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John Holtrop

13 May, 2007

Over the last ten years we have seen the popularity of cruising multihulls grow from a backyard novelty to a well-engineered cruising platform that can carry four couples in total luxury. They have done exceptionally well in the charter industry, where they now outnumber mono hull boats in some ports. The safety record for these newer, cruising multihulls, is excellent. Builders from as far away as South Africa routinely deliver their boats "wet", sailing thousands of miles with out incident, and the charter boat industry also has a good safety record. Hopefully, this safety record will remain good as more multihulls are selected for offshore cruising.

Even though the newer multihull boats are extremely stable, they can capsize. Last fall a large cruising cat, headed for the Seattle boat show, was lost off the Oregon coast. The boat was a modern design, with a professional skipper and two crewmen. We will never know exactly what went wrong, but the accident underscores the need for tools that will quantify and manage the risk of capsize. This is a broad field that includes vessel preparation, weather concerns, crew experience, and stability details unique to the particular vessel.

The purpose of this article is to introduce a technique for estimating the wind strength necessary to capsize a multihull, with respect to a specific vessels dimensions, weight, and sail plan. The final product of this exercise is a table, unique to your particular vessel and personal risk level, that contains the maximum wind velocity you can operate in with a reasonably high margin of safety. The table starts with light air conditions (full sails and a cruising spinnaker}, and ends under bare poles facing hurricane force winds

The first step in this process is to throw away the traditional 0 to 180 degree "STATIC STABILITY" GRAPHS. We all know that multihulls have tremendous initial stability. What a skipper needs to know is how close is the boat to capsize and what tactics are available for keeping the boat right side up. One way to estimate stability is to look at the wind strength that exists when the windward AMA is pulled out of the water. This wind speed is called "CAPSIZE VELOCITY" or CV, and it is assumed that in this condition, the slightest additional gust will capsize the boat. CV is composed of four basic boat dimensions: Displacement, Beam,, Center of Effort, and Sail Area. A high CV, say 60 to 70 knots, minimizes the risk of capsize. Lower CV numbers indicate a higher capsize risk ( FIG 1 ).  

When the overturning moment (Wind Force times Center of Effort) is larger

than the Restoring Moment (Displacement Force times Beam / 2), the boat

will capsize. The wind speed at this point is the boats Capsize Velocity (CV).

                        CV = (295*Displacement*Beam / (Sail Area * LOA))^.5

                                                                Fig 1

 

 

The simple equation in FIG. 1 can be used to compare the stability of two different boats, evaluate the effect of water ballast, or help sort out what sail combination is best. Anything that makes the CV larger is good, from a stability viewpoint.

Raising the CV lowers the risk of capsize, but the changes often come with a performance penalty. Increasing displacement and decreasing sail area are two common ways to raise the CV. On an offshore passage, the prudent skipper will update the CV associated with the vessel, apply a safety factor ( 2 or 3 /1 ), depending on the vessel) and instruct watch standers on what action to take, if the wind should approach that velocity. A more complete description of CV is found in Fig. 1. Even though the CV calculation is a simple estimate, if we apply it evenly to a large number of boats its possible to identify important trends. A few hours surfing around the internet yielded basic information on 512 multihulls, which I assembled into an Excel Data Base. The first task was to sort the database for CV. I was surprised at the spread, from a high of 80 knots to a low of 20 knots. The results are shown in FIGURE 2.

The sorted data in fig. 2 is interesting. A quick glance seems to show the small boats clustered around the lower values of CV, however a few big boats have low CV numbers. The next two Figures list 25 boats that have the highest value of CV, and another 25 having the lowest. Weight plays heavily in these figures.

    The average CV for the "hard to capsize" boats , Fig. 3, is 72. The average for the more tender boats , Fig. 4, is 27. This spread can be seen in Fig. 5. If we select a displacement of 25000 lbs, we can see boats with CV as low as 28, and as high as 70. Before you buy, or charter, a cat, you should calculate some basic CV numbers and compare them with respect to your expected use. Boats with a small CV are generally higher performance, but easier to capsize. Your choice!

Fig. 5 shows the relationship between CV and Displacement. Like figs. 3 and 4, the heavy boats have high CV values and the smaller boats have low CV values.

a 4/1 aspect ratio catboat rig as a base line, and is sorted on "change". The biggest payoff is the BI plane rig (two masts, arranged transversely), seen in some racing multihulls. The CE is lowered 8 feet, raising the CV almost 9 knots. Other advantages of the BI plane rig include smaller sails, which are easy to raise and lower, and the option of running with just one sail. The second highest payoff, 4.5 knots, is to simply reef the sail 10 %. In number three position is water ballast. Adding two 24 cubic feet tanks ( 3000 #) is a major job, but not impossible. A four-foot long, raised landing in each AMA would be large enough for 1500# of water. One nice thing about water ballast is you can pump out the tanks when its safe to do so, and regain some performance ( the SA/DISP ratio goes from 18 back up to 21 )

The remaining changes, like increasing the beam, are best left to the manufacturer. Implementing all the changes raises the CV to 67 knots, a 12 percent gain.

Fig. 7 (not available at this time) shows how CV data can be compiled into a table that minimizes capsize risk by calculating a maximum wind speed for each sail combination, from spinnakers to bare poles. Its not perfect, but it’s a place to start and may develop into something better as we learn more about what goes into safe cruising multihulls.

I hope this article will spark some discussion on how to best improve multihull safety starting with the design, then on to the manufacturer, and finally the end user. reducing the probability of a capsize, but you should remember that this is only one aspect of a successful and safe voyage. Weather issues, crew experience and compatibility, boat performance, acceptable risk levels, and dozens of other factors must be considered. My advice: Learn all you can, filter out the BS, and go for it!

End

BIO

I‘m a retired mechanical engineer, with over 30 years experience in weapons design and testing for the US Navy. I keep busy, and broke, taking care of SPICE, a 39-foot cutter, berthed at Channel Islands Harbor, California, MOLLY, a 17-foot trailerable gaff rigged catboat, and a HOBIE 14 with a very small CV. You can contact me at  john@johnsboatstuff.com

Glossary

AMA : The name native’s use for the outer hulls of a multihull boat

Aspect Ratio: The ratio of sail height to length (luff / foot)

Bi Plane Rig : Two masts, arranged side by side, the same distance from the bow

Capsize : The boat has rolled 180 degrees to a stable, but upside down, condition

Catboat rig: A single mast, well forward, supporting one large mainsail.

CE : The Center of Effort is the centroid of a sail or sails.

CG :the center of gravity is the point where the vessel will be stable in roll, pitch, and yaw

Cutter :A two jib sloop. The second jib (stay sail) is often self tending and easy to use in strong winds.

Data Base : A tool for organizing information in three or more dimensions

Displacement: The weight of the boat as measured on dry land

Graphs :A method for visualizing information with two dimensions, x = horizontal, y = vertical

Knots :Velocity, at a speed of one knot, a boat will travel one nautical mile in one hour

LOA : Length overall

Moment : The product of a force times a distance. Moments try to rotate things.

Overturning Moment : The product of the wind force times the center of effort

Pitch :rotary motion around the transverse lines ( the bow goes up and down when pitching)

Restoring Moment : The product of the displacement times the beam divided by two

Roll : rotary motion around the fore and aft lines. Sometimes called heeling

Sort : Arrangement of data according to a user defined rule

Spread : The difference in Y-axis data when the X-axis is held constant

Stable : In balance, not moving

Trend : A cluster of data that suggests a preferred arrangement

Windward : The side facing into the wind

Yaw : rotary motion around the vertical axis ( the bow swings side to side )

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