Marine Performance Technology Exchange
|Copyright © 2004 HydroComp, Inc. All rights reserved. -|
The numerical prediction formula for propeller thrust and torque are typically based on propeller models (e.g., B-series, Gawn, Kaplan) with a constant pitch distribution, which means that the pitch is constant from root to tip. (One exception to this was in the 4-bladed models for the B-series, which had a pitch reduction at the root.)
Many contemporary propellers, however, have a variable pitch distribution, meaning that the pitch is different along different locations from root to tip. Therefore, in order to use the standard series for calculations, it is necessary to determine a nominal hydrodynamic pitch. We use a "radius-chord moment average" calculation to find this.
This calculation is quite simple - it is the integration of section pitch x chord x radius divided by the integration of chord x radius. This formula accounts for the chord length and radius of any particular blade section. Sections at larger radii near the tip travel at a higher velocity so they have more influence, but a section of no chord (e.g., at the tip) has no influence. Any of the simple integration techniques (e.g., Simpson's rule, trapezoidal) make this an easy calculation to program into a spreadsheet.
Recent analysis of cupped propellers here at HydroComp has resulted in some interesting new insights about how they perform. This analysis was conducted as part of our validation studies for the new PropExpert 2004, and includes performance data from technical trade literature as well as from sea trials. (For an introduction to cupped propellers, please review the HydroComp technical paper MacPherson, D.M., "Small Propeller Cup: A Proposed Geometry Standard and a New Performance Model", SNAME Propellers/Shafting '97 Symposium, 1997 that can be found on the Knowledge Library page of our web site.)
Cupped propeller performance mimics a propeller with a larger pitch. For example, a cupped propeller with 200 mm pitch might have the comparable thrust and power demand of a propeller with a 250 mm pitch. Predicting performance of a cupped propeller with an appropriate "effective pitch" has proven to be a usable technique - with some caveats. Using an "effective pitch" as a model for a cupped propeller reliably predicts performance only when boat speed is high and cavitation is modest.
Change in performance with boat speed - You can adequately predict cupped propeller performance with an "effective pitch" if the boat speed is high. It makes sense that such calculations would be acceptable, since cupped propellers are most often used on fast craft, and most of the study of cupped propellers has focused on operation for these vessels.
However, we have found that thrust and torque predicted by a single "effective pitch" is too high at low boat speeds. For example, the 200 mm cupped propeller might act like a 250 mm pitch for a motor yacht, but it will act more like a 230 mm pitch for a work boat pushing a barge. It also appears that cupped propellers are more efficient than their uncupped counterpart when operating in high-thrust conditions at low boat speed - even though the opposite is true for operation at high boat speed.
[Hydrodynamic conclusion: cupped propellers are typically less efficient than comparable uncupped propellers at high J - which we have always known - but are more efficient at low J.]
Cup's effect on cavitation breakdown - Perhaps the most interesting thing we learned from the analysis is that we get more thrust and power breakdown from heavy cavitation with cupped propellers. In other words, a cupped propeller will lose more thrust and power at high levels of cavitation than the corresponding uncupped propeller, and will also tend to have breakdown occurring somewhat sooner. In addition, there appears to be an increase in efficiency of a cupped propeller at very high levels of cavitation. This means that our current predictions of power demand for heavily cavitating cupped propellers are likely too high.
[Hydrodynamic conclusion: under high levels of cavitation (e.g., boat-speed cavitation numbers of 3 or less), thrust and torque breakdown is greater than for the comparable uncupped propeller. Even so, cupped propellers become more efficient under heavy cavitation, even at higher J. The principal improvement seems to come from torque reduction.]
So what new insights can we conclude about cupped propeller performance? First, the "effective pitch" approach to predicting cupped propeller performance is reasonable for fast craft at top speed or cruising - which is where we find most cupped propellers.
Second, suitably cupped propellers offer a way to gain efficiency for high-thrust, low-speed situations, like towing or acceleration. There is also an associated increase in RPM for the same thrust loading, allowing the derivative benefit of additional engine power (at the higher RPM) for towing or acceleration.
Third, the existing prediction models are inadequate for cupped propellers when they are under high levels of cavitation. The existing models tend to underpredict the extent of cavitation breakdown. In other words, they will overpredict the thrust and power for a particular boat speed and RPM.
These conclusions are based principally on tests of 3-bladed propellers, although a small number of 4-bladed propeller tests were evaluated, the results of which suggest that they follow the same trends. HydroComp is extending this study with the goal of developing a better prediction model for cup and cavitation. Look for the results of this work later in 2004.
HydroComp’s Technical Director presents at IBEX
In this presentation co-authored with Jeff Bowles
of Donald L. Blount & Associates, Don MacPherson will offer reliable
speed prediction pointers. He will illustrate some of the more common
shortcomings found in speed prediction methods and in software using
certain methods. He will also point out strategies that can be applied to
virtually all small-craft speed prediction.
You can discuss product capabilities with HydroComp staff at these upcoming trade shows:
The following is a list of current program versions and dates. If you have a current MSU subscription,
you can click on the appropriate link below to go to the update download
page. (Note: users of SwiftTrial and SwiftCraft are on a perpetual subscription.)
Seminar in the works
We would like to take this opportunity to thank those who attended our Fundamentals of Resistance and Propulsion seminar held earlier this year in Seattle, WA. We are currently planning another Propeller Sizing seminar tentatively to be held in Florida early in 2005. We welcome your comments, recommendations, and interest in this event.
|HydroComp, Inc. is the 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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