TWENTY-FIRST CENTURY PROPELLER TECHNOLOGY

by Dave Gerr, N.A.

Did you know that there is no standard for propeller machining accuracy in the United States---none at all? Did you know that it's not at all unusual--- infact quite common--- for blades on a brand new propeller to be siginficantly out of pitch, both in respect to each other and in refertence to the stamped pitch? Yes, even on a bright, shiny wheel that looks, oh, just right, sweet and sharp. Did you know that until just recently, there was virtually no costeffective way to check on all this? My guess is that you didn't; most folks don't. Indeed, many mysterious vibrations, performance, and engine problems are a direct result of this lack of propeller precision.

Amazingly -and regreattably- all the above is true. Happily, at last, there's a top notch- solution. A clever Australian named Terry Ryan invented an automatic, computerized, propeller-measuring instrument. He founded a company called Propeler Dynamics, based in Southport, Queensland, Australia, and named his brainchild- a gizmo that sells for $60,000 and up- "Prop ScanŽ." For the first time ever, you can inexpensively have your propeller's geomentry checked for accuracy and for consistency between blades. How inexspensively? Believe it or not, for free. The nearest U.S. Prop Scan measuring system is located at a prop shop called Black Dog Propeller, presided over by Larry Carlson and his wife, Allison, and ably assisted by their three fine dogs. I was luckey enough to take a trip down there and see what they were doing.

Before I explain their Prop Scan however, perhaps I ought to fill you in the results of their work. A typical case was a fellow with a 34-foot Bertram, that was running "just fine." He thought there was no vibration- just the usual background engine and water noise. Larry convinced this fellow to check his wheels for free [which is standard- you just pay for shipping] and- as usual- found considerable difference in pitch betweem blades. [Keep in mind, that these props looked superb and ran well enough; without Prop Scan, there was just no way to know precisely.] Larry and his top prop man, Gene Thomas, trued the blades and reinstalled the prop. They said they wouldn't charge for the prop work [as opposed for the free measurment] unless the owner was satisfied. Well the Bertram's owner fired up the engine and backed out of the slip and instantly looked stupefied. He exclaimed that the boat was so quiet he wasn't even sure it was running for a moment.

Another, more extreme example was the case of a fleet of 40-foot pilot boats that had been delievered from England for use in the Baltimore area. Initially, these craft had props made in Morocco and engines manufactured in Europe. They had a top speed of about 22 knots and vibrated most unpleasently. The pilot's association switched to Cummins C-series engines and U.S. made wheels. This upped the speed to 24 knots and did ease the vibration. Black Dog was then asked to check the new wheels with Prop Scan and true them up. The result was a gain of another 3 knots [now 27 top] and still less vibration.

Small boats gain too. Gene [Black Dog's master prop technician] related that a typical outboard ski boat picked up 6 knots top speed after a full prop scan and adjustment. Then, with the computerized data on these wheels stored in their computer, Black Dog took the props back again and added camber [a moderate and spread out form of cup] to the blades for a futher increase of 1/2-knot. All this with the same load. same engine, same horsepower, and indentical fuel consumption.

Of course, extra speed can occasionally be a disadvantage. One California tour boat operator gave hour-long tours through the day. A standard cruise was timed to end up back at the dock at, say, 1:55. After Black Dog's full Prop Scan treatment and reconditioning, this tour operator found himself back at the dock at 1:45 [running at the same RPMs he had previously], with a full 10 minutes left to kill on his "one hour" tour! He also found that, where he used to fuel up every 7 days, he was now filling up every 8 days.

What Prop Scan does so quickly and so accurately is measure pitch. Diameter is easy enough, just measure across the span of the blade tips and you've got it. Pitch is different. Like any rotating object, the inner part of your propeller [near the hub] travels much less distance each turn than the tips. If you had a 16-inch diameter prop, its tips would be traveling along a 50-inch circumference each trip round. By contrast, the root of the blades, right by the hub, would only be traveling an 11-inch circumference each revolution.

This is one heck of a difference. Since you don't want the tips of the blades to push the boat ahead more than the inner parts of the blades do, you have to give the tips a steeper angle so they'll end at the same place as the blade roots every turn. Carrying this principle all the way down the lenght of the blades gives them their characteristic twist. It is good to remember that the pitch of a propeller is not the same as its blade angles. These angles very all along to keep pitch constant. This also makes pitch very hard to measure or even to visualize clearly.

Actually, the principle's fairly easy. You can make crude measurement of pitch using variations of the following. Take, for instance, your spiffy Oscill-Eightor's propeller. Place it face-up [with the aft faces of the blades up] on a flat table. You'll need a reference line, so take a grease pencil and, with a 12 inch ruler, draw a reference line along the face of one one of the prop blades, along its center from hub to tip.

Now measure out from the center any convenient distance and make a tick mark. Oscill -Eightor's props are 24 inches in diameter, so we'll make a measurement at, say, an 8-inch radius. Draw a transverse line across the blade face [at right angles to the radius line] through the 8-inch radius mark.

Take any straightedge and lay it on this transverse line so that its bottom edge runs off the prop blade and just touches the tabletop. Place a carpenter's square against the straighteadge anywhere along the high side. Note the vertical height from the tabletop at that point, and measure the base- the distance along the tabletop from the point of the straightedge, on the table, to the bottom cornor of the square. For Oscill-Eightor's wheel we've found a height of 6-3/4 inches and a lenght along the tabletop [a base] of 13 inches. We're ready to pitch. Pitch=6.2832xRadiusxHeight/Base. [This formula's the same no matter what method is used- our simple hand method, or the sophisticated Prop Scan.]

So, 6.2832x 8in. radiusx 6.75in. height/13in.base=26.1 inch pitch

This process is straightforward- and a useful basic check- but it's not very accurate. First off, because- as you will quickly find- it's hard to get precise measurements off the prop's complex shape.

Second, you can't always fit a straightedge onto and between the blades, and will have to use ingenuity to measure complex props.

Third, the straight line really is an imprecise approximation. It really should be a circumference of a circle, the radius of your measurement. With a bit of practice you can measure out from the hub with a ruler and make tick marks on the blades. Connect the points, and you can pencil in a portion of a circle on the blade. Take the height measurements from both ends of the circle on the blade and measure the length along the circle's segment, using that as the base in the pitch formula. I find I can usually come within about an 1/8th of an inch this way.

Fourth- and most important- all these measurements only give you the heights at the leading and trailing edges of the blades. If there's cup or shaping to the blade [which is ususlly impossible to detect by eye] this will throw your measurements way off.

In the old days a device known as a pitchometer would be inserted into the prop's shaft bore. The prop technician would rotate its arms to numerous fixed points along the blade and drop a vertical guage to measure heights. This is more accurate then our simple hand method but to get many, many accurate measurements across each blade took hours each time. It just wasn't financially practical for a prop technician to measure a prop with great accuracy, set it on the work table, true it up, then return and check again, true it up, and repeat the process until a precise standard was reached.

photoŠTipper Davis 1996

Larry at the Prop Scan Computer

You can see in the photo that Prop Scan allows just this. The first time I watched Gene taking a prop measurement I thought he was just setting up the machine. In fact, in just a matter of a minute, or so, he'd completely defined the entire blade geometery, with a precision of 0.001 inch! What's more, the measurements were recorded in the computer permanently for ready reference.

Gene could determine instantly, by inspecting the displayed pitch curves, not only what the pitch was at each section of each blade, but where exactly he wanted to adjust. He could then go to work on the blades and return the prop to the Prop Scan as often as needed to refine his progress.

With the extreme flexibility and accuracy of the Prop Scan, Black Dog can certify their propeller work to an offical standard. What Standard? ISO 484/2. Though the U.S. doesn't have a propeller standard, Europe does, and it's this International Standard Organization guideline that Black Dog uses. With Prop Scan they can certify ISO Class-1 [high accuracy] with ease. This is an extremely precise standard with pitch varying from spec and between blades no more than plus 2.5% and minus 1.5%.

Incredibly, many U.S.-manufactured props don't even meet the minium ISO Class-3 [wide tolorance] of plus 6% and minus 4%! Many are extremely inaccurate and the cause of many otherwise unexplained engine and vibration woes. Indeed, with Prop Scan, Black Dog can even exceed the ISO S-Class standard. Such props are considdered "very high accuracy."

How much does this cost? Well, again, the Prop Scan measurement is free. The reconditioning work is not. Because there's such a wide varity of propeller types, and diversity of work to be done on them, each job must be costed out.

As a rough guide, though, Larry gave me the following estimates. If your Oscill-Eightor had, say, a 22-inch Nibral, 3-bladed prop, then to completely recondition and true it up [without damage repair, or a pitch change- more than 1 inch adjustment] would run about $245 each. On the other hand, if you'd run Ocill-Eightor up on the beach and mangled her blades, then wanted a full rebuild, with pitch change and also decided to clip the blades to reduce diameter an inch, it'd run about $440 each prop. As far as I can tell, this would be all money very well spent, and I'll be recommending Prop Scan to most of my clients.

There is, incidentally, no real limit to the size prop that Prop Scan can measure. Terry Ryan has units that can measure propellers well over 100 inches in diameter. Some can even make the measurements underwater, without hauling the ship. Both the U.S. and Canadian navies are currently begining to use Prop Scan to improve ship proformance, reduce fuel consumption, and to save drydocking fees.