Sailboat Parameter Calculations & Comparisons (English Units)

Description: This spreadsheet automatically calculates various ratios and other parameters allowing you to quickly compare characteristics between similar sailing boats. For valid results compare boats whose lengths do not vary more than two or three feet from each other (i. e. the 38' to 40' length range). Also the boats should have the same type of keel configuration (long keel, fin keel, ballasted, centerboard, etc.), same type of service (racer, cruiser, racer/cruiser).

There are several advantages and benefits to this calculative approach.  Some of these advantages are listed below.

• First this method saves time.  Many of computations are quickly generated behind the scenes on your input data.   Also to modify, simply change a value in the spreadsheet and it will automatically and immediately recalculates all affected values.

• Second this approach provides clear and neat documentation. 

• Third this approach is cost effective because the calculative approach is already developed for you, research time is minimized to the familiarization of concepts when necessary and not to time consuming development activities.

• Fourth this template is kept simple, it contains no Excel macros and there is no Visual Basic code utilized in it's creation.  Also advanced Excel features such "Goal Seek," "Solver" and "Scenarios" are not used.

• Fifth, because this is a spreadsheet and not a program, the users can easily modify it to suit their particular needs.

Electronic Document Type:  Microsoft Excel spreadsheet           Cost: $20 US funds
 

Number of Pages: Inputs/Outputs Sheet 1 page, Instructions Sheet 1 page, References Sheet 1 page, and Use Terms 1 page.                   

Inputs:  

• Length Over All, LOA, feet (for hull only, does not include pulpits etc.)

• Length on Waterline, LWL, feet

• Hull Beam, Maximum, B, feet (for hull only, does not include rub rails, etc.)

• Weight or Displacement, D, pounds, try to get the same weight condition for all sailboats, whether light, half load or fully loaded

• Ballast Weight, pounds, in keel and in hull. (Often not available, input as "NA" if this is the case)

• Sail Area, SA, square feet, area of fore triangle, plus area of main and half the mizzen if applicable

• Wetted Surface of Underwater Portions of the Hull, square feet, for condition under evaluation.  Include the keel and rudder wetted surfaces with this amount.  (Often not available, input as "NA" if this is the case)

• Weight Density of Water, g, pounds per cubic foot (64 for salt water, 62.4 for fresh water)

Outputs:  For each set of inputs the following outputs are automatically generated by this template:

• Displacement Length Ratio, where DLR = (D/2240) / (LWL/100)³, long tons / cubic feet. Generally the vessel with the lower value will be the faster vessel. But ideal values depend on the speed length ratio that the vessel is operating at. Design lanes for DLR as a function of speed length ratio are given in References B-B, B-J, B-O and B-R. The other references may also mention optimal values for a given speed.

• Sail Area Displacement Ratio, where SADR = SA / (D/g)^2/3. The SADR is a measure of the power available to push the load (the displacement). Generally, the higher the value the faster the boat, provided the boat is stiff enough to handle the larger sail areas. Reference W-E, www.sailingusa.info, states Cruising Boats have ratios between 10 and 15, Cruiser-Racers have values between 16 and 20, Racers have ratios above 20 and high performance racers have ratios above 24.

• Length Beam Ratio, where LBR = LOA / B. High values indicate large form stability, faster speeds (if light boat) and larger interior volume. Low values indicate gentler motions and normally safer blue water performance.

• Ballast Ratio, where BR = Ballast / D. This is an indicator of stability, but it is not a very accurate one. This is because this ratio does not differentiate between bulb ballast at the bottom of the keel and ballast in the fin keel or hull. Since the location of the ballast is not taken into consideration only boats with similar ballast arrangements should be considered. The sail boat ballast weight may be available for comparison with other vessels. Reference W-E, www.sailingusa.info, states the average value is 0.35. Another source says that a value of 0.33 is average and a value of 0.40 is considered quite stiff.

• Hull Speed or Max. Displacement Speed, where V = 1.34 x LWL^1/2, knots. This value is based on a speed length ratio (SLR) equal to 1.34. This SLR is considered the maximum that a hull can go in the displacement mode. At this speed length ratio the length of the wave generated by the hull is equal to the length of the hull. However, this barrier speed does not apply to all sailboat hulls. Hulls that are very light with flat exit angles and wide beams may exceed this value and go into semi-displacement or semi-planing or even planing mode. Also hulls that are very narrow and light, like racing catamarans, can exceed this speed.

• Sail Area Wetted Surface Area Ratio, where SA / WS. Reference A-B (Brewer by the Numbers, by Ted Brewer) states: The sail area/wetted surface area ratio is simply the sail area divided by the area of the hull that is below the LWL. It should include the keel and rudder areas. This frequently neglected ratio is the major determiner of boat speed in light and medium air. In these conditions, wave-making resistance is minimal and surface friction is the primary drag component. Ratios below 2.0 indicate poor performance in light air. Ratios of 2.2 to 2.4 predict good light-air performance, while a ratio of 2.6 would indicate a boat designed specifically to sail in very light wind. Wetted surface is a difficult number to obtain, but the concept is important.

• Capsize Factor, where CF = BOA / (D/g)^1/3. This parameter is an indication of a vessel's ability to resist capsizing in a violent storm. Reference W-A, design.htm and estimati.htm (by John Holtrup) states: This is a seaworthiness factor derived from the USYRU analysis of the 1979 Fastnet Race, funded by the Society of Naval Architects and Marine Engineers. Values less than 2 are good. This formula penalizes wide boats for their high inverted stability and light boats because their violent response to large waves. This value does not indicate or calculate stability. 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; both undesirable attributes in a cruising boat. All multihulls, some modern coastal cruisers and many racing designs have problems meeting this criteria.
  Reference A-B (by Ted Brewer) states the following: Capsize Screening Ratio (CSF): Some years ago, the technical committee of the Cruising Club of America came up with a simple formula to determine if a boat had bluewater capability. The formula compares beam with displacement, since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. The formula is the maximum beam divided by the cube root of the displacement in cubic feet, or B/ DISPL cf. The displacement in cubic feet can be found by dividing thedisplacement in pounds by 64, of course. The boat is acceptable if the result of the calculation is 2.0 or less but the lower the better. For example, a 12-meter yacht of 60,000-pound displacement and 12-foot beam will have a CSF number of 1.23, so would be considered very safe from capsize. A contemporary light displacement yacht, such as a Beneteau 311 (7,716 lb, 10-foot 7-inch beam) has a CSF number of 2.14, and a Dufour 38 (14,300 lb, 12-foot 7-inch beam) comes in at 2.07. Based on the formula, while they are fine coastal cruisers, the latter two yachts may not be the best choice for ocean passages.

• Comfort Ratio, where CR = D / [0.65(0.7LWL + 0.30LOA)B^1.333]. See References A-B by Ted Brewer and Reference W-A, design.htm by John Holtrup for more details on this parameter. This term was developed by yacht designer Ted Brewer. Large numbers indicate a smoother, more comfortable motion in a seaway. This parameter favors heavy boats with plenty overhang and narrow beam. These factors slow down a boats response in heavy weather. Higher values indicate steadier motion in ocean waves and reduced crew fatigue. These characteristic are contrary to many modern racers and racer / cruisers. This criteria is based on real blue water experience and not what just looks good in a boat show. For offshore cruising the minimum value recommended for this parameter is 25, the optimum is in the range from 30 to 40, and the maximum recommended value is 50.

Recommended Reading:

• Books

  • Reference B-A: Arthur Edmunds, Designing Power & Sail, page 193, 1998, Bristol Fashion Publications, Harrisburg, PA.

  • Reference B-B: SNAME, Principles of Naval Architecture, Volumes I and II, 1988, Society of Naval Architects & Marine Engineers, Jersey City, NJ

  • Reference B-C: Dave Gerr, Propeller Handbook, International Marine, 1989, Camden, Maine.

  • Reference B-D: C. A. Marchaj, Seaworthiness, the Forgotten Factor, Chapter 4 - Boat Motions in a Seaway, 1986, International Marine, Camden, Maine.

  • Reference B-E: Edward M. Brady, Marine Salvage Operations, Cornell Maritime Press, 1960, Cambridge, Maryland.

  • Reference B-F: Dave Gerr, Nature of Boats, International Marine, 1995, Camden, Maine.

  • Reference B-G: Howard I. Chapelle, Yacht Designing and Planning, 1971, W. W. Norton & Company, Inc., New York, NY.

  • Reference B-H: Norman L. Skene and Francis S. Kinney, Skene's Elements of Yacht Design, 1973, Dodd, Mead & Company, Inc., New York, NY.

  • Reference B-I: Juan Baader, The Sailing Yacht, Second Edition, 1979, W. W. Norton & Company, Inc., New York, NY.

  • Reference B-J: Lars Larsson and Rolf E. Eliasson, Principles of Yacht Design, Second Edition, 2000, International Marine, Camden, Maine.

  • Reference B-K: Pierre Guttelle, The Design of Sailing Yachts, 1984, International Marine Publishing Company, Camden, Maine.

  • Reference B-L: Robert G. Henry & Richards T. Miller, Sailing Yacht Design, 1965, Cornell Maritime Press, Inc., Cambridge, Maryland.

  • Reference B-M: K. Adlard Coles & Peter Bruce (editors), Adlard Coles' Heavy Weather Sailing, 30th edition, Chapter 2 Sailing Yachts in Large Breaking Waves, pages 11-23, International Marine, Camden, Maine.

  • Reference B-N: C. A. Marchaj, Sailing Theory and Practice, 1964, Dodd, Mead & Company, New York, New York.

  • Reference B-O: C. A. Marchaj, Aero-Hydrodynamics of Sailing, 1979, Dodd, Mead & Company, New York, New York.

  • Reference B-P: Douglas H. C. Birt, Sailing Yacht Design, 1951, Robert Ross & Co. Limited, Southampton, UK.

  • Reference B-Q: Andrew G. Hammitt, Technical Yacht Design, 1975, Van Nostrand Reinhold Company, New York, New York.

  • Reference B-R: D. Phillips-Birt, The Naval Architecture of Small Craft, 1957, Hutchinson & Company, London, UK.

• Papers

  • Reference P-A: Robert G. Henry and Richards T. Miller, Sailing Yacht Design - An Appreciation of a Fine Art, pages 425-490, SNAME Transactions, Volume 71, 1963 issue, Society of Naval Architects & Marine Engineers, Paramus, NJ.

  • Reference P-B: Richards T. Miller and Karl L. Kirkman, Sailing Yacht Design - A New Appreciation of a Fine Art, pages 187-237, SNAME Transactions, Volume 98, 1990 issue, Society of Naval Architects & Marine Engineers, Paramus, NJ.

• Articles

  • Reference A-A: Ted Brewer, Is Your Boat Stable?, http://www.boatus.com/goodoldboat/stability.htm, Article
    from Good Old Boat magazine: Volume 3, Number 2, March/April 2000.

  • Reference A-B: Ted Brewer, Brewer By the Numbers, www.boatus.com/goodoldboat/brewerformulas.htm, Article
    from Good Old Boat magazine: Volume 3, Number 2, March/April 2000.

  • Reference A-C: Roger Marshall, Design By the Numbers, Motor Boating & Sailing magazine: September 1981.

  • Reference A-D: Roger Marshall, Design Calculatons, The Design Process Part III, Article from Boatbuilder Magazine, November/December 2004.

• Web Sites

  • Reference W-A: John Holtrup, Several articles including: "Design Basics," Fuzzy Logic," "Estimating Stability," "Plots from Data Base," "Dynamic Stability," and "Best Offshore Cruising Boats" - updated 20 June 2000," www.johnsboatstuff.com/technica.htm.

  • Reference W-B: Michael Kasten, "Sail Area Ratios," http://www.kastenmarine.com/sail_area_ratios.pdf, 2001.

  • Reference W-C: Dan Pfeiffer, "Sailboat Design Ratios," http://dan.pfeiffer.net/boat/ratios, 2003.

  • Reference W-D: SailingUSA.info, "Angle of Vanishing Stability," http://www.sailingusa.info/cal_avs.htm and formulas.htm, 2001.

  • Reference W-E: SailingUSA.info, "Keelboat Course - Design & Stability," http://www.sailingusa.info/design_winds.htm, 1999 - 2002.

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Minimum System Requirements: Windows 95/98/NT/2000/XP/Vista/Windows7

Sample: A sample of an output page is shown below.