# Bow foils installed on ferry

@portofdc So the main purpose of bow foils or fins is to convert wave energy directly into propulsive thrust (to reduce fuel consumption) and in order to make use of the wave energy, the fins would need to be located in the area of maximum vertical translation. What is vertical translation? And how can a fin fitted below the bow convert wave energy into propulsive thrust?

Granted I didnât scrutinize the press reports but what Iâve read leaves me with a question. Itâs not specifically clear to me how the winglets physically provide both stability and propulsion. Electric motors slaved to gyros for stability, maybe, but propulsion? To do so, the wings have to provide lift and translate it to forward motion. How is the âvertical translationâ actuated? Mechanized energy demands power and hardware. Have they figured out a way to help assist propelling the vessel forward using its own forward motion? Please help.

I imagine that it is the porpoising motion of the fins caused by the wave motions that actually provide propulsion. I donât think the foils themselves have to rotate. Researchers have previously proven that the flapping motion of a hydrofoil can convert wave energy into propulsive energy.

Although in the case of the diagram, the foil was allowed to flap, the same forces could be generated by the free motions of fixed foils on a porpoising bow.

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Thatâs what propels fishes and divers with fins. I think Leonardo had that figured out but his is a quantum leap if it can be made to work. I think itâs an interesting concept on the road to the as yet unattained holy grail of perpetual motion. The movement of the winglets would have to adapt to ever changing motions of the bow with the exact amount of resistance and react with precisely the correct amount of movement to create a significant amount of lift. They would need to be responsive within milliseconds like an aircraftâs auto pilot. Not exactly free or perpetual but if they can make it work, a worthy endeavor.

These foils come out and locks. No motion, or change of attitude once deployed.
I donât pretend to understand the underlying theories, but I would assume that the people at Havyard Design do. They have incorporate these foils in their design for the new ships under construction for Havila Kystruten, which will join Hurtigruten on the Coastal Express route from Bergen to Kirkenes vv. in 2021, as referred to in the original article:

They will be competing on who can make the round trip with least fuel consumption and emission, as required in the contract with the Norwegian Government.

For those who have any hope of understanding this, here is an abstract of a Master Thesis on the subject:
https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/2575374
Much of this goes over my head, but Iâm ready to hear from those who can break it down in understandable English.

If thatâs the case I donât expect dramatic results at the race finish.

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Every litre of fuel save count I imagine. Besides there is the bragging rights.

Well yes, you do seem to value those.

Yes in the competition to gain or retain reputation in the Coastal Express service there is a lot to gain from bragging about being the most âgreenâ and offering the smallest carbon footprint to the travellers and cargo owners. (But than you knew that, right?)

I liked your first version better. More imaginative and to the point.

In the case I cited, the flapping of the foil is not caused by external excitation like a fish. Instead, it hangs freely and and the flapping is a result of the waves. But the same paper discussed early attempts of using fixed foils to induce wave driven propulsion on a small boat. It referred to one that achieved a speed of 5mph just with fixed foils. So it certainly can be done. Whether they are able to do it economically is a different question. And of course theyâll have to overcome all the skeptics that donât believe. All things naval have always been bound by tradition more than incited by innovation.

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I just posted this in another thread, but maybe repeating here will be appropriate:

Now donât say that this is just more Norwegian bragging because it refers to a Norwegian issued report.
Just read it with an open and unbiased mind and try to forget that I linked to it.

You have me pegged wrong. I support their efforts and wish them success. Read my previous posts.

Bow foils produce net thrust when thereâs a sea running, but net drag in smooth water. There may be ways to mitigate this by twisting the foil and/or altering the angle of attack; but this is hard, so rather than go down that rabbit hole it seems good to simply retract the foil when running in smooth water. This also keeps you from punching a hole in the dock when coming alongside.

From an external link quoted above:

Quote â It has been known for over 150 years that vessels with wings â or foils â in the bow can be partly or solely propelled by wave energy. The foils generate lift when the vessel is moving up and down in waves, and the lift typically has a forward thrust component larger than the drag.

Put simply, wave power is directly converted into propulsive power.

A vessel in waves experiences additional resistance compared to calm-water operation. For wave lengths similar to the hull length, this additional resistance will be amplified due to resonance, implying large heave and pitch motions. Wavefoilâs retractable bow foils dampen these motions so that the additional resistance decreases.

So how much fuel can you save? That depends on the ship, its speed, the foil size and location and the wave conditions. For a 100 m long ship operating in a wave-rich area, with constant brake power equivalent to a calm-water speed of 14 knots, research shows (sic) that an average fuel saving on the order of 20% can be achieved. Lower brake power gives higher percentage fuel saving.

Fuel savings are not the only benefit of bow foils. Increased comfort, reduced vibrations, less spray, increased operability and the ability to maintain higher speeds in waves for a given power are some other perspectives.â unquote.

Sounds good? 20% fuel savings!!!

Anyway, frictional and wave resistance of ships follows different scale laws, so when doing ship model tests in still water you have to split the resistances accordingly. If you add weather â model tests in regular/irregular waves â matters become even more complicated.

In my opinion it looks like the bow foil will simply act as a brake when reducing pitching and will thus increase total resistance/fuel consumption. The forces acting on the foils are also depending on the vertical accelerations forward. In stormy weather such accelerations will, in my opinion, rip off any fixed of movable fins fitted below the bow. Anti-roll fins fitted midships always increase the resistance.

Thereâs only a short hop across to Trondheim from your place. Why not go there and explain to them that they do not know what they are doing and wasting their money.

Iâm sure they will recognise your superior knowledge of all things maritime and beyond and welcome your expert advice.

Why would I bother with people at Trondheim selling bow foils? I would not fit them on my ships under any circumstances. We save fuel by careful route planning and slowing down in severe weather.

Unfortunately Iâm not that familiar with this particular system, but I can understand the basic principle and how it converts the pitching motion into net thrust. If it can demonstrate fuel savings as well as a reasonable payback time, Iâm sure the market is open for new innovations. Perhaps itâs not suitable for all ships (thinking my projects for example), but hardly anything is truly universal.

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I find fin a finer word to describe this folly foil. The extended fin rotates 90Â° from vertical to horizontal positions and then adjusts the pitch to produce thrust, when the ship moves. In severe weather the fin must be retracted into the fore peak. Details of the central, retractable hub controlling the fin would be interesting. Is it electric hydraulic? Also details of maintenance. Do you pull the whole thing and fin up on deck to fix any problems?

The use of the word foil doesnât really fit in this context so it may just be a poor translation. Foils on boats are defined by Mirriam Webster as extended struts or fins to lift a hull clear of the water as speed is attained. The latest generation of AC boats use foils:

Foil seems like a good fit.

# airfoil

### noun Aeronautics.

any surface, as a wing, aileron, or stabilizer, designed to aid in lifting or controlling an aircraft by making use of the air currents through which it moves.

Key is itâs like a wing, provides lift, for example a parafoil.

The entomology of âfinâ is a little more obscure.

Old English finn âfin,â from Proto-Germanic *finno (source also of Middle Low German vinne, Dutch vin), perhaps from Latin pinna âfeather, wingâ (see pin (n.)); or, less likely, from Latin spina âthorn, spineâ (see spine).