Boat Information Library

The
file attached should be an HTML spreadsheet with the design parameters and
calculations for Capsize Ratio and some other good ratios for some boats between
30 and 41 feet, with the F210 & F235 added by request. The 30 to 40 foot
range seems to be the most critical for the Capsize Ratio since the beam is
pushed out to get more room in the boat. Most of these parameters come from the
design basics article on the J. Holtrop website and are defined at the end of
this section.

I have added some columns on the right to show the
stability calculations mentioned in Yachting World. These came from the Wolfson
Unit at Southampton U. The only term not easily available to the boat owner is
DCB(depth of canoe body. You would need the lines of the boat to measure it, but
I have gotten pretty good correlation by using the a value of 0.35 for the block
coefficient(CB). You can estimate DCB in feet with the following
equation.
dcb = displacement/(beam x lwl x BC x 62.2)
Multiply by 0.304 to
convert to meters.
The equation for the stability range is

Range =
110 + 400/(beam^2/[BR x DCB x (DV^.333) - 10])

BR =
ballast/displacement

DV = displacement/1025

All of the units must
be in the metric system.

Based on stability curves that I have seen
published in boat tests, the equation overpredicts the range by about 5 degrees
for boats in the 110 to 130 range and even more for the "traditional/classic"
design, e.g., Island Packets, Cabo Rico etc. The Contessa 32, which was one of
the benchmnark boats in the Yachting World article has a range of 157 deg
compared to the 179 in my table. Obviously 179 is not realistic.
Using the
design data for my boat, an Oceanis 370 and some boat design equations, I was
able to generate a stability range plot. Righting arm Gz in feet is plotted
against heel angle. The first plot shows the range for the basic hull(solid
line) with the spec displacement. The Gz comes from the form of the hull. The
dotted line shows what happens when you add the ballast at the correct distance
below the waterline.

The second plot starts with the hull + ballast curve
and starts to distribute the displacement like a real boat. First the weight of
a conventional rig with non furling mainsail was added, which raises the center
of gravity and reduces stability - a pound up high is worth many pounds below
the water line. Then for those with furling mainsails(me) another 50 lbs was
added(lower dotted line)and then another 50 lbs was added(dashed line) to see
how sensitive the stability was to rig weight. Finally 1000lbs of cruising junk
was put aboard to get the lowest solid line. In this case it looks like the
furling main does not have a big effect. If the boat some boat design equations,
I was able to generate a stability range plot. Righting arm Gz in feet is
plotted against heel angle. The first plot shows the range for the basic
hull(solid line) with the spec displacement. The Gz comes from the form of the
hull. The dotted line shows what happens when you add the ballast at the correct
distance below the waterline.

The second plot starts with the hull +
ballast curve and starts to distribute the displacement like a real boat. First
the weight of a conventional rig with non furling mainsail was added, which
raises the center of gravity and reduces stability - a pound up high is worth
many pounds below the water line. Then for those with furling mainsails(me)
another 50 lbs was added(lower dotted line)and then another 50 lbs was
added(dashed line) to see how sensitive the stability was to rig weight. Finally
1000lbs of cruising junk was put aboard to get the lowest solid line. In this
case it looks like the furling main does not have a big effect. If the boat is
under power, the difference in rigs will be greater because the classic main
comes down, the CG of the furling main stays high.

I also did the
plot for filling my starboard water tank with 45 gallons of water. If it is on
the windward side it acts like water ballast as the boat heels and increases
peak Gz. If you get rolled, you want water in one tank because it reduces the
inverted stability some.

Since most of the Oceanis hull shapes are
similar, the plots for other Beneteaus should have a similar range, but the peak
Gz may be higher for bigger boats, or if the ballast to displacement ratio is
greater.
Design Basics

back to technical
articles

The following ratios and factors are commonly used by
naval architects and boat designers to help them predict how a specific design
will perform. The selected values represent those found on a typical offshore
cruising boat, where safety and comfort are of primary importance.

In
addition to the definitions below, plots showing how these factors relate to the
database boats can be found at the Data Base Plots page.

DISP /
LENGTH RATIO = disp/2240/(.01*lwl)^3 Probably the most used and best understood
boat evaluation factor. Low numbers (resulting from light weight and long
waterlines ) are associated with high performance and quick response. Cruising
designs begin around 200 and can go up to the high 300's. Many racing boats are
below 100. My approach is to start with a minimum of 230, optimal of 280 - 320,
and maximum of 370. This will give us boats with a nice blend of weight (for
good load carrying capability and seaworthiness) and reasonable performance. My
numbers may seem high to some, but they are in agreement with the experience of
many blue water sailors.

SAIL AREA / DISP RATIO = sail
area/(disp/64)^.666 This is a ratio of power to weight, calculated using a 100%
jib. Most monohull designs range between 16 - 18. Racers can be much higher,
motor sailors lower. We want a cruiser with enough power to sail well but not so
much that the crew is fatigued by constant sail changes or worried about having
a fragile oversize rig. I selected 14 as the minimum, 15 - 17 optimal, and 18
maximum.

VELOCITY RATIO = 1.88*lwl^.5*sail area^.33/disp^.25 /
(1.34*lwl^.5) The numerator of this equation is an empirical formula that
relates high speeds to a long lwl, large sail area, and small displacement. The
denominator is the traditional Hull Speed term. Their ratio is a measure of how
well our ideal cruising boat will perform under sail. A well designed boat
(adequate sail area and light weight hull) will have values are between 1 and
1.1. All out racing machines will be as high as 1.8. I selected 1.0 as the
minimum, 1.04 - 1.08 as optimal, and 1.14 maximum. One should never under
estimate the pleasure, and sometimes safety, of owning a fast
boat.

CAPSIZE RISK = beam/(disp/(.9*64))^.333 A seaworthiness factor
derived from the USYRU analysis of the 1979 FASTNET Race, funded by the Society
of Navel Architects and Marine Engineers. Values less than 2 are good. The
formula penalizes wide boats for their high inverted stability and light weight
boats because of their violent response to large waves. All multi hulls, some
modern coastal cruisers and many racing designs have problems meeting this
criteria. Since safety is a very important feature in a cruising boat, I
selected values less than 1.8 for full credit Anything over 2 scores
zero.

COMFORT FACTOR = disp/(.65*(.7*lwl+.3*loa)*beam^1.33) This is 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 lots of overhang and a narrow beam. These are all factors that slow down a
boats response in violent conditions, which is a major factor in reducing crew
fatigue. This design philosophy is contrary to many modern racer / cruisers. A
value of 30 - 40 is recommended for a cruising boat. Racing designs are
typically less than 30, and a full keel, Colin Archer design, could be as high
as 55. Ted?s recommendations were used for the optimal values, with a minimum of
25 and a maximum of 50.

LENGTH TO BEAM RATIO = loa / beam This ratio
appears to be lower now than in older designs. Wide boats are popular today
since they provide form stability and large interior volume. On the downside,
excess beam contributes to poor balance, distorted heeled waterlines, and high
inverted stability. Since stability and volume are not so difficult to achieve
in larger boats, the L/B ratio increases with LOA. (plot of L/B Vs
LOA)

Title:

Boat Data for Capsize
Calculations

Model(s):

Any Sailboat

System(s):

Safety

Author:

John E Lueke

Copyright:

This document Copyright (c) 1999 by author(s)
above, all rights reserved. Reprinted here by permission for
non-commercial, personal use only. May not be redistributed without
express written permission of the author(s).
This notice applies only to this specific document
and not any other materials (web pages) to which this document may
link.

Disclaimer:

Opinions expressed here are solely those
of the authors of the document. This information is provided for
recreational purposes only and the need for due diligence to consider the
results of acting on this information rests on the user. No expertise or
suitability to any purpose is implied, use this information at your own
risk.

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