Speed through the water

[David of Essex]

Recently fitted Raymarine ST 60 tri data. The speed through the water was showing 14knts and the speed over ground on gps was showing 18knts. I was travelling with the tide at that time not against it. Should the two speeds be equal?
David

 

[Poignard]

No - you had 4 knots of tide helping you

 

[hlb]

So, given that you seemed to have 4 knots of tide, that is correct.

 

[jimg]

The speed through the water is exactly that and is what the ST60 is showing.
The GPS is showing your speed over the ground.
For example if the tide is flowing at 4 knots in EXACTLY the same direction as your travel then you speed over the ground is 14 + 4 = 18knots.
You can check the accuracy of the ST60 by making a couple of runs in opposite directions over a known distance.
In various places that are marked on a chart you will find " measured miles" although they may be actually or less than a mile!

 

[David of Essex]

Thanks for replies, found it a bit confusing. If doing a pasage plan, do you take sog or through water to determine how long to reach distance?.
David

 

[Observer]

Imagine you want to go from point A point B, a measured distance of 10 miles and your speed through the water is 5kts so approx. passage time is two hours.

Starting at point A, you plot (on a chart) the first hour's tide then the second hour (speed and direction or set and drift). Then plot the line from the end point to the destination (B). The bearing of that line is course to steer and the length is the distance you will actually need to travel through the water (which will be a little more or less than the straight line distance).

So the answer is you use speed through the water but applying the tide vectors will then give you your (theoretical) speed over the ground.

You will learn all this on RYA theory courses.

 

[ian38_39]

Or if you are in a fast planing boat doing a short hop your speed over ground will be pretty damn close.

Ian

 

[Solitaire]

Imagine you want to go from point A point B, a measured distance of 10 miles and your speed through the water is 5kts so approx. passage time is two hours.

Starting at point A, you plot (on a chart) the first hour's tide then the second hour (speed and direction or set and drift). Then plot the line from the end point to the destination (B). The bearing of that line is course to steer and the length is the distance you will actually need to travel through the water (which will be a little more or less than the straight line distance).

So the answer is you use speed through the water but applying the tide vectors will then give you your (theoretical) speed over the ground.

You will learn all this on RYA theory courses.

You are doing the dot to dot error of calculating course to steer! You do not join the end of the tidal vector to the destination point B. The vector is calculated based on boat speed; so you measure 5 knots (the boat's projected speed) on a pair of dividers and then from the end of the tidal vector arc a point along the DR line and then join the end of the tidal vector to that mark. It may be below or beyond the final destination depending on boat speed in relation to tide direction. Which is why the DR (Dead Reckoning) or ground track line should always be drawn through the final destination point.

The CTS is then got by laying the plotter along the new line - called the water track - and shown as a single "V" mark along the new line.

The new speed is then measured from your start point to the point where the arc has crossed the DR line. That is the speed but measured as Speed Over Ground because in this case the tide is pushing you along. So it will be greater than the 5knts you based your original calculations on. The new speed will then allow you to work out how long it will take in reality.

 

[Observer]

You are doing the dot to dot error of calculating course to steer! You do not join the end of the tidal vector to the destination point B. The vector is calculated based on boat speed; so you measure 5 knots (the boat's projected speed) on a pair of dividers and then from the end of the tidal vector arc a point along the DR line and then join the end of the tidal vector to that mark. It may be below or beyond the final destination depending on boat speed in relation to tide direction. Which is why the DR (Dead Reckoning) or ground track line should always be drawn through the final destination point.

The CTS is then got by laying the plotter along the new line - called the water track - and shown as a single "V" mark along the new line.

The new speed is then measured from your start point to the point where the arc has crossed the DR line. That is the speed but measured as Speed Over Ground because in this case the tide is pushing you along. So it will be greater than the 5knts you based your original calculations on. The new speed will then allow you to work out how long it will take in reality.

Oops - quite right. I haven't practised dividers and plotter planning for a while.

 

[Solitaire]

Oops - quite right. I haven't practised dividers and plotter planning for a while.

The old saying of, "If you don't use it you lose it" syndrome.

 

[sarabande]

umm, you have prompted me to try a couple of Marc St Hilaire sights....


Lost it first time, and only 5' out the second.

 

[Hurricane]

I know I'm probably going to get shot down in ribbons here but IMO - just let the autopilot do it - thats what its for.

If you havent got an autopilot - steer to the GPS - if necessary using a rolling road.

I know that David is technically correct and thats what we all learn in order to pass the RYA exams but I bet that virtually all mobo skippers just use the electronics.

Easy peasy - oh year - and dont give me that - "what if it breaks down" krap - just have a backup.

 

[Observer]

umm, you have prompted me to try a couple of Marc St Hilaire sights....


Lost it first time, and only 5' out the second.

What - you fixed your position to 5' and you think that's bad? That's better than GPS!!

 

[David of Essex]

Some excellent post that will be of a benefit to all. Solitaire you are right in saying ''If you don't use it you loose it''. I did the Day Skipper Theory 2 years ago, forgot a proportion within 6 months, 2 years down the line ???
Some one put a refresher coarse together with drinks after and I would be willing.
David

 

[hlb]

I forgot most of it before the course ended, and most of the rest within a week or two. Cant say I've ever needed any of it.

Strangely though I remember all the things the course never taught, but much more important.

 

[sailorman]

Recently fitted Raymarine ST 60 tri data. The speed through the water was showing 14knts and the speed over ground on gps was showing 18knts. I was travelling with the tide at that time not against it. Should the two speeds be equal?
David

set the ST 60 up in still water to the gps reading ( see manual )

 

[Solitaire]

I know that David is technically correct and thats what we all learn in order to pass the RYA exams but I bet that virtually all mobo skippers just use the electronics.

I agree with you. But that's again not the point. The OP was about somebody who could not work out why the log and the GPS were reading different things!

You have shed loads of experience and yes, I use the electronics. Yesterday I delivered a client's boat to Dartmouth ( he didn't have time to get it there for the start of his holiday) - got on the boat and left at 06.30, plotted a quick route as going down the Hamble, turned right, kept the land on the right (into the Solent before anybody says I would then be going up Southampton Water!) and hay presto 3.75 hours later was in Dartmouth. Why the quick route? To give the auto pilot something to do!

If I wanted to check tides I quickly did it using the plotter, showed my crew how we could see see tidal streams in real time and all that. But some have to understand what it is they are looking at. I bet when you started you knew FA as well, I know I did!

My point was simply to make clear something that is a common mistake when doing a course to steer. The vector is created by tidal stream and boat speed, not joining the point at the end of the tidal stream to the destination!

 

[Kawasaki]

Some excellent post that will be of a benefit to all. Solitaire you are right in saying ''If you don't use it you loose it''. I did the Day Skipper Theory 2 years ago, forgot a proportion within 6 months, 2 years down the line ???
Some one put a refresher coarse together with drinks after and I would be willing.
David

Yeh but if You include Drinks You will forget again

 

[David of Essex]

Life's about learning not remembering

 

[Hurricane]

I agree with you. But that's again not the point. The OP was about somebody who could not work out why the log and the GPS were reading different things!

You have shed loads of experience and yes, I use the electronics. Yesterday I delivered a client's boat to Dartmouth ( he didn't have time to get it there for the start of his holiday) - got on the boat and left at 06.30, plotted a quick route as going down the Hamble, turned right, kept the land on the right (into the Solent before anybody says I would then be going up Southampton Water!) and hay presto 3.75 hours later was in Dartmouth. Why the quick route? To give the auto pilot something to do!

If I wanted to check tides I quickly did it using the plotter, showed my crew how we could see see tidal streams in real time and all that. But some have to understand what it is they are looking at. I bet when you started you knew FA as well, I know I did!

My point was simply to make clear something that is a common mistake when doing a course to steer. The vector is created by tidal stream and boat speed, not joining the point at the end of the tidal stream to the destination!

Yes, David - I fully accept all your points and understand what you are saying about the obvious lack of understanding that the OP made clear.

But......

You (and I dont mean you personally, but you, the industry) do make things sound like a "black art".
And really, the things you teach are most of the time approximations.

The fact is (and this is a message to all newcomers)
IT AINT THAT DIFFICULT

As HLB says its the things that we AREN'T taught that are important - how many times have any of us mobo owners set off across the channel and calculated our courses to steer? - I suspect that you could count the ones that have on the fingers of one hand.

Gome on guys - GET REAL

Just a bit of common sence and anyone can handle it - dont be put off by all this theory - in reality we all end up using electronics (which as I've said many times before - doesnt seem to be taught anyway)

Again, David, I'm not pointing a finger at you or your team - it was very refreshing to hear that you have actually addressed the issue internally and are offering your own courses on how to handle the electronics.

Perhaps we need the rest of the industry to follow your lead.

Sorry, I have just had a really nice bottle of wine with my dinner and the emotion is coming out - I'll shut up now.