During the past 20 years that I have been involved in the boating industry, my understanding of propellers amounted to: they worked or they didn't. If they didn't work we took them to the local propeller shop and had it fixed. I relied on the insight and intuition of that particular shop's "guru," to analyze the problem and devise a cure. Some times it worked, sometimes it didn't. If it didn't---well, we tried something or another propeller shop.
Although this process seemed like black magic, it really stemmed from the experience a few cleaver individuals. When asked how they devised the cures they would sight rules of thumb and past experiences with other propeller problems If I was lucky, we would find someone who could come up with a cure for my customers' problem that worked well the first time. Unfortunately success and quality were inconsistent.
During the early days, science and technology just hadn't managed to infiltrate the propeller industry. Admittedly, there are some very fine universities that have done some excellent work with propeller theory. Indeed, it is from this theoretical work that most of the documented major advances in propeller technology have been born.
During the late summer of 1994, as the owner of a boat yard, I began to have seriously problems with both new and repaired propellers coming to my shop. Research lead me to discover that in the United States, specific universal standards for propellers did not exist, and still do not exist today. "What you got" is "what you got." During this period of heightened frustration I discovered that The International Standards Organization (ISO), Geneva, Switzerland, did have a set of standards for the manufacture and repair of propellers. I also located a prop man on the other side of the world who had invented a device for accurately and easily measuring propellers. Now, while "Measuring Propellers" sounds quite easy -- it is actually very difficult.
To measure the diameter of a propeller one merely measures from the center of the hub or boss to the tip of a blade and doubles the result. Pitch is an altogether different matter. Pitch is the distance that a propeller will move through a perfect media (like a block of butter) is one complete rotation. A propeller's pressure face, where pitch is measured, must conform exactly to the helical path of whatever the pitch is. In other words we are attempting to measure a portion of a screw thread.
Blade angle also complicates the issue. You will note that the angle of the blade on your propeller changes (becomes less) from the root to the tip. Interestingly enough the blade angle of every propeller starts out as 90_ (that is parallel to the shaft) at the center of the boss or hub becoming less toward the tip of the blade. This is because although the propeller as a whole travels the same distance through the water as the boat, various parts of the propeller travel at vastly different rotational distances and speeds.
Most propellers we deal with are characterized as "Simple Pitch Propellers". Simply put, this means that the pitch along a given radius should be the same from the leading edge to the trailing edge. From the root of the blade to the tip the pitch can, and often does change. Checking (not measuring) for simple pitch on a given radius on a single blade is simple. Simply scribe a radius on the blade, take a straight edge and lay it on the blade along the arc of that radius. On a simple pitch propeller the straight edge will touch the blade over the entire arc. Of course you'll have no idea how this compares to the other blades on that propeller or how that compares to the other prop on a twin screw vessel.
Measuring (not checking) pitch is somewhat more complicated. There are several methods for measuring pitch. There are "Pitch Bevel Gauges" which are proprietary devices that measure an angle from the leading to trailing edge along a chosen radius. The gauges simply convert angle to pitch by assuming the leading and trailing edges are points lying on the helical path and that the blade in between follows suit. This isn't always the case.
Another method is the "Pitchometer" which measures the drop of a blade along a particular radius over a rotational angle and geometrically converts this measurement to pitch. This method is as accurate as the equipment used often to as much as _0.1". Again the assumption is that the blade is following the helical path---before, in between, and after the 2 measured points. This isn't always the case.
There is also the "Pitch Block" which is best characterized as a mirror image of the pressure face of the propeller blade. The blade is placed over this pattern and checked for fit. This is of course only as accurate as the pattern used and has an obvious drawback in that those portions inside the margins of the blade are hard to check for proper fit.
There are even more obscure methods such as "bubble protractors" and "proprietary level gauges." The problem is none of these various methods give the same result. A 20-inch pitch prop at one shop would not be a 20" pitch prop at another shop using a different measuring method. Accurate repairs were akin to the ability of a village blacksmith. The repair person would hammer out a bent blade to where he thought it should be and no one could argue because of the inconsistency of the measurements.
Utilizing these various methods between different shops could easily lead to differences in measured pitch of as much as an inch and a half. Within a given shop, this is not a problem but it could easily lead to heated discussions between shops. To confuse us even more there is a school of thought amongst the theorists that advocates measuring pitch along an imaginary line through the center of the blade from the leading edge to the trailing edge.
Of course none of this helped me the confused boat yard owner understand what was going on. Nor was it any help dealing with propeller problems. I was still at the mercy of the local propeller "guru."
Frustration led me to search the world for a better and more accurate method. My search ended in Australia where I found Terry Ryan the developer of Prop Scan a micro processor based system that dominates the propeller industry "down-under."
Prop Scan consists of a rotating table on which the propeller sits, combined with a distance traveled probe that travels over the pressure face of the propeller. There are optical counters in the rotating table and distance traveled sensor. While measuring, the system actually follows the helix scribing a circle over the surface of the propeller. The computer calculates the pitch of the propeller continuously, for every degree of rotation to 0.001" of pitch.
Prop Scan is capable of measuring propellers as small as two inches in diameter up to ship size propellers. It will measure as few as 1 or as many as 7 blades. It will measure a single radius or as many as 10 radii. The system will then take these measurements, compile them and compare them to ISO 484/2 Classifications, all of this to 0.001", and all the work is saved in the computer for future reference.
ISO 484/2 is a classification system set up by the International Standards Organization for the Repair and Manufacture of Ship's Screw Propellers. While the U.S. is not a signatory to this particular standard it none-the-less, allows us to discuss with the customer the quality of their propellers. ISO defines four different classes of propellers. Class 3 which is defined as a Low Accuracy Propeller where the mean measured pitch of the blades have to be within 4% of each other. Class 2 is defined as a Medium Accuracy Propeller where the pitch of the blades must be within 1.5% of the other blades. Class 1 is a High Accuracy Propeller, blade pitch must be within 1%. Finally, Class S, which is a Very High Accuracy Propeller with pitches within 0.75% of each other. Most propellers are Class 3 or worse. We regularly see pitch variances between blades of a single propeller and between propellers on twin screw vessels of as much as _3/4". This means that there can be as much as 1 1/2" of total pitch difference between two propellers!
Prop Scan allows us to measure and assess a propeller in a matter of minutes. A good shop with a calibrated pitchometer can do the same thing. Lay out four radii on each blade, locate 5 points on each radius, measure the pitch between each of those points, calculate the pitch of each radius, calculate the pitch of each blade, calculate the mean pitch of the propeller, and then calculate the relationship of all those pitch measurements. Realistically, for a 24" 3-blade propeller this would take most of a day at the end of which they would really have no idea what the actual shape of the blades were. We not only get these relationships immediately after measuring the propeller but the data is also shown to the technician in a graphical form. With these graphs we can see the actual shape of the blades and the relationship between the blades. Now the technician can see exactly where and in which direction to manipulate the blades to achieve the desired result.
What this has done for the propeller repair business is to set it apart from the village blacksmith's shop. The new equipment can not only measure a propeller's pitch and accurately assess the condition of the propeller but it can also show the customer the results. This is all before anything is done to the propeller. The equipment is so efficient at this that we don't even charge to do this assessment. Measuring and analyzing are free.
If a propeller company does nothing more than make the pitch of each of the blades the same and the pitch of the port and starboard propellers the same we would improve the efficiency of the propellers. Combine this with customer provided performance data which allows the proper calculation of design and pitch changes and the performance increases again.
All work is documented. This allows the technician to show the customer exactly what his propeller looked like before the job was started and to show him the finished result. The new equipment takes all the guesswork out of the process. The customer sees exactly what the technician sees.
What this means to the customer is that he can now have a set of propellers that are matched both in pitch and blade shape. This alone increases the efficiency of the propellers. Additionally, the information about the propellers is saves in the computer and the set can be duplicated at any time. This would allow the manufacture of a second set of props that are exact duplicates of the first set. Essentially, the customer could have a matched set of four propellers. Should he damage one of the props he would only have to replace that single prop with a spare, send the damaged back to the shop and they could remanufacture it to once again be a part of the set.
Combining the measured data from the propellers and the customer's performance information, this equipment is able to fine tune the design criteria to further improve performance.
