Minimum speed using calculus I've been trying to answer this question , specifically part c) 

I've done part a) and b) where $U = \frac{28\sqrt{15}}{15} $ and $ \theta = 53.3 $ degrees .
For the last part I need to work out the minimum value of v. As soon as I saw the word 'minimum' , I thought I had to use calculus to work it out. I've seen the worked answer to this but calculus is not used; he says the minimum value of v is when the vertical velocity is 0 which I do understand. My working out for this does not yield the correct answer to which I do not understand why hence I can only conclude something is wrong with what I'm doing.
My method was to find the value of $\theta$ for which $v$ is a minimum. I did this by forming an equation in $v$ from the standard equation of motion $v = u + at$ where in my case:
$v = usin(\theta) - gt$ 
Then 
$\frac{dv}{d\theta} = ucos(\theta) = 0$
Then solve that which gives $\theta$ as $\frac{\pi}{2}$ 
Then I plugged that into my original equation to get a value of v for which I thought was minumum but I ended up with something like $-12.37$ which is incorrect.
The correct answer IIRC is 4.32 m/s.
Can someone show me how to derive the correct answer using calculus or show me where I went wrong ? I suspect it's my equation of v. 
 A: The velocity of the ball in the horizontal direction is constant until it hits the ground. The velocity of the ball in the vertical direction varies, and becomes $0$ when the ball reaches its maximum height. 
The speed at time $t$ is $\sqrt{v_h^2(t)+v_v^2(t)}$, where $v_h(t)$ is the horizontal component of velocity at time $t$, and $v_v(t)$ is the vertical component. Since $v_h(t)$ is constant, the minimum speed is reached when $v_v(t)=0$. 
Thus the minimum speed is the speed in the horizontal direction. This is $u\cos\theta$.
A: If you want to use calculus you have to calculate the speed $s$, with respect to time. A bit of trig shows that $s(t) = \sqrt{\text{verticalspeed}(t)^2 + \text{horizontalspeed}(t)^2}$ the horizontal speed is constant and the vertical speed can be calculated using $v=u+at$.
Once you've found out all the values of the constants, you can differentiate $s$ and find the critical points as normal.
I should also say that treating $\theta$ as your variable is where the mistake came in. $\theta$ is fixed in this problem, you even worked out it's value! Your independent variable should be time $t$.
A: The energy equation is given by
$$
U = mgh + \tfrac{1}{2} mv^2
$$
So you obtain
$$
mgh + \tfrac{1}{2} mu^2 = mg(h-8) + \tfrac{1}{2} m [2u]^2
$$
whence
$$
3 u^2 = 16g
$$

The height is given by
$$
h(t) = 8 + u_y t - \tfrac{1}{2} g t^2
$$
and $h(2) = 0$, thus
$$
u_y = g - 4
$$
As
$$
u^2 = u_x^2 + u_y^2
$$
we find for point $B$
$$
u_x^2 = \frac{64}{3} g - [g-4]^2
$$

Note that
$$
\cos(\theta) = \frac{u_x}{u}
$$
so
$$
\cos^2(\theta) = \frac{64 g - 3[g-4]^2}{16g}
$$

The minimum velocity is $u_x$
