What is minimal drift?

[Myrtonos]

A vessel of any kind will tend to drift unless anchored or moored, there can be no true stops on the water. A boat could hover in one place above a riverbed and still be propelled upstream and maintain steerage due to flow. So given all that what does minimal drift mean, though I do know that the largest vessels take more than a mile to minimize drift.

 

[achris]

What is the context of 'minimal drift'? ie, what sentence did you hear it in?

Vessels can 'hover' in one spot for days, even weeks, without moving (accurate to a few feet). It's call Dynamic Positioning. It involves some very accurate positioning systems, like DGPS, syledis, Artemis, a tort wire, or a 'fan-beam'. This position data is feed into a computer (or computers) which controls the vessels thrusters.

Chris....

 

[smokeonthewater]

by it's self "minimal drift" doesn't mean anything more than what it says.....What is the back story here?...

 

[Ned L]

Hmm, ... Maybe he mashed an incorrect term with a wrong partial definition?? Might he be wondering about minumum draft??

 

[Outsider]

I'm still trying to figure out how one 'hovers' over a riverbed and can still go upstream ...

 

[Ned L]

Yea,... So wrong in so many ways. Maybe he is thinking about no speed through the water, but still movement over the bottom by drifting DOWN stream (more typically). And of you are drifting with the current you are making way over the bottom, but not through the water, so you are not making steerage way and have no control over the boat.

 

[Myrtonos]

What is the context of 'minimal drift'? ie, what sentence did you hear it in?

Okay, first of all, bicycles and especially cars take longer to come to a stop than a pedestrian, and car sized boat at planing speed takes even longer to come to mininal drift than a car going at the equivalent of three times that speed, whilst the former doesn't really acheive a true stop until achored or moored.

Vessels can 'hover' in one spot for days, even weeks, without moving (accurate to a few feet). It's call Dynamic Positioning. It involves some very accurate positioning systems, like DGPS, syledis, Artemis, a tort wire, or a 'fan-beam'. This position data is feed into a computer (or computers) which controls the vessels thrusters.

They may hover in one place above the river/sea bed, but this is not necessarily a true stop unless anchored or moored.

I'm still trying to figure out how one 'hovers' over a riverbed and can still go upstream ...

I'm puzzled as to why you might still be trying to figure it out, because a vessel hovering over a river bed must go upstream in relation to the water flowing over the bed, which itself has dynamic properties such that true stops cannot exist on the water. When was the last time anyone here ever saw traffic lights or stop signs at river or canal junctions?

Yea,... So wrong in so many ways. Maybe he is thinking about no speed through the water, but still movement over the bottom by drifting DOWN stream (more typically). And of you are drifting with the current you are making way over the bottom, but not through the water, so you are not making steerage way and have no control over the boat.

It's not clear to me whether mininal drift means this or whether it means slow enough relative to the banks of the river and riverbed that it can be anchored or moored, hence the question.

 

[southkogs]

I've never heard the term "minimal drift" applied toward a principle or effect in boating. On water with a current, you'd always have an issue of relative velocity (much like airspeed vs. groundspeed in an airplane). But wind can also cause drift on water that has no current, so it's not a singular concept. But you also have crosswind and drift issues that affect you as well.

I think achris answers much of that in his post above.

 

[Myrtonos]

And wind can push a wheeled vehicle, but not sideways without skidding. In fact the dynamic properties of water alone create the issue of relative velocity, as in two boats passing each other on a bend in a river, differences in water speed aren't proportinal to differences in ground speed, they might for example go at the same water speed but different ground speed.

When you walk across the road, you reach the kerbside directly across from the entry point, same applies when riding a bicycle or drivng any other ground vehicle across the road, that is with the nose of the vehicle straight towards the other side. This is not the case with a boat heading straight across a river of any width, with the bow pointing towards to other side, but rather at an angle in the downstream direction. If a boat does travel straight across a river, it will be at an angle in the upstream direction. Or, when a bow rider is overtaking a ship on a river less than a mile wide, its bow will point away from the ship while passing it, whether in the same direction or opposite direction, due to draw between the vessels.

 

[tomhath]

I've never heard the term "minimal drift" either.

Normally when there's a current you consider two things together: Set and Drift. Set is the direction that the current is coming from, and Drift is the speed. Once you know the set and drift you can calculate your actual course and speed made good by adding or subtracting vectors (usually the course and speed you are steering, although in a small boat the wind's direction and speed might also be a factor).

Obviously if you're concerned about the relative velocity between two boats near each other you can ignore the current unless you have reason to believe it's different in the two places.

 

[Ned L]

Ok, so if we presume that the OP is wondering how long it takes for a boat to pretty much come to a stop from (we'll say) cruising speed; it will loose most of its speed pretty quickly (drop down off of a plane rapidly). Once it drops to below hull speed it will keep moving along for quite a while. Exactly how long depends on the boat.
Oh, and "large vessels" by the way take a lot more than a mile to drift to a stop. A fully loaded VLVC or ULCC at cruising speed put into full reverse can take up to 15 miles to come to a stop. If they see you in front of them it is too late for them to avoid you. The start slowing down a hundred miles or more from shore.

 

[Myrtonos]

Normally when there's a current you consider two things together: Set and Drift. Set is the direction that the current is coming from, and Drift is the speed. Once you know the set and drift you can calculate your actual course and speed made good by adding or subtracting vectors (usually the course and speed you are steering, although in a small boat the wind's direction and speed might also be a factor).

Is it true, even on rivers (even those less than a mile wide), that the set isn't always along the course of the river?

Obviously if you're concerned about the relative velocity between two boats near each other you can ignore the current unless you have reason to believe it's different in the two places.

And a very common reason is bends in the river, where the flow increases with distance from the inner bank. And when a boat heads across a bend in the river, even a wide one, it is not only on an angle in the downstream direction, but that angle increases with distance from the inner bank, and the drift difference between the bow and stern, left on its own, turns the bow of the boat in the downstream direction.

Ok, so if we presume that the OP is wondering how long it takes for a boat to pretty much come to a stop from (we'll say) cruising speed; it will loose most of its speed pretty quickly (drop down off of a plane rapidly). Once it drops to below hull speed it will keep moving along for quite a while. Exactly how long depends on the boat.
Oh, and "large vessels" by the way take a lot more than a mile to drift to a stop. A fully loaded VLVC or ULCC at cruising speed put into full reverse can take up to 15 miles to come to a stop. If they see you in front of them it is too late for them to avoid you. The start slowing down a hundred miles or more from shore.

But you cannot acheive a true stop on the water. I'm guessing that the ships you mentioned, put into full astern propulsion, can take up to 15 miles to come to minimal drift. I sure hope that jet skiers and pleasure boaters stay out of shipping channels when the ship comes in.

It seems that drift can be used in two, if not three different contexts, drift of a vessel relative to either the ground and/or the water, and in the context of flow.

 

[hungupthespikes]

When was the last time anyone here ever saw traffic lights or stop signs at river or canal junctions?

Venice, Italy has them, same with Amsterdam.

http://www.tripadvisor.com/Location...1172-i22411418-Grand_Canal-Venice_Veneto.html

Carful... in the states we do use them around construction/dredging operations and traffic lights are used for locks and draw/swing bridges.

It's not clear to me whether minimal drift means this or whether it means slow enough relative to the banks of the river and riverbed that it can be anchored or moored, hence the question.

Minimal drift is ambiguous without qualifying. Drift to stop, holding a position, bearing vs course vs direction vs heading vs track????? :noidea:

 

[southkogs]

... But you cannot acheive a true stop on the water...

I would contest that: it may not be easy to achieve a true stop (ground relative) on the water, but it's possible with or without sophisticated equipment. Almost any body of water has enough movement to it, that "true stop" is going to have to measured relative to a fixed (ground) point. Again: airspeed vs. groundspeed.

Anchored properly in a lightly flowing river, a boat should settle into a fixed position relative to the ground. True that the water would still be moving relative to the boat and ground, but the boat is still actually stopped (no velocity or energy on a vector).

 

[tomhath]

Is it true, even on rivers (even those less than a mile wide), that the set isn't always along the course of the river?

Sure - eddies, whirlpools, channels, tides, all kinds of things can change both the direction and speed of the flow.

 

[UncleWillie]

Ok, so if we presume that the OP is wondering how long it takes for a boat to pretty much come to a stop from (we'll say) cruising speed; it will loose most of its speed pretty quickly (drop down off of a plane rapidly). Once it drops to below hull speed it will keep moving along for quite a while. Exactly how long depends on the boat.
Oh, and "large vessels" by the way take a lot more than a mile to drift to a stop. A fully loaded VLVC or ULCC at cruising speed put into full reverse can take up to 15 miles to come to a stop. If they see you in front of them it is too late for them to avoid you. The start slowing down a hundred miles or more from shore.

I believe the unit of measure is not in Miles but in Ship Lengths. 15 Ship lengths, Not 15 Miles.

SOLASII-I Reg 28
"Means of going astern"... to being the ship to rest within a reasonable distance from maximum ahead service speed shall be demonstrated and recorded.
See also Res MSC.137(76) applicable for ships > 100m and all chemical tankers and gas carriers. Stopping ability at the test speed (85% of the maximum engine output): "The track reach in the full astern stopping test should not exceed 15 ship lengths. However this value may be modified by the administration where ships of large displacement make this criterion impracticable, but in no case exceed 20 ship lengths.

A 100 m ship needs to be able to stop in full reverse in <1500 m, A little under one mile

Of course if your ship is over 5000 feet in length..... The largest ULCC ever built WAS 1504 feet.

A total loss of power on a large ship may allow it to drift ahead for 15 miles, but then again,
it will never come to a complete stop without the external forces of wind or current.
The water drag will continue to slow it forever but will never bring it to a complete and absolute stop.

 

[H20Rat]

The water drag will continue to slow it forever but will never bring it to a complete and absolute stop.

Sir Isaac Newton and the laws of physics might disagree with you... The boat contains a certain amount of kinetic energy. Moving through the water imparts drag, which subtracts energy. At some point your drag absorbs all kinetic energy and you stop. Otherwise you just invented the first perpetual motion machine. Keep in mind drag from water IS an external force... Think of it the other way, you are saying at some point that drag goes away entirely. That is the only way it could continue moving for infinity. Outside of a vecuum, drag never disappears.

It is absolutely the same as a vehicle that you let coast on a perfectly flat piece of concrete. Its going to coast for a long time, but it will eventually come to a complete and dead stop. The mass of the vehicle and the medium it is traveling through (air or water) make zero difference, drag always wins eventually.

 

[Outsider]

because a vessel hovering over a river bed must go upstream in relation to the water flowing over the bed

Uhhh, no. A vessel 'hovering' over a riverbed is by definition stationary in relation to the riverbed. True, water may be flowing under the hull, but it's water moving, not the hull. Practical definition would have 'upstream' be the direction water is moving from, 'downstream' is the direction water is moving to. The vessel isn't going either direction if it's in a true 'hover'. Be that as it may, mayhap you can define which direction a vessel is moving when hovering over the bed of a true tidal river ... :faint2:

 

[dingbat]

I'm an Engineer, and all this matters why????

 

[superbenk]

I'm an Engineer, and all this matter why????

Been wondering that myself.