Air Date December 11, 2012
At IBEX 2012 in Louisville, Tim Coats (Carderock Naval Surface Warfare Center) took part in a panel discussion about Shock Mitigation. For this week’s show, Coats visits ProBoat Radio to continue this campaign of educating both industry and academia about this topic.
To listen to the archived recording of this show, click here.
Tim, you’ve been briefing for several years on a hot and popular topic; that is the topic of Shock Mitigation, and even most recently at the International Boat Builder’s Exhibition and Conference (IBEX) in Louisville, KY, 4 October. At IBEX you seemed particularly intent on sharing information that you wanted industry and academia to understand about this topic, and your passion for this was clearly visible. Where does this passion for Shock Mitigation come from?
Answer…Basically it’s about mission effectiveness and safety for those who serve our country operating and riding high-speed planing craft/boats, whether in a foreign land or in the littorals within our own borders.
Operating or riding a high-speed planing boat is a hazardous occupation. There was a self-reported injury study (Ensign, 2000) of the SWCC community. It found that 65% of responding operators sustained a boat-related injury, 89% of which occurred in the first two years of operation, and 33.6% of which were to the lower back. I received this data from Eric Pierce of NSWC Panama City, so if you want more or recent data he is your POC. But the point is that these boats are fast and if you’ve never ridden one of these boats at 40+ knots in six-foot seas it is really hard to convey to you how this feels. It can be violent.
So, it should be clear for the purposes of this discussion, I am concerned about the operators and passengers that occupy these craft. Furthermore, there seems to be a general lack of understanding of the fundamental physics associated with severe craft motion. So my pursuit has not been focused so much on a technology, but a phenomenology (study of the phenomena) associated with severe wave impacts (call them wave slams) for the purpose of documenting the fundamental physics and providing that information to the COI so they can make educated and informed decisions on their technology developments, whether we are talking about hull structure, ride control, or seats. Furthermore, this phenomenology also provides insight into testing, data analysis, and knowledge for the smart buyer from an acquisition point of view.
So, what have you learned from this phenomenology? That is, if you could leave one golden nugget for the audience today based on what you have observed in your work, what would it be?
Answer…can I leave 2? Craft responses in rough water are not truly random, and the accelerations of interest with respect to crew safety cannot be summarized to a single number like“20g impacts”.
OK, well now you’re going to have to explain that, and do those two nuggets have anything to do with each other? And wait a minute. I am certain I have heard a time or two, someone talking about some pretty high Gs on these craft. Your “20g” comment there conveys some doubt on your end.
Answer…let’s start with the ‘non-random craft response’ nugget first. In a nutshell we understand from data analysis that no matter what the sea state, or the speed and heading, each slam event can be mathematically modeled by a half-sine impulse. This is particularly useful when certifying equipment ruggedness or the shock attenuation capability of a shock mitigating technology. That brings me to the second nugget, my criticism of the claims made concerning 20g impacts. This so-called 20g is a result of a number of factors; placement of accelerometers, post-processing acceleration data, and interpretation of the data. ”I experienced a 20g slam”, or “my shock mitigating technology reduced a 20g slam by x-percent” are common statements heard today and quite frankly they are a fallacy…argumentum ad populum…everybody is adopting the phrase and many believe it to be so.
You seem pretty dogmatic there with that last phrase. Would you care to clarify?
Answer…Yes. What I am talking about is the fundamental differences between the forces that are of interest to crew safety and structural vibration; impulse loads compared to high-frequency responses manifested as millimeter vibrations. This is fundamental engineering mechanics, but not necessarily easy to apply in craft motion data collection and analysis. Please bear with me as I try to explain this from two points of view. First, a historical perspective, and then I’d like to address this with some very simple kinematics and Newtonian mechanics.
So first some history…Naval Architects should recall that Savitsky in 1964, reported on some model testing using the prismatic planing hull… constant deadrise, constant beam, and constant trim for the entire wetted planing area. What is important about this is that these models were rigid wood models, essentially rigid blocks of wood that would exhibit little, if any high frequency vibration and certainly none that would be detected by the analogue sensors used in 1964. Second noteworthy point is that during these times data acquisition did not afford one the luxury of capturing very high frequency (low displacement, low energy) vibrations that can occur during a wave impact. Therefore, it is no wonder the works in the open literature through the late 1970s focus on accelerations associated with rigid-body mechanics and not vibration. Whether or not they determined from first principals that it was proper to exclude vibration content really does not matter, the fact is they were constrained by their model fidelity and limitations of the data acquisition. The first principles arguments would come later.
An example to illustrate the point comes from a general guidance for selecting vertical accelerations for structural design, where Savitsky and Koelbel in the early 1990’s include personnel effects where they suggest that 3.0 g impact accelerations produces extreme discomfort, and physical injury may occur at approximately 5.0 g. If during a time leading up to the 1990’s a 3.0 g impact was thought to cause extreme discomfort, why do we now have reports of passengers surviving 20 g impacts? Are there any Naval Architects out there using 20g as a load factor in their design process? Is anybody getting an ABS certification for 20g? The answer is that during that time the data acquisition provided mostly acceleration response at lower frequencies, whereas now with sensors that have significant increases in frequency bandwidth, the acceleration record includes very minute structural deflections vibrating very rapidly.
During a wave slam, the deck may experience simple harmonic motion, and in this case the first or primary mode should result in maximum vibration displacements. As the deck plate vibrates up and down very quickly, the accelerometer is basically on a sinusoidal ride, a trampoline so-to-speak, going up and down very quickly. I have data showing prominent modes at 25 Hz, meaning that accelerometer was being moved up and down as fast as 25 times per second. As the accelerometer is moving up, it is decelerating until its velocity is zero when the deck plate stops to reverse direction. At that moment the velocity is zero but the acceleration has reached maximum. And this continues extremely fast until the vibration dampens. Here is the relationship between the displacements, velocities, and accelerations. Classical dynamics tells us that the velocity is the first derivative of displacement with respect to time, and so the resulting velocity sinusoid is the first derivative of the sine curve representing the displacement. A second derivative will provide the acceleration. Therefore, acceleration is simply the first derivative of the velocity sinusoid, or to put it another way, (VERY IMPORTANT) it is the instantaneous slope at any given point along the velocity curve. This is what one measures with an accelerometer. So, one should understand that significant accelerations can be measured from vibrating structure with no significant momentum, meaning that little energy may be associated with the response even though the accelerations (or “Gs”) seem high.
Conclusion…the 20g is likely a vibratory response, and most of its content carries very little energy.
So how does one arrive at the “right answer”? In other words, how are you analyzing the data that perhaps others are not?
Answer…Well, I should say I believe many are properly looking for the low frequency response, but the efforts are inconsistent across the community rather than collaborative. We have developed primarily for Naval Architects an approach to identifying the peak accelerations of interest. We call this approach “Standard G”. A number of people have worked on a computer code that performs the StandardG process, but our latest algorithm was programmed by a Virginia Tech Professor, Leigh McCue, and we currently have this code available to all upon request. I call this our beta test. We, the Combatant Craft Division have partnered with the USNA in holding and distributing this code. Professor John Zseleczky is our configuration manager. If one would like to use the code, just contact John and he will send our StandardG package that includes the executable code and associated documentation. Of course one can always call or email me for further clarification if needed.
Basically, StandardG can be described in three main steps…The first step after demeaning the data is to filter it, keeping in mind that a filter frequency should be chosen after a careful consideration of a Fourier spectrum of the data, and if it is the lower-frequency content that is important, the data should be filtered accordingly. For the class of boats we’ve studied to date under this wave-impact phenomenology, I have found that a low-pass filter frequency of about 10 Hz will filter out the vibration leaving only the acceleration amplitudes of interest associated with the wave-slam event…keep in mind I’m talking about deck accelerations.
The identification and quantification of the peaks should be based on physics rather than numerical methods such as a zero-crossing algorithm which is a numerical method that can result in numerical artifacts. So after the data has been demeaned and filtered to reduce the data, then the second step is to compute the Root-Mean-Square (RMS), the significance of which is the value of an acceleration record that correlates well with the lower amplitude values associated with forces due to buoyancy, hydrodynamic lift, drag, and gravity. So physically speaking, any peaks associated with these phenomena are considered irrelevant so data points equivalent to or less than the RMS are discarded.
The third step involves a wave-encounter period criterion. The average time period between wave slams can be shown to be on the order of 0.5 seconds or greater for speeds up to 50 knots and significant wave heights greater than about a half meter. The majority of the peak accelerations caused by wave-slam events will therefore be greater than 0.5 seconds apart, and so it is recommended that a horizontal time threshold be set equal to 0.5 seconds. In terms of an algorithm, check to see if any given peak is the largest within one time threshold of where it is, discarding all peaks in that ½-second window except the largest. This will ensure that the peak acceleration associated with each significant wave slam will be extracted by the algorithm.
So using this “StandardG” provides the correct G-loads?
Answer…yes and no. Using StandardG consistently in this manner results in a database of peak accelerations that correlate directly to the impulse load associated with the craft impacting the water. However, these are not “G-loads”. They are simply accelerations. Remember, like I said a few moments ago, according to classical dynamics acceleration is the instantaneous change in velocity with respect to time with units of distance per time-squared and is solely a structural response caused by the shock pulse (or impulse). Simply normalizing the acceleration (ft/s2) by the gravitational constant (32.2 ft/s2) does not somehow turn the acceleration into a load. Referring to these accelerations as G-loads is not consistent with classical engineering mechanics. A ‘G-load’ is that acceleration that creates a quasi-static load on a known mass as some multiple of the gravitational constant. Local transient vibration as we measure on a deck or a seat on a boat does not produce this kind of quasi-static load.
Fighter pilots and dragsters, for example, will experience a “g-load”; a quasi-static load on their body giving them the sensation of being “pressed” hard into their seat. Similarly, a centrifuge at an amusement park will produce such loads. This is the ride where people stand against a wall of a spinning cylinder and the floor drops out from underneath leaving the riders “forced” to the wall by what we may call a “g-load”. Some might refer to this as a centrifugal force. In order to lift one’s arms, legs, or head off the wall, they would need to exert a centripetal force greater than the “g-load”. These examples are not at all the same phenomena associated with wave slams. So it should be clear that wave-slam accelerations are not g-loads. So you can’t just pull the acceleration time-history from the deck of a craft, multiply it by craft mass and claim a load.
Ok, before we run out of time, let’s try to wrap up with a clear and deliberate recommendation to our listeners on the application of this Standard G. You noted we can get a copy of Standard G and so we don’t have to write our own code. Can you give us the POC information, and what software do we need to run the code? And once we have a functioning Standard G, and we compute meaningful peak accelerations associated with the wave impact rather than the higher frequency vibrations, how would they be used?
Answer…John Zseleczky is our StandardG configuration manager. Simply email him with StandardG somewhere in the subject line. His email address is email@example.com. You will need a free software download called ‘OCTAVE’. You can also run this quite easily using MATLAB.
The code was intended to provide A1/n values compatible for Naval Architects worldwide. But as part of the process it provides an objective identification of the most relevant acceleration peaks. This code (and process) can also be used for new ride quality approaches…also for computing acceleration peaks used in injury models…or if you need to perform validation of seakeeping and CFD codes…
Maybe one of these days soon I’ll try to hold an industry day at Carderock for any and all who would like a tutorial through the process to make sure we all understand how to use and apply it.
NOTE: If you have questions about this topic, please send to firstname.lastname@example.org and we will post the responses here at a later date.