If power corrupts, too much horsepower corrupts absolutely, as does poor deployment of weight or hull shape. Dynamic instability has multiple causes but the unsteadiness that is manifested as erratic motion is mostly caused by overpowering or overloading a boat, by having a center of gravity too far forward relative to the boat’s center of planing surface, or by excessively convex bow hull sections. Most often, a combination of these factors is the cause.
Dynamic instability usually appears at high speed, so it’s often considered a problem specific to high-performance craft. But I’ve seen it at speeds as low as 15 knots on everything from 16-foot runabouts to 100-foot motoryachts. The symptoms can be subtle exaggerations of normal pitch, roll, and yaw, but can be violent enough to cause severe damage to the boat and injury to those aboard. Worse, although there are usually warning signs, dramatic motions can occur suddenly, on a boat that has heretofore been well mannered.
Dynamic instability is most clearly evident in placid waters, since confused seas mask the problem. The motion can be either a steady state or oscillatory. The yacht can roll to one side and stay there, refusing to right itself until the throttles are pulled back-chine sticking or keel skiing-or it can continually sway from side to side-chine walking-at certain speeds. For pitching, the steady condition includes tipping down at the bow, both minor-bow diving-and catastrophic-stuffing- and the oscillatory condition is often porpoising, slamming up and down in a violent rhythm.
Erratic yawing is not usually seen as an oscillatory condition, except where a mistuned autopilot or an inexperienced captain zigzags a “hard to handle” boat across the sea. More common, and often with a following sea, the boat slews to one side, leading at best to an uncomfortable roll, and at worst to a complete broach- turning end for end-or a capsize.
Remember that kitchen-sink experiment in highschool physics, holding a spoon loosely with the convex side to the water stream, watching it get pulled into the stream rather than pushed away? Now, think of the rounded sections of the bow as the backside of the spoon. When those sections are pushed underwater, by bow trim or excessive tab, by overpowering or overweight, or by surfing down a wave in a following sea, that same suction occurs on one or both sides of the bow. It’s the same as the spoon, only on a larger scale, creating tremendous forces that act on the hull.
So what to do? If the boat is new, take it back and let the dealer or manufacturer handle the matter-you should not have to cope with their mistakes. If it’s your problem, caused by repowering (and increased speed),adding weight forward, or removing weight aft, or by adding a cockpit to the stern (shifting the relationship between center of gravity and center of planing surface), there are a few possible remedies. Sometimes hull appendages, such as struts, rudders, bottom strakes, and even transducers, contribute to instability, as do poorly shaped propeller tunnels, but these are more difficult to diagnose and cure. Be aware that they can be part of the problem. Since instability is often caused by a combination of factors, it can sometimes be managed by a combination of solutions, many of which involve little cost.
The first solution is the easiest and least desirable: Slow down. That’s hard to swallow for someone who’s just spent a small fortune on new engines. Adding bow wedges or hull bottom strakes may help break the suction on some boats, but the reality is that a boat designed for a top speed of 15 knots was not meant to go 30.
Next, remove any excess weight. Be careful that in removing stern weight, you don’t shift the center of gravity forward. If you can’t remove the weight, try shifting it toward the stern, by relocating heavy equipment, and by running with stern tanks full and forward tanks empty. It’s hard to modify the hull shape at the bow, but you may counteract the down force forward with greater down force aft. If you have trim tabs, try resetting them with a bit of up-angle at their fully retracted “bow up” position. This pulls the stern down and thus the bow up, hopefully getting those curved hull sections out of the water.
You can also have your boatyard create the same effect by fastening fixed up-angle trim planes to the transom, or on a fiberglass boat, by grinding a “negative wedge” into the last few inches of the hull bottom, reinforcing the interior with additional resin and glass to restore the laminate thickness and strength.
Dynamic instability is serious. You may need to enlist a marine consultant, but don’t ignore the problem.
