Tacking or Gybing

[paul1960]

I first want to say I have never been on a real sailboat so I may be totally off base in the reasoning of my question but here goes. I have been playing a sailboat simulator called sailaway that is I think pretty realistic in many ways. I have been puzzling over tacking or gybing. From a convenience stand point I can see where your best option may be one swing out and one swing back into your mark but it seems like you should be able to optimize that using the pythagorian theorem provided you keep you tacks/gybes at 45° to the hypotenuse (course). I've been looking for some information on this online but either my search skills are lacking or no one is talking about it. The formula is a² + b² = c² where c is the hypotenuse and a and be would be legs of the tack/gybe. So say if you are making 6 knots on the hypotenuse straight downwind and at 45° to either side you can make 8 knots how far out do you gybe before gybing back. There has to be a point where the 2 knots of extra speed is lost to the extra distance traveled. Trying to wrap my head around that if any of you have solved it I would appreciate an explanation.

[44'cruisingcat]

Sounds like you're talking about what people call "tacking downwind".

It can be a faster way to sail downwind byt only on pretty quick boats.

[UNCIVILIZED]

Every boat has a best angle to sail in relation to where the wind is coming from, & at what wind speed. There are data tables called Polars (Polar Projections) for most boats which show you what the best course to sail is based on the above, & what boat speed to expect when sailing to the Polar’s recommendations.

And ignoring current for the moment, if you follow them, you’ll get the best Velocity Made Good (VMG), & or Course Made Good (CMG), as well as perhaps Speed Over Ground (SOG).Which, VMG is the BIG one, as it tells you how well you’re making progress towards your destination (the end point of your hypothetical triangle).And VMG accounts for tacking, & gybing.Thus it tells you how well you’re doing overall, progress wise, & not just simple boat speed. The higher your VMG, the faster you get where you’re going.

That said, if you’re fighting a lot of current on one tack, & little to none on the other, then obviously your VMG will be higher on one tack than the other, even though your speed through the water is the same. So one tack is definitely "favored". And in theory, you try & stay on a favored tack as much/long as possible. Hopefully until you reach a point where the wind or currents shift, & you can then switch to the other tack without taking too much of a beating on VMG. Or if you're lucky, the winds or currents will shift enough so that this other tack is now the favored one.

The bottom line is that sometimes when going upwind, it pays to sail a few degrees below being fully close hauled. Since the gain it'll give you in boat speed more than makes up for the extra distance traveled. And the same is true downwind. With relatively few boats anymore being their fastest sailing Dead Downwind (DDW). But rather they'll sail with the wind at 130-150 degrees relative, & gybe a few times. Since the extra speed gained by sailing those "hotter" angles (hotter than DDW) more than offsets the extra distance sailed.

Here are a couple of sites listing polars for a number of boats. And they show you how fast you should be going on any point of sail, & when to switch from a jib to a spinnaker for best speed.Also, many boats, & classes of boats come with Polars, or they’re easy enough to get copies of them.
http://l-36.com/polar_polars.php
http://www.blur.se/boats/

[Blue Stocking]

-- and if uncivilized explanation didn't give you something to think about, add this !


As you sail in the direction from which the wind is coming ( upwind ) the natural wind speed is added to your boat speed, and is called apparent wind. This apparent wind is what creates flow ( lift ) as it flows across the sails. In the case of the recent AC35 cats, which I have had sailing in my back yard for 2 1/2 years, you will notice that the mainsail ( wing ) moves very little off centre. That is because they are always sailing upwind, even when sailing downwind.

[Dave_S]

Go at it backwards - You can calculate a chart to tell you what speed increase you need to achieve at what angle to maintain your velocity made good then change angles untill you are running at the greatest speed above the figures on your chart.

[GrowleyMonster]

I just go wing and wing, crossed sheets to tiller, and take a nap.