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?
Best Answer
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.