muchy_
Well-Known Member
Hello all. What would one expect the differences in draft to be between fresh water and salt water?
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Draft
- Thread starter muchy_
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MainlySteam
Well-Known Member
Very little. The density of sea water is about 1.02 that of fresh so if one was a box shaped barge drawing 1.8m in salt water then one would draw about 40mm more in fresh.
However, most boats are not box shaped and a 1.8 m draft yacht certainly is not, so one has to work out the area of ones water plane at the waterline, see how much extra water would be displaced in freshwater for the boat's weight and then what extra draft that works out to using the waterplane area. But, for small displacement craft the difference will be small.
If you draw close to 1.8 m (as I think I saw you mentioning the French canals in another post) then an approximation could be that as the keel probably does not contribute much buoyancy, just look at your canoe draft (the draft excluding the keel). If your canoe draft is 1 m (which is very much more than most modern yachts under say 45 ft) then again approximating to just a square box in fresh water you will draw another 20mm. Taking into account that your boat is not a square box your increase in draft will be considerably less than 20 mm.
For all intents and purposes the increase in draft can therefore be ignored on small pleasure boats for working out safe passage, even for canal navigation, as the bottom will be much more irregular than that.
John
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However, most boats are not box shaped and a 1.8 m draft yacht certainly is not, so one has to work out the area of ones water plane at the waterline, see how much extra water would be displaced in freshwater for the boat's weight and then what extra draft that works out to using the waterplane area. But, for small displacement craft the difference will be small.
If you draw close to 1.8 m (as I think I saw you mentioning the French canals in another post) then an approximation could be that as the keel probably does not contribute much buoyancy, just look at your canoe draft (the draft excluding the keel). If your canoe draft is 1 m (which is very much more than most modern yachts under say 45 ft) then again approximating to just a square box in fresh water you will draw another 20mm. Taking into account that your boat is not a square box your increase in draft will be considerably less than 20 mm.
For all intents and purposes the increase in draft can therefore be ignored on small pleasure boats for working out safe passage, even for canal navigation, as the bottom will be much more irregular than that.
John
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MainlySteam
Well-Known Member
I think mistaken identity on the canal bit!!! However the idea is still the same.
John
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John
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Mirelle
N/A
Very good answer, anyway.
<hr width=100% size=1>Que scais-je?
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Twister_Ken
Well-Known Member
Roughly speaking
If seawater is 2% more dense than fresh then would the draft increase by 2% going from salt to fresh?
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If seawater is 2% more dense than fresh then would the draft increase by 2% going from salt to fresh?
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MainlySteam
Well-Known Member
Re: Roughly speaking
Hi Ken
No, because it is the volume of water displaced that increases by 2%. This extra water to be displaced is the boat's area at the waterline multiplied by the extra depth the boat is required to sink to displace the extra (lighter) water. If the boat had perpendicular sides and a flat horizontal bottom then the draft would increase by 2%, but that is not the case if (as is usually so) when the boat's sides are not perpendicular and the bottom is rounded or vee'd as part of the total displacement is from these smaller volumed sections.
A way to see that it is the area at the waterplane that matters and not the draft is if one imagines a horizontal tubular hull and adding weight (by Archimedes, adding weight has just the same result as reducing the density of the fluid the hull is floating in). As it is loaded up it sinks less and less per unit of added weight until it is half submerged because for each unit of increase in draft it is displacing a greater volume of water than the last unit of draft increase did. But then, as it is loaded further beyond that, it sinks faster and faster per unit of added weight.
Another way to see it is if one takes a fin keeled yacht having some but very liitle buoyancy from the keel and sail it into freshwater and measure the increase in draft. Then remove the keel and without increasing the weight of the boat double the draft of the boat by making the keel a skinnier but deeper fin, but with no change in the fins buoyancy contribution in salt water. When one then sails the boat back into fresh water from salt, one will not find that the increase in draft is twice what it was before - it will be the same as before.
John
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Hi Ken
No, because it is the volume of water displaced that increases by 2%. This extra water to be displaced is the boat's area at the waterline multiplied by the extra depth the boat is required to sink to displace the extra (lighter) water. If the boat had perpendicular sides and a flat horizontal bottom then the draft would increase by 2%, but that is not the case if (as is usually so) when the boat's sides are not perpendicular and the bottom is rounded or vee'd as part of the total displacement is from these smaller volumed sections.
A way to see that it is the area at the waterplane that matters and not the draft is if one imagines a horizontal tubular hull and adding weight (by Archimedes, adding weight has just the same result as reducing the density of the fluid the hull is floating in). As it is loaded up it sinks less and less per unit of added weight until it is half submerged because for each unit of increase in draft it is displacing a greater volume of water than the last unit of draft increase did. But then, as it is loaded further beyond that, it sinks faster and faster per unit of added weight.
Another way to see it is if one takes a fin keeled yacht having some but very liitle buoyancy from the keel and sail it into freshwater and measure the increase in draft. Then remove the keel and without increasing the weight of the boat double the draft of the boat by making the keel a skinnier but deeper fin, but with no change in the fins buoyancy contribution in salt water. When one then sails the boat back into fresh water from salt, one will not find that the increase in draft is twice what it was before - it will be the same as before.
John
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