23-Apr-98
Text Box: Some general information on this spreadsheet: If you have any comments, find any mistakes, add any new features, or more data, etc., I'd appreciate hearing from you. My e-mail address is jholtrop@iwvisp.com. ********************************************************************************************************************************** * Input data for a new boat on the "DATA BASE" page, by opening "Data, Form, New" and filling in the blanks. * Filter the data by opening the criteria box at the top of a column heading and selecting the desired variable or range. The box's are turned on or off by selecting "data, filter, auto filter or show all" * Sort the filtered data base by selecting any cell in the data base, then open "Data, Sort", select the parameter (LOA for example), and "OK". * Lengths are in decimal feet, sail area is for 100% jib (.5*I*J+.5*P*E), and weights are in pounds. CALCULATED PARAMETERS DISP / LENGTH RATIO = disp./2240/(.01*lwl)^3 Dimensionless, if you ignore the constant "2240" that converts displacement from pounds to long tons. ".01" is another constant that scales the result. Probably the most used and best understood evaluation factor. Low numbers (resulting from light weight and long waterlines ) are associated with high performance. Depending on who you ask, cruising designs begin around 200 and can go up to the high 300's. Many racing designs are below 100. The general trend for new designs is towards lower ratios and higher performance. The trade off is more violent motion in storms, which requires constant attention to steering and sail trim, resulting in crew fatigue. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666 Dimensionless. "64" converts displacement. to cubic feet . This is basically a ratio of power to weight, calculated using a 100% jib. Most monohull designs range between 16 to 18. Racers can be much higher, motor sailors lower. The ratio is independent of boat length (see "chart" page). HULL SPEED = 1.34*lwl^.5 Dimensions of "Length" to the 1/2 power. An empirical formula, generally regarded as the highest practical velocity for a displacement boat ( in KNOTS ) assuming a reasonable power input (2-3 hp per ton). The higher the speed, the "longer " the hole the boat makes in the water. A short boat falls into this hole at lower speeds. An enormous amounts of power (50-100 hp / ton) is required to "climb out" of this hole and transition to higher speeds ( planing ). Large overhang (the difference between loa and lwl) helps by tending to make shorter boats appear longer, but interior volume is lost. VELOCITY RATIO = 1.88*lwl^.5*sail area^.33/disp^.25 / (hull speed) Sort of dimensionless (knots/knots). The numerator of the equation calculates potential maximum speed, using an empirical relationship. Boats with a generous sailplan and light displacement will have a velocity ratio greater than 1. Under powered or extra heavy boats will be less than 1. BALLAST / DISP = ball/disp Dimensionless. One indicator of stability, but the center of gravity, center of buoyancy Vs heel angle, and total weight is needed for a complete picture. Values range from a low of .25 to a maximum of .5. Another way to estimate stability is to divide the boat's roll period (seconds) by the beam (meters). Values less then 1 are "stiff". Values greater than 1.5 are considered "tender". LOA / BEAM RATIO = loa/beam Dimensionless. This ratio measures the fineness of the hull. Fine hulls, 3.0 - 4.0 and higher, are long and slender which promotes easy motion, high speed (low drag), and good balance when heeled. Newer designs favor wider hulls which have larger interior volume, sail flatter, and have high down wind speed potential. One note of caution when making comparisons, longer boats tend to be finer then short ones. This effect is plotted on the "chart" page. CAPSIZE RISK = beam/(disp/(.9*64))^.333 Dimensionless. An empirical factor derived by the USYRU after an analysis of the 1979 FASTNET Race. The study was funded by the Society of Navel Architects and Marine Engineers. They concluded that boats with values greater than 2 should not compete in ocean races. Values less than 2 are "good". The formula penalizes boats with a large beam for their high inverted stability, and light weight boats because of their violent response to large waves, which are both very important during violent storms. It does not calculate static stability. Some modern coastal cruisers and many racing designs have problems meeting this criteria. An interesting note, the study concluded that static stability was relatively unimportant in predicting dynamic capsize. Beam and weight were much more important factors. Wide boats give waves a longer lever arm to initiate roll and light weight boats require less energy to roll over. COMFORT FACTOR = disp/(.65*(.7*lwl+.3*loa)*beam^1.33) Dimensions of "Length" to the 2/3 power. An empirical term developed by yacht designer Ted Brewer. Large numbers indicate a smoother, more comfortable motion in a sea way. The equation favors heavy boats with some overhang and a narrow beam. These are all factors that slow down the boat's response in violent waves. This design philosophy is contrary to many modern "racer / cruisers", but it is based on a great deal of real blue water data, not just what looks good in a boat show. A value of 30 - 40 would be an average cruiser. Racing designs can be less than 20, and a full keel, Colin Archer design, could be as high as 60. BOAT COMPARISONS The spreadsheet can be used to compare or rank boats according to how CLOSE they fit seven "FUZZY" variables. These variables can be adjusted by changing their appropriate values in the INPUTS page. Suggested starting points for Offshore, Coastal, and Racing suitability are provided, along with three fuzzy hedges (levels of precision), SOMEWHAT CLOSE, CLOSE, and VERY CLOSE. Another option is to use a specific boat for the input "baseline" numbers, select a hedge level, and input the calculated values. This will return all boats similar to the baseline, which can be interesting if you want more information about a boat your not familiar with. For a boat to be "compatible" with the selected baseline every DOC (DOC is short for degree of compatibility, and is a number between 0 and 1, 1 meaning perfect compatibility, 0 meaning no compatibility) must be greater than zero. Linguistically this means the compatible boats have a compatible loa, and sa/disp ratio, and disp/lwl ratio, and comfort factor, and capsize risk, and velocity ratio, and loa/beam ratio. If any one DOC is 0, the design is not compatible with the inputs. When the variables on the INPUTS page are correct, press F9 and recalculate the spreadsheet. Then sort on the AND column (decreasing) and see what boats are FULLY compatible with your inputs (AND values greater than zero). High values indicate good compatibility. If you do not find any compatible boats try relaxing one or more of the hedges to SOMEWHAT CLOSE. If you find to many matches, trim them using the VERY CLOSE hedge. The values selected for the hedges (a percentage) is based on a somewhat subjective procedure, but it appears to work well. What I did was collect data from the following well known cruising boat designers; Alberg, Alden, Crealock, Brewer, Hess, Hood, Mason, and Perry. I then averaged the values they used in their designs (representing over 90 boats) and selected the high and low for each DOC variable. This is the maximum range that these designers allow. Dividing the range by the average value expresses the range as a percentage of the average. I then divided this percentage by "8" and declared that percentage to be "very close". "Close" is defined as twice that percentage, and "somewhat close" is four times the percentage. Additional details and results can be found at http://www1.iwvisp.com/jholtrop