Tides and Forward Progress

[xfedex]

Have you all decided that the time spent in a current that is pushing you is significantly shorter than time spent in the same current going in the opposite direction? Seems true for moving in water, air, or magma. ^_^

[AusCan]

I would guess more fuel is sometimes used traveling in high current areas simply because people are used to cruising at hull speed.

With a 4kt current against you, hull speed may only be 2 knots, but people are impatient and try to do 3 or 4 knots, dramatically increasing fuel usage.

[FlyWright]

Originally Posted by markpierce

No, forgot about it. Also had my hands full steering through the waves and current swirls. And I even flubbed taking the picture of The Real McCoy car ferry parked alongside Mare Island. Under repair again?

To avoid the dreaded thread creep, I started a Real McCoy ferry discussion here.

http://www.trawlerforum.com/forums/s...tml#post120669

[koliver]

Originally Posted by FlyWright

I believe Marin is right and I'll attempt to prove it mathematically.

Imagine cruising a 2 NM course, round trip at 6 kts with no current. Each leg will take 20 minutes (2/6=.333x60=:20) for a :40 round trip.

Now cruise the same course with 2 kts of current with you and against you. Leg 1 is into the current yielding a 4 kt speed over 2 miles and it takes you 30 minutes. (2/4=.5x60=:30)
Leg 2 is completed at 8 kts with the push from the current and takes you 15 minutes.(2/8=.25x60=:15) Adding leg 1 and 2 is :30+:15=:45.

The same course traveled with the 2 kt current took 5 minutes longer. The reason is that when the current is helping you, you have less time to benefit from it. When it slowing you down, the effect of the current hits you over a longer span of time.

That's this engineer's take on it anyway.

Al:

You haven't included the known headwind factor. When I started boating, I learned pretty quickly that the wind (I was sailing) always comes from the place you are trying to get to. Now that I am in a Stink Pot, that factor has changed just slightly, to include currents, which are always running hardest against you when you want to go.
This is relevant, because we all suffer from a tendency to get anxious about the adverse current and decide to go, regardless of the adverse current, and to laze around the anchorage when the current is running our way, so as to miss most of that speed with us when we finally get moving. On average we miss more opportunities to go with a current and we go anyways, despite an adverse current, so there is never a "cancelling out".
All of this on top of the mathematical certainty that it takes longer to travel in a current than in still water.

[Marin]

Originally Posted by manyboats

SS,

I read all of Marin's post and am still trying to figure out what he actually said but I kept on saying ... yes ... yes ... yes until he got to the dog stuff. I don't get the doggoned dog stuff.

Well, the first thing you need is a dog. If you don't have that you're pretty much dead in the water with the rest of the concept.

It all started when I referenced something as being so basic that even my dog knows it. Then someone came back with how they don't ask their dog for advice, and it went on from there.

[RT Firefly]

Greetings,
Aw, darn! and I thought I had explained it so well.....

[Marin]

Originally Posted by manyboats

SS,


Marin I read your entire long post and yes if you spent a lot of time studying currents and altered your course much of the time you could I'm sure raise your speed above normal cruise and if it's that important to you then OK w me but I would rather enjoy the scenery.

No argument there, at least not for that part of the world.

My point was not that you can reduce your trip time by using the currents to raise your SOG. The only point I was trying to make is that the currents along the Passage are such that the slowing and speeding effect they have on your boat when you go up or down the coast do not automatically cancel each other out over the duration of the trip. In fact, they rarely do for all the reasons we've been talking about.

Our boat has a theoretical range of some 600 miles. But the currents along the Passage will not permit us to go 600 miles. Someone else said they would because over that distance the slowing effects of the current going one way would be cancelled out by the speeding effects of the currents going the other way. So we could go 600 miles.

That is an incorrect assumption, because of the nature of the currents and what influences them along this coast PLUS what FlyWright figured out. Hence all the posts.

[Nomad Willy]

Wuff

[Marin]

Originally Posted by manyboats

Wuff

There you go.

[jeffnick]

Originally Posted by xfedex

Have you all decided that the time spent in a current that is pushing you is significantly shorter than time spent in the same current going in the opposite direction? Seems true for moving in water, air, or magma. ^_^

This was my point. And if you wait for favorable conditions you are indeed adding to the length of time your journey will take over a route with no variation. Tides with inlets WILL slow you down, in either direction.

[FlyWright]

Originally Posted by koliver

Al:

You haven't included the known headwind factor. When I started boating, I learned pretty quickly that the wind (I was sailing) always comes from the place you are trying to get to. Now that I am in a Stink Pot, that factor has changed just slightly, to include currents, which are always running hardest against you when you want to go.
This is relevant, because we all suffer from a tendency to get anxious about the adverse current and decide to go, regardless of the adverse current, and to laze around the anchorage when the current is running our way, so as to miss most of that speed with us when we finally get moving. On average we miss more opportunities to go with a current and we go anyways, despite an adverse current, so there is never a "cancelling out".
All of this on top of the mathematical certainty that it takes longer to travel in a current than in still water.

You're right, koliver. I was only addressing the current in reply to the OP. But I have also experienced this phenomena which we in our flying circles affectionately called homebound headwinds. Leaving town we'd experience headwinds and the uninitiated copilots would assume they'd cancel out on the way home a few days later. Eventually they caught on that homebound headwinds were just a fact of life most of the time. Looking back, it was a much bigger deal when we were flying King Air 300 turboprops. In the Learjet 60, it barely fazed us...we laughed in the face of headwinds much the way Moonstruck chuckles at head current.

[FlyWright]

Originally Posted by jeffnick

This was my point. And if you wait for favorable conditions you are indeed adding to the length of time your journey will take over a route with no variation. Tides with inlets WILL slow you down, in either direction.

Sometimes you leave early for the best current, sometimes you leave late. Maybe those early and late departures cancel out. I don't have a formula for that, though.

[Marin]

We do the best we can to take advantage of the current on our runs through the islands. But more often than not, it seems, the most advantageous currents occur before we want to get up, or can get up to Bellingham, or waiting for them will get us home too late.

In these cases we just go when we want to go, knowing we'll have to "put up" with the slow pace along the way per FlyWright's calculations.

In our boat with our engines we don't bother trying to reduce the effects of an adverse current by adding more power. We just go slower over the ground and count the eagles in the trees or something.

[Art]

For goodness, goodness, sake fellows... settle down, please!

Result/difference of running against or with currents can be spun in conversation as broadly as the result/difference between right and wrong can be spun in political debate... as to which one more often takes control. Both sets of circumstances mentioned offer 180 degree directional-differences to one or the other of their independently available sides. But... of the two circumstances neither side of either need be followed (used) all or even any of the time as long as conscious, planned decisions are followed.

If you plan to leave at a correct time in same direction of a current then the effects of increased “land speed” will be yours. If you do not... then fighting against a current produces slower "land speed". It ain’t rocket science.

Even if you decided to travel in same direction for hundreds upon hundreds of miles, wherein the current would reverse direction approx every 12 hours and 25 minutes... when current changes direction simply pull aside and anchor or moor or dock until the current’s direction is again in your favor. You don’t even need to know current change time specs in a region... once you see the current changes direction simply keep your eye on the clock to predict the 12 hour and 25 minute current change schedule. Open your eyes and see when the current direction changes to your travel direction. Calculate your cruise speed, the average speed of the current and deduce how long you should wait to begin following the current so that when you reach a certain time of cruising you meet the final portion of the current where it will reverse direction. Then wait appropriate time span till it is again time for you to travel with the current. It is important that you calc your cruise speed in regard to avg current speed so you don’t overtake the current traveling in your direction (or you’ll begin to fight a current). Basically it becomes X hours travel (depending on your “through-water-travel” cruise speed) and then 12 + hours rest to wait for current direction change so you again can time your leave for maximum “land speed” travel by cruising with the current.

Of course, if you have appointment schedules that mandate not taking time to relax in wait for current to be in your favor, and you are going the same direction travel for hundreds upon hundreds of miles (mentioned above) then you will be doing an approximate 50/50 "land speed" split while cruising with or against currents.

[Marin]

Your theory will work where you are. It won't here, to a large degree, because for much of the coast there is no way to know exactly what the currents will do in a specific channel or pass because there are far too many of them to put in the current tables. And it's safe to say that in many of them current data has never been collected.

So all you have available for calculations along much of one's route is are very basic, average current speeds and directions. Local currents can be totally different than these averages and even be going the other way depending on the geography.

You can get in the ball park by using the data available, no question. But it's a big ball park. It's why our boat's range of 600 some miles is actually much less than 600 miles in the waters from here up through SE Alaksa.

[RickB]

The authors of posts #41 and #42 need to do a rethink as do a few others.

Unless you arrive at your destination before the current changes direction you spend just as much time going one way as the other. The distance you move in that time changes but not the duration. If there are two boats traveling on reciprocal courses in the same mass of water they both spend the same amount of time in that mass.

If a boat is floating motionless in a moving mass of water it moves across the ground at the same speed as the mass of water, there is no relative motion. If a boat is floating motionless in a stationary mass of water there is no relative motion

If a boat moves through a mass of water the relative motion to that mass of water is what determines "hull speed" and the amount of power required to maintain that relative motion. Hull speed has nothing whatsoever to do with speed across the ground, only the relative speed of the hull through the mass of water that may be moving in the same direction the boat is headed, the opposite direction, or may not be moving at all.

You guys made more sense arguing about anchors.

[AusCan]

RickB

Exactly - That is the point I was making. Trying to do your still water hull speed when running against the current will eat up extra fuel. Its a no-brainer.


This is when the old pitot tube speed indicators are handy to to judge a steady efficient cruising speed. (although they don't give an accurate "land" speed)

[Giggitoni]

What's next? Length contraction and time dilation? Is this why I feel younger when I arrive at my destination after a long day at breath-taking speeds in my GB? Or are the rest of you just older?

[RT Firefly]

Greetings,

[RickB]

Originally Posted by AusCan

Trying to do your still water hull speed when running against the current will eat up extra fuel. Its a no-brainer.

Talk about no brainer ... JFC

Hull speed is the speed of the hull through the water. I know it is a difficult concept for some but here it is again ... hull speed is the speed of the hull through the water.

If that mass of water the hull is moving through is moving at 1 knot or 100 knots it doesn't matter, it is the relative speed of the hull through the water that defines hull speed.

Hull speed has nothing to do with how fast you get from A to B, it is not related to speed over the ground, it is only a measure of how fast the hull is moving through the water in which it is floating. That water can be moving from ahead or astern and it doesn't make any difference to the hull or the engine. It takes the same amount of power to move a hull through the water at 10 knots into a 50 knot current as it does to move it at 10 knots with a 50 knot current.

If you were halfway across the Pacific with no landmarks in sight and a 50 knot current developed from ahead or astern you wouldn't know about it unless you looked at your GPS and the boat would never know about it. The only thing that changes is the time it takes to go between A and B.

Good grief ...