Can any land-based plane be converted to a seaplane by installing skids? For example, is it possible to convert a Boeing 747 to one that is capable of landing and taking off on water using pontoons or planing hulls?

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    $\begingroup$ Not if you are concerned with the effect of salt-water on otherwise untreated parts. There's a good reason that most sea-planes have engines that are fairly high from the water. This just won't work with jets and under-slung engines because they'll suck in sea-water. Put them on huge pontoons and now you have an easily capsizable airplane. $\endgroup$ – Ron Beyer Mar 30 '17 at 19:33
  • $\begingroup$ @RonBeyer it may suck in salt water but would it get a rise flying through rain? the skids could also be on the wings. $\endgroup$ – user20435 Mar 30 '17 at 19:38
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    $\begingroup$ @Muze your questions are often unclear, ambiguous or invite many possibilities. They often sound like a general discussion rather than a precise, to-the-point question. Here at Stack Exchange, we look for questions that have a narrow scope and focus on one specific topic. I've edited this question (hopefully without distorting your meaning), but I'd encourage you to work on your question quality if you wish to keep your ask question privilege from being banned again. $\endgroup$ – kevin Mar 30 '17 at 19:38
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    $\begingroup$ What do you mean "a rise flying through rain"? Sucking in some rain water isn't even close to the amount of sea (salt) water being sucked in from an almost unlimited supply a few feet below the engine. Sea planes don't land on "skids". Sea planes have pontoons or planing hulls, but not skids. $\endgroup$ – Ron Beyer Mar 30 '17 at 19:40
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    $\begingroup$ @kevin I will try harder. thanks $\endgroup$ – user20435 Mar 30 '17 at 19:45

Well "any" is an awfully broad statement, but the answer is "Theoretically yes." -- You would just need floats that displace enough water to provide buoyancy for the aircraft at its maximum gross weight.

For a 747 that would be impractical: We're talking about a plane with a maximum takeoff weight in the neighborhood of 700,000-900,000 pounds depending on model and variant, and that would be an ENORMOUS set of pontoons. The form drag alone would be prohibitive, nevermind the structural work required to attach the pontoons in such a way that they won't be ripped off at takeoff speed in the water or blow off the plane in cruise flight.

For smaller aircraft it's certainly do-able: You can take a Piper Cherokee 180, remove the landing gear, and install floats (Piper kit #756-746) to make it into a seaplane:
Piper PA-28-180 Piper PA-28S-180
Note the structural bracing for the floats, which attach both at the main landing gear point and the nose landing gear point to enable them to take the loads imposed by taxiing through water. There are also a bunch of additional seals and blanking plates installed as part of the seaplane kit to prevent the aircraft from taking on water.

Note however that just because something is possible doesn't mean it's a good idea, and the PA-28S is an example of that: The low-wing design of the Cherokee coupled with its single door on the right side makes the seaplane version an exercise in impracticality.

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    $\begingroup$ Really good answer, but the PA28s door is on the right side, not on the left side ;) $\endgroup$ – pcfreakxx Mar 30 '17 at 20:12
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    $\begingroup$ @pcfreakxx No, your OTHER left! (I fly one of these things so you think I'd know what side the door is on, but no.) $\endgroup$ – voretaq7 Mar 30 '17 at 20:13
  • $\begingroup$ Me too and you can even see it on the first picture: It's the right side :D $\endgroup$ – pcfreakxx Mar 30 '17 at 20:21

You could, but why would you want to.

Yes, in theory, an 400,000 kg airplane like a 747 could be fitted with floats capable of displacing the 400 cubic meters of water required to keep it afloat, albeit with a decrease in performance. It's not that unrealistic; if a 747 can carry a space shuttle on its back it stands to reason that it can haul a pair of floats as well. As to whether current naval architecture can create a structure light enough for this yet strong enough to allow for water operations remains to be seen.

But since a 747 already has a 7000+ mile range and can reach any number of large terrestrial airports on a single tank of gas, there really is no need to fit the aircraft with floats for water operations.

  • $\begingroup$ Nitpicking: Didn't the space shuttle contribute lift as well as drag while riding on the 747? $\endgroup$ – DJohnM Mar 31 '17 at 15:50
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    $\begingroup$ yes, then again so do pontoons. The point being that if a 747 can carry a large, heavy object like that outside of its aerodynamic envelope, it is reasonable to believe that it could operate with large pontoons for sea operations as well. $\endgroup$ – Carlo Felicione Mar 31 '17 at 17:33

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