Find the distance $\overline{CF}$ in the diagram below where $ABDE$ is a square and angles and lengths are as given:
[asy]
markscalefactor=0.15;
size(8cm);
pair A = (0,0);
pair B = (17,0);
pair E = (0,17);
pair D = (17,17);
pair F = (-120/17,225/17);
pair C = (17+120/17, 64/17);
draw(A--B--D--E--cycle^^E--F--A--cycle^^D--C--B--cycle);  
label("$A$", A, S);
label("$B$", B, S);
label("$C$", C, dir(0));
label("$D$", D, N);
label("$E$", E, N);
label("$F$", F, W);
label("$8$", (F+E)/2, NW);
label("$15$", (F+A)/2, SW);
label("$8$", (C+B)/2, SE);
label("$15$", (D+C)/2, NE);
draw(rightanglemark(E,F,A));
draw(rightanglemark(D,C,B));
[/asy]
The length $\overline{CF}$ is of the form $a\sqrt{b}$ for integers $a, b$ such that no integer square greater than $1$ divides $b$. What is $a + b$?
1. **Identify the coordinates of points \( F \) and \( C \):**
   - Given that \( ABDE \) is a square with side length 17, we can place the square in the coordinate plane with \( A = (0,0) \), \( B = (17,0) \), \( D = (17,17) \), and \( E = (0,17) \).
   - Point \( F \) is given such that \( \overline{EF} = 8 \) and \( \overline{AF} = 15 \). Since \( \angle EFA = 90^\circ \), we can use the Pythagorean theorem to find the coordinates of \( F \):
     \[
     \overline{EF}^2 + \overline{AF}^2 = \overline{EA}^2 \implies 8^2 + 15^2 = 17^2 \implies 64 + 225 = 289 \implies 289 = 289
     \]
     Therefore, \( F \) is at \( (-\frac{120}{17}, \frac{225}{17}) \).

2. **Identify the coordinates of point \( C \):**
   - Point \( C \) is given such that \( \overline{DC} = 15 \) and \( \overline{BC} = 8 \). Since \( \angle DCB = 90^\circ \), we can use the Pythagorean theorem to find the coordinates of \( C \):
     \[
     \overline{DC}^2 + \overline{BC}^2 = \overline{DB}^2 \implies 15^2 + 8^2 = 17^2 \implies 225 + 64 = 289 \implies 289 = 289
     \]
     Therefore, \( C \) is at \( (17 + \frac{120}{17}, \frac{64}{17}) \).

3. **Calculate the distance \( \overline{CF} \):**
   - Using the distance formula between points \( F \) and \( C \):
     \[
     \overline{CF} = \sqrt{\left( \left( 17 + \frac{120}{17} \right) - \left( -\frac{120}{17} \right) \right)^2 + \left( \frac{64}{17} - \frac{225}{17} \right)^2 }
     \]
     Simplify the coordinates:
     \[
     \overline{CF} = \sqrt{\left( 17 + \frac{240}{17} \right)^2 + \left( \frac{64 - 225}{17} \right)^2 }
     \]
     \[
     \overline{CF} = \sqrt{\left( \frac{289}{17} \right)^2 + \left( \frac{-161}{17} \right)^2 }
     \]
     \[
     \overline{CF} = \sqrt{\left( 17 \right)^2 + \left( -9.47 \right)^2 }
     \]
     \[
     \overline{CF} = \sqrt{289 + 289}
     \]
     \[
     \overline{CF} = \sqrt{578}
     \]
     \[
     \overline{CF} = 17\sqrt{2}
     \]

4. **Express \( \overline{CF} \) in the form \( a\sqrt{b} \):**
   - Here, \( a = 17 \) and \( b = 2 \).

5. **Calculate \( a + b \):**
   \[
   a + b = 17 + 2 = 19
   \]

The final answer is \( \boxed{19} \)