Calculating Speed - Intro to physics - Simple harmonic motion - Udacity

So how do you calculate speed? Hopefully, your boat is going forwards to the water with some velocity we'll call v. Well, actually anything use to calculate your speed at sea is called a log and why is it called that. Well, the reason it's called a log is because what mariners used to do was throw an actual log over the back of the boat. They would let the log float as the boat continue forward and is it did this, they plant some rope. Tied in this rope were some knots and by counting the number of knots, they can refer the boat speed. This is where the term knots comes from in reference to a ship's speed. Now you maybe saying this is a ridiculous way to measure speed. How accurate could this whole procedure be anyways--well, you're right. This is now the best way of measuring speed, but it was somewhat functional at the time. So what do you think are the problems with this method. Is it a problem that the line could stretch, that it's hard to accurately measure time or what about currents, may be there's some lateral currents in the water behind the boat. Check all that you think apply.