Skip to main content

Dynamic instability: It’s as bad as it sounds

If you’ve ever been around the class of racing boats known as speed skiffs, developed along the New Jersey shore, you know that you can hear them coming long before you see them. There’s a lot of exhaust noise, of course, but the distinguishing sound comes from their traditional lapstrake hulls slapping the water, bass-drumming down the waterway.

Suction pressure where the buttocks curve up can cause dynamic roll instability; a spoon under a faucet is a simple way to understand how this suction works.

Jersey speed skiffs alternately rise and fall in a rhythm known as porpoising, named after the classic motion of the bottle-nosed marine mammal. The driver and throttleman sit in an open cockpit behind the engine, above the propeller shaft, with their backs against the transom. That’s the only location where they won’t be beaten to death, quite literally, from the incredible impacts and associated G-forces closer to the bow.

Farther north, Maine-iacs supercharge V-8 engines to race lobster boats, not unlike what Southerners did both afloat and ashore in the rum-running days of Prohibition that led to today’s NASCAR. Although the lobster boats and rum-runners don’t usually porpoise as much as Jersey speed skiffs, they do exhibit other bad behavior, such as broaching, barrel rolling and bow diving.

All three classes of boats share a working heritage and an evolution that led to such problems. The speed skiffs derived from sailing and rowing Sea Bright and Jersey skiffs, double-enders designed to be launched and recovered off the beach. Their tapered sterns were replaced by transoms when gasoline engines came along in the early 20th century, but the mid- and forward sections remained largely unchanged. Lobster boats were originally designed to be powered by low-horsepower engines that drove them at or slightly above displacement speeds. The rum-runners started as open working launches of moderate speed. In all three cases, however, competition — whether racing for bragging rights or to beat the other guy to market or to outrun federal agents — led to speeds the original designers and builders never considered.

With the added horsepower came more weight and often a different center of gravity, as well as increased speed that was sometimes incompatible with those existing hull shapes. And there you have the four causes — more weight, relocated center of gravity, increased speed and incompatible hull shape, sometimes acting alone but more often together — of various bad behaviors underway. These behaviors are collectively known as dynamic instability.

Dynamic instability is a phenomenon that affects not only racing and repurposed working boats, but also many pleasure boats on the market today. Sometimes they come from the factory that way, with the problems not arising until the boat is loaded up by the owner and placed into service. In other cases, an existing boat that was just fine is repowered in search of more speed or has a cockpit added in search of more space, leading to the problem.

In any case, it’s unwise to ignore the symptoms of dynamic instability — the results can be deadly. They are often subtle exaggerations of normal motions, such as roll, pitch and yaw, but in other cases they can appear suddenly and dramatically. Ironically, such motions appear more in calm water than in a seaway, as waves tend to interrupt the buildup of forces necessary to affect the hull to a noticeable extent. On the plus side, this enables you to sense small indicators that something doesn’t seem quite right and do something about it before there’s a casualty.

I first experienced dynamic instability when I was a teenager, on my older brother’s plywood cabin cruiser. Above about 20 knots, she would lay to one side or the other, running on half of the hull bottom between the keel and chine. She could not be brought upright again except by slowing almost to idle speed. I didn’t give it much thought at the time. Frankly, it was my brother’s boat, thus his problem.

A Viking 62 model undergoes tank testing.

Twenty years later, as a professional with a degree in naval architecture, I wasn’t a whole lot more prepared when a large yacht I was aboard decided to “submarine,” literally taking a dive toward the bottom. The water level had passed the foredeck and was climbing up the cabin front and onto the flybridge when the captain finally pulled back the throttle and the boat popped back to the surface, leaving both of us wide-eyed, puzzled and more than a little rattled. This time, it was my problem.

At the time, it was hard to find any published data on dynamic instability as we went in search of a solution. Designers and builders were understandably reluctant to share such information, but a breakthrough came when the Coast Guard, with a new rescue boat, and the Navy, with a new special-ops boat, both experienced significant “anomalies” related to dynamic instability as the boats entered service. Suddenly, there was intensive government research into the problem, with the causes and suggested cures quickly — for such projects — pinpointed and implemented, and published soon thereafter.

The whole thing went back to Daniel Bernoulli, that 18th century guy you may have studied in high school physics. In a classic kitchen sink experiment, you dangled a spoon with its back in a stream of water from the kitchen faucet. The water did not push the spoon away as you’d expect. Rather, the spoon was sucked into the stream of water, and the more you opened the faucet and increased the water flow, the farther in it went. It all had to do with the shape of the spoon and the flow of the water creating suction — “negative pressure” in scientist-speak — on the spoon, the Bernoulli Effect. The same force, greatly amplified, acts on hulls at any speed, and if the speed is too high for the hull shape, particularly at the bow, odd things can happen to the boat’s performance.

Dynamic instability can be steady-state, as with my brother’s boat lying to one side and staying there, or oscillatory, as with the Jersey speed skiff’s porpoising. It can occur in any direction — vertically, transversely or fore and aft — as variations of the better-known roll, pitch and yaw. Combinations of state and direction make for a wide variety of weird motions. For instance, if the lying to one side is oscillatory rather than steady-state, the boat rolls from side to side repeatedly in what is called chine-walking. Likewise, bow diving, as we experienced on the large yacht, is just the steady-state manifestation of porpoising. Though dynamic instability usually occurs at higher speeds on larger, heavier boats, I have seen it as low as 15 knots and on boats as small as 16 feet, so don’t think you’re exempt if you don’t have a big high-powered yacht.

If you suspect you are experiencing the symptoms of dynamic instability, there are a number of things you can do. If the boat is new, simply call in the dealer or manufacturer, tell its representatives what you suspect and ask them to investigate and correct the problem. You should not have to deal with their mistakes. If, however, the problem arises on an older boat that you’ve repowered, modified or overloaded, you’re in the hot seat.

The simplest solution is often the most unpleasant one: Just slow down. That’s not easy to do when you’ve just shelled out big bucks for new engines, plus accessories and labor, but sometimes there’s no other way. Many older boats with hull shapes designed for 20 knots simply will never perform well at 30 or 40 knots. And by the way, the structure is often not strong enough to survive such speeds, either.

After that downer, I’m happy to offer some hope if the problems are not extreme. The next-simplest thing to do, after slowing down, is to remove unnecessary weight that often builds up over the years. Consider leaving spare props and shafts ashore if you’re not going on a world cruise. Leave spare parts, filters and oil (lube and hydraulic) ashore if you can, and clear out the stateroom and galley lockers as much as possible. I know it’s a pain to reload for each trip, but that way you’ll take along only what’s necessary, leaving the excess at home.

Relocating concentrated weights farther aft — batteries, generators and water heaters are good choices — will shift the center of gravity to alleviate the problem on some boats that are bow-heavy, either through added weight or through the addition of a cockpit. You can also effectively shift that center by using fuel and water from forward tanks first, or by leaving them empty if your trip doesn’t require that much capacity.

This gathering of classic boats reveals how different hull shapes handle the same conditions.

Reshaping the hull is a fairly major undertaking, but you can often fix the problem by fooling the water. Dynamic instability is caused by suction forces (negative pressure) over large portions of the hull, usually at the bow. By interrupting that flow, you can drastically reduce the affected surface area and, thus, the total suction force pulling the bow down.

This can be done in a number of ways, but in most cases, it involves adding small wedges at the bow or widening the chines forward. Wedges, similar in shape and cross-section to spray strakes, are added at the bow, sometimes as extensions of, or parallel to, existing strakes, other times at an angle to them. Widening the chines with fiberglass or metal flat bars or with proprietary “strake enhancers” will often do the trick, as well.

At the stern, reset your trim tabs so they are slightly elevated when fully retracted, creating a “stern down” force that will lift the bow. With outboard or I/O power, trim the drive out to accomplish the same thing.

If you don’t have adjustable drives or tabs, the same result can be achieved by adding “negative wedges,” either stock non-movable tabs or custom fabrications attached to the transom. You can also create a negative wedge by grinding a small area across the full beam of a fiberglass hull bottom/transom intersection, but reinforcement must be added inside the hull to compensate for the lost thickness and strength.

In many cases where any one of these solutions is not adequate, you’ll find that a combination of them will do the trick. Informed trial and error is the best approach and will often suffice. If not, hiring a professional may be in order. Contact a boatyard, surveyor or designer, making sure the personnel are familiar with the ins and outs of dynamic instability, and let them devise a solution.

The one thing you don’t want to do is to ignore the indicators, hoping that things will get better. That’s not the way to go when the safety of your family and friends, as well as the survival of your boat, are at stake.

This article originally appeared in the August 2015 issue.