True Wind Discussion Redux
- Thread starter Scott T-Bird
- Start date
The polars for my boat include both true and apparent angles. While I haven't sailed with them yet, it appears to me that they would form a good check by looking at them both. Having instruments I don't know yet, I will likely be using the apparent and speed to figure out if I am in the ballpark.
A big reason why this discussion has gone on so long is that we have different folks talking about different things and folks use terminology differently. A boat’s performance is speed through the water, of course. That is what the polars measure, etc. I read the article. What the author is trying to explain is that current has to be considered when measuring boat performance to the polars. The water flowing over the keel is affected by the current so the windspeed and direction relative to the current’s speed and direction are what matters in this case. This is a separate subject.
6 weeks ago I would have agreed with you Will. That is certainly in their mind. But now I think that depends on the local club racer or the elite racer.
Sure being in the lead at the finish line means you win the race. But I think the elite crowd thinks more about making their boat go as fast as they can. Why else would you see first place finishes with a boat 10 minutes or an hour ahead of the next guy. It is not to just beat the second place boat. It is to demonstrate that on this day at this time we made our boat go that fast.
And yes they care about the win.
Mark you had my "Like" right up to this point.
Reading the original post Scott was talking about polars, wind speed then how to measure that.
These are connected and when trying to identify if your boat is reaching the Polar speeds as identified for your boat you need to know the speed of the boat relative to the water.
Then Scott slipped in the concept fo VMG (velocity made good) and racers. This adds a new dimension to the equation. Now not only do we need to consider the speed of the boat on the water we want to maximize the movement of the boat relative to two fixed places on the globe. Thus the conversation of current and the use of SOG, COG, GPS (which cares less about you being on the water or the beach).
What we ask computers to do for us is to give us instant readout of the two equations STW and SOG and calculate our best action relative to the wind across the boat.
For most of us with our monohull displacement boats is true wind. But as boats have evolved we have learned that as the boats speed up apparent wind becomes a more important metric.
I don't have a clear understanding of this term. Never heard it until joining SBO, in fact. Since then, I've heard different definitions. The one I'm sticking with, so far, is velocity DDW/DUW. It isn't to a point downwind or upwind, but rather to your progress in the direction of or against the wind.
I know it is used most often in reference to the downwind or upwind mark on a race course, but I didn't think that is part of the definition.
-Will (Dragonfly)
VMG is really about measuring how effectively you’re moving toward a point. We talk about measuring VMG in two ways - pure geometric distance to the mark, or speed straight into or away from the wind. Now, when you’re inside the lay lines, measuring speed directly to the mark turns out to be an inaccurate way to figure out how well you’re actually progressing to that mark. Imagine this - you’re 5 boat lengths from the starboard layline, on port tack, but 50 boat lengths from the port layline. Your VMG directly to the mark looks really bad, because your pointed about 85 degrees off it. So you tack, and your VMG goes up to almost your full boat speed, because you’re only 5 degree off your target. Thing is, whether you tacked or stayed on port tack until you got to the layline, you’re really still making the same progress to your destination. So instead we use VMG directly upwind to measure our sailing performance and calculate the time it will take to get to a destination. Once you’re outside the layline the model changes - sailing away from the mark becomes lost distance, so your VMG should be calculated based on the mark only.
If you watch televised sailing races they often have a line drawn across the bow of the lead boat perpendicular to the wind. VMG is basically how fast that line is progressing up the course, and isn’t affected by whether the boat is sailing on port or starboard tack, inside the lines.
Chartplotters often can do this math for you as well. When navigating to a point, our B&G plotter can show both “VMG” (geometric to that point) and “sailing VMG” which accounts for the boat’s polars to figure out whether the boat is inside the laylines, and if it is will show upwind/downwind VMG.
VMG can be to a point or my last chartplotter could produce a display that was VMG to wind which could also include a negative number when moving downwind. To make it simple to get straight, headed dead down wind your boat speed would equal the negative value of the VMG to windward.
"The term Velocity Made Good (VMG) is often confused with term Velocity Made good on Course (VMC), and often used interchangeably.[1][2] VMC is the speed component in the direction of a mark." Velocity made good - Wikipedia
"In a sailboat – VMG is the velocity component of the boat in an upwind (or downwind) direction. The distinction is made because sailboats inherently can not head in an upwind direction but it is important information to know." What is VMG?
-Will (Dragonfly)
"In a sailboat – VMG is the velocity component of the boat in an upwind (or downwind) direction. The distinction is made because sailboats inherently can not head in an upwind direction but it is important information to know." What is VMG?
-Will (Dragonfly)
Wow after reading all these reply's to the OP made my head spin. I am glad I just sail for pure pleasure, and don't care much about speed.
Then again that is just me.
Then again that is just me.