Propellers By the Numbers: Propeller Dimensions

How Propeller Dimensions Affect You Quick!  Name all the different propeller dimension terms you need to specify the exact shape and size of a propeller: Propeller diameterNumber of bladesPitchBlade areaBlade skewBlade rakePropeller section seriesRotation speed / directionNumber of propellers Not bad, but the hard part is remembering how each item affects propeller performance.  How do the different pieces...

How Propeller Dimensions Affect You

Quick!  Name all the different propeller dimension terms you need to specify the exact shape and size of a propeller:

1. Propeller diameter
2. Number of blades
3. Pitch
4. Blade area
5. Blade skew
6. Blade rake
7. Propeller section series
8. Rotation speed / direction
9. Number of propellers

Not bad, but the hard part is remembering how each item affects propeller performance.  How do the different pieces fit together?  This article provides a brief guide on the various aspects of a propeller, highlighting general expectations for each item.  Even though the descriptions are brief, it still adds to an extensive wealth of wisdom, collated from multiple sources.  Print a copy of this article; keep it handy to serve as a starting point the next time you need to select a propeller.

1.1 Quick Start:  Selection Information

Don’t want to read the entire article?  Just the basics then.  Here is the information you should provide to a propeller designer to allow them to select your best propeller.

1. Design service speed
   1. Any other major speeds and relative time spent in each
2. Effective power required at each speed
3. Propulsion arrangement (single screw, multiple screw)
4. Immersion of propeller shaft
5. Any ice class requirements

2.0 Propeller Diameter

Generally, you want the largest propeller diameter possible.  What limits the propeller diameter?  Propeller immersion.  The bottom of the propeller generally aligns with the bottom of the ship.  The intent is that the ship hull should protect the propeller.  This also makes it easier when the ship needs to get drydocked.  It helps all around if the propeller is never below the lowest point on the ship.

The top of the propeller gets set by the waterline.  Your typical is propeller should always be fully submerged.  So the top of the propeller gets set by the lightest draft waterline.  On passenger vessels, we see very little variation.  But for cargo vessels, there may be significant variation in the waterline between fully loaded and light loaded.

2.1 Propeller Clearances

Propeller diameter also gets limited by required clearance between propeller tips and surrounding hull structure.  The main goal for propeller clearance is to prevent any vibrations with the hull.  Recommendation from naval vessels:  vertical clearance between propeller tip and hull should be 25% of propeller diameter.  This is just a general rule of thumb, but naval vessels rarely experience propeller hull vibrations as a result. [1, p. 303]  On merchant vessels, this rule may be relaxed to 15% of propeller diameter.  In any case, the goal is to keep the propeller tips outside the boundary layer of flow over the hull.  CFD analysis can help identify the exact thickness of your hull boundary layer.

You must also consider longitudinal clearances.  Often designers put a skeg in front of the propeller.  Forward clearance between the propeller disk and the skeg should be approximately 30% of propeller diameter. [1, p. 303]

3.0 Number of Blades

A propeller with one blade would be the fastest and most efficient propeller if it weren’t for the fact that the vibration could not be contained. The reason for its great speed would be the lack of blade drag caused by metal in the water. As blades are added to propeller hubs, the drag is increased. However, the ability to grab more water and decrease the time frame between load and unload is shortened. This means the propeller will “feel” smoother as it grabs water or loads the blades. The number of blades does many other things to the performance and control of a vessel. Performance type hulls tend to show the greatest response from adding and subtracting blades. Propellers typically come with 2 to 6 blades.

-BBlades [2]

Regardless of the number, you generally want an odd number of blades.  This is a concern for vibration.  The key here is to consider the path of the blades as they rotate around.  Propeller blades create a wave of pressure pulses as they pass by a structural element, such as a rudder or shaft strut.  You want to ensure that only one blade interacts with structural elements at a time.  The symmetry of even bladed propellers works against that goal.

4.0 Propeller Pitch

Pitch is the distance that a propeller travels forward in a single rotation, if it were threading through a solid material.

It is very important that both, pitch and diameter, are properly calculated. If for any given HP the pitch is too big, the propeller becomes heavy and demands more power than the engine can reach and vice versa, if the pitch is too small then we have a light propeller that wouldn’t absorb the engine’s full power.

-Edgar Reyes [3]

Pitch to diameter ratios typically start around 0.6 for highly loaded propellers, like tugboats.  And they can go up to around 2.0 on high speed motorboats.  [1, p. 176]

5.0 Blade Area

In normal ship design, the blade area is also the pushing surface that transfers all the forward thrust into the ship.  So we need a large blade area.  How large?  This gets governed mostly by cavitation.  Although blade efficiency also plays a part.  A propeller with a smaller blade area must pack in more lift per area.  Generally, when you convince a propeller blade to pack in more lift, the penalty is extra drag.  That leads to greater torque on the propeller and lower efficiency.

We also favor larger blade areas to avoid problems with cavitation.  This should be the first check when considering smaller blade areas.  When we try to squeeze extra lift out of a propeller blade, we increase the pressure drop on the blade face.  That quickly leads to cavitation.

So why don’t we just create massive blade areas and avoid the entire issue?  Designers need to balance this blade area against the strength of the propeller.  A larger blade area creates larger stresses on the propeller blade.  These can easily break the blade.  The designer needs to counter this with thicker blade sections, purely to maintain blade strength.  But we generally don’t want super thick propeller blades.  Remember that the blade shape is also the key to generating lift.  A super thick blade section will be less efficient.  The blade strength ultimately limits the size of the blade area.

This careful balance between blade area, section shape, and blade strength are why most designer stick close to designs based on standard propeller series.  And part of that standardization required three different ways to measure the blade area.  We can not fit all the complexities of a twisting 3D shape like a propeller onto 2D paper.  So propeller designers created three different ways to represent the blade area. (Figure 5‑1)

• Projected Area
• Developed Area
• Expanded Area

5.1 Projected Area

Projected area is the area of the blades, projected onto the plane of the propeller (normal to the thrust vector). [4]  This misses the 3D shape of the propeller.  Sometimes we use projected area for a first measurement when considering cavitation.

5.2 Developed Area

For the developed area, we take each blade section and untwist it to bring the section pitch to zero.  It makes the blade flat, but each section still has thickness and skew.  This allows us to see the true shape of the blade and distribution of chord lengths at each blade section.  This is one of the most commonly used views seen on propeller drawings. [4]

5.3 Expanded Area Ratio

The expanded outline is not really an outline.  It does not show the true geometric shape of the propeller.  It consists of several sections stacked up.  At each section, the propeller pitch is removed and the section is stretched out so that the chord of each section is flat.  This is not extremely practical.  But propeller designers can use this to show the stacked up section shapes, locating the position of each section on the expanded area diagram. [4] (Figure 5‑2)

6.0 Blade Skew

Blade skew primarily reduces vibrations and propeller noise. (Figure 6‑1)  These problems occur due to the pressure pulses that happen as a propeller blade passes some piece of ship structure, like a rudder.  Blade skew smooths out that transition.  Rather than the entire blade passing at once, we start with the small area at the root of the propeller.  The area slowly increases as more of the blade passes the structure.  This gradual change in blade area smooths out the pressure pulses of passing blades.

7.0 Blade Rake

You generally don’t see blade rake (Figure 7‑1) on commercial propellers.  This is because a raked propeller blade reduces the overall propeller efficiency.  But on smaller boats with outboard engines, blade rake is very common.  The raked propeller blade helps to prevent ventilation of the propeller.  With a ventilated propeller, air gets sucked down from the water surface.  When the air runs over the blade surface, you lose thrust.

The raked blade uses the centrifugal force of the propeller motion to push the water against the blade surface and reduce ventilation.

8.0 Propeller Section Series

Generally, designers do not reinvent the propeller with every application.  We developed numerous standard series of propellers to pick from.  A designer typically picks a standard series to best fit the propeller application.  They may then tweak the propeller design a little, but most propellers remain close to one of the standard series designs.

What goes into a standard series?  It is all about the shape of the propeller blade, and the experimental testing.  Each standard series is a range of propeller sizes, all based on a single geometric template.  Each series includes variations for diameter, blade area, and propeller pitch.  Some even include variation for skew.  The key is that each of these series were tested in a lab to determine their exact performance characteristics.  Some possible names you might here when referring to propeller series [4]:

• Wagenigen B-series
• Ma-series
• Newton-Rader series
• KCD series
• Meridian series
• Kaplan series (ducted propellers)
• MARIN series (contra rotating propellers)
• SSPA series (contra rotating propellers)
• DTMB series (skewed propellers)
• Wagenigen ducted propeller series
• Gutsche and Schroeder series (controllable pitch propellers)

As you can see, the list of available series is numerous.  Each with their own strengths and weaknesses.  It takes a full-time propeller designer to appreciate the nuances of each series.  Propeller blade series are a lot like buying new clothing.  You first need to find the series that fits your vessel and operating conditions.

Most propeller series were tested for a select set of conditions.  They do not cover the entire range of possible speeds and loading requirements.  So a propeller designer will first eliminate any propeller series that do not cover your specific needs.  That often leaves only one to three options.  From there, it is a simple comparison to see which one delivers the best performance for your application, considering all the tradeoffs.

9.0 Rotation

Just as the word suggests, rotation determines the direction a propeller revolves around the axis of the propeller shaft. Rotation is determined by looking at the propeller from the rear of the boat. A right hand propeller will turn clockwise therefore moving down to the right hand side. Contrary, a left hand propeller will rotate counterclockwise or down to the left while looking at the propeller from the rear of the vessel. Changing rotations on single, twin, or triple engine applications can affect the boats performance too.

-BBlades [2]

If you have dual propellers, the two propellers will be set to rotate in opposite directions.  This prevents the stern from walking sideways due to any propeller rotational forces.

9.1 Rotation Speed

Rotation speed is an output of propeller design, not an input.  The rotation speed and pitch are the two primary tools that propeller designer uses to select the right fit for you.  Once constrained within a specific propeller series, the designer will pick the rotation speed and pitch to get the most efficient propeller possible.  Slower rotation speeds are generally more efficient.  Normally, propeller rotation speeds reside the range of 90 – 250 rpm for commercial vessels.

But the designer also needs to match the propeller with the engine.  An engine does not develop full power and full torque right from zero rpm.  It slowly builds up the available power and torque as the rpm’s increase.  The propeller designer must adjust the propeller speed to ensure that the demanded power and torque does not climb faster than the engine can deliver.  The engine gearbox is the main tool that they use to match propeller speed demands with engine availability.

10.0 Number of Propellers

In many cases, we prefer at least two propellers.  This is for redundancy.  But ultra-efficient ships, like container ships often just have one propeller.  Less propellers means less drag in the water.

Additional propellers are another way to pack in more blade area if your propeller diameter is limited.  But generally, I would not consider going higher than four propellers, with exceptions for a few special ships.

11.0 Conclusion

1. Propeller diameter
2. Number of blades
3. Pitch
4. Blade area
5. Blade skew
6. Blade rake
7. Propeller section series
8. Rotation speed / direction
9. Number of propellers

Face it, these terms are almost meaningless with just a definition.  But this article added the appropriate context.  Normal ranges for each item.  And how the values affect your overall performance; what is good or bad.  The dimensions of a propeller design coalesce from meaningless jargon into the stepping stones along the design path for your perfect propeller.

12.0 References