Hydrofoils have a very high lift drag ratio. You also have to remember the wetted area of the hull produces significant drag too. Lifting most or all of the hull out out of the water can enable dramatically higher speeds, at the cost of stability and complexity. The biggest America's Cup catamarans were roughly the scale of a 7 story apartment building, but were capable of lifting their entire mass on a foil the size of an ironing board. Because of the density of water, foils can generate a lot of force as lift.
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Most seem to involve a wing or fin extending out from the hull in some way.
That seems like it would introduce drag and greatly limit speed.
Hydrofoils have a very high lift drag ratio. You also have to remember the wetted area of the hull produces significant drag too. Lifting most or all of the hull out out of the water can enable dramatically higher speeds, at the cost of stability and complexity. The biggest America's Cup catamarans were roughly the scale of a 7 story apartment building, but were capable of lifting their entire mass on a foil the size of an ironing board. Because of the density of water, foils can generate a lot of force as lift.
Oh this kind of thing? https://wavefoil.com/
I'm assuming they've tested it and it's not entirely fraudulen and does in fact give more speed than drag?
I'm not really sure the existence of a thing really indicates efficiency or usefulness, that's not a high bar.