Marine Performance Technology Exchange
|Copyright © 2004 HydroComp, Inc. All rights reserved. -|
Hydrofoils are simply foils operating in the water. They are found in many different places on ships - fin stabilizers, struts and propellers. In some cases, such as propeller and stabilizers, their role is to provide lift. In other cases, they are simply used to reduce drag, as with struts. One important aspect of every hydrofoil is the shape of its leading edge. Speaking in general terms, some are sharp, others are blunt. So, why would we choose one over the other?
If flow is symmetric about the foil, you can minimize drag with a fine, pointed leading edge. However, if flow is asymmetric - we use the term "oblique flow" - then a sharp leading edge can be a detriment as it may induce separation and turbulence. This is particularly common when a hydrofoil is found in rotating flow, such as with propellers and struts.
Let's consider the case of a propeller strut. The angle of the rotational flow into the strut will change widely with differing propeller and boat speeds. Therefore, we design a rounded leading edge - with a radius about half of the thickness, for example - to be more forgiving and avoid separation in this varying flow field.
I'm having model testing done for a new project. How much confidence can I have in the results?
Assuming that the question refers to the prediction of drag (and possibly trim), the answer really depends on the purpose for the testing. Model testing of all vessel types at one of the well-known professional establishments is generally quite reliable, and testing at many academic institutions can also be perfectly acceptable for most purposes. Regardless of the precision that you need, there are some things you can - and should - do to insure that you fully understand the development of the results and their potential impact on the suitability of the testing to your project.
Test equipment - Every towing tank has its own personality. All mechanical equipment is slightly different, so it should come as no surprise that no two tanks will get exactly the same result. Generally, however, validation studies will have been conducted so that the staff at each tank will know what minor corrections to make to insure proper analysis.
You should always ask about the test equipment, and for any calibration documents. The measurement of drag and speed use devices such as load cells, force blocks, and tachometers. All devices suffer from some amount of precision error and drift over time. These devices must be calibrated periodically. It is obvious that the reliability of the testing is directly affected by the calibration.
The physical layout of the tank can impact the testing and should be reviewed. The maximum allowable size of the model is a function of the size of the tank. Smaller models will have a potentially greater error caused by the larger scale correction from model to full scale. Tanks which are too shallow or narrow require corrections to be applied. These are called blockage corrections.
Blockage correction - As a model moves through a tank, the limited water depth and width in the tank actually causes a channel or blockage effect, whereby the water needs to actively move around the hull as it passes. (Welcome again, Mr. Bernoulli.) This has the effect of increasing the relative speed between model and water. In other words, you need to apply a small increase in velocity to the measured speed to determine the effective model speed.
How important is the blockage correction of speed? First, remember that drag is a function of velocity squared. So, accurate speed is critical. You might be surprised to learn how little it takes to have a measurable effect on effective model speed. For this we look to the ratio of the model-to-tank sectional area. If the ratio is less than one-half percent, the effect is minimal. However, if it is greater than one percent, you will have a change in the effective speed which should not be ignored. For displacement hulls where the drag curve changes significantly with speed, this speed correction is important for a proper analysis and expansion to full scale. For planing hulls which are fully on plane, the drag curve can be relatively flat, so a blockage speed correction may be less critical.
Expansion technique - Virtually all towing tanks use the ITTC-1978 expansion technique or a variant of this technique. (The one notable exception to this is the U.S. Navy, which uses an older, traditional technique.) Most of these tanks submit reports to clients where the drag is already expanded to full scale, so you should ask for the expansion formula - including the correlation allowance and the form factor (if this was used). Use of the form factor is part of the expansion method, but not using this actually gives a slightly conservative answer. Correlation allowance, however, suffers from a somewhat hit-and-miss application. While contemporary formula are available, some tanks use a correlation allowance originally proposed some fifty years ago. Others don't use any at all. Often, the correlation allowance is used to compensate for the roughness of a model.
Model roughness and precision - It is hard to comment on the precision of the model, but of course, a more accurate model is better. The surface should be very smooth and consistent. It should have turbulence stimulators which conform to the tank's current practice.
Planing hull testing - Model testing of planing hulls presents a special set of challenges. Planing performance, as you know, is dependent upon the trim of the hull. In other words, drag and trim are inter-related, so the dynamic trim must be modeled as closely as possible. Virtually all contemporary models are allowed to heave and trim during testing. The trim of full scale planing hulls, however, are affected by lift and drag of appendages, wind, trim tabs, and most importantly, the shaft line thrust vector.
Some of the more advanced facilities have the ability to provide active thrust vector control to better represent the direction of forces. (Remember, towing is a horizontal process. Actively applying an upward force vector to a towed model is no small task.) In the bulk of planing hull testing, however, the hull is towed about an assumed center of gravity (CG). You need to remember that these results are applicable only to this case of a horizontal force vector through the CG. All other conditions will need to be evaluated numerically. (For example, you can apply this kind of thrust line correction using the aligned prediction technique in NavCad. You can also evaluate the effect of wind and appendages in a similar fashion.)
Appendages - Models are often towed with significant appendages, such as skegs or even rudders. It is important to note that the expansion of hull and appendages occur at a different scale. (Their Reynold's numbers are different, leading to different friction coefficients.) A numerical technique, called appendage stripping, can be used to numerically remove the appendage drag and derive an effective bare-hull drag. Then, the two components can be expanded at their own scale. Of course, the appendage drag is just a prediction, but validation studies have indicated that the expanded results are more accurate with this technique.
In closing, do not be afraid to ask whatever you feel is necessary to better understand how the tank developed the test results. After all, you are the one ultimately responsible for the performance of your design.
In 1984, HydroComp was formed and itís mission was to "provide naval architects and ship builders with resistance and propulsion analysis solutions of the highest technical excellence." Since then, HydroComp has developed a significant international presence providing technical and consulting services, commercial software, and a progressive internal R&D program. HydroComp staff is continually at the cutting edge of contemporary knowledge and expertise.
HydroComp is justifiably proud of twenty years of enthusiasm by its staff and dealers, dedication to its original mission, and the loyalty of more than 500 companies worldwide. Thank you for your continued support!
You can discuss product capabilities with HydroComp staff at these upcoming trade shows:
With the success of our recent propeller seminar in Florida, we have received numerous requests to hold another seminar later this spring on the West Coast and possibly Europe.
Typical seminars include user training for NavCad, PropExpert and PropCad, as well as for basic and advanced principles of marine propulsion. You do not need to have the software to participate in these seminars - especially on the first day, where the focus is on fundamentals and principles.
HydroComp tailors seminar topics to address the needs of a specific audience. They are designed to allow participants the opportunity to interact with the instructor, ask questions, and bring your own projects for discussion. In an effort to provide personal attention, we intentionally keep the number of attendees relatively low. If your company would prefer a customized seminar for your staff, contact us at .
We recently sent out surveys to determine interest in a seminar and if you received a survey from us, please take a moment to send back your reply to us as soon as possible. HydroComp is proud to support the Juvenile Diabetes Research Foundation (www.jdrf.org) and will donate $10 for every completed survey. If you wish to receive a survey, please contact us.
Also, please contact HydroComp if you are interested in joining us for a seminar this spring, or if you might be interested in hosting one of our seminars in your area.
|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.|
|About HydroComp||Knowledge Library||Ordering|
Home ¶ Software ¶ Consulting ¶ Knowledge ¶ Support ¶ Company ¶ Search ¶ Blog
Copyright © 2004 HydroComp, Inc.
Durham, NH USA. All rights reserved.
To contact us, email or call (603)868-3344.