The question for all of you scientists is the following scenario: while motoring at 2,800 rpms in salt water, Long island Sound, the the boat does about 6.5knts through the water. In fresh water, the Hudson River, the boat traveled at 6.9knts at 2,800 rpms. I know about speed over ground. This question is only concerning speed through the water. Is the answer that salt water is more dense and therefore more resistant to boat speed through the water? Any thoughts or explanations would be interesting. Thanks.
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spped
first, unless you are well north of poughkeepsie(?), there is no fresh water in the hudson. second, the difference in density is way to small to make a difference in your speedo. you are using a speedo for this reading, and not a gps, no?
first, unless you are well north of poughkeepsie(?), there is no fresh water in the hudson. second, the difference in density is way to small to make a difference in your speedo. you are using a speedo for this reading, and not a gps, no?
Boat Speed
I have noticed that current will affect 'through the water' speed. In other words, if I am bucking a current my speed is a few tenths higher.
I have noticed that current will affect 'through the water' speed. In other words, if I am bucking a current my speed is a few tenths higher.
boat speed
the question is based on the boat's knot speedo. I am not factoring in current as the boats movement through the water should not be affected by the current. Yes speed over ground is affected by current but not the actual speed through the water. The salt line for the Huson River is about West Point at this time of year.
the question is based on the boat's knot speedo. I am not factoring in current as the boats movement through the water should not be affected by the current. Yes speed over ground is affected by current but not the actual speed through the water. The salt line for the Huson River is about West Point at this time of year.
Salt vs Freshwater Speeds
On page 2 of this pdf is a short explanation of the effects of salt vs freshwater on boat speed. While this document talks about larger commercial craft, it would seem to indicate that there should be little noticeable difference. But take a look also at page 1 of this, where they discuss shallow water speed effects. Don't know about the water depth differences you might have seen between LI Sound and the Hudson, but it's something to be aware of.
On page 2 of this pdf is a short explanation of the effects of salt vs freshwater on boat speed. While this document talks about larger commercial craft, it would seem to indicate that there should be little noticeable difference. But take a look also at page 1 of this, where they discuss shallow water speed effects. Don't know about the water depth differences you might have seen between LI Sound and the Hudson, but it's something to be aware of.
Crikey!
Is it the viscosity of the salt water that's been slowing me down? I knew there had to be a reason.
Is it the viscosity of the salt water that's been slowing me down? I knew there had to be a reason.
Viscosity of fresh water in the Hudson?
It's hard to imagine that the chocolate colored brackish water in the Hudson could be less viscous than salt water in LIS. Maybe the depth does have something to do with it. I assume that the Hudson (south of Newburgh)generally runs deeper than most locations in LIS near the west end.
It's hard to imagine that the chocolate colored brackish water in the Hudson could be less viscous than salt water in LIS. Maybe the depth does have something to do with it. I assume that the Hudson (south of Newburgh)generally runs deeper than most locations in LIS near the west end.
speed
bob, if and when you get a definitive answer, i'd love to know. in the meantime it reminds me of what one of my few really good teachers used to say, "there are and infinite amount of wrong answers."
bob, if and when you get a definitive answer, i'd love to know. in the meantime it reminds me of what one of my few really good teachers used to say, "there are and infinite amount of wrong answers."
Not on a sailboat
Due to the presence of your keel and your hull shape you will not experience shallow water effect. This effect is felt by flat bottom vessels, as all merchant vessels of any size are. It has very "interesting" effects on steerage on those vessels. They tend to shear off without warning. So the old adage applys: Beware of large Greek vessels making knots in shallow water. The only shallow water effect you're going to feel in your sailboat is much more familiar, "I think we're aground". Never seen a lick of difference in speed, fresh versus salt. Of course your draft will be greater in fresh water as fresh is less "buoyant". Salt water has a specific gravity of 1.026, fresh is 1.000, and brackish as found in both the Hudson and inshore on the Sound somewhere in between. Your fresh water allowance (incr. in draft entering fresh water) is roughly 1/4" per foot of draft. Only a rough approx. as each vessel is different. Shallow water effect takes place when the depth of water is approximately 1.2 times the vessel's draft. Comparing under hull forms between a merchant ship and sailboat will show why this is not of concern to the sailboat.
Due to the presence of your keel and your hull shape you will not experience shallow water effect. This effect is felt by flat bottom vessels, as all merchant vessels of any size are. It has very "interesting" effects on steerage on those vessels. They tend to shear off without warning. So the old adage applys: Beware of large Greek vessels making knots in shallow water. The only shallow water effect you're going to feel in your sailboat is much more familiar, "I think we're aground". Never seen a lick of difference in speed, fresh versus salt. Of course your draft will be greater in fresh water as fresh is less "buoyant". Salt water has a specific gravity of 1.026, fresh is 1.000, and brackish as found in both the Hudson and inshore on the Sound somewhere in between. Your fresh water allowance (incr. in draft entering fresh water) is roughly 1/4" per foot of draft. Only a rough approx. as each vessel is different. Shallow water effect takes place when the depth of water is approximately 1.2 times the vessel's draft. Comparing under hull forms between a merchant ship and sailboat will show why this is not of concern to the sailboat.
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