Marine Performance Technology Exchange
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Do propeller tunnels mean a loss of efficiency? How do they affect performance? Can the effects be modeled? Tunnels (also called "propeller pockets") are recesses into the bottom of a hull (typically a planing hull) used to help reduce its operational draft. There are two ways in which propeller tunnels affect vessel performance - changes in drag and changes in propeller performance. Some of these effects can be modeled, others require use of a "common sense" margin.
While there is very little published technical information about propeller tunnels, there is one significant body of work undertaken by the U.S. Navy [Koelbel, 1979]. We refer to this work for systematic test results and trends in comparing tunneled hulls to the same hull without tunnels.
Changes in drag Most of the effect of tunnels on drag are due to its additional wetted surface and changes to the boundary layer. The results of the Navy testing suggests a nominal 5% or so increase in drag. Having said this, there is also some anecdotal evidence that some of this increase is due to changes in trim and that the drag penalty can be much smaller with a properly designed tunnel.
Changes in propulsion This is where you will find the greatest influence of propeller tunnels. The testing indicates a reduction in relative-rotative efficiency and thrust deduction (although this effect is small). There is typically a marked increase in wake fraction as the friction due to the closer proximity of the propeller to the hull will slow the water velocity. This, in turn, typically leads to an increase in hull efficiency. (This depends of the relative relationship between wake fraction and thrust deduction, however, so this is not always the case.) The increase in wake fraction also means you'll typically need a lower pitch.
In summary, a reduction in overall efficiency is likely - although the loss of efficiency is very small at modest speeds (FV < 3), small tip clearances, and modest tunnel depth (< 40%D).
A comment on tunnel design Proper design is very important to a successful tunnel application. We would like to point out a well prepared reference on this subject from Donald Blount and presented at the FAST '97 conference.
Validation study of Delft sailing yacht model
During the course of assistance to a new user of SwiftCraft, we investigated how the various semi-displacement prediction methods fared for the Delft Model 25 [Gerritsma, 1991]. There were some very interesting conclusions:
Performance prediction tools, such as NavCad and SwiftCraft, rely on a "parametric" description of the hull. They use dimensional and non-dimensional parameters such as length or prismatic coefficient rather than the 3D geometry of the hull.
Deriving this data is a relatively simple task with a set of lines and hydrostatic output. However, this task can be made simpler by a direct data exchange with hull design or hydrostatics software. HydroComp has developed the following data imports:
Cost-effective power prediction for users of FastShip, FastYacht and PHASER
HydroComp and Proteus Engineering are looking to the new SwiftCraft software as a cost-effective power prediction tool for Proteus Engineering customers. To celebrate the launch of SwiftCraft, users of FastShip, FastYacht and PHASER can take advantage of a limited-time special offer. For more information about this SwiftCraft discount offer, please email Bruce Hays of Proteus Engineering at firstname.lastname@example.org.
We are extremely pleased to welcome the first nine users of SwiftCraft - all internationally recognized leaders in their disciplines.
|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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