It seems that every day, a new startup or small autonomous hardware company is introducing a drone, or rather, an unmanned surface vessel (USV), into the market with wonderful prose and claims to its ability. When looking a bit deeper, it appears most of these companies are extoling the virtues of the autonomy and the ability to do XX or YY, all remotely. All of it sounds good, but after scratching beneath that veneer, will we see any actual naval architecture going on there?
When looking at the “solutions” many companies are making, one wonders whether a naval architect was actually involved in the design of the vessel. Most seem to be a bunch of very clever and sophisticated hardware for the mission control side, all squeezed into a small platform, which is a three-dimensional shape that happens to look like a hull.
Whether it’s a five-metre or a 50-metre monohull, the hydrodynamics are the same, which begs the question, how much actual high-speed hydrodynamics is being understood, or merely nodded at for the sake of generated publicity or a “wow factor”? It appears that there is a desperate attempt by hardware engineers to showcase their very cleaver hardware in anything that floats, just to get attention and grants or government contracts.
Recent “solutions” are that of high-speed stepped monohulls. The first question that comes to mind is why adopt a steeped monohull? Steps were first created on the hulls of flying boats to prevent the hulls from being “stuck” to the water, owing to the negative pressures under the planing section of the hulls, since the whole objective of a flying boat is that it must exhibit a low take-off speed on a short run.
This modus operandi assisted the rough water take-off, as the less time spent slamming, the better for the structure. Thus, a step in the hull bottom provided this solution.
Interestingly, it has been documented that adding a step to a flying boat made the structure more complex, and added both weight and air resistance. In addition, a slight reverse rocker near the step assisted in keeping the hull in the water longer (i.e., more lift) until the speed required to lift off has been reached, allowing the pilot to concentrate on the take-off and any potential porpoising rather than any fine control. But it was seen as a suitable trade-off, the usual design compromise. So, the objective is clear; add a step to help the flying boat get sufficient lift as fast as possible and in a short enough distance to permit take-off.
When looking at steps for a high-speed boat, the objective is subtly different: decreasing the wetted surface area, which reduces the viscous resistance. This also enables the effect that is wanted by most stepped designs and also yields a lower running trim. Research conducted by the US Navy in 2012 showed a requirement for an optimum running trim of 2.5 being the target for research into steeped hull designs, based on LCGs ranging from 25 to 35 per cent L. More recent systematics series of steps in hulls at the University of Napoli (2022) clearly indicate a link with lower running trim with steps, and that a single step is sufficient given “typical” parameters. The models were run with the LCG at 33 per cent L.
So, to put this into context, an LCG of circa 30 to 35 per cent L means the weight is far aft, which is nothing especially groundbreaking in the high Froude number performance of high-speed vessels. Simply walk past any marina with these ultra-high speed pleasure boats and you can see the transom nearly submerged and exhibiting a large static trim when at rest.
This brings us to the statement of requirements (SOR) of the vessel. What is the USV’s objective?
Looking at the current plethora pf press releases, they all seem to be heavily armed with ordnance for offensive duties and/or interception. A small five-metre-ish USV, if on a single mission, hitting a target of around 200 metres should not be rocket science in that sense. It would be like throwing a dart at a barn door (hard to miss).
If this is the SOR, what would be gained by adding a step or steps other than more complexity to the hull structure, and possibly weight, which was the same conclusions as with flying boats? And when viewing these ultra-high speed hulls, they appear to be at near level trim when static. This does not suggest the correct LCG location for ultra-high-speed cruising.
If we look at the more recent offerings with what appears to be an SOR of operating for several days, up to 400 nautical miles offshore, looking beyond the headlines citing average speeds of four knots, the purpose of a step seems somewhat pointless. But with the added mix of sprint speeds of up to 50 knots, this brings into question the rationale behind such a vessel. For context, 50 knots on a five-metre vessel is an extremely high Froude number/Taylor quotient requiring significant hp/ton, since the two design objectives that the vessel needs to satisfy are poles apart. Going four knots in a five-metre or even slightly larger vessel is considerably different from 50 knots.
The naval architecture and the hydrodynamics between these two design objectives could not be further apart. For example, the simple fact of where to place the LCG for slow speed is at variance with high speed, and vice-versa, not to mention going 50 knots on a five-metre vessel can result in anything that is post 12g accelerations with large vertical displacements at the bow. Shock loading on equipment therefore also becomes critical.
The design compromises required for four knots would render the higher speed objective, on the same platform, as a serious compromise. It would be like buying a tractor and then wondering why it struggles when it is being driven around a race track.
Just because it is “possible”, does it mean it’s worthwhile? Dual design objectives are compromised vessels from the outset, even on conventional size vessels, so why make it more difficult on a smaller vessel? This leads to the how much real naval architecture has been done versus the needs for publicity, clicks, likes and show, merely to get the “wow factor”. Is the baby being thrown out with the bath water?
Then we have USVs that are a bit larger, in the 10- to 15-metre range with heavy ordnance on-board. Whether it be missiles or cannon, all seem to use electro-optical/infrared or forward-looking infrared systems for target acquisition.
On vessels quoted at again, at high speeds of 45 knots, given the vertical displacement of the bow/hull in anything other than a “slight” sea state, it begs the question how the system copes with searching the sky then suddenly searching the seascape, and then the sky and the sea again constantly, in vertical accelerations of 12g or more. Riding on any high-speed monohull, whether stepped or otherwise in a developed sea, will be extremely uncomfortable, especially with steps designed to maintain a low running trim with the bow slamming into waves constantly.
Infrared target acquisition is indeed impressive, but I’m curious as to how it distinguishes and finds targets when its “eyes” are constantly alternating between the sea and the sky in an extremely short time period with vibrations and violent slamming in high accelerations and an environment that is prone to attack by any exposed dissimilar metal. Those advertised as “steps” may assist in reducing the running trim, but does it help with increasing the rate of slamming in anything but flat calm seas? In offshore racing boats, an experienced helm operator can adjust owing to reading the sea and direction; can a USV do the same?
If we decide to go ahead and fit a system than can address that, there are several on the market designed to reduce motions. However, none will be able to cope with the violent accelerations and extremely low periods between such slamming events, as the hardware to cope with such will most likely be greater than the displacement and size of the vessel it is housed on.
So, what about when loitering or stopped? Existing small systems help the vessel reduce the rolling and minimal pitching (at best), but not the heave.
Meanwhile, the USV becomes a sitting duck whilst attempting to acquire a target. The assumption must be that if such a USV is deployed, there would be a similar one hunting it, too.
Thus, did a considerable amount of actual high-speed naval architecture go into the design of such vessels in trying to satisfy competing demands? Or was the vessel developed merely to acquire publicity, funding, and/or social media likes? What is on paper may sound great, but high-speed hydrodynamics do not lie.
