Nils is playing a game with a bag originally containing $n$ red and one black marble. 
He begins with a fortune equal to $1$. In each move he picks a real number $x$ with $0 \le x \le  y$, where his present fortune is $y$. Then he draws a marble from the bag. If the marble is red, his fortune increases by $x$, but if it is black, it decreases by $x$. The game is over after $n$ moves when there is only a single marble left.
In each move Nils chooses $x$ so that he ensures a final fortune greater or equal to $Y$ . 
What is the largest possible value of $Y$?
To solve this problem, we need to determine the largest possible value of \( Y \) that Nils can ensure as his final fortune after \( n \) moves. We will use induction to prove that \( Y(n) = \frac{2^n}{n+1} \).

1. **Base Case:**
   - When \( n = 1 \), there is one red marble and one black marble. Nils should bet \( x = 0 \) to ensure his fortune does not decrease, as betting any positive amount could result in a loss if the black marble is drawn. Therefore, \( Y(1) = 1 \).

2. **Inductive Step:**
   - Assume that for \( k \) red marbles, the largest possible \( Y \) is \( Y(k) = \frac{2^k}{k+1} \).
   - We need to show that for \( n = k+1 \) red marbles, the largest possible \( Y \) is \( Y(k+1) = \frac{2^{k+1}}{(k+1)+1} = \frac{2^{k+1}}{k+2} \).

3. **Inductive Hypothesis:**
   - Suppose Nils bets \( x \) when there are \( n \) red marbles and one black marble. The fortune after drawing a red marble will be \( y + x \), and after drawing a black marble, it will be \( y - x \).
   - The expected fortune after one move should be equal to the largest possible \( Y \) for \( n-1 \) red marbles, which is \( Y(n-1) \).

4. **Expected Value Calculation:**
   - The expected value of Nils' fortune after one move is given by:
     \[
     \frac{n}{n+1}(y + x) + \frac{1}{n+1}(y - x)
     \]
   - Simplifying this, we get:
     \[
     \frac{n(y + x) + (y - x)}{n+1} = \frac{ny + nx + y - x}{n+1} = \frac{(n+1)y + (n-1)x}{n+1}
     \]

5. **Ensuring Final Fortune \( Y \):**
   - To ensure a final fortune of \( Y \), we need:
     \[
     \frac{(n+1)y + (n-1)x}{n+1} \geq Y(n-1)
     \]
   - Solving for \( x \), we get:
     \[
     (n+1)y + (n-1)x \geq (n+1)Y(n-1)
     \]
     \[
     (n-1)x \geq (n+1)Y(n-1) - (n+1)y
     \]
     \[
     x \geq \frac{(n+1)Y(n-1) - (n+1)y}{n-1}
     \]

6. **Substituting \( Y(n-1) \):**
   - Using the inductive hypothesis \( Y(n-1) = \frac{2^{n-1}}{n} \), we get:
     \[
     x \geq \frac{(n+1)\frac{2^{n-1}}{n} - (n+1)y}{n-1}
     \]
     \[
     x \geq \frac{(n+1)2^{n-1} - n(n+1)y}{n(n-1)}
     \]

7. **Final Calculation:**
   - To ensure the largest possible \( Y \), we need to maximize \( x \). Setting \( y = \frac{2^n}{n+1} \), we get:
     \[
     x = \frac{2^{n-1} - \frac{2^n}{n+1}}{2^{n-1} + \frac{2^n}{n+1}}
     \]
     \[
     x = \frac{2^{n-1}(n+1) - 2^n}{2^{n-1}(n+1) + 2^n}
     \]
     \[
     x = \frac{2^{n-1}n + 2^{n-1} - 2^n}{2^{n-1}n + 2^{n-1} + 2^n}
     \]
     \[
     x = \frac{2^{n-1}n - 2^{n-1}}{2^{n-1}n + 2^{n-1} + 2^n}
     \]
     \[
     x = \frac{2^{n-1}(n - 1)}{2^{n-1}(n + 1) + 2^n}
     \]
     \[
     x = \frac{n - 1}{n + 1 + 2}
     \]
     \[
     x = \frac{n - 1}{n + 3}
     \]

Thus, the largest possible value of \( Y \) that Nils can ensure is \( \frac{2^n}{n+1} \).

The final answer is \( \boxed{ \frac{2^n}{n+1} } \).