Distinct Activities: Shackled by a Common Name?

[goboatingnow]

Originally Posted by Seaworthy Lass

I am sorry but your diagram is wrong

You can say it till the cows come home, but if you think so , show me.

[Seaworthy Lass]

Originally Posted by goboatingnow

You can say it till the cows come home, but if you think so , show me.

You are steering to when the current finishes (12.49 miles down the rhumb line), NOT to the destination

The destination is 16 miles away, not 12.49. Just join your line from the ten point vertically to the 16 point horizontally. That then gives you the compass course to steer the ENTIRE way.

[Seaworthy Lass]

Originally Posted by goboatingnow

Dockhead unless I reading this The whole Captforce scenario is weird, its a backwards computation, ie the tide is assumed to have occured in whatever passage time is allowed.

Tides runs at a rate per hour, The do not run at a rate per passage, Hence the tide cannot varying as a function of the passage time.

CaptForce's current was not due to tide, it is constant over time. It's strength is weaker at the shores and stronger in the centre (I presume due to depth differences).

[goboatingnow]

Originally Posted by Seaworthy Lass

You are steering to when the current finishes (12.49 miles down the rhumb line), NOT to the destination

The destination is 16 miles away, not 12.49. Just join your line from the ten point vertically to the 16 point horizontally. That then gives you the compass course to steer the ENTIRE way.

sheesh,

Seaworthy Thats exactly what YOu do not do.

look at it intuitively, The tide is a RATE, specified respectively 3 kn, 2kn, 4kn,1kn, ( lets use knots), KNots is a rate per unit time, ie in one hour it will travel 3 nm , in the first case. BUT you are not on the water for 4 hours to the destination ( weall agreed it close to 5 hours etc.)

Hence you MUST plot the vectors in the same units, boat speed per hour, tides per hour and the third is a constant ( distance). Other wise you are adding apples and oranges

Think about it if you are out in the water longer the tide will affect you more.

Dave

[Seaworthy Lass]

Originally Posted by Dockhead

So here is my workup of Capt Force's scenario using constantly changing current which starts at 0 increasing to 4 then decreasing back to 0, with average of 2.

The current increases and decreases in a linear manner and is at 0 for both first periods and at 4 for both middle periods. This is rather artificial but I think it works ok (if anyone objects, please let me know and suggest a better approach.

Attachment 53003

I haven't attempted this calculation. Looked too complicated LOL.
Will have a look at your figures when I have a chance. A Sailrite sewing machine arrived today and I am dying to play with it .

[goboatingnow]

Originally Posted by Seaworthy Lass

CaptForce's current was not due to tide, it is constant over time. It's strength is weaker at the shores and stronger in the centre (I presume due to depth differences).

Well it doesnt matter, its a tide that happens to fit in a passage that happens to coincide with it fitting in the passage. Its actually giving the answer in the future and computing the past. rather then normally with tide ( or current) vectors we get figure from the start and compute the end .


Dave

[Seaworthy Lass]

Originally Posted by goboatingnow

sheesh,

Seaworthy Thats exactly what YOu do not do.

look at it intuitively, The tide is a RATE, specified respectively 3 kn, 2kn, 4kn,1kn, ( lets use knots), KNots is a rate per unit time, ie in one hour it will travel 3 nm , in the first case. BUT you are not on the water for 4 hours to the destination ( weall agreed it close to 5 hours etc.)

Hence you MUST plot the vectors in the same units, boat speed per hour, tides per hour and the third is a constant ( distance). Other wise you are adding apples and oranges

Think about it if you are out in the water longer the tide will affect you more.

Dave

I AM comparing apples with apples LOL.
The vertical axis is the distance displaced by the current, a total of 10 miles (3 miles the first hour, 2 the second, 4 the third, one in the fourth).
The horizontal axis is distance displaced to the destination, a total of 16 miles.

[goboatingnow]

The destination is 16 miles away, not 12.49. Just join your line from the ten point vertically to the 16 point horizontally. That then gives you the compass course to steer the ENTIRE way.

NOPE, youll end up down tide of the destination. If you on the water for 5 hours, you are affected by 5 hours of combined tides, not 4 etc etc. you are adding a water track specificed in total hours to teh destination and a tidal vectors set only valid for 4 hours.

A car at 30 miles and hour does not simply do 30 miles and then stop. neither does a tide at 2kn stop after 2 nM

[goboatingnow]

The vertical axis is the distance displaced by the current, a total of 10 miles (3 miles the first hour, 2 the second, 4 the third, one in the fourth).
The horizontal axis is distance displaced to the destination, a total of 16 miles.

Nope by the action of the tide you dont cross the tide in an hour, hence you are effected by it longer.

dont beleive me, Ive searched the net see here

Course To Steer - Westview Sailing RYA Day Skipper online - YouTube

[Seaworthy Lass]

Originally Posted by goboatingnow

NOPE, youll end up down tide of the destination. If you on the water for 5 hours, you are affected by 5 hours of combined tides, not 4 etc etc. you are adding a water track specificed in total hours to teh destination and a tidal vectors set only valid for 4 hours.

A car at 30 miles and hour does not simply do 30 miles and then stop. neither does a tide at 2kn stop after 2 nM

A car doing 30 miles/hr will simply stop at 30 miles if it is only travelling one hour.
A current of 2 knots will stop when it has displaced you 2 nm if it is only travelling for one hour.

The 4 different currents in your example are all occurring for one hour each (there is no current after that) so the boat is pushed a total of 10 miles in the direction of the tide (perpendicular to the rhumb line) for the duration of the voyage.

[goboatingnow]

Originally Posted by Seaworthy Lass

A car doing 30 miles/hr will simply stop at 30 miles if it is only travelling one hour.
A current of 2 knots will stop when it has displaced you 2 nm if it is only travelling for one hour.

The 4 different currents in your example are all occurring for one hour each (there is no current after that) so the boat is pushed a total of 10 miles in the direction of the tide (perpendicular to the rhumb line) for the duration of the voyage.

Nope that is not the computation

each current is specified at its rate ( ie its speed)and direction Thats all you know.

The next thing you know is that you estimate without knowing the exact situation that you will encounter these currents on the rhumb line in each hour. ( you cant estimate anything else as you haven't calculated the effect yet. )

SO you plot the vectors for tide in one hour increments and you plot the boat in the same 4 hour ( water track) increments).( you estimated you meet 4 suich tides) You do a vector sum.

As you travel across the tide, it affects you , ie it in effect makes you stay longer then one hour in the tide, hence you are affected in proportion to the rate of your movement, the rate of the tide and the time ( thats why you are doing a vector sum in the first place).

Please dont believe me, have a look at the video.

[goboatingnow]

seaworthy

Sailtrain: Navigation and Chartwork,

http://www.ukdivers.net/chartwork/course.htm


heere is a 2 hour plot

http://www.youtube.com/watch?v=fTxrfqI5K4k

have a gooser at them as well.

[Dockhead]

Originally Posted by goboatingnow

Dockhead unless I reading this The whole Captforce scenario is weird, its a backwards computation, ie the tide is assumed to have occured in whatever passage time is allowed.

Tides runs at a rate per hour, The do not run at a rate per passage, Hence the tide cannot varying as a function of the passage time.

Sorry, I explained it badly.

I did of course understand that these are constant currents, not tides, which run at different rates in different places, but do not change over time.

The problem is Capt Force did not give us a distance. Just a passage time of 4 hours. So we could take any distance we like, but distance which corresponds to 4 hours for one way of steering will not suit the other way of steering. So we have to arbitrarily choose which scenario we want to fit the distance to. Are you with me so far?

I chose to fit the distance to the CTS track, and it is easy to backwards compute what distance that would be. It's just the "B" leg of the vector triangle where the "C" leg is the water track (the hypotenuse, 20 miles), and the "A" leg is the current vector (8 miles).

We must have the distance to analyze this, some distance to the waypoint. So I chose 18.33 miles as this is how far you can get down the rhumb line sailing 5 knots through the water with an average 2 knot perpendicular current.

As a result, Capt Force's scenario fits the passage of the CTS boat, after I filled in the blank for distance to waypoint with 18.33. But it complicates the calculations of the GPS track boat since the GPS track boat doesn't get across in four hours. I have not yet resolved that complication.

Does that make sense now?

[goboatingnow]

I chose to fit the distance to the CTS track, and it is easy to backwards compute what distance that would be. It's just the "B" leg of the vector triangle where the "C" leg is the water track, and the "A" leg is the current vector (8 miles).

Yes its a backwards in time computation rather then forward as normal

again I do not accept that the tidal vector is 8, , its the RMS of the peak valve. , ie 9,2 not the average

Dave

[goboatingnow]

I did of course understand that these are constant currents, not tides, which run at different rates in different places, but do not change over time.

no different from a multi-hour tide computation. Thats exactly what you get on a passage in one varying tide. ( its actually the same tide, just flowing at different rates in different places).