Marine Performance Technology Exchange
|Copyright © 2003 HydroComp, Inc. All rights reserved. -|
What makes a propeller "sing"? Some propellers produce an audible high-pitched tone which has come to be known as "singing". More of an annoyance than anything harmful, the hydrodynamics of singing have been a source of debate for decades.
The most widely-held opinion is that the tone is produced by alternating vortices which roll off of the trailing-edge of the blade. Singing is produced if the vortices develop into a well-behaved system (e.g., a continuous steady "train" of eddies) and the frequency of this "train" is in the audible range.
The most frequent cure for a singing propeller is the popular "anti-singing edge". This is a chamfer applied to the trailing-edge to promote separation of the vortices.
For a more indepth review of singing propellers, review the following HydroComp report:
Half angle of entrance
What is the best entry angle for a boat's operating speed? Entry angle (referred to as the half angle of entrance) is a significant player in vessel wave-making drag. In simple momentum terms, think of it as the cause of how quickly the mass of the water needs to get moved out of the way. A high angle also can also lead to longer flow paths and subsequently higher local water velocities (a' la Bernoulli), leading to higher viscous (frictional) drag.
Small is not always better, however. From a design standpoint, you would likely have to increase vessel length to achieve a very small angle, with a corresponding increase in the overall wetted surface. Shape with reverse curvature of the waterplane (as you might find with very small entrance angles) can increase what some refer to as the "shoulder wave", actually leading to more drag. So you always have to compromise when searching for the "best" entrance angle for minimum total drag.
A quick literature search did not quickly find anything like a curve of "best" angle versus speed. This is not a surprise, as there are so many inter-relating factors, and it would be difficult to pull out this one variable.
Measurement of this angle is somewhat subjective
- it is not the measurement right at the bow. For example, you can take a bow
shape, tug the stem forward and get a very acute local angle - without any
measurable effect on drag. (The opposite example is with a small radius at the
bow, such as from a small conical development. The measured geometric angle
right at the bow is 90 degrees, but of course this is not its effective entrance
angle.) You need to measure the angle some distance away from the centerline to
avoid the local stem shape. We recommend drawing a line parallel to the
centerline 10% of the maximum waterline beam (i.e., 20% of the half-breadth) off
center, then take the tangent of the waterplane at that point. This gives a
repeatable, quantifiable, measurement and avoids all of the small insignificant
local shape right at the bow.
Thrust and power factors for propeller material and ISO class
Does propeller material or ISO class affect my choice of thrust and power factors? Material and class have no real impact on thrust and power factors (i.e., the Kt and Kq multipliers in NavCad, T & P factors in PropExpert and SwiftCraft). These are used to correlate thrust and power performance principally for the differences of geometry between your propeller design and a standard series design (such as the Gawn or B-series). For example, if you were to build a true Gawn propeller, the thrust and power factors would be 1.0 (i.e., no correction). However, if the geometry were different from the tested series propeller (as we typically see with the relatively heavier blade and edge thickness on commercial propellers), the factors would be adjusted to reflect the change in thrust and torque that you see in the the real world.
You could make the argument that there needs to be some correction for propellers of less precise ISO classes. Assuming that the original propellers which made up the series were of high precision (which is to be expected for a laboratory model), then you might expect a propeller with poor pitch accuracy between the blades to be less efficient. You therefore might increase the power factor by 1%-2% to account for this possibility.
Regarding material properties, the only effect a change in material might have on performance would occur if the material deflects in service. Stronger materials will change less under load. You also might see a change in performance if the surface texture of the material is measurably different.
The following reports from HydroComp's web site will give you more information on thrust and power factors:
HydroComp has begun using a new numbering system for our software releases. In the past, products were given a specific release number (such as NavCad 4.6). The new numbering system simply appends the release year to the product name (as is the case for this year's release, NavCad 2003). Development of this type of technical software is evolutionary, with frequent, small updates, rather than large updates which might be years apart.
We hope the new numbering system will help users track their update history and their Maintenance, Support and Update authorization. For example, all users with a 2003 subscription of NavCad will be authorized for NavCad 2003. To use NavCad 2004, they will need a 2004 subscription.
New web site improves support and information
HydroComp will shortly be unveiling our newly redesigned web site. In addition to a more visually-appealing look, it provides more support and information to existing users, service clients, and visitors.
A section entitled Knowledge will be devoted to the distribution of technical information on various hydrodynamic topics. A new Software summary page will help direct those seeking software information to the proper products. Potential service clients can learn about our Application Report program - an inexpensive way to enhance your R&D resources.
You can discuss product capabilities with HydroComp staff at these upcoming trade shows:
SwiftTrial is HydroComp's newest product - an easy-to-use and inexpensive utility to document and evaluate boat performance trials. The companies listed below are a few of the users who span the entire spectrum of maritime industries.
|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.
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