Flying On A Moving Ship?

[SiderisAnon]

I've run into a physics of flight question in my new campaign, and I'm hoping some of you out there can help me figure it out. It has been a lot of years since college physics, and I'm not sure if I've got this right. (I realize I could hand-wave it all away as magic, but that doesn't sit well with me or most of my players.)

There are several flying races in my campaign. Two are medium sized, one is tiny sized. I'm using the tiny sized as an example because I've got a player who is going to be playing one.

A pixie is on a ship at sea, which is moving along at a good clip. The pixie takes flight from the deck and hovers. I know that if the ship is accelerating, once the pixie is not in contact with the deck, the ship is no longer accelerating the pixie along with itself, so the pixie is going to either be left behind or get hit by the raised wheel deck when it catches up with her. If the ship turns, it will turn right out from under the pixie.

What happens if the ship is neither accelerating or turning and stays at a steady speed? Unless the pixie's fly speed is sufficient to keep up with the ship, wouldn't the ship leave the pixie behind? (Or smack her into the rear wall?) From what I remember of physics, I believe that is exactly what would happen to a bird let fly from the deck, but since it's been a while, I might not be remembering correctly.

Would it be technically true that if the ship was moving faster than the pixie could fly, the pixie stays with the ship briefly because she was already moving at the ship's speed. Then as wind resistance and other factors slow the pixie down to her normal flight speed, the ship starts pulling away from her? (From her perspective, the ship is moving ahead.)

The same problem would occur in a wagon or a train. (There are steam engines in parts of my world.) The conveyance will accelerate away, leaving the pixie behind, unless she is standing on the deck or sitting in a chair, because there is no force acting on her to keep her with the train or wagon; right?


If my understanding of the physics is true, I am thinking that pixies in my world dislike ships and trains and wagons because of this. They can only make small hops to fly around, unless the vehicle is moving slowly, because otherwise they end up sliding toward the back of the vehicle.

So can anyone confirm that the physics are right or tell me what the correct physics are?

 

[mmadsen]

Most college physics problems simplify away air resistance, which is not trivial for something like a hummingbird or pixie, which has relatively big wings and little mass.

 

[Celebrim]

You've got it basically correct, though in an typical age of sail or earlier themed campaign, it wouldn't matter much.

Not only is a typical sailing ship probably slower than the pixie by a good deal, but the pixie would have the benefit of the same tailwind that the ship benefits from.

Most wagons would be bouncing along even slower than that. A pixie could fly circles around an ox cart. A chariot or an advanced carriage might be hard to keep up with if the horses were at a gallop, but then so would someone on a horse.

Advanced steamships (or trains) are a different matter. It could prove difficult to keep up with an advanced ship, especially if it was moving into the wind.

 

[Bagpuss]

As mentioned ships wouldn't be to significant, if they are sailing with the wind then the pixie would get blown along with it, so easiest to ignore it. If it's tacking into the wind or using a cross wind then there could be an issue, but then the ship probably wouldn't be moving as fast.

In an enclosed space, then the air inside the train or ship is getting pushed along with it so it won't matter. But if they were above deck or on top of a train or other fast moving vehicle that doesn't use wind propulsion then there would be a problem.

 

[JohnSnow]

Without getting into the details too much, the problem with using a tailwind to move is that you're running away from your motive force. So if a tailwind pushes you at 5 knots, and your ship moves at 5 knots, your real tailwind speed is now...0 knots, and you stop. That doesn't happen in the real world, because the water resists motion, so you still end up with a slight (but not great) forward momentum. To make up for this, ships running downwind have to let out extra sail to generate more pressure. They also tend to run slightly crosswind to make up for this. On a square-rigger, repositioning the sails for different angles of wind is a LOT of work, requiring a large crew.

Most sailing vessels move best with a mild crosswind. Which plays havoc with your hovering pixie. Now, while it's true that more primitive ships didn't handle headwinds well because they lacked either proper sails or proper hulls, some of them did okay (e.g. - Viking longships, which had good hulls, or lateen-rigged dhows, which had impressive sails). In the real world, a well-keeled hull to prevent side slippage and the lateen (as opposed to square) sail weren't brought together on the same vessel until the 19th century. This is despite the fact that both existed, independent of one another, in different parts of the world, 1000 (or more) years earlier. Combine the two, and you can sail almost directly into the wind (as a modern sailboat does).

That's probably more than you wanted to know about the physics of sailing ships.

My suggestion to simplify your life: don't have a flying race that can do nothing but hover. Let them make short hops or bursts of flight until they develop true flying at a level you deem appropriate.

My two cents.

 

[Celebrim]

Without getting into the details too much, the problem with using a tailwind to move is that you're running away from your motive force. So if a tailwind pushes you at 5 knots, and your ship moves at 5 knots, your real tailwind speed is now...0 knots, and you stop. That doesn't happen in the real world, because the water resists motion, so you still end up with a slight (but not great) forward momentum. To make up for this, ships running downwind have to let out extra sail to generate more pressure. They also tend to run slightly crosswind to make up for this. On a square-rigger, repositioning the sails for different angles of wind is a LOT of work, requiring a large crew.

Most sailing vessels move best with a mild crosswind...

Great comments. However, on a ship-rigged vessel, the best speed is going to be close enough to a tailwind ('running free') as to make little difference. Your point is certainly true on a latteen rigged vessel, which will probably be 'reaching' to get its best speed, but if we presume something like medieval technology, mostly we will be dealing with square sails that basically only travel in the way that the wind is going.

 

[Pbartender]

Great comments. However, on a ship-rigged vessel, the best speed is going to be close enough to a tailwind ('running free') as to make little difference. Your point is certainly true on a latteen rigged vessel, which will probably be 'reaching' to get its best speed, but if we presume something like medieval technology, mostly we will be dealing with square sails that basically only travel in the way that the wind is going.

I'd always learned the best point of sailing for a square-rigged ship is, in general, a wind from either of the aft quarters.

A square-rigged ship running free can't use all its canvas... The aft sails would steal the winds from the fore sailsmaking them practically useless. So, while you get the most out of the sails you can use, you can only use a third or a quarter of your sails to actually catch the wind.

While a quartering wind allows the ship to use all its sails and present the geatest surface area of canvas to the wind, while still getting reasonable forward momentum from the wind's push... You are, perhaps, halving the amount of push you are getting from the wind by going just a bit crosswise to it, but at the same time, you've got three or four times as much sail to get a push from.


As far as the flying pixie is concerned... Anything with a movement rate of 30 or 40 feet per round will be able to keep pace with any type of sailing vessel without effort, and will be able to outpace it by hustling. Pixies, with a 60 ft fly speed and good maneuverability should be able to fly rings around any D&D sailing ship.

 

[Rothe]

....
What happens if the ship is neither accelerating or turning and stays at a steady speed? Unless the pixie's fly speed is sufficient to keep up with the ship, wouldn't the ship leave the pixie behind? (Or smack her into the rear wall?) From what I remember of physics, I believe that is exactly what would happen to a bird let fly from the deck, but since it's been a while, I might not be remembering correctly.
...

You have it right except for this part. Ignoring wind that may be pushing the pixie around more than it can counter, if the ship does not accelerate once the pixie leaves the deck then the will not leave the pixie behind.

The ship is at velocity v when the pixie leaves the deck. The pixie accelerates straight up in a direction perpendicluar to v and stops, so the pixie still has veolcity v. Absent a force to change v, the pixie will retain velocity v. This is Newton's first law of motion. As the ship also does not change velocity the relative position of the pixie to the ship will not change.

 

[SiderisAnon]

My suggestion to simplify your life: don't have a flying race that can do nothing but hover. Let them make short hops or bursts of flight until they develop true flying at a level you deem appropriate.

Sorry if I wasn't clear there. I meant the pixie in the example CHOSE to hover. The pixies have full flight from a very early age. I was using the hover example to simplify what the pixie was doing.

 

[Pyrex]

From a practical standpoint, there are three solutions:

1) The pixie must expend 0' of movement each round to keep up with the ship.
2) The pixie must expend 0'< n < ShipSpeed' in movement each round to keep up.
3) The pixie must expend ShipSpeed' in movement each round to keep up.

Neither 1 nor 3 feels reasonable to me, to (imo) the answer must be 2.

Absent storm conditions, I'd probably require the pixie to expend 0.5*ShipSpeed' of movement per round to keep up.

So if the ship was sailing at 30'/round, any flying creature within ShipLength' of the ship needs to spend 15' of movement/round to maintain their relative position. The 'free' (or 'remaining' if you prefer) 0.5*ShipSpeed (15') represents them being "swept along" by the wind propelling the ship.