Stability & Comfort Design Factors

Stability and comfort characteristics with sailboats are relatively easy. The more stable the boat, the faster and more comfortable it is. Since a key component of this stability is the vertical center of gravity (VCG-the center of all of the weight which makes up the boat), designers and builders work really hard at keeping high structure light. The other component of stability is the hull shape when upright and at various angles of heel. More beam equals more stability, but also creates drag. So there is a lot of refinement that goes into how the hull interfaces with the water.


This is an optimal stability curve. You can see that the boat does not lose its stability until it is heeled past 130-degrees-with the deck 40 degrees past horizontal. Then look at the amount of area under the curve with the boat inverted and compare it to that with the boat right side up. This ratio, very little inverted stability means the boat will come back quickly. In all probablility, the wave which creates the problem will also right the boat.

You would think that a powerboat would be a lot simpler. Since there is no sail to carry, stability (except as it applies to waves) is not an issue. But life on the water is never this easy. We’ve been working long hours ever since we became serious about selling BEOWULF, and most of the time has been spent on the issue of comfort and safety in heavy weather.

There are two aspects of power boat stability which have to be addressed: First is the initial stability and roll period. These are comfort issues. The second is the capsize resistance of the vessel. This is a function both polar moments and the limit of positive stability (LPS-the heel angle at which the boat will capsize, from which it may, or may not recover).

In the sailboat business, capsize resistance is a major design and construction issue. Starting with the 1979 Fastnet Race disaster, considerable real world and theoretical R&D has gone into the subject. There are all sorts of standards on which you can call, to assess the capsize resistance of a given vessel.

For us, resistance to capsize starts with the ability to control the boat well enough in big waves that the problem can be avoided in the first place. This means enough power to make headway into the seas, and/or good steering control running with the waves. And if a knockdown by a breaking sea cannot be avoided, then we move on to how the wave energy is absorbed.

There are two components in this. The first is that the boat is not locked into position by the deck edge, hull, or keel/rudder combination. We want to see the boat skidding sideways after the wave impact, allowing it to absorb the wave energy over time-which limits how far it will heel. All our sailboat designs have been designed in this fashion, and we can see no reason to change the approach now.

The second issue is LPS-just how far can the boat heel before it will keep going (which relates to how quickly it will come back).

The powerboat industry must assume that there are no capsize risks, because as far as we can see, 99% of all powerboats-right up to 100,000-ton bulk ore carriers-will capsize and not recover, given the right set of circumstances. One of the facts which startled us when doing research for our book Surviving the Storm, was the calm manner in which the professional seamen we interviewed discussed what would happen if the right wave caught them at a vulnerable angle. The norm for most yachts, ships, and military vessels is a maximum heel angle of 65 to 70 degrees, after which the boat keeps going and does not come back.They accepted that their ships would capsize and everyone would die. One of our former clients told us of a 50-knot gale he weathered, hove to behind a parachute anchor, in a heavy displacement 60-footer. He said he felt they would not have survived much worse conditions. We’re too conservative and have too much respect for the sea to tempt the odds in this fashion. Sure, if you have propulsive power you can head into the waves when it gets really gnarly, but what happens if you lose power?

It is therefore a given for us that any unsailboat on which we venture offshore is going to have good capsize resistance. At the top of this page you’ll see the type of stability curve we’ve been working towards.

There’s another side to the stability question. That is comfort. This is where it gets tricky because the same factors which create good capsize resistance conspire to make a boat with a quick, less comfortable motion. Sailboats don’t feel this because they have their sails to steady them, plus very high “polar moments” from their rigs which slow down the roll period. In addition, their keels and rudders slow things down even more.

Unsailboats have to use a combination of active fin stabilizers, paravanes, and bilge keels to get rid of their motion. In addition, designers work to reduce the stability of the boat to slow down the roll period (it is this reduction in initial stability which causes the problems with capsize resistance and recovery). A further problem is caused by the need for all the stability generating systems to have a minimum amount of boat speed flowing by in order to do their work. So if the boat is slowed down, the active stabilizers or paravanes do not work as well.

Now for a little stability theory. There is a term designers use called GM, which is derived from the waterplane shape of the hull (the metacenter-M and the vertical center of gravity. Wider hulls have a higher GM. Lowering the VCG raises the GM. Both of these create a more stable boat with faster roll period (which is hard on tummies!).

Once you understand how these elements work together, it is easy to see why unsailboats have such a problem with capsize resistance. Unsailboats typically have beamy waterlines, with very full ends (for more interior volume in which to live). This hull shape creates a lot of stability and that would create a fast roll period, if the VCG was low enough to insure a good range of positive stability. To reduce the roll period the VCG is raised. This reduces GM, slows down the roll period, and makes the boat more comfy in normal sea states. That is why trawler manufacturers tell their owner’s to add weight up high-like dinghies on the highest deck, to slow down the roll period.

What nobody is discussing is that this also makes the boat a lot less safe in heavy weather.

Is there a way around this? Yes, but it takes a totally different approach to design and construction.

Lets go back to roll period for a minute. The amount of motion your body senses is a function of the quickness of the motion, and how far you (or more precisely the space between your ears) are from the pitch center of the boat. The closer you are to the pitch center, the less you will feel whatever motion is happening. This is why you hear stories of crews sleeping on the sole in heavy weather-because they are closer to the pitch center so the relative motion is reduced.

In the normal unsailboat layout, maximizing interior space is the design goal. This works great at the dock. But all those layers put you further and further away from the pitch center both vertically, and along the length of the hull-exascerbating the way the boat’s motion feels to the crew.

Look at it this way. The motion you would feel while standing watch twenty feet above the pitch center with a six-second roll period is the same as you feel at ten feet above the water with a three-second roll period. But in terms of capsize resistance, the vessel with the three-second roll period might have a range of stability of 130 degrees while the six-second vessel might only be 70 or 75 degrees. Which configuration would you prefer crossing the Gulf Stream in a Norther, or trying to make it onto Bay of Islands, New Zealand with a compression zone gale howling about your ears?

There’s another subject we need to look at. This is the “polar moments” to which we earlier referred. PM is developed from weight times the square of its distance from the pitch center (typically taken as the CG of the entire boat). Polar moments provide a powerful resisting force to the wave energy trying to capsize the boat. It is one of the major factors to prevent capsize in the first place.

What this means is that because of the squared function of distance in the performance of polar moments, a small amount of weight far away from the pitch center has the same impact as much larger amount of weight closer to the pitch center.

Here’s an example. You would get the same amount of polar moments from 1000 lbs of weight 10′ above pitch center as you would from 169 pounds at 25′ above the pitch center. Of course you still have to consider the overall VCG of the boat to determine the limit of positive stability. But this is only a first power function (i.e. weight times distance) as opposed to how polar moments are computed with a second power number (weight times the square of the distance). From this you can see that you can get big increases in polar moments, which slow down the roll period and increase wave impact resistance, while at the same time maintaining a reasonable LPS angle-if weights are carefully arranged.

How the weights are configured involves a lot of computation, computer analysis, and plain old experience.

As already mentioned, the traditional power-boat is locked into interior arrangements with significant distances from the pitch center and the absolute requirement for a slow roll period as a result make it impossible to create an overall package that has good capsize resistance. Big ships get away with this, for the most part, by their shear size relative to the wave energy. But yacht-sized vessels don’t have these scale effects working for them, and so are at a higher risk in the event of heavy weather.

If this was the end of the story we’d hang up our cruising dreams right now-or go back into a sailboat. But there is an entirely different approach that can be taken, and for that we need to talk a little bit about how you design fast, comfortable ocean-crossing sailboats.

There is a careful balancing act in cruising sailboat design between stability, drag, weight, and the vertical center of gravity. What we’ve always tried to optimize is a balance between a slippery, low drag hull which goes fast with small amounts of horsepower (supplied by the sails or engine), an accommodation plan optimized for the time spent at sea (because if you are not comfortable on passages, the odds are you won’t be making many ocean crossings), with structure and systems that keep the vertical center of gravity low. When you get this formula right the end result is great fun to sail, very quick, and extremely comfortable. You also end up with a design that does very well in heavy weather.

What we’re doing is bringing this same approach to our unsailboat. Rather than start from the normal powerboat view of the world, we’ve been using our sailor’s view, and experience.

You are probably asking yourself how does this manifest itself? Let’s start with the hull. If we use the same sort of slippery hull that has been so successful for us over the sailing years-modified just a little for the different weight distribution of the unsailboat, we have a very, very efficient platform with minimum horsepower requirements to move it at very fast speeds.

Next, employing a similar interior layout to that of the sailboats we are used to-large engine room aft, big forepeak forward, and living quarters centralized, we end up with an interior in which nobody ever sleeps or sits more than 10′ from the pitch center of the boat when at sea. If we keep the watch-standing station low-just high enough to have good visibility and no higher, the maximum vertical distance from the pitch center is just a hair over six feet. In short, the two of us would be living very, very close to the pitch and roll center, so that our perception of whatever motion the sea was imparting to the boat would be minimized.

As previously mentioned the vertical center of gravity has a big impact on this. Hull shape-the way the boat floats when it is knocked down-also is a major factor. The more topsides you have, and the narrower the deck beam, the better the capsize resistance and/or the recovery therefrom. That’s the major force behind the look of our Deerfoot, Sundeer, and Beowulf series of cruising boats. And why they have compiled such a good safety record over the years. And this same approach works for an unsailboat.

In fact, by giving our unsailboat a bit more freeboard, and having a larger deck house (we’ll talk more about this later) we can obtain such powerful righting forces when knocked down that we’re able re-arrange weights in a way as to slow down the roll period, while still retaining a limit of positive stability in excess of 130-degrees (compared to the motor vessel norm of 70-75 degrees).

There’s one more factor at work here. That’s the arrangement of weights throughout the boat. In terms of motion there are areas where weight works for you, and others where it works against you. What we’re going to do is build a high-tech, strong-but-light hull, deck and house structure using a combination of Kevlar, Carbon Fiber, E-Glass, heat-cured epoxies, and a variety of cores to create a light yet extremely tough structure. We’re also going to use our usual highly efficient approach to systems. This combination yields a total cruising displacement which is light enough that we can come back into the hull, and add weight in areas where it works to soften the motion-either in the form of lead ballast, salt water, or fuel. In the end, we should have a boat which is both extremely comfortable, damage tolerant, and which gives us a margin of safety in heavy weather.

How do we know all of this will work? There are several ways. First, we have a large database of offshore experience with our sailboats on which to draw. We know what makes these boats so comfortable, and this gives us targets towards which we can work. Next, by employing a form of CFD (computational fluid dynamics) we can test different hull shapes, displacements, and distributions of weights to find out what gives us the most comfortable configuration in a variety of sea-states. We’ll share more about this process in the next installment.

Posted by Steve Dashew  (March 28, 2003)

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