Orca3D News

Sunday, September 12, 2010

Planing Analysis: Resistance Design Margin, and LCG, Porpoising, and Resistance

Question: When I run the planing analysis I am guessing at the Resistance Design Margin. Is there any way to more accurately estimate what that efficiency should be to compensate for appendage and wind drag? 
Answer: There are various ways to account for drag due to wind, appendages, and other factors such as waves. Some of these methods are purely empirical, while others may be classified as “semi-empirical” in that they are based on first principles but use some empirical data. A semi-empirical approach for estimating wind drag might be to compute the projected transverse area above the waterline (including hull above the waterline, pilot house, windshields, etc.) and use this together with the relative wind speed (including absolute wind speed and vessel speed) and an appropriate drag coefficient (such as might be found in Hoerners “Fluid-Dynamic Drag” or other technical papers). More fully empirical approaches might just recommend using a percentage of the bare hull drag whose value depends on the type of planing hull (e.g. center console vs sportfish vs cruiser).

Ideally you would have some past data for similar vessels to base your estimates on. If not, a few good references which you might find useful for guidance include:
  • http://www.hydrocompinc.com/knowledge/library.htm
  • Blount, D. & Fox, D., “Small-Craft Power Prediction,” SNAME Marine Technology, Jan 1976.Hadler, J.B., “The Prediction of Power Performance on Planing Craft,” SNAME Transactions, 1966.Hoerner, S.F., “Fluid-Dynamic Drag,” 1965.
Question: On my vessel the LCG is far enough aft that the planing analysis indicates that the vessel may porpoise. It seems that locating the ideal LCG so the vessel won't porpoise is determined by trial and error. Is this correct? Also, though the sensitivity index is less than 1, it recommends decreasing the LCG from transom which would only exacerbate the porpoising. How do I find the "happy middle ground"? 
Answer: Orca3D uses the HydroComp prediction library for the planing analysis including the porpoising stability check. Three different evaluation algorithms are used in the library [Savitsky, 1976][Celano, 1998][Colton, 1990]. The well-known Savitsky algorithm is a based on a limiting "critical trim" indicator, as is the Celano algorithm (an implementation of earlier work [Day, 1952]). The Colton algorithm uses a relationship between the center of pressure (relating to LCG) and a speed coefficient to indicate regimes of stability and instability. The planing analysis library evaluates all three indicators and presents the likelihood of porpoising as Stable (none indicate porpoising), Check (some indicate porpoising) or Unstable (all indicate porpoising). Given this background, it is difficult to say exactly how you would need to adjust LCG to achieve the desired result, so it does become somewhat of a trial and error situation. However, I would expect there to be some consistent (though perhaps not linear) trends. Often times the change in design parameters required to improve one aspect of performance, such as porpoising stability, is at odds with what would be needed to improve another aspect, such as resistance. Usually a compromise must be reached. 
Also, the sensitivity index for LCG is merely an estimate of the relative importance of LCG to the computed resistance and has no relation to porpoising stability. The sensitivity study just performs the resistance calculation repeatedly with small perturbations in each of the relevant input variables to determine their relative impact on resistance.