Stupid racing question - Navigation

[tommays]

It is chicken and egg

It depends on the GPS as the new ones do it different and dont require a costly system

The S10 calculates the heading from the downwind reference point to the upwind reference point. This heading is assumed to be the wind direction. Since the only information derived from the reference points is this heading, the position of the reference points in absolute terms does not matter. The only thing that matters is the position of the upwind point relative to the position of the downwind point.
The S10 takes GPS readings every second to get your actual speed as well as the heading of your course. The device then multiplies your speed with the cosine of the angle between your heading and the wind direction to extract the component of your speed that is aligned with the wind direction. This process is repeated every second. Some averaging is then performed to make the output smoother and easier to interpret. The results are displayed on the LCD screen.

[jarcher]

Originally Posted by tommays

The S10 calculates the heading from the downwind reference point to the upwind reference point. This heading is assumed to be the wind direction.

And therefor VMG is treated as velocity directly into or directly against the wind. It's merely coincidental that the windward mark happens to be up wind

I suppose this is better than nothing, but wind does shift. I guess if you want to steer to VMG then its better than nothing.

[SEMIJim]

Originally Posted by jarcher

Well VMG to mark vs VMG to the wind are referred to differently in different places. Here is one example:

Why VMG Matters - a knol by John Navas

Ockam and Nexus both use VMG to mean directly into or against the wind, and I have found that this is common when referring to racing.

Several months ago a sailor posted another solution, using a standard GPS: At the start, and as you round each mark, set a waypoint quite a way beyond the next mark (IIRC he suggest a couple thousand miles), on the same bearing as the bearing to the next mark. That way you don't encounter the problem, mentioned in the cited article, of your VMG decreasing as you approach the mark, because you'll never get even close to the far-off waypoint.

Exactly how you go about doing that, I'm not sure. A nice addition to GPS software would be a "VMG on course" (?) display, where the GPS would do this for you. That way you could still display bearing, distance, etc. to the actual mark, but the course VMG would be calculated based on the far-off pseudo-waypoint. Otherwise, I guess you'd need two GPS'.

I found the article I described above: VMG & GPS: VMG USE MADE EASY (“VMG FOR DUMMIES”)

Jim

[SEMIJim]

Originally Posted by stardvd555

They'll usually give you a bearing and distance from the previous mark or starting line if its a drop mark course. Check your SI's.

That's not the problem with the GPS method. The problem is figuring out how set a waypoint way off in the distance on that bearing. (I haven't tried it yet, so maybe it's not all that difficult.)

Jim

[maxmunger]

The race course is always posted by RC. Either a series of marks or an Olympic course. Either a diagram or a list/map of set marks is provided in the SI's.
Put the waypoints in any little GPS and it will guide you to all of them. With critical boat info including current effect.

[jaakennuste]

SailCruiser Pro polar data screen

We always get pre-defined coordinates of each mark. Navigator enters the coordinates into the navigation system. Knowing the wind direction and course, it's possible to calculate target speed, if correct polar data is also entered into the system.

[jarcher]

Cool software, I'll check it out.