davidfox
Well-Known Member
Bilge increases your options on the east coast, ask Maurice Griffiths
Moody 346 - Bilge Keel, opinions?
- Thread starter absit_omen
- Start date
Birdseye
Well-Known Member
[ QUOTE ]
I believe that the vmg is calculated by log and wind, and don't understand why GPS comes into this at all
[/ QUOTE ]
No Nick. VMG is "velocity made good" in the direction of the destination.
You can be doing 5 knots in a direction at right angles to the direct course to your destination, and your VMG is zero because you are getting no closer. If you are doing (say) 5 knots on a course at 45 degrees to the direct line then the VMG is 5 x .7071 = 3.53 knots, because in an hour you will be 3.53nm nearer the destination (assuming you havent gone past it).
None of this is anything to do with either log or wind, except that given a clear head and a pocket calculator, you can estimated VMG by doing 2 EP's a few minutes apart and working it out from that. Basic vector maths.
I believe that the vmg is calculated by log and wind, and don't understand why GPS comes into this at all
[/ QUOTE ]
No Nick. VMG is "velocity made good" in the direction of the destination.
You can be doing 5 knots in a direction at right angles to the direct course to your destination, and your VMG is zero because you are getting no closer. If you are doing (say) 5 knots on a course at 45 degrees to the direct line then the VMG is 5 x .7071 = 3.53 knots, because in an hour you will be 3.53nm nearer the destination (assuming you havent gone past it).
None of this is anything to do with either log or wind, except that given a clear head and a pocket calculator, you can estimated VMG by doing 2 EP's a few minutes apart and working it out from that. Basic vector maths.
tome
Well-Known Member
Aha, the voice of reason. And vmg will require you to select the correct waypoint.
A little trick. Put in a waypoint half way across in the middle of the shipping lanes. It's good for morale, it's good for vectors and it will make a better navigator and skipper of you. Use it to plan your sleep pattern and watches, after all you'll have time to go on display and can work backwards to the start of the first shipping lane.
Oops, posted this on the wrong forum but will leave it anyway
A little trick. Put in a waypoint half way across in the middle of the shipping lanes. It's good for morale, it's good for vectors and it will make a better navigator and skipper of you. Use it to plan your sleep pattern and watches, after all you'll have time to go on display and can work backwards to the start of the first shipping lane.
Oops, posted this on the wrong forum but will leave it anyway
S
Skyva_2
Guest
All true, but I was referring to velocity made good to windward, sorry if that was not clear, and I think Nick was also referring to that.
I supppose the GPS would be an alternative means of looking at it, must try that, if I can set up a waypoint to windward.
I supppose the GPS would be an alternative means of looking at it, must try that, if I can set up a waypoint to windward.
BlueSkyNick
Well-Known Member
Fully understand the theory - did maths and physics A'level - when they were still hard !!
I obviously need to learn how to use the technology on the boat ! Off for a sail today, so will give it a go.
Thanks for your guidance
I obviously need to learn how to use the technology on the boat ! Off for a sail today, so will give it a go.
Thanks for your guidance
Twister_Ken
Well-Known Member
"Fully understand the theory - did maths and physics A'level - when they were still hard !!"
Respect!
I did geography, botany and zoology, when they were still about colouring in pikshures.
Respect!
I did geography, botany and zoology, when they were still about colouring in pikshures.
Sailfree
Well-Known Member
Am I missing something here. Certainly the Raymarine series of instruments show wind log etc (these are based on the sea not the ground). They work independant of any GPS and calculate VMG on how close you are to the direction of the wind therefore when you are heading into the wind to any extent it assumes you want to now how much you are going upwind and give vmg. THis vmg is a rough calculation as it cannot take any account of tide or leeway. As someone said if you are at 90 deg to the true wind the vmg from the raymarine will be zero. Now if you are using your GPS and heading to a waypoint it take the direct distance you are from it and divides it by the "recent" historical progress towards it (this does factor in leeway ,currents etc as it is relative to the ground).
Or have I completely misunderstood something?
Or have I completely misunderstood something?
Twister_Ken
Well-Known Member
"Or have I completely misunderstood something?"
I've no idea, but I've a feeling I have. VMG has to be over the ground if its to be of any real use, cos unless you know that, you won't know which is the paying tack, except you probably will if you can read a tidal atlas.
And anyway, it must be more useful to know VMG to a waypoint, rather than to the eye of the wind, cos very rarely do you want to go EXACTLY where the wind is coming from.
Is this one more thing we shouldn't really scratch our heads about, unless we're racing?
I've no idea, but I've a feeling I have. VMG has to be over the ground if its to be of any real use, cos unless you know that, you won't know which is the paying tack, except you probably will if you can read a tidal atlas.
And anyway, it must be more useful to know VMG to a waypoint, rather than to the eye of the wind, cos very rarely do you want to go EXACTLY where the wind is coming from.
Is this one more thing we shouldn't really scratch our heads about, unless we're racing?
