Sailboat Parameter Calculations & Comparisons
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,
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
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
$20 US funds
Pages: Inputs/Outputs Sheet 1 page, Instructions
Sheet 1 page, References Sheet 1 page, and Use Terms 1 page.
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)
For each set of inputs the following outputs are automatically generated
by this template:
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.
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.
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
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
or Max. Displacement Speed, where V = 1.34 x LWL1/2,
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
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.
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.
where CR = D / [0.65(0.7LWL + 0.30LOA)B1.333].
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.
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,
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,
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,
Reference B-P: Douglas H. C. Birt,
Sailing Yacht Design, 1951, Robert Ross & Co. Limited, Southampton,
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,
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.
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.
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,"
Reference W-B: Michael Kasten, "Sail
Area Ratios," http://www.kastenmarine.com/sail_area_ratios.pdf,
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
Reference W-E: SailingUSA.info, "Keelboat
Course - Design & Stability,"
http://www.sailingusa.info/design_winds.htm, 1999 - 2002.
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