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Name: Allison
Status: student
Grade: 9-12
Location: CA
Country: N/A
Date: April 2007

Question:
Does a sailboat move faster in a freshwater lake or in the ocean, assuming all other conditions are the same?



Replies:
Allison,

If we were to consider only the salinity of the water, then we would have to note that salt water has a higher density than fresh water and therefore the boat would be expected to move slower through this fluid. However, the higher density of salt water would also mean that it has a higher buoyancy effect and less of the boat would be submerged in salty water and would therefore present less drag/resistance. I do not know which of these two would have a greater effect - I think it would depend on the keel/boat design.

Greg (Roberto Gregorius)


Great question, but the 'all other conditions are the same' part is more difficult than you might think. So I am responding with more questions, not answers Sorry; but welcome to Science! :)

Salt water is more dense, hence less of your boat will be submerged and drag will be changed (not necessarily reduced) the geometry of your boat determines this. So you can add weight to your boat so that it is submerged the same (but then its mass differs), or you can have it ride higher, and have its drag profile change. But keeping 'all else equal' is challenging indeed!

Second, the viscosity and surface tension of water change with salt, also affecting drag. Temperature is a huge factor as well, and salt bodies may be very different in temperature than fresh water bodies. Generally, the higher the viscosity, the more drag (and hence lower speed), but with large/fast objects like sailboats, turbulence plays a huge role, and it's very hard to predict the impacts of even small changes.

Overall, I would suspect the boat will be close to the same or very slightly faster in salt water (a wild guess) based on buoyancy, but I will defer to sailors or other experts for a better answer.

Burr Zimmerman


Allison,

I think the primary question comes down to one of water density. On this factor, I am willing to bet most sailboats may be slightly faster in salt water than in fresh water.

A given volume of saltwater will have more mass than the same volume of fresh water. At first glance, this may seem to favor the freshwater boat, as it will need to move less mass aside in it is passing. However, the boat's displacement is always the same, so the saltwater boat will ride higher in the water, and need to move the same total mass of water out of its way in passing. This means that the actual distance which XXXXX Kilograms of water will need to be moved is smaller in saltwater! Thus it would take less energy to move the boat through the water.

This is by no means a perfect answer, as the sailboat's design itself will also be a factor. Ships or boats which ride to high in the water may tend to bob around more, which is really detrimental to overall top speed. Imagine riding a bicycle with a really wobbly front wheel!

Ryan Belscamper



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