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In general, if a seaplane can fly X distance, can it travel that distance on the surface of the water as if it was a boat or ship? In other words, this would be equivalent to taxiing all the way to a destination. E.g. if a seaplane has the range to fly from New York to Amsterdam, could a pilot simply choose to taxi all the way across the Atlantic Ocean and casually slip into the harbor?

If this is not generally possible, what rules define what sort of sea journeys a seaplane could undertake? Is there a coefficient of inefficiency (e.g. if a seaplane can fly X miles, it can taxi X/2 miles due to increased drag from the water)? Are the vast majority of seaplanes not set up to handle the stresses of a long-distance sea trip, making them likely to result in a shipwreck (er, seaplanewreck)?

This is not the same question as Is it possible to sail a seaplane? . That question is asking about sailing a seaplane using sails without any engines. I am asking about "sailing" a seaplane using the engines and whatever other tools are available for taxiing.

This question is about the physics and engineering issues - not about licensing. I am fully aware that taxiing a seaplane across a body of water might make the trip fall under boat regulations and require a sea captain's license, but I'm not asking about that.

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    $\begingroup$ Water is something like 8 times more viscous than air. It wouldn't have enough fuel to make the same trip floating as it would flying. $\endgroup$ – FreeMan Oct 30 '17 at 18:17
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    $\begingroup$ Not in the water, but just above it would be the best way. $\endgroup$ – Ron Beyer Oct 30 '17 at 18:23
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    $\begingroup$ @FreeMan It would if it would travel 64 times as slow. $\endgroup$ – Koyovis Oct 31 '17 at 0:51
  • $\begingroup$ Another factor here is that the engines (and probably the props) aren't efficient at low speeds, so even neglecting the higher water friction, you'd need a lot more fuel to travel a given distance. $\endgroup$ – jamesqf Oct 31 '17 at 19:21
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Not very well. If we look back to the 1930's, the age of the large flying boat airliners, they were designed to take off and land in minimal sea states - no more than 3 foot swells. Typical usage would be in large rivers or protected bays, and the flying boat was used because long runways weren't available. Nor did those airliners tend to stay on the water for long, usually they were brought onto land with beaching gear if they weren't in service.

The problem is - an aircraft on water is highly susceptible to changes in wind - it is, after all designed specifically to use flowing wind to remain airborne, so gusts of wind while on the water will have a stronger effect on an aircraft than they would on a ship. As well, the aircraft's hull in the water tends to anchor it, subjecting the airframe to stresses from wind in varying directions, that it wouldn't encounter while flying.

Amphibious aircraft and the pure flying boats, left on the open ocean, tend to become damaged to the point of becoming non-airworthy.

Couple of examples:

While on lifeguard duty off of Truk during WW2 and landing to pick up a shot down pilot, Lt John Dowdle's OSU-2 Kingfisher had overturned due to a gust of wind. Lt John Burns also lost his Kingfisher due to the hull springing leaks from being pounded by waves, while he rescued Dowdle and the downed pilots. Both Kingfishers were lost, but their crews plus the airmen they had rescued were saved by submarine USS Tang, in an epic lifeguard mission that saw Tang pick up 22 aircrew.

The Bermuda Sky Queen, a Boeing 314 formerly known as the Pan American Capetown Clipper, ran short of fuel while flying from Foynes, Ireland to the US. It did survive for over 24 hours on the ocean before the 62 passengers and crew could be rescued, but after that much time on the open ocean, the plane was coming apart from the pounding of the waves (the tail section was starting to come loose) plus damage from colliding with the Coast Guard cutter picking up the people. As a result, the Bermuda Sky Queen, the last airworthy 314, had to be scuttled.

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    $\begingroup$ I once learned how to drive a motor boat and I agree with this answer. Going from the harbor area to some outer islands then further out to the Pacific, the difference in swell is huge. The largest practical obstacle is not efficiency - a seaplane would likely disintegrate / sink before it runs out of fuel. $\endgroup$ – kevin Oct 31 '17 at 5:31
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    $\begingroup$ @kevin I own a motor boat and take it out on the Indian Ocean for fishing. It is clear to every boat driver that small boats are not the best.for crossing oceans. Sea state is an issue, but only part of the question. $\endgroup$ – Koyovis Oct 31 '17 at 8:37
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    $\begingroup$ @Koyovis the main question is can a seaplane make a long distance sea voyage, this answer provides a valid reason why it couldn’t, looking from a structural point of view. It’s a different view from your answer but equally valid (possibly more valid). $\endgroup$ – Notts90 Oct 31 '17 at 9:13
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In the water: it could reach its fiinal destination, if:

  • it would travel very slowly, as slowly as a boat;
  • it could use water for creating thrust instead of air;
  • the sea state would be calm.

enter image description here Image source

The main drag resistance on a boat is the surface wave (wake) drag, much higher than the submerged incompressible drag deeper down. Wake drag increases dramatically at a certain speed-to-length ratio, like hitting a brick speed wall. That is why hydrofoils work: they lift the vessel out of the water and eliminate wake drag.

The other bit about range efficiency is in the propulsion. Highest efficieny is obtained by accelerating a high mass stream to a lower velocity, and water has a density that is 1000 times that of air. Let that ships' propeller lazily turn about to propel the long supertanker to a slow speed, and there is not much that can beat that efficiency. Perhaps a road train running at 5 km/h. The flying boat propelling air cannot do it.

Speed is the governing factor in travel efficiency. Aircraft only exist because people want to be somewhere now.

Are the vast majority of seaplanes not set up to handle the stresses of a long-distance sea trip..?

Flying boats need to fly and are not designed for handling violent seas. They sit relatively high in the water and would topple over if they had no outrigger floats - picture that set-up in a storm and it is easy to see a wing break off or the craft capsizing.

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From here looks like an airboat (propeller driven boat) is not very efficient mean of transportation, using somewhat 0,1 l of fuel per minute (so 6 l per hour) when moving at roughly 2 knots (so 3,7 km/h), so only 0.6 km per liter. Assuming we have somewhat 160 l fuel tank, this is enough for about 100 km journey.

If it the "airboat" is actually a seaplane, probably would rather fly. It may be even less efficient as has not been designed for taxiing on water over hundreds of km.

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Is it possible using the engines/propellers, at taxi speed, if you have a mean to refuel.

As the fuel consumption will be about the same per unit of time, the cruise speed vs taxi speed ratio is meaningfuel, you will need to refuel 15 or 20 times.

You don't use anymore lift, and the craft is not anymore an aircraft. This possibility would be silly. The solution is to increase speed by lifting the hull with hydrofoils.

enter image description here
Lisa taking off with hydrofoils, source

The actual lifting surfaces are now the hydrofoils (air/hydro foils), they create lift from the water flow. Since the craft is only used over water, it can be optimized, and the wings can be shortened.

enter image description here
Source: Wikipedia

As a hydrofoil craft gains speed, the hydrofoils lift the boat's hull out of the water, decreasing drag and allowing greater speeds.

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    $\begingroup$ I think the question is about whether one can use an existing seaplane to cross an ocean, not whether it is feasible to design a craft specifically to cross the ocean. $\endgroup$ – kevin Oct 31 '17 at 5:22
  • $\begingroup$ @kevin: I clarified. $\endgroup$ – mins Oct 31 '17 at 7:44

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