Our last discussion covered some of the hull instabilities that can occur at speed and how, from a design perspective, we stay away from those. But let’s be honest, it is not unheard of that changes in vessel design occur after the initial hull design is complete. Owner and builder decisions are made throughout the build and are often necessary or beneficial. Sometimes decisions made late in the game can result in an execution that can cause performance issues. None of us want performance issues. Let’s look at a few potential pitfalls on the build/execution side.
Repowering is a common occurrence that can extend the useful life of a yacht and keep it competitive in the market. Often, it goes off without a hitch. But let’s consider an example. An older hull is designed for particular engines and a top speed of, say, 30 knots. Years later, the new owner wants more speed, installing much more power. The new engines can be significantly heavier. Because of this, bigger props are needed and the engines are shifted forward to both fit in the engine space and to maintain shaft angle. Oh, and we add more fuel since the burn numbers on the new motors are much higher. The results? We have a more heavily loaded running surface, a center of gravity more forward and we are running significantly faster—meaning she runs much flatter (less trim) at top speed. Another possible piece of this repower scenario is shifting the rudder and propeller aft to maintain our shaft angle. This can induce rudder ventilation, and cause any number of bad and unpredictable behaviors. Now, a boat that ran perfectly well for years becomes squirrely, can’t track and leans over, or worse. Mitigating steps must take place.
Execution of hull details can have a profound effect on running and handling in our high-speed sportfishing machines. On a planing craft, we want undisturbed water flow across the hull bottom and clean separation where the flow detaches from the hull surface. The two main locations of water flow separation are the chines and the transom. Others would include the outer edge of the strakes. (Transverse steps are another location, but we’ll discuss those at another time.) We want these edges to be as sharp as possible, minimizing the potential for water to hang on. High-quality custom builds will normally sport very sharp edges here. But sometimes the radius of that edge can get a bit too large. In production hulls, it is often a result of limits to how sharp the edge in a mold can be. Most often, a non-sharp edge in these places simply results in slightly more drag. But if the hull is marginal with regards to dynamic instability issues, round edges at the chine and/or transom can be enough to move the hull from operating with no issues to exhibiting very unpredictable behaviors.
Execution of hull details can have a profound effect on running and handling in our high-speed sportfishing machines.
We discussed asymmetry previously (say, for example, from prerelease from a mold or faulty fairing on a cold-molded hull). But asymmetry can also occur from appendages. Intakes, transducers and sonar fairings are a few of the appendages that are installed on the bottom. If large enough, like a sonar fairing, or if multiple items are placed on only one side, pressure differences occur, and sometimes this is enough to cause unwanted heel. One test I participated in was on a boat that leaned over above 30 knots, and the bow dropped with lean (quite violently) when dropping throttles from top speed. The cause was the propeller tunnel air induction system, which was only working on one side. If the issue, like lean over, always happens to one side, it could point to some sort of asymmetry being the cause.
Sometimes, it is the placement of appendages that can have an impact downstream. Disruption of flow can result in unwanted air. Water flow on a hull bottom is mostly buttock flow; it runs straight aft. If an appendage is installed in line with propellers, or rudders, the air bubbles will flow aft and may cause cavitation or rudder ventilation.
Special care must be taken when the underwater exhaust is chosen. The exhaust ports must be placed properly to minimize the potential for these issues. Even strakes can be the culprit here. So as you can see, proper troubleshooting can be challenging in these cases.
If the rudders are being ventilated, multiple behaviors can result, including rapid stern lift/bow drop, extreme heel, almost instant change in yaw (in turning) or even turning in the opposite direction from the helm input. The good thing here is that rudder ventilation can be mitigated by a few relatively painless fixes.
Obviously, several design problems can result in unacceptable hull behavior. But even a good design can be hampered by poor execution of certain hull form features. The devil is often in the details. An important facet to achieving the superior product we all want is exceptional communication between the designer or naval architect and the builder before and throughout the build process. More than naught, this results in a hull that never needs troubleshooting.
