standards.
Computerized equipment has likewise greatly improved the precision of propeller repair. What used to be mainly a matter of the art and skill of the repairman with a drop gauge pitchometer, a mallet and pitch block has given way in a lot of repair shops to equipment like the Hale Measurement Recording Instrument Propeller Measuring System (Hale MRI) which is distributed by Michigan Wheel Corporation.
Don Jackson, Executive Director of the National Marine Propeller Association (NMPA) says he knows craftsmen who can achieve high repair standards without the use of computer-aided equipment. But he also says it would probably take them considerably longer than a computer-aided technician and spending the necessary hours to manually achieve extremely high tolerances may be cost prohibitive.
An MRI system shows how and where each blade is out of specification by determining deviation at hundreds of points. That information is plotted on a color-coded graph that also displays a curve showing how the prop should look. After repairs, the prop is re-scanned to gauge the results.
Gary Linden, of Linden Propeller Co. in Dubuque, Iowa has been on the cutting edge of the advancements in computerized repair technology for some time. He was the first in the Midwest to use a Hale MRI and now has pushed the envelope further by designing and using another piece of sophisticated equipment.
Linden and his uncle, Jack Linden, a retired mechanical design engineer and a boater, have built a bending table that uses hydraulic pressure to reshape props much more gently, quickly and accurately than banging on them. Called the Prop Press 360, they are close to receiving their patent and are now marketing the equipment to other repair shops.
The machine has caught the interest of the folks who make the Hale MRI. Randy Hale says they have designed a remote arm that will connect their MRI scanner with the Prop Press 360. That will eliminate the need to scan a prop on one machine then move it to another for bending before moving it back to the scanner for follow-up readings. With the two machines connected by the arm, a prop can be locked down, scanned, re-shaped, and rescanned without moving it. A repairman will be able to tune each blade to perfection before working on the other blades. That is a huge time-saver because to do it right Linden says, “we don’t eyeball anything” so a prop might easily have to be tuned and re-scanned a dozen times.
When a prop comes in for repair, it is first stamped with a number that is used for inventory control and also has the shop’s code. The imprints show how many times the prop has been repaired and who did the work. After getting a hydrochloric acid bath, oxidation is removed with an abrasive clean-up wheel so the prop can be easily inspected for pitting and cracks.
Linden determines each blade’s diameter through an elaborate patterning process that begins with pounding down any damaged or curled edges to create the fullest degree of diameter. By scribing a series of arcs and using a pattern of a new blade to compare with the damaged blade, he determines where new material needs to be welded on to achieve the correct diameter. A tungsten inert gas (TIG) welder and naval bronze filler rods are used to build up the diameter. After cooling, excess material is sanded off.
From there, the scanner gives him a clear picture of where and how much each blade needs to be re-shaped. It also tells him if a blade has been knocked off center and needs to be re-spaced. That is common when the prop has had a significant impact.
The re-contouring and re-positioning process is the heart of the “black art” of propeller repair because even with high-tech equipment, it comes down to whether the technician has the instinctive touch, feel and knowledge to know how his efforts affect the prop. Maintaining the proper camber across each blade is one of the highly evolved skills that require practice and experience. The very best master technicians have such a finely honed sense of the blade’s contours that it sometimes seems like they use a bit of magic.
After the prop is bent and re-scanned, the process is repeated until the results are satisfactory. With the Hale MRI, every pitch can be measured to five decimal points. The last step is to balance the prop.
Jackson says Linden is different from some other shops because he “is a bit of a finicky old lady about his work. He really cares about what goes out of his shop,” which is why he personally does all the dirty, stinky, smelly and difficult work on bronze props and also inspects all props before they leave his shop.
For props with less than obvious terminal damage, deciding to repair or replace is best determined once the prop is scanned. Linden says he can re-size a prop by adding up to about one-inch in diameter and adjusting a pitch that is plus or minus the inches. Beyond those numbers, the cost of repairs gets prohibitive so the prop needs to be replaced but it can be re-sized and pitched as close as possible to serve as a back-up.
The cost of prop repair is a function of time and material. That is why, says Jackson, the use of a scanner and bending table, although not cheap equipment, can bring prices down for a boat owner by helping a repairman turn out more repairs with a greater degree of precision in a shorter period of time.
When considering re-propping a boat, Linden recommends an owner take some steps to ensure the data fed into the computer is as accurate as possible to avoid the “garbage in- garbage out” syndrome. The first step is to check the boat’s tachometer readings against those from a portable photo tach reading of the flywheel, prop shaft or harmonic balancer. Make sure the carburetor linkage is allowing full fuel flow and then check the engine’s timing. Verify that the maximum spark advance is being attained at the specified rpm and even check the fuel tank vents to make sure they are not clogged with critter or buffing pad debris.
While propeller repair is a mix of art and science, it does not take a rocket scientist to understand that poorly sized or tuned props can be a major problem for a heavy houseboat under load. Perhaps it is time to give a little more thought to the big wheels that move your boat through the water.
For more information:
Linden Propeller Co.
Michigan Wheel Corporation
Propeller sizing analysis, Hale MRI
National Marine Propeller Association