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We are about to sail to arctic seas to check out a boat that has been attacked. I am trying to figure out an escape if we get attacked. What I am wondering is: if my cleric is in the back of the boat can he cast Gust of Wind directed at the sails successively in an attempt to escape an ambush?

The spell description says the gust "blasts from you" and lasts a minute. I don't think it matters that I would be moving with the boat. I would think I could get 13 minutes of wind as I am level 9 Tempest Cleric. It is a pretty strong gust, but I would think they could adjust the sail to work.

There is floating ice in the area to hide behind: I am hoping that a short sprint might let us get away. Probably have the druid cast Fog Cloud behind us to further mess them up.

Does this sound viable or am I missing something?

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    \$\begingroup\$ Don't forget you could also point the Gust of Wind at the ambushing ship's sails to slow them down or smash one ship into the other! \$\endgroup\$ – RobertF Oct 22 '15 at 15:12
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Sounds viable.

On the water...

I think your read that your motion doesn't matter is right. It's not analogous to the ineffective method of pointing a fan (from the stern) at the sails because... well, magic. In real life mounting a fan at the stern and pointing it at the sails doesn't work (well...1) because of the "reaction force" generated when the air is pushed forward.

But this spell clearly doesn't create a reaction force: the spell description contemplates pushing creatures away from the caster, but doesn't call out any "pushback" the caster feels. (Because he doesn't. Because magic.) So then "a line of strong wind... blasts from you." The plain reading of wind blasting from you is to say that "the air near you moves with some speed relative to you." You stand at the stern, look at the sails, and magically air behind the sails starts hitting them as a strong breeze emanates from you.

(How strong? The spell description gives us creatures in the area having difficulty walking (2'->1' typical "difficult terrain" rules). I equate this with the descriptor "effort needed to walk against the wind" given in the Beaufort Scale in the entry for "high wind, moderate gale, near gale." But the description also calls it a "strong wind," so perhaps we should look at the weaker Beaufort "strong breeze." Even then, it's 21-27 kts, or 35-46 fps.)

Now, hopefully you were running when you cast the spell, and you've just given yourself a significant boost. How significant a boost? That gets real complicated, real fast; @KorvinStarmast's answer does a good job of pointing out the tip of that iceberg.

1 - I really didn't want to get too much into the weeds on IRL fan-boating vs. magic fan-boating. The weird quirk being that IRL fan-forward works but is less efficient than fan-backward, whereas in magic-land the reverse is true!


At the table...

If you want to play out the chase at all, I recommend you discuss these plan with your GM ahead of time. Expecting your GM to be able to deal with windspeed differentials, point-of-sail issues, hull speeds, &c. on the fly may not yield you the chase scene you may be looking for.

Even if you don't want to game out the chase, you should give your GM a heads-up anyway. Even if you're content with a "good thinking, you get away with no trouble" your GM should have the opportuity to ponder how this will impinge upon your spell slots.

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    \$\begingroup\$ Pointing a fan at a sail does work. See this question physics.stackexchange.com/questions/135548/… \$\endgroup\$ – Gusdor Oct 22 '15 at 8:15
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    \$\begingroup\$ @Gusdor Of course, the answers to that question points out that it would be far more efficient to just have the fan pointing backwards instead. \$\endgroup\$ – eirikdaude Oct 22 '15 at 11:51
  • \$\begingroup\$ Fans experience a reactionary force. Fonts of divine power do not. Because magic, as nitsua60 pointed out. \$\endgroup\$ – Slacklord the Terrible Oct 22 '15 at 14:18
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    \$\begingroup\$ @Gusdor I've taught both sailing and physics for multiple decades. It's sooooo hard not to produce an inappropriately-long post/article/online course in response to this question. =) \$\endgroup\$ – nitsua60 Oct 22 '15 at 14:54
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    \$\begingroup\$ Definitely agree about giving the DM a heads-up. After all, if the first time he hears about this plan is when you do it, you may end up arguing about whether or not it works. I mean, there's a reason you felt the need to ask this question. \$\endgroup\$ – jhocking Oct 22 '15 at 18:23
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This can work

Besides "talk to your DM" your idea is viable. What you and the DM need to work out is "How strong is a gust of wind?"

Conservative Estimate

From the spell descriptions, a good estimate is that it is blowing at least ten mph (though I prefer the nautical knots, the book uses 10 mph which is a bit less). How did I arrive at that?

That much wind, at least 10 mph, will disperse a Fog Cloud. (PHB, p 243 spell description). Fog is similar enough to "dispersing gas or vapor," which the Gust of Wind spell description says it will do. (PHB. p. 249).

Generous Estimate

The Gust of Wind could be something like a strong breeze, Beaufort scale 25-30 mph, but the book doesn't say that it uses that scale. - idea from nitsua60's comment to this answer - See if your DM will buy that. Since the spell description indicates that it pushes people back, or makes walking into the wind difficult, that larger number is a valid argument.

What does this do to your boat?

If you get at least a 10 MPH tailwind boost that your opponent doesn't have, and a strong wind isn't already blowing, you get just under a quarter of an hour's lead on him (13 minutes per your spell budget), so we figure out now: How far is that?

That depends. How fast can your boat go?

Depending on how granular you DM is with sailing boats and how they work, hull length is the prime factor in maximum possible speed, so you may not get a 10 MPH boost compared to your pursuing boat, but some fraction of that: 1/4 to 2/3.

A conservative and playable estimate would give you a half-mile to one-mile lead, if you are pursued by a boat without your advantage. However, if a strong wind is already blowing (in excess of 25 knots) you may both be at your max speed already, and the extra wind may only make handling your boat more difficult.

If you are both tacking more or less into the wind, at an angle, your tail wind may allow you to increase your speed, depending upon how you direct it ... how many sails does your boat have, and what is its rigging?

Present both to the DM, be happy with Conservative Estimate

You are running with more wind than your pursuit, so getting a lead on your pursuer should be an easy case to make with your DM.

A larger lead might be a tougher sell, depending on how rigorous your DM is about ships and their speeds in real world detail. The other potential inefficiency is that the sail area versus spell area may require a deduction. Per spell description, the gust is 60 feet long and ten feet wide ... that shape may or may not hit 100% of the sail area.

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I generally don't approve of applying real-world Dynamics to D&D but...

Don't cast the spell into your sails, cast in in your hold. The point of the sails is to catch the wind effectively, which is generally not inside your ship. However your ship will catch the wind just as effectively belowdecks as above, assuming you orient the spell properly. This prevents the enemy from having line of effect to your spell (or you!), and allows you to construct a controlled environment designed to maximize the amount of wind you absorb (which is very important if you want to optimize your escape plan).

This wind-energy plug from the University of Oregon contains some basic info we can use to figure out how much faster our ship will be going when we apply the magic turbo booster:

For average atmospheric conditions of density and moisture contant:

Power per sq. meter = .0006 V^3

Now, you're sailing in a Polar sea, so you are indeed very very close to 'sea level' and density should be about right. Counter Intuitively, dry air is actually denser than humid air (I just learned this because of this question, props for that), so your very low moisture content will actually add with your very cold conditions to even more significantly increase your density. The density of dry air at 0 degrees celsius at sea level is 1.294 kg/m3, which is about 10% more than 'average' sea level conditions. The coefficient in the above equation goes like m^2 so this adjusts it to about 1.21 X as much. All in all, we have:

Change in Power/m^2 = .0007 (Change in Relative Speed)^3

so, when you use the spell, we need to know only how much the local wind speed changes, and how much area is hit orthonormally by the wind. I think a Beaufort 'strong breeze' is a reasonable assumption.

The Gust of Wind spell affects a Volume, not an area, so the amount of area we affect is merely a function of how good a 'sail' we can have the spell hit. Ideally our 'sail' mechanism will be a set of 10 ft X 10 ft (or larger) rigid bodies in sequence with syncopated holes in said bodies for line of effect. The boat will thus receive the noted power for each fin in the sail.

Now, there are two possibilities here:

1) the windspeed of Gust of Wind is relative to you/the boat so if the boat speeds up the wind will speed up relative to the ocean as well.

2) the windspeed is always relative to the Plane

In the second case, you will basically end up travelling at the speed of the gust of wind for the duration of the spells, as the V in the formula rapidly goes to zero with a fair number of sail-fins.

In the first case, the change in speed results in a new, faster, wind speed, and successive castings can efficiently accelerate you to arbitrarily high speeds with sufficient sail-fins (though the integrity of your boat will become an issue, as the drag of the water goes like V^2 above a certain speed).

In either case, this is an effective escape plan.

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  • \$\begingroup\$ -nitpicky-, your "second case" only works on a catamaran type hull. I like the answer. \$\endgroup\$ – KorvinStarmast Oct 22 '15 at 12:20
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It absolutely does not matter whether the wind produces a reaction force on the caster.

If the wind does not produce a reaction force (i.e. the caster induces the air to move, magically violating conservation of momentum), the caster can simply cast the spell at the sails, as originally intended.

However, if the wind does produce a reaction force on the caster, the caster can simply stand on the boat and cast the wind backwards, opposite the direction she intends the ship to move in. This should produce an identical level of impulse on the boat.

Therefore, no matter what the nature of momentum for the spell is, you will definitely be able to speed up your boat.

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    \$\begingroup\$ The OP asked, "Hey, I want to do X. Will it work." The answer March Ho gave is correct, "It might work depending on physics of spell casting ... if it doesn't this will instead." \$\endgroup\$ – J. A. Streich Oct 22 '15 at 15:06
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    \$\begingroup\$ @J.A.Streich “You can get impulse from the spell” is a far cry from “yes, it will actually be enough impulse, because [evidence], to be effective for escaping.” This answer does not contain anything addressing the point of the question. It would be improved by including an attempt to answer the question itself, instead of merely weighing in on the reaction/reactionless controvery in other answers. \$\endgroup\$ – SevenSidedDie Oct 26 '15 at 17:47
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This really depends on a couple factors. I happen to be a sailor in real life, so I hope this is taken in the right spirit.

Assuming the spell does not create a reactive force (I.e. The cleric isn't the source of the wind but is simply rather causing the wind to blow), this should create some drive.

On the other hand. Sailboats have a speed limit, based on how long they are at the waterline. If there is already enough wind to drive the boat at full speed, additional wind will not have any appreciable affect. However and alternatively, if the wind is dead calm, a sustained gust can bring you from a dead stop to full speed.

In the real world, it will take a lot more than 13 minutes to make any gain ahead of a pursuer. Again, if the wind is already blowing a gust won't gain you much. Let's say for this example, you gain one additional nautical mile per hour (close enough to a regular statute mile for this conversation) that mean it will take a whole hour to make a 1 mile headway against your pursuer. On the water, that's really not very far to "get lost".

Now as said earlier, if the wind is dead, then I think this could be useful in getting away from another sailboat giving you some distance between you.

And finally...the longer the boat, the faster it can go, so a long slim boat will go faster than a shorter fat boat, sometimes by a lot. My 23 ft sailboat (16ft at the waterline) can only go 5, maybe 6 knots if it's being pushed really hard, whereas a 65 foot vessel with a 50 ft waterline can easily reach 9 or 10 knots if there is enough windage.

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I agree with the other posters about it should work because magic. Assuming that your DM doesn't go for it (because reasons), and pursuit is in the form of a sailed ship, you have another sail-based option: cast Gust of Wind on your pursuer's sails, reducing their effectiveness.

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    \$\begingroup\$ If you check the range of the spell you may find that this answer doesn't quite help with the problem statement. "I am hoping that a short sprint might let us get away." To get away probably means to increase the distance between the two boats, so the range at which the cleric can target the other boat soon limits how far apart he can keep the two boats. Suggest considering the range of the spell and the intended use of it from the question again. \$\endgroup\$ – KorvinStarmast Oct 22 '15 at 20:41

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