When discussing catamaran design, one of the most common questions among owners and prospective buyers is: “Would extending the hull length increase performance?” The answer is nuanced and closely tied to another critical design feature: rocker.
I’m sitting in my office with a prospective catamaran buyer. From our vantage point in the marina here in the north of the Netherlands—at what feels like the very end of the world, or as we like to see it, the beginning—catamarans of all types, sizes, and ages pass by, arriving from every corner of the globe. Among them, we can spot a few older designs moored alongside the newer models, and naturally, the topic of waterline length came up. Since this is a question I’m frequently asked, I reached out to a few naval architects to gain a clear and well-explained perspective. The following is a summary of what emerged from that discussion.
Understanding Waterline Length and Rocker
The waterline length (WL) of a catamaran—essentially, the length of the hull that sits in the water—is a primary determinant of theoretical hull speed. Longer waterlines generally allow higher speeds without planing, following the classical hull speed formula:
Vhull≈1.34×WL (ft)
Rocker, on the other hand, refers to the curvature of the hull along its longitudinal axis. Hulls with more pronounced rocker curve upwards at the bow and stern, while straighter hulls have minimal curvature.
High rocker: Shorter effective waterline, easier turning, better maneuverability, softer ride in waves.
Low rocker: Longer waterline, higher potential speed in calm water, but slightly reduced agility.
Performance Catamarans and Light Carbon Builds
Performance-oriented catamarans and lighter carbon constructions typically feature relatively low rocker. This design choice ensures that the hull is already optimized for speed: the straight hull maximizes waterline length and efficiency through the water. Extending the hull further often adds minimal benefit because the original design already easily exceeds the theoretical hull speed. Once planing, waterline length is irrelevant.
Even so, certain owners of models like the HH44 occasionally consider hull extensions, particularly because the stern is abruptly truncated. A modest increase in length here could theoretically enhance the waterline without significantly affecting handling—but it remains a marginal gain compared to the benefits already provided by the low-rocker design.
Heavier Polyester Catamarans
On more conventional cruising catamarans—such as Lagoon, Outremer, Bali, or Nautitech models—the situation is different. These boats, built from heavier polyester, inherently sit deeper in the water and feature more natural rocker. This rocker improves comfort, handling and buoyancy when loaded, but reduces the effectiveness of simply extending the hull. In these cases, adding length does little for speed, as the heavier weight and rocker already limit the potential gains. Also, the extension following the rocker line, would exit the water. To prevent this it would have to counter the angle of the hull line, bend down and add resistance.
Explocat 52
I remember the days when I was developing the Explocat with Garcia. On hull number one, I was extremely focussed on light weight (as far as possible when you’re building aluminium) because to keep her fairly fast, we wanted as little rocker as possible in that weight categorie. On later -heavier boats- I noticed the weight caused the boat to sink in quite a lot. More rocker would have increased the buoyancy and prevented that, but in both cases it killed the performance. So it’s also depending on consistency: the shape of your hull defines her weight carrying capacities, so go easy on those extra’s!
The Bottom Line
On performance catamarans and light carbon designs, low rocker and straight hulls already deliver easy planing capabilities. A carefully considered hull extension might help in edge cases but is generally unnecessary.
On heavier polyester cruising cats, more rocker and weight diminish the potential benefits of hull lengthening. Any gain in speed would be minimal, and the trade-offs in construction complexity and cost usually outweigh the benefits.
In essence, the decision to extend a hull cannot be taken in isolation. Designers balance waterline length, rocker, weight, and intended use. Performance catamarans with low rocker already make the most of these factors, whereas conventional cruisers rely on rocker for comfort and manageability, rendering hull extensions largely redundant.
Earlier designs
So, to bring this back to those older designs that initially triggered the question: yes, there can be some benefit in extending the waterline. Twenty years ago, computers and software simply weren’t as advanced as they are today, and the same will likely be true twenty years from now—future tools will allow even better designs. But particularly with designs from the 1990s and 2000s, there is still room for improvement. If those hulls have a relatively straight rocker, extending the waterline can indeed make sense. That said, it’s always best to consult a naval architect, as they can provide advice tailored to each specific situation. It can be worthwhile, and I would be very interested to hear whether that proves true in practice. I’m happy to continue this conversation, as it’s a fascinating topic that personally intrigues me greatly.