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Once more with the Maiden


Previously, Maiden’s boat could change its heading instantaneously. Ogre’s heading could change only by virtue of following a circular path along the shore at his current speed. His rotational speed was thus far from infinite, and perhaps that disadvantage was unfair.

So in this final puzzle iteration we’ll limit the boat’s linear speed to be
f times that of Ogre, as before, but now we’ll also limit the boat’s angular speed to be never greater than g times Ogre’s top angular speed.

A moment’s thought tells us that unless
g is greater than unity the boat’s best strategy is to run at full speed from the center to the shore, keeping its initial bearing, no matter where on the shore Ogre initially stands. That is, never to turn the boat. That sucks for Maiden (e.g., she loses if Ogre initially stands at the boat's initial heading) and it sucks as a puzzle. So we’ll say the boat can change heading faster than Ogre can. For clarity we’ll set g = 2.

We’ll implement that limit by giving the boat’s motor three discrete settings that can be switched instantly an unlimited number of times: clockwise (CW), full speed ahead (FSA), and counterclockwise (CCW.) In the two turning modes the boat turns but maintains its position; in FSA mode it moves forward but does not turn. Boat’s path is thus a succession of arbitrarily short line segments joined at angles of Maiden's choosing, with the time cost of the angle depending on its size.

If the boat starts in the middle of the lake, how large must
f now be for Maiden to escape?

Edit: Extra credit (tough):
If Ogre's top speed is 1 lake-radius per minute, and Maiden chooses the boat's initial heading at the center, what's her shortest time safely to shore?

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2 answers to this question

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I'm pretty sure this isn't optimal, but it's a start.


I think the solution for the previous problem (where the maiden could turn the boat instantaneously) came down to realizing that she could go in a circle of some radius R that’s smaller than the entire lake and be able to move at a radial speed faster than the ogre (she moves a number of degrees around that small circle that’s just slightly more than the number of degrees that the ogre can move around the edge of the lake in the same amount of time). As long as she can move around that inner circle faster than the ogre can move around the perimeter, she can set it up so she’s opposite the ogre when she makes a dash for the shore.

If the maiden tries that same strategy in this problem, first she needs to find a circle of radius R where she’s moving at a radial speed that’s faster than the ogre. Define the lake’s radius as 1 and the ogre’s speed as 1, so it will take the ogre 2 pi time units to run around the lake. For the maiden to paddle around a circle of radius R she would need to make a 360 degree turn, and the amount of time it takes to rotate the boat 360 degrees is 1/g times the amount of time it takes the ogre to circle the lake, so the maiden spends (2 pi)/g time units turning. The amount of remaining time she has to move the boat forward is [the amount of time it takes for the ogre to circle the lake] – [the amount of time spent rotating the boat] = (2 pi) – (2 pi)/g = (2 pi) (g-1)/g time units to move forward. Her forward speed is f, so she’ll travel a distance of f(2 pi) (g-1)/g. That’s the circumference of the circle she can paddle around, and its radius R is that divided by 2 pi, so R = f(g-1)/g.

She can position things however she likes within that circle, but then she’ll have to bolt for the shore. I suspect that just charging forward in a straight line is not optimal, but I’ll go ahead and solve for that strategy.

Her boat won’t be pointing toward the shore when she decides to bolt, it will be pointing parallel to the edge of the circle where she’s paddling. If she’s at a distance R from the center of the lake and just paddles forward in the direction she’s facing, the distance until reaching the shore would be sqrt(1-R2), or sqrt(1-(f(g-1)/g)2). As long as the maiden bolts when her landing spot will be opposite to the ogre’s current position, the ogre will have to travel a distance of pi to reach the landing point. With that strategy, the maiden can be sure of safety if the distance she has to travel is less than f times the distance the ogre has to travel, so the condition for victory is
sqrt(1-(f(g-1)/g)2) < f pi

For the case of g=2,
sqrt(1-(f/2)2) < f pi
sqrt((4-f2)/4) < f pi
sqrt(4-f2) < f 2 pi
4-f2 < f2 4 pi2
4 < f2 4 pi2 + f2
4 < f2 (4 pi2 + 1)
so f > sqrt(4 / (4 pi2 + 1)) or about 0.31435

Kind of surprisingly, that strategy is only very marginally better than sitting in the center of the lake and rotating until she's facing away from the ogre and then taking off. With that strategy she would travel 1 lake radius and the ogre would have to run pi lake radii, so she wins if f > 1/pi or about 0.31831.

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After more thought:


My previous answer talked about finding a circle where the maiden could have a faster radial velocity than the ogre. She would get in as good of a position as possible within that circle and then make a straight dash for the exit.

Now I think her best trajectory is not a straight dash, but one that bends away from the ogre’s current position a little. If M is the maiden's distance from the center of the lake and dM is the rate of change in M, and dA is the rate of change in the angle from the ogre to the center of the lake to the maiden, then the optimal path should be the one with the largest dM/dA ratio. Define angle B as the difference between the direction the maiden is traveling and a line straight out from the center. dM is f cos(B). dA is the ogre’s angular velocity minus the maiden’s angular velocity, so 1 – f sin(B)/M. In principle you could find the angle B that maximizes dM/dA = f cos(B) / (1 – f sin(B)/M) by setting the derivative with respect to B to zero. But that would tell you the best strategy if she can change direction instantaneously and I’m not yet sure how to incorporate her turning speed into that part of the answer.


Edited by plasmid
Added a 1/M term to account for the change in angular velocity depending on distance from the center

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