BrendanS
Well-known member
I already have! /forums/images/graemlins/grin.gif
Morning office update
- Thread starter TigaWave
- Start date
Dipper
Well-known member
Excellent. That was perfect but since I wasn't on the helm I was starting to worry. Same as real life really.
I'll leave you to get on with it now. I hope you don't feel I have interfered too much. My last bit of advice is don't sail into a flat patch. Better to deviate a bit and keep the wind.
I'll leave you to get on with it now. I hope you don't feel I have interfered too much. My last bit of advice is don't sail into a flat patch. Better to deviate a bit and keep the wind.
Phoenix of Hamble
Active member
Only, you need to be careful, as the VMG is for your speed relative to dead upwind, or dead downwind, and that might not be the way you want to go!, so sometimes a lower VMG is better!!!!
BrendanS
Well-known member
Not at all. I'm learning loads through feedback, and I love that aspect. It's great to be taught new things!
BrendanS
Well-known member
Can you explain that in words of one syllable to a novice? I'm still trying to get the hang of when vmg is better, or boat speed is better?
fireball
New member
VMG is no good if it isn't in the direction of the course .....
Dipper
Well-known member
Ok.
First grab the corners and drag to make it bigger.
It’s like a circular graph where you read your course (relative to the wind) off against your predicted speed. They don’t bother showing the top (because you can’t sail that close into the wind) and they don’t bother showing you the left side (because it is a mirror image of the right side).
Your TWA (True Wind Angle relative to your moving boat) is shown around the circumference (ignoring positive and negative values) and the coloured lines are the different wind speeds. You can see that the boat behaves subtly differently depending on the wind speed. Some wind strengths have a double top speed, a bit like a double high tide.
There is a thin black line at the current wind speed with a black circle/dot on it showing your boat speed. If you change your course either by TWA or Course, the dot will move one way or another.
Using the black line, you can see what course gives you your maximum straight line speed but it may not be in the right direction. The black line will mimic the coloured lines as the wind changes but you can use the coloured lines as a guide to future wind strengths/courses if you know this is going to change for your boat.
On the diagram, wind from 180 degrees is directly behind you and wind from 90 degrees is at right angles to your beam. Sailing with the wind at TWA 180 (dead astern) is very inefficient and the diagram shows faster speeds as the angle decreases. Exactly the same with heading into the wind except at 0 degrees you will stop! There is obviously a cut off point when going upwind or downwind where the speed gained starts to be lost by being too far off course. The trick is to find this perfect point which varies a bit with wind strength. The VMG reading helps this. If you are beating to a mark or having to gybe downwind to reach it, then use the VMG reading to get your maximum speed in the direction you want. If you can make the mark without tacking or gybing, ignore it. In real life, you could put the next mark as a waypoint in a chart plotter and set up a VMG on that but there isn't that facility here.
I hope this helps.
First grab the corners and drag to make it bigger.
It’s like a circular graph where you read your course (relative to the wind) off against your predicted speed. They don’t bother showing the top (because you can’t sail that close into the wind) and they don’t bother showing you the left side (because it is a mirror image of the right side).
Your TWA (True Wind Angle relative to your moving boat) is shown around the circumference (ignoring positive and negative values) and the coloured lines are the different wind speeds. You can see that the boat behaves subtly differently depending on the wind speed. Some wind strengths have a double top speed, a bit like a double high tide.
There is a thin black line at the current wind speed with a black circle/dot on it showing your boat speed. If you change your course either by TWA or Course, the dot will move one way or another.
Using the black line, you can see what course gives you your maximum straight line speed but it may not be in the right direction. The black line will mimic the coloured lines as the wind changes but you can use the coloured lines as a guide to future wind strengths/courses if you know this is going to change for your boat.
On the diagram, wind from 180 degrees is directly behind you and wind from 90 degrees is at right angles to your beam. Sailing with the wind at TWA 180 (dead astern) is very inefficient and the diagram shows faster speeds as the angle decreases. Exactly the same with heading into the wind except at 0 degrees you will stop! There is obviously a cut off point when going upwind or downwind where the speed gained starts to be lost by being too far off course. The trick is to find this perfect point which varies a bit with wind strength. The VMG reading helps this. If you are beating to a mark or having to gybe downwind to reach it, then use the VMG reading to get your maximum speed in the direction you want. If you can make the mark without tacking or gybing, ignore it. In real life, you could put the next mark as a waypoint in a chart plotter and set up a VMG on that but there isn't that facility here.
I hope this helps.
Dipper
Well-known member
We may come to regret this advice! /forums/images/graemlins/grin.gif
Phoenix of Hamble
Active member
Yeah, as said above.... the VMG is toward the wind (or away from it)
As an illustrtaion.... if the course you require is at 90 degrees to the wind, then optimum VMG is 0.0kts!!!!
So.... to be technically 100% accurate, you need to calculate your own VMG for the course you require.... which is a whole load of maths..... /forums/images/graemlins/grin.gif
As an illustrtaion.... if the course you require is at 90 degrees to the wind, then optimum VMG is 0.0kts!!!!
So.... to be technically 100% accurate, you need to calculate your own VMG for the course you require.... which is a whole load of maths..... /forums/images/graemlins/grin.gif
Whitelighter
Active member
Thanks all,
Not sure it will help my position in the short term, but I understand it so hopefully will get a little more 'in the game'.
Up to 568th now /forums/images/graemlins/smile.gif
Not sure it will help my position in the short term, but I understand it so hopefully will get a little more 'in the game'.
Up to 568th now /forums/images/graemlins/smile.gif
Dipper
Well-known member
I can't see what your current wind direction/strength is but as a start, try setting your TWA at minus 140 (you should already have a negative TWA). Let it settle then adjust one way or the other by one degree increments and see if the VMG reading on the polar diagram improves. Since you are going downwind, the figure will become a larger negative. If your VMG drops, try the other way and you should find a high point.
Remember the minus sign each time (if you are on that tack) or you will gybe round. Been there. Done that. Several times. /forums/images/graemlins/blush.gif
Edit: You have to interpret the results carefully if the wind strength is changing (up or down) as your VMG will also change due to this.
Remember the minus sign each time (if you are on that tack) or you will gybe round. Been there. Done that. Several times. /forums/images/graemlins/blush.gif
Edit: You have to interpret the results carefully if the wind strength is changing (up or down) as your VMG will also change due to this.
Whitelighter
Active member
I take it the dots along my intended course are mark my prohected position in hours 1-6?
Dipper
Well-known member
Yes. They are at hourly intervals so when you go faster the line stretches.
Whitelighter
Active member
currently at a TWA of -144.1 degrees, making 6.6knts with a VMG of -5.34knts
seems to be ok from what I understand
seems to be ok from what I understand
Dipper
Well-known member
One other thing on the polar diagram is the % figure. If you change course abruptly or run aground, this drops depending on how this might affect your speed in real life. Then it slowly recovers. The worst thing about accidental gybes is not that you've gone off course but that you drop down to say 87% of your original speed and it can take several minutes to pick up again. Minor course changes don't seem to affect it.
Whitelighter
Active member
ok, worst I have seen so far is 98%
Whitelighter
Active member
one more thing, but on the forcast, i can see the wind arrow getting bigger = more wind, but how can I tell the forcast wind speed on different parts of the course? is there a numerical value I can get somewhere?
Dipper
Well-known member
You can move your mouse cursor (the cross) over the course and find additional information in orange writing on the black bar at the top. I got caught out at first because the wind direction and speed only relates to the time of the last update and may not be right. That's why I don't know what the wind direction is where you are. I can only see what it was when it was last updated. This also applies to the wind arrows - they become immediately out of date as soon as they are updated so you have to mentally extrapolate speed direction by looking at the next 3 hour window and see what the trend is.
The mouse cursor cross also gives you the bearing and distance of the cross from your boat. Handy if you don't want to bother with the ruler. So you can place the cross on the next headland , get these readings and use it to judge when to gybe/tack.
The mouse cursor cross also gives you the bearing and distance of the cross from your boat. Handy if you don't want to bother with the ruler. So you can place the cross on the next headland , get these readings and use it to judge when to gybe/tack.
Whitelighter
Active member
Thanks.
Sorry for turning the thread into a how to play one. Getting right into this now. Think I might be restricted in the amount of work I do if I can get the client on the system at work /forums/images/graemlins/smile.gif
Sorry for turning the thread into a how to play one. Getting right into this now. Think I might be restricted in the amount of work I do if I can get the client on the system at work /forums/images/graemlins/smile.gif
Dipper
Well-known member
Correction - the wind is shown in pale blue.