This was another point I had forgotten to mention: While hydrofoils are said to avoid wave tubulence by being underwater, it only works for small waves. Once the waves get above a certain size, they really screw up a hydrofoil boat. I vaguely remember a story of a hydrofoil car ferry boat that found use restricted to calm days, due to this factor.
There were quite a few pretty large hydrofoil craft in the Soviet Union, but I think mostly used in smaller bodies of water, with smaller waves.
Remember also that giant ships are supported in buoyancy across a large surface area
Hydrofoil attachment points will require significant reinforcement built into the hull structures
Bigger surfaces means more drag. You can compensate a bit by increasing the aspect ratio and you could finish with something closer to a plane. It might fly, but you would end up with more drag than a floating or planning boat.
Calm seas are needed for piercing foils, but fully submerged foil with automated control as the boeing jefoil can follow waves (this is called contouring) even better than conventional archimedian or planning ships. This asset might explain why this boat is still in use.
Here is a nice article (and blog) on the subject Lâextinction dâune autre race de dinosaures, les hydrofoils de grande taille ! â Foilers !
There are some interesting takeaways on this. I read the article it was very good. There is a summary at the end explaining the issues facing hydrofoils, and indeed they seem to have gone out of fashion lately.
I am adding the english translation here:
- [1] Low efficiency under certain conditions (even electronically regulated machinery)
- [2] Fragility (reveal of propulsion systems, foils, cavitation, etc.)
- [3] American warships suffered from problems of seawater pumping (cooling), resistance of piping, autopilot sensors (parasites, a height sensor per radar was tested as an alternative), main engineer with a âshortâ life of âshortâ life.
- [4] It is extremely difficult to avoid the cavitation of the foils above 50 knots. It is then necessary to use supercavitating foils and a low incidence, but this generates a high drag.
- [5] Construction and maintenance costs
- [6] Consumption (to be compared with the oil crisis of the mid-1970s)
- [7] Incurment (width, depth)
- [8] Savings not really necessary and not so large a gap compared to new hulls (NGV: powerful engines, hydrojet propulsion, lightweight aluminium hull with a profile allowing for light regaping)
- [9] Transport of difficult loads, vehiclesâŚ
- [10] Increased detectability of military vessels (higher vessels on the horizon)
- [11] Certification/insurance (who wants to certify, insure this type of equipment, too many unknowns, risks?).
So lets analyze regarding CargoKite;
[3] and [10] seem military related or modern technology would alleviate.
[1] [6] [8] are related to fuel economy which would not be super important for a kite driven vessel. The question is; are we having problems with motor technology or with the drag of the vessel itself? How much is related to these mainly passenger vessels designed to go very fast?
[4] Not relevant for a vessel cruising at 10 - 20 knots
[9] would not be relevant for a container ship
[7] Not so relevant as these vessels are much smaller than existing ships.
So this leaves actual relevant problems
[2] Fragility
[5] Construction and maintenance cost
[11] Vertification/insurance
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Lets do a quick analysis of a vessel carrying 10 containers. The mass of the containers could be 300 ton, and the whole vessel maybe 600 ton. Cruise speed could be 10 m/s [almost 20 knots].
The lift of the wing would be given by
L = \frac{1}{2} \rho v^2 C_L S
Assuming water density 1000, lift coefficient 1.0 we thus need a lifting area of S = 117\mathrm{m}^2.
Wetted surface area per kg transported mass is: S_w = \frac{2 S}{m} \approx 0.39\ \mathrm{m}^2 \mathrm{per\ ton}
Some quick ChatGPT calculations for the Ever Given cargo vessel puts that number at S_w = \frac{2 S}{m} \approx 0.070\ \mathrm{m}^2 \mathrm{per\ ton}
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Now I am treading very thin ice here, but in my experience, inductive drag is far larger than skin drag for a foil. So we could assume that the Cargokite may have much larger drag per transported cargo mass compared to the large container ship that only deals with skin drag, and also the skin drag of the large container ship is lesser relative to the smaller foil vessel (due to square cube law I guess)
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I realize now where I went wrong when comparing drag coefficients earlier in the thread; the drag coefficient of a container ship is relative to cross section of the submerged hull (and transported mass scales by that area multiplied by \sqrt 2 because the length of the vessel should also scale). The drag coefficient of a lifting foil is relative to wing area, which again is proportinal to transported mass. So a foil vessel could never compete in terms of drag per transported mass to a large container ship.
The foil vessel would have to compete on other attributes; travel speed, using abundant [free] wind power or being renewable and so onâŚ
Skin friction drag is only a minor part of a ship total drag. Most of it is wave making drag and form drag.
From https://www.perseverancenb.com/post/hull-drag
I think the mass scale with area**(3/2) not sqrt(2) (otherwise it wonât be an issue).
But otherwise you get my point.
I am being to rough and quick here, I realize I am being more stupid than I really am. Anyways, you are right of course about wave drag. But, that depends on hull length. For huge cargo vessels, they are travelling to slow for this too have an impact. Skin drag accounts for 50 - 60 % of total according to ChatGPT. So the analysis may hold well anyhow.