Scanning any news feed or posts being fed on social media, one may be forgiven for thinking that placing a foil or rather a hydrofoil (as it is submerged in water) onto a vessel is new.
It would be very easy for any non-naval architect or even young naval architects below a certain age to think that foils have just been invented. To compound the explosion of interest around foils, are the claims made by using them.
Foils, or more correctly hydrofoils, work, they have their uses, and when used correctly can be very beneficial and enhance the performance of a vessel. The science, the hydrodynamics behind a foil, is nothing new and very well documented.
The purpose of the hydrofoil is to partially or fully lift the hull out of the water. Hydrofoils generate lift from the increase in forward speed, where the amount of lift generated is proportional to the square of the speed. It is a function of the area of the hydrofoil and its thickness, which influences the drag; nothing is for free.
When the hull is partially or fully lifted, the angle of incidence of the foil or amount of area that is immersed in the water will dictate the type of foil arrangement, as this affects the total amount of lift that is generated.
With a hull that is partially lifted out of the water (vessels like the Hysucat are examples of this, where the hull is fully lifted out of the water), conventional hydrofoils are either fully submerged.
A "jetfoil" is an excellent example, or where the foils are surface-piercing. Classic Rodriquez hydrofoils are good examples of this method.
Hydrofoils have been used on vessels since the 1950s, notably by the Russians and then the Italians. Their acceptances, into both the commercial and military sectors, began in the 1970s with the Boeing Jetfoil in 1974.
When designed correctly, the concomitant effects of using a fully submerged hydrofoil can be a reduction in vessel motions. Since the only contact with the water is the strut that links the foil to the main hull, the effects of the waves are near non-existent.
Again, the Boeing Jetfoil is an excellent example of this, with its reputation of being an extremely sea kindly ride in rough seas. But beyond the odd jetfoil dotted around the world, and a sprinkling of Rodriquez hydrofoils operating from the 1970s, hydrofoils, despite their hydrodynamic advantages, never really captured the market.
Then came the flood of renewed interest in the 1990s. Thumb through any FAST or RINA conference of the 90s and one will see endless papers on hydrofoil boats, all in one way or another demonstrating or claiming many benefits and uses. These include less resistance and/or greater speed, lower motions, and superior seakeeping compared to vessels without foils. One such company literally bet their future on it: Kvaerner Fjellstrand with its Foilcat.
So the question today one must ask is where are these superior vessels now? Sadly, the Foilcat literally bankrupted the company that made it. The Boeing Jetfoil has been updated, but with a price tag of around US$50 million per unit, hardly a viable option. Lastly, the other offerings from the 1990s have all slowly passed into the annals of history.
Even the US and UK militaries pursued these claims for better performance with the Pegasus-class missile boats and HMS Speedy. After several years of operation to evaluate the claims and in-service operations, maintenance, etc, both countries' navies concluded that these are unsuitable options for their role, for similar reasons.
Oddly enough, the overall high fuel consumption as a mismatch between design and mission profile was one of those reasons.
Anyone who has been on a hydrofoil when it has come off its foils in a heavy sea state and attempting several times to get foilborne again will perhaps understand why a hydrofoil is viewed as a solution looking for a problem, after they reach for the seasick bag.
The Boeing Jetfoil was limited to significant wave heights of 2.3 metres, since above this wave height, it is near impossible to get foilborne again. Passengers are then left to wallow around in a box, which is neither a safe nor comfortable experience.
Then of course there is the very simple square cube law. A foil profile is simply an area that provides a known amount of lift for that amount of area at a given speed. That lift is to support the weight of the vessel, which is a 3D shape.
Thus, as the weight increases, it increases in three dimensions by the cubic power, whereas the foil is by the square. There comes a point where the weight of the vessel is greater than the lift available, hence the limited the size of vessel. Since a foil is also bound by the laws of material properties, there is also a limit for its thickness and span.
And yet, as one flicks through any social media feed, it appears that none of the lessons from past experience of introducing hydrofoils are learned or even known, since many claims are as if this is new and unique or a brand new invention, even being labelled as a "green solution".
Whether powered by battery or diesel engine, the hydrodynamics of a hydrofoil vessel remain the same, just as with any conventional vessel.
So, is it a matter of having better materials? Well, then, let’s investigate that claim.
The only influence this may have is the thickness of the foil, to reduce its drag profile, thereby increasing the lift/drag ratio. So if the span and area remain the same, there is some benefit from less drag. That is the objective of any naval architect.
But the bold claims made for such a reduction appear to be highly questionable given the actual reduction in drag vs speed or being "green" in nature. On the other side, if the span can be increased, owing to improved strength, one begins to fall back to the square cube law.
Is this to say hydrofoils are pointless? No, not at all; everything has its place.
Like any appendage or device installed on a vessel, it has a purpose and ostensibly a niche market. Take the Hysucat boats invented by Professor Karl-Gunter Hoppe, for example. These vessels have an excellent performance profile, but a niche market due to their being generally smaller size vessels.
As the vessel size increases, there is the size of the foil required for the lift, which plays into the thickness, the drag, and the square cube law. But the Hysucats seem to be the exception rather than the rule in that they don’t appear to claim to be the one solution for everyone, or an innovative and "green" solution, as others appear to be.
The question that should be asked is, if hydrofoils are such a revolution, when those same claims from 30 years ago were made (and again before that in the 1950s) about superior performance and seakeeping, why don’t we see fleets of them the world over?
