Motion of a boat in a stream We are doing problems involving boats sailing from one shore to the other. Usually, in such problems we are told the velocity of the boat with respect to the water and the angle at which the boat sets out. We are then told the speed and direction of the stream (from ground). 
My question is why do we assume that the flow of water effects all boat paths equally. For example, if one boat is much more massive than the other and both have the same velocity with respect to water and both set out at the same angle, how is it that the stream deviates the path of both by the same amount? Shouldn't the more massive one have deviated less? But we don't include mass in any of our calculations. 
What am I missing? 
 A: You are given the velocity of the boat with respect to the water.  You are also given the movement of water with respect to the ground.
You don't assume that the water affects all boat paths equally.  If you are told the velocity of the boat with respect to the water, you know how much the water affects the movement of the boat, because you are already given the speed the boat moves relative to the water.
A: In these questions, they assume that the stream is much much bigger than the boat. Hence the boat will go along with the stream regardless of its size. You don not have to assume anything as such questions will never deal with numerous boats each having different masses.
A: These problems tend to oversimplify the system being analyzed. So, it is certainly not always true that boats of different masses are affected equally. 
But that isn't to say it can't. If we want both boats to move at same velocity then we need the net force to equal zero, which requires that the force of the water pushing on the boat is equal in magnitude to the drag force. Assuming the dominate drag force in the water is from the quadratic term, we have:
$$F_{water} = -F_{drag}$$
$$P_{water} *(Area) = cv^2$$
where the area is the cross sectional area, P is the pressure supplied by the water, and c is a constant that happens to be proportional to the cross sectional area. The velocity for a given object is then:
$$v = \sqrt{\frac {P_{water}(Area)}{c}}$$
Since the area and c are both proportional to the cross sectional area, and an increase in mass results in a change in cross sectional area (Archimedes Principle), then I imagine it is possible the speed would remain unchanged for different masses. It all depends on how exactly c changes. 
