Sailboat Resistance: Salt vs. Fresh Water
Date: April 2007
Does a sailboat move faster in a freshwater lake or
in the ocean, assuming all other conditions are the same?
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
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.
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!
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Update: June 2012