Hi everyone, To thank Peter Brennan for his help and invaluable information on this fine boat, I want to share with everyone the following ratios that I have calculated on the boat. We are planning to ours around the world in about 18 month and are in the process of refitting the S/V StarShine to accomplish this (SSB, EPIRB, home made watermaker see the excellent article in good old boat issue 28, liferaft , add a second reef to our full batten main, set up a cutter rig, windlass, etc ...). Many people would not think to undertake such a voyage with this boat but after making all the calculations, I have found our boat to be similar in specification to some fantastic blue-water cruisers even if the boat is a little on the light side (According to John Holtrop's MS Excel vessel database our boat compares to some pretty impressive boats i.e. Valiant 40, Oyster 391, Malo 37, Malo 39, ... via a fuzzy logic selector, but I would take all this with a very large grain of salt!) . Finally before I put the numbers down for you, according to Nigel Calder, most if not all manufacturers use a form of light load when establishing displacement, therefore is would be reasonable to add about 2000 lbs to the 14,000 displacement. I have therefore provided both per spec numbers and adjusted (realistic) numbers for comparaison. Enjoy ! O'day 37 CC (1981 lead keel) Masthead Marconi Sloop LOA: 37 feet Mast: 47 feet, keel steepd LWL: 30'4" Beam: 11'2" Ballast: 6000 lbs Displacment: 14,000 lbs Draft: 5' Sail area: 594 sq feet SA/Displacement ratio: 16.4 Displacement/LWL ratio: 224 LOA/Beam: 3.3 Capsize Risk: 1.8 Ballast Ratio: .42 Hull Speed: 7.4 (According to Nigel, on a boat with a D/LWL of around 200 you can go up to 1.5 X lwl raised to .5) in this case the hull speed could be as high as 8,2 knots Vmax/Vhull (velocity ratio): 1.08 Moment of Inertia: 479,313 Period in seconds: 3.12 Stability Index: 0.92 Comfort Factor (Ted Brewer): 26.7 Period/Beam: 5.26 Roll Acceleration: 0.091 g (in gravitational force) What all this means is that the boat is a little light but because it has a narrow beam compared to new boats, it is very stiff and should provide a fairly seakindly behaviour in a seaway. The only part that I am still researching is the actual manufacturing process of the hull (what kind of glass and how thick) and how the deck is joined to the hull. This will tell me if my hull will withstand the constant pounding of 10-15 meter waves that I could encounter crossing the capes (just joking ;-) ). So here are the modified numbers LOA: 37 feet Mast: 47 feet, keel steepd LWL: 30'4" Beam: 11'2" Ballast: 6000 lbs Displacment: 16,000 lbs (added 2,000 lbs for gear and people) Draft: 5' Sail area: 594 sq feet SA/Displacement ratio: 15.2 Displacement/LWL ratio: 254 LOA/Beam: 3.3 Capsize Risk: 1.72 Ballast Ratio:3.3 Hull Speed: 7.4 Vmax/Vhull (velocity ratio): 1.05 Moment of Inertia: 605,002 Period in seconds: 3.5 sec Stability Index: 1.03 Comfort Factor: 30.6 Period/Beam: 5.93 Roll Acceleration: 0.072 g (in gravitational force) This makes the boat look at lot better. According to Nigel Calder, I am lowballing my weight estimates. Be careful though, we only have 1% of LWL of sinkage available which is 3 inches of play before the boat becomes dangerous. To sink the boat by one inch requires approximately 1143.3 lbs (pounds per inch of immersion=LWL X Beam at waterline X .70 X 5.33). You can read up on the different ratios in Nigel Calder's Cruising Handbook and the excellent site http://www.johnsboatstuff.com/Articles/estimati.htm by John Holtrop. The following is a direct copy from John Holtrop's fantastic web site to help explain some of the value of these ratios: 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 that favor higher performance. The trade off is that a light boat will exhibit more violent motion in a heavy seaway or storm. This 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 and 18. Racers can be much higher, motor sailors lower. The ratio is independent of boat length (see DATA PLOTS page). HULL SPEED = 1.34 * lwl^.5 Dimensions of velocity (knots). Derived from the speed of a wave under gravity forces, generally regarded as the highest practical velocity for a displacement boat assuming a reasonable power input (2-3 hp per ton). As a boat's speed increases, the wave it creates becomes longer, creating a trough that moves aft. At hull speed, the trough will be as long as the waterline length, creating a "hole" that the boat just fits. An enormous amount of power (50-100 hp / ton) is required to "climb out" of this hole and transition to higher speeds ( planing ). Boats with lots of "overhang" (the difference between loa and lwl) will appear longer as they settle into the trough, and have slightly higher hull speed than their lwl would indicate. Keel type, underwater shape, heel angle, and wetted surface will effect power requirements for a given speed, but have nothing to do with hull speed itself. VELOCITY RATIO = 1.88 * lwl^.5 * sail area^.333 / 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. A poor indicator of stability by itself, since details of the center of gravity, center of buoyancy Vs heel angle, and total weight are needed for a complete picture. Values range from a low of .25 to a high of around .5. LOA / BEAM RATIO = loa / beam Dimensionless. This ratio measures the fineness of the hull. Fine hulls, having ratios of 3.0 to 4.0 and higher, are long and slender which promotes easy motion, high speed (low drag), and good balance when heeled. Many newer designs favor wider hulls which have larger interior volume, sail flatter, and have high reaching and down wind speed potential. One note of caution when making comparisons, longer boats tend to be finer then short ones. (see CHARTS 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 (SNAME). 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 (low roll moment of inertia) to large waves, which are both very important factors during violent storms. It does not indicate or measure 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. This factor is related to LOA, longer boats have higher comfort factors, and is linear with respect to the roll period. MOMENT OF INERTIA ("I") = disp^1.744 / 35.5 Dimensions of lb.ft.^2. A rather simplistic empirical term developed by SNAME for the Fasnet race analysis. Large values resist rolling forces. The moment of inertia is very sensitive to the distance items are from the CG. A heavy rig can greatly increase "I" with little impact on displacement. ROLL PERIOD (T) = 6.28 * ((disp^1.744 / 35.5) / (82.43 * LWL * (.82 * beam)^3))^.5 The roll period is based on the moment of inertia, waterline length, and beam. The term (.82*beam) has been substituted for the waterline beam due to lack of data. Using (.82) results in a close match for the few boats with measured periods. Simply stated, a sailboat’s roll period, in seconds, is inversely proportional to its stability. Tender boats have long periods, stiff boats have short periods. The roll period is very easy to determine, you simply grab a shroud and push / pull until the boat is rocking over a few degrees. Then measure the time it takes for ten full cycles , and divide by 10. The general rule of thumb is that boats with periods less than 4 seconds are stiff and periods greater than 8 seconds are tender. The roll period is related to LOA and strongly related to COMFORT FACTOR. ROLL ACCELERATION = (6.28 / T)^2 * RADIUS * (ROLL ANGLE * 3.14 / 180) / 32.2 ( units of G's) In Marchaj's book, SEAWORTHINESS, THE FORGOTTEN FACTOR, chapter 4, "Boat Motions in a Seaway". The author presents a graph of roll acceleration ( in G's ) Vs four physiological states; Imperceptible, Tolerable, Threshold of Malaise, and Intolerable. Malaise starts at .1 G, Intolerable begins at .18 G. Spending much time under these levels of acceleration reduces physical effectiveness and decision making ability through sleep deprivation. The radius term assumes an off center berth located 1.5 feet inboard from the maximum beam. The roll angle is assumed to be 10 degrees. G levels above .06 are considered undesirable for offshore cruising conditions. Several light weight, beamy designs have G levels above .4, definitely "intolerable" for any length of time. PERIOD / BEAM = T * (g / beam)^.5 This is a dimensionless expression that is closely related to the disp./lwl ratio, and very closely related to the comfort factor. T is the period (seconds), g is the gravitational constant (32.2 ft/sec^2), and beam is in feet. Have fun !!!