Many years ago, before I left high school, I read a book about a famous Dutch ocean sailor who was asked what his preferred size for a safe ocean crossing sailboat was and, without hesitation, he responded 44 feet.
I worked as a yacht designer and found time and time again that 44 feet is a weird magic number for sailboats. If they are smaller, there is a certain level of camping involved, but once they hit 44 feet they become a real boat; you can carry what you need, go wherever you want, and have a little bit of creature comfort for the crew.
You can go bigger, but it will not make the boat much more capable, it will just make it easier to fit towel heaters or a granite counter or get a bit more speed.
There are many such rules of thumb in design and sometimes it makes more sense to follow a rule of thumb than to reinvent the wheel.
It is possible to design anything with first principle approaches, but since boats and ships are very complex systems, they actually tend to come together with certain realities that become hard to beat.
Full displacement vessels like tugs cannot beat hull speed.
As the Neu van Hemmen line shows, certain vessel types just have inherent speed limits and there are also limits to reasonable speeds for catamarans.
One can build very fast catamarans, but once a catamaran has to do something, speeds will go down due to payload induced drag.
In theory a small catamaran should be able to go as fast as larger catamarans, but they hit a practical speed limit due to an inability to fit bigger engines and carry more fuel, while still carrying an acceptable passenger load. Therefore, in real life, short passenger catamarans tend to be slower and longer passenger catamarans tend to be faster.
This plot illustrates that limit. It is a collection of actual lengths and speeds of a collection of catamaran ferries.
The circles are actual and published speeds of vessels of various lengths. The yellow circles come from published data by one famous catamaran ferry designer, while the darker blue circles are another famous designer.
The red and lighter blue circles are vessels well known to us and show their actual speeds.
In the 27 m length range there is a green triangle and an orange triangle. The green triangle at 32 knots was the specification speed for a passenger catamaran by a first time passenger catamaran designer and the orange triangle at 28 knots was the actual speed of the vessel.
This very simple plot provides a ton of information and issues to ponder, such as:
1. The dark blue designer tends to get more speed out of its designs.
2. But maybe the yellow designer tends to design with higher passenger capacities or they are more seaworthy or longer range vessels.
3. We added some trend lines, but these lines have dubious validity since a 0 m long ferry will inherently have a zero speed and there is a practical minimum size for passenger ferries (which may be in the 10 to 12 m range) with its own maximum practical speed for which we found no data.
4. In the 27 m range there are no ferries that exceed 30 knots.
5. For ferries to run in excess of 30 knots they need to get to the 40 m range.
6. It is easy to go slow, but hard to go fast. As such, the upper limits have particular significance for designers.
So what is the story with that green triangle? In theory everything can be made faster, but this was not a racing catamaran; it was simply a typical catamaran ferry of typical capacity. Yet somehow the designer convinced himself that 32 knots was attainable, while in the real world no such catamarans exist.
Self delusion may be mostly harmless, but it becomes a problem when it ends up in a specification, and when the Owner and the builder end up in a tussle because of it.
So here is the moral of the story. As engineers we often have to venture into unknown territory. That is OK, but we work in the real world, and when you are asked to do real things, don’t forget to make reality checks with the real world.
The plot was made by our high school intern Riley Flanagan, while I provided some inside info, the dark blue and yellow circles were available on the internet. It took this high schooler about five hours to make the plot. If the designer would have made the same effort, thousands of dollars would have been saved.
I also asked Riley to plot passenger capacity versus length. It adds little to the above argument, but also makes interesting points to a clever designer and we provide it as a public service.
Design is applied plagiarism. Copying is wrong, but using existing information to come up with better solutions is central to design. Don’t ignore the real world.