>> Why are hydrofoils inherently impractical?

Limitations on sea states. This gets dangerous when the waves very suddenly start striking the hull. The hull doesn't impact the bottom of the wave but mid-wave, slamming into it in a way a normal hull never would. Limitations on cargo capacity (too heavy and it won't 'fly') which is why large hydrofoils are normally limited to passenger services. Increased draft at low speeds (ie when not flying). Moving parts below the waterline are always a fouling hazard. When that delicate foil hits something it doesn't end well. Outboard engines are mounted in a way to survive such events by folding backwards. A lifting foil cannot do that.

These all lead to a dangerous senerio. Say you are driving a hydrofoil in increasingly bad conditions. Because you are 'flying' slightly above the waves you don't notice that they are getting worse. Then one slams into the hull at full speed, or something breaks, and you can no longer 'fly'. The non-foil boat would have turned back long ago, but because you were flying above most of the waves you are now deep into a situation your craft was never meant to handle.

> The homepage claims superior ride and comfort in bad sea states.

For a limited definition of bad sea states--the FAQ says the limit before you might unexpectedly land is about 1m wave height, which wouldn't be a serious issue for an ordinary boat almost 8m in length [1].

That said, I do think there's a place for hydrofoils on sheltered bodies of water. There is (or was, 10 years ago) a regular hydrofoil ferry service that runs between Petrozavodsk and the island of Kizhi [2], which cuts travel time dramatically compared to a traditional ferry.

[1] https://www.rms.nsw.gov.au/maritime/safety-rules/before-boat...

[2] https://en.wikipedia.org/wiki/Kizhi_Pogost

Condor Ferries used to run hydrofoils between the Channel Islands, the UK and France. https://en.wikipedia.org/wiki/Condor_Ferries. However they retired their last hydrofoil in 1993.

https://www.youtube.com/watch?v=wJLnJx9SzR8

https://thesarnian.com/guernsey-history/condor-1-hydrofoil-f...

I went on one once. It was disappointing. Going above decks was no longer allowed, so it was a bit like a very bumpy airplane ride with everyone sitting in their seats and nervously eyeing the sick-bags.

Unless it changed recently, there is a regular hydrofoil line linking Vienna to Bratislava on the Danube. I rode it in 2015, it's a wonderfully kitsch piece of Soviet era kit, but does this beast fly!

I rode one 20 years ago to islands off the coast of Greece. One of the trips was rough though due to choppy seas. I recall seatbelts were required and barf bags were utilized by a few passengers.

At sea, sure, but on a large river like the Danube it'extremely smooth, hardly any wave :)

Every hit the wake from a larger boat? That a river is generally flat doesn't mean that you won't occasionally hit a freak wave.

Regardless of type of boat, waves need to be considered and managed. Most of the time speed needs to be reduced. The same for Candela, when a wave is to high it works to operate just like a regular boat.

Rode one between HK and Macao. Turbine driven, airplaine seat with seat belt fasten !!

Thanks for finding that. A 1m wave height limit makes it basically useless in the North Pacific.