Boat Speed: Salt versus Fresh Water
Country: United States
Date: Fall 2009
Do boats travel faster in saltwater or fresh water, and why?
There are a number of factors that MAY come into play; however, I
doubt that they would overwhelm other factors. First, and most importantly,
you did not say "What kind of boat?" -- sailboat, racing skiff, motor boat,
I am not using this to criticize you, but rather to illustrate a
point. Think about "the problem" or "question" such as you proposed, I have
a suggestion. In a quiet (dark) room, sit down in a comfortable chair and
"run" the experiment over and over in your head. Pencil paper in hand. Eyes
closed --let you mind run free. Scribble key words that you can refer to
later, but do not get distracted "taking notes", just key words. This part of
the scientific method is seldom if ever taught in school. It takes some
practice to become skilled in this technique, but if you let your mind run
free not thinking about the problem, while thinking about the problem, ideas
may come to you as you re-run the mental DVD.
Example: If the boat is sinking, it is not going to go anywhere very
fast. So how deep the boat rests in the water will have a big effect
regardless of the type of water. Where does that thought lead you? If "deep"
is bad maybe "shallow" is good. So for maximum speed you want the boat to
float like a cork -- right up in the surface. Yes, but there is a limit.
What is that limit? Keep going.
With regard to fresh versus salt water. Salt water is more dense
than fresh water, so a boat is more likely to float more easily in salt
water, but there are so many factors involved it is difficult, without
carefully controlled experiments to distinguish the difference (I think).
The only thing that could affect the speed of a boat in salt water,
compared to running in fresh water, is that the slightly increased
buoyancy of salt water would make the boat float a little higher. This
means that there would be less of the boat's hull in the water, and
result in a little less "drag" underway. As a result the boat may be
able to go slightly (and I do mean only slightly) faster in salt water,
then in fresh water. In reality, the difference would be nearly
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Update: June 2012