Propeller slip, expressed as a percentage, is defined as the difference between the distance that a propeller of a given pitch should have moved forward (as the result of a full rotation) and the distance that it actually moved forward. For example, given a propeller with a pitch of 10, if while operating the boat it is somehow observed that the boat moves forward only 9 inches with each revolution of the propeller, then the slip is 10%.

This point exposes a weakness of the strategy used by JavaProp. The only way I can think of to measure propeller slip with near absolute accuracy is to measure the boat speed exactly with radar equipment, and calculate backwards using the RPM reading at that speed.

Most users of JavaProp do not have radar equipment. It will suffice to say that most pleasure and performance boats experience a propeller slip between 10-13%. Unless you have measured your propeller slip exactly, you must estimate this value. What this means to JavaProp users is that the accuracy of numbers generated may be incorrect by up to, say, 3% due to this fact alone. This is still better than the +/- 10% accuracy of most speedometers, however.

That said, I will tell you what I know about propeller slip. A perfectly tuned propeller that is operating in totally undisturbed water will still slip, because water is not a solid medium. In most applications, propellers are not perfectly tuned, and do not operate in undisturbed water. Things that cause a propeller to slip include turbulence caused by the transom or hardware underneath the boat, imperfections in the blades, and general hydrodynamics (turbulence caused by the blades themselves). In general, V-bottom boats tend to disturb the flow of water to the propeller more than flat bottom boats. Some boats use a set-back device that positions the propeller further back from the transom, allowing a cleaner flow of water to the propeller. I have heard that two blade propellers used on hydroplane hulls have achieved efficiency in the 5-8% range, but I do not have any references to support that. Hydroplane hulls are designed such that no portion of the boat touches the water in a way that disturbs the flow of water to the propeller. There are several good books available on the topic of propellers. See your dealer for references.

Speed

Boat speed, as described here, is relative to the surface of the body of water on which the boat is traveling. The calculation of boat speed relative to land is certainly possible, and is especially useful in navigation applications, but is beyond the scope of JavaProp. Many JavaProp users will be interested specifically in determining the top speed of their boat.

Example of using JavaProp to decide what propeller pitch to use

In the following example, I will use values from my own boat to determine what performance to expect from a different propeller. You may wish to work along with this example, entering these values as you go. I entered these values:

• 6200RPM, obtained by reading the tachometer at near full throttle
• 21 Pitch Propeller, obtained by reading the value stamped on the propeller hub
• 1.87 Gear Ratio, obtained by reading manufacturer's documentation
• 13% Propeller slip, a good guess for a semi V bottom boat

You may wish to open JavaProp and this help file in seperate browser windows, to avoid having to jump back and forth between reading this text and entering values.

Speedometer accuracy and top speed

Increasingly, segments of the boating community that previously were not interested in top speed (e.g. tournament ski boat and family runabout owners) are now developing a keen interest in top speed. Boat manufacturers certainly agree with the previous statement, evidenced by the appearance of top speed capability quoted on most every full-page advertisement for family style boats. Accurately measuring top speed with a boat speedometer is a special case of measuring speed in general.

Like most gauges, speedometers tend to give the most accurate readings in mid-range values, and the least accurate readings near the minimum and maximum values. That is to say that boat speedometers are not especially accurate at any speed, but accuracy only gets worse when measuring top speed.

For example, many family runabouts are designed with watersports in mind. Since the primary intended use of such vessels involves speeds between 18MPH and 36MPH, it is in the customer's best interest that boat manufacturers equip these vessels with speedometers that are most accurate in the speed range of primary use. A speedometer with a range of 0-50MPH would be a good choice for such an application, but would not be the best choice for accuracy at a top speed of, say, 48MPH.

The RPM reading at top speed is generally not at the maximum range of the tachometer (in most applications, a tachometer leaves a large portion of the range for red-line values, where readings should seldom or never be seen).

Accuracy of Calculated Values

Fields may display calculated values that show lots of digits to the right of the decimal point. This is not intended to imply that this program is useful for calculating values to such accuracy, but rather to show what numbers are being used in the calculation.

The accuracy of JavaProp for your application will most likely be limited by the sum total of:

• Error in estimate of propeller slip
• Inexact pitch of propeller
• Tachometer error

Functional differences from WinProp Calculator

Those users familiar with WinProp Calculator have probably noticed that JavaProp is nearly identical. As a result of several user comments, I changed a couple of minor details regarding allowable ranges (or more correctly stated, I removed bounds on input values). Here are the reasons for lifting range restrictions:

• Gear Ratio
  High performance craft (e.g. flat bottom V-drives) commonly run an overdrive transmission, and therefor have gear ratios less than 1.
• Propellor Pitch
  At least two individuals had propellers with a pitch of less than 10.
• Propeller Slip
  The information I originally gathered was for applications that allowed variation in thrust angle (i.e. stern drives and outboards). Maximum propeller efficiency is less on certain applications (e.g. those with fixed angle propshafts), and therefor slip is greater than the 10-13% range.

Copyright Notice

By agreeing to the following terms, you are hereby granted rights to use JavaProp, version 0.9, free of charge.

JavaProp, and this associated help file, is provided on an "AS IS" basis, without warranty of any kind, including without limitation the warranties of merchantability, fitness for a particular purpose and non-infringement. Furthermore, you, and not BigWave Software or the author, assume the entire risk of using this software, including, but not limited to, loss of data, loss of time, and incorrect purchase decisions. If you do not agree to these terms, please do not use JavaProp.

About BigWave Software

BigWave Software is a sole proprietorship. Software consulting services are available on a limited basis. For more information, send e-mail to:

pandg@ix.netcom.com

In case you're interested in the name...

About the name

Some 10 years ago I traveled with friends to an island off the coast of Mexico, for the purpose of surfing. The name of the island is Todos Santos. We stayed on the island for a week, in the midst of a nasty winter storm. Although the group sported some very talented athletes (if I recall, two were professionals), no one was brave enough to attempt to ride the surf on the West side of the island. We instead stayed on the East side (the surf was actually wrapping around the island, and producing ten foot breakers on the wrong side!). Four days into the trip, after things settled down from the storm, a few of us decided to paddle out on the West side. In all honesty, even the largest waves on that island are not particularly challenging due to the slow, rolling nature. Because I was not very talented compared to my surf-wizard peers, and because I still was one of the few to take to the large surf, I was dubbed Pete (BigWave) Tenereillo. I don't do much surfing these days, and I didn't do anything very spectacular to earn the title anyway, but it is a very cool name wouldn't you say?