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FAQ: Measures of Seaworthiness?

 

How does the C34 come off in terms of quantitative measures of seaworthiness, such as sail area/displacement and capsize risk?

Cruising World (April, 1998) published an interesting article by John Holtrop ("Crunching Numbers for a Quality Cruiser"). The article outlines a quantitative methodology for assessing the seaworthiness of a cruising yacht for blue-water crusing. In what's appended, I've applied Holtrop's approach to my C34 (tall rig, wing keel, '88) and interpreted the numbers. Please note that Holtrop's definition of optimum and acceptable ranges reflects his admitted bias toward heavy, narrow cruising yachts with considerable loa/lwl overhang.

In brief, this analysis suggests that the Catalina 34 is a fast, "stiff," and safe yacht for coastal cruising, and can probably handle offshore use, but may not be as comfortable for prolonged sailing in heavy seas as yachts designed specifically for blue-water cruising. (These yachts would, in turn, be quite inappropriate and uncomfortable for coastal cruising.)  Once again, note that Holtrop's analysis embodies assumptions which, by his own admission, are not universally held. This is just a starting point for understanding and comparing cruising yachts.

The following table lists the C34's measurements, and gives Holtrop's calculated measures of the vessel's seaworthiness. You'll also find my interpretation of the data.


Measured Characteristics of the C34

LOA

34.5

LWL

29.9

BEAM

11.8

DRAFT

4.2

DISPLACEMENT

12550

BALLAST

5600

SAIL AREA (100%)

554


Calculated Characteristics of the C34

Parameter (click to see an explanation) Catalina 34 Optimum Range for Blue-Water Cruising Acceptable Range for Blue-Water Cruising Interpretation

DISP/LWL

210

278-323 233-368 Outside of "acceptable" range, but reflects Holtrop's biases (see his article for an explanation of these)

SA/DISP

16.47

15.34-16.66 14.01-17.99 Optimum

BAL/DISP

0.45

Less than 1 (stiff) Greater than 1 (tender) Optimum
Vm/Vh

1.09

Greater than 1 (fast) Less than 1 (sluggish) Fast

CAPSIZE RISK

1.96

Less than 1.8  Less than 2.01 (is "Good") Good

COMFORT

23.3

30.6-39.4 21.9-48.1 Within acceptable range

L/B

2.94

3.0 and higher (fine hulls, long and slender) Less than 3.0 (beamier hulls, better downwind, sail flatter) Close to the optimum

V HULL

7.33

(theoretical hull speed)    

Measures of Seaworthiness

--J. Holtrop

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. 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 high performance. The trade off is more viloent 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

HULL SPEED = 1.34*lwl^.5 Dimensions of Length to the 1/2 power.

Another 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

Dimensionless. 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 devide 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 interor 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.

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 initate 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.


Important: The opinions expressed here are those of the individual contributors to this page, and not those of the Catalina 34 National Association or Catalina Yachts, Inc. Additionally, this material has not been reviewed by Catalina Yachts, Inc. for technical accuracy. This page's maintainer cannot guarantee the accuracy of this information or the desirability of suggested modifications or upgrades. Please obtain assistance from a competent marine mechanic or boatyard prior to making any significant modifications to your vessel.   

 

Last modified by Phil Imhof, Wednesday, August 11, 2004 . Copyright 2001 by Catalina 34 International Association.  All rights reserved.