Marine Performance Technology Exchange
July 2003
Copyright 2003 HydroComp, Inc.  All rights reserved.  - 
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Four blades or five?
  

We recently had a discussion with a client about a propeller sizing project, where a four-bladed propeller was being considered due to concerns not directly associated with performance (cost, delivery, and the traditional use of four-bladed propellers on boats of this type). There were no also apparent performance problems with the sizing from a purely thrust and power stand-point. However, there is "more than meets the eye" with this project.

This project illustrated a number of interesting technical issues, so we wanted to share it with you. The boat was a 33 ft (10 m) single-screw motor yacht running 20 knots, with 430 hp (320 kW) and a 24 in (610 mm) propeller. As you might expect with so much power, the propeller was cavitating heavily. A four-bladed propeller had been selected with a significant amount of cup (PropExpert's "Medium" cup).

The initial sizing was not bad from a straight speed/power standpoint, but we reached a different solution - a five-bladed propeller with much less cup. The path we took to a solution was different because we evaluated the number of blades during the process. Software does not choose the number of blades for you - you need to make these decisions on your own. Our technical decisions were based on the following considerations: 

  • Cupping does not improve efficiency, so use as little as you can. PropExpert's "Medium" cup is actually pretty heavy in the commercial world. (It is based on a U.S. Navy standard, which was the only published standard at the time PropExpert was designed.) In fact, it is off of the scale of available commercial cup gages for a 24" prop. We typically limit cupping in our own consulting work to PropExpert's Very Light or Light, with the occasional Light Medium. (See Report 136 on our Technical Reports page about this.)
     
  • You need to be concerned about 4-bladed props on single-screw applications because of the potential for blade impulses behind a deep skeg. With both the top and bottom blades of a four-bladed propeller passing through the disrupted water flow (some refer to this as being in the "shadow" of the skeg), you have higher total impulse amplitudes and lower frequencies than with either a three- or five-bladed propeller.
     
  • One important issue, which is never given enough consideration, is engine-gear-prop resonance. For example, the whole propulsion is "in phase" from a vibratory standpoint if you have a four-bladed propeller with a 2.0-to-1 gear and an eight-cylinder engine. This combination is unfortunately all too common.

Adding these together, we suggested a five-bladed propeller giving us more blade area (requiring less cup) and the elimination of potential blade impulse or resonance problems.


The three modes of cavitating performance

What happens if you pack more power into a boat without changing the shaft line or stern geometry? At some point, more blades and BAR will not help - and the nature of how the propeller generates thrust changes significantly.

Propellers begin to cavitate when there is too much thrust for the propeller to carry. Virtually all propellers for modern boats run with some amount of cavitation - and some run with tremendous amounts. How can a heavily cavitating propeller do its job? To answer this, we will need to look into the three modes of cavitating performance - sub-cavitating, trans-cavitating, or super-cavitating.

The analysis of propeller performance is very well behaved for the sub-cavitating mode. A large body of research is available for propellers in this mode, and most propeller analysis and sizing software is based on sub-cavitating performance.

In comparison, very little work has been done over the years for the trans- and super-cavitating modes. One reason for this is that propellers operating with high cavitation are often unstable, and small changes in thrust loading or water flow can result in large changes in RPM. Cupping a propeller further complicates the underlying hydrodynamic analysis. For these reasons, the prediction of propeller performance when operating in trans- and super-cavitating modes is not always reliable.

While there are no consistent prediction models available for highly-cavitating propellers at this time, there are guidelines which can be of assistance. We refer you to our recent technical report (Report 137 - The Three Modes of Cavitating Performance) for a more indepth discussion of this topic.

 

 
SwiftTrial 

HydroComp has released a new product - SwiftTrial. SwiftTrial is a tool to assist in the documentation and analysis of sea trials. Conventional test data, such as speed and RPM, can be analyzed to reveal important information about a boat's performance. Overload and acceleration tests can also be recorded.

It is not enough to document a trial - you must learn what is really happening with your boat, engine and propeller. At only $695, SwiftTrial gives you a cost-effective tool to reveal the true performance behind the numbers. Please contact us for additional details.


Scan Converter  

A new add-on for PropCad

Scan Converter is a new add-on module for HydroComp's PropCad propeller CAD software. The role of Scan Converter is to take scanned blade surface data (in XYZ or radial format) and convert it into a PropCad design file. With carefully scanned data of the pressure surface and the corresponding thickness, Scan Converter can develop the full geometric representation of the propeller blades. This would allow a user to quickly and accurately make improvements to an existing product or to create new variants of the propeller.

HydroComp developed Scan Converter in response to user requests for a tool to document and compare a propeller to an original PropCad design, as well as to create PropCad design files for existing wooden patterns. Scan Converter would also allow a user to extract important design characteristics about legacy product models, or to precisely determine significant changes to a propeller after damage or repair. Please contact us for additional details.

   

people    

 
Training

Companies from virtually all sectors of the marine industry - boatbuilding, boat design, propulsion equipment, academia, government - were present for our two recent marine propulsion training seminars. One participant noted that "continuing education in this business is rare", so we are pleased to be able to provide a valuable service to the industry. 

Please contact us if you would like to know about upcoming seminars, or if you would like to find out about holding a seminar at your location. We are in the planning stages for our next seminar about propeller design and PropCad, so let us know if you would like more information about this session. 

Interested in on-line short courses? Let us know.

  

HydroComp, Inc. is a leading supplier of software and services for marine performance prediction, propulsion analysis, and propeller design. For more information, visit www.hydrocompinc.com or one of the pages listed below.
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Copyright 2004 HydroComp, Inc. Durham, NH USA. All rights reserved.
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