Using a Compass to Determine When to Tack

[theuselessone]

I heard about a technique in which you use a compass to determine when to tack during a race. It involves something along the lines of taking a compass heading or bearing to a mark before the race. Then, during the race, if you're compass heading/bearing goes outside some value, you tack (possibly also used on the downwind leg too). Can anyone fill in the details?

I did a search and didn't find anything on this.

Much appreciated,
theuselessone

[celenoglu]

The finish line is never completely to the wind. It is always 10 degrees or more off the wind. You should know the limits of your boat. Although it is believed that tack angle is 90 degrees, some tack 80 degrees and some 100. Assuming the target is directly to the wind, you can guess your tack angles. If Your tack angle is directing you less than anticipated, that means you should keep on that tack less than the other tack.

[puddinlegs]

Not quite so simple as that, but I think what you're getting at is:

Is the starting line perpendicular to the true wind direction?

Sometimes it's pretty clear which end of the line is favored (closer to the first mark), but can be checked by luffing head to wind, taking a compass reading, then reaching down parallel to the start line... a little arithmetic, and you're there. Wind, wind direction, and velocity are fluid of course, so once you're off to the windward mark, you want to sail the shortest distance. There are any number of other variables including tidal and/or land influence, as well as tactical considerations that will determine where to go and when to tack. On top of that, there are your boat's own polar information concerning ideal target speeds for particular tacking angles, etc... Some instrument packages will be programmable and will calculate boat targets, VMG, etc... About the basics though, there's a ton of information on this stuff, and if you can find an old copy of one of Stewart Walker's books, he explains things as well as anyone. In the end, if there were a compass that told everyone when to tack, everyone would have one! What you've stumbled on is both the idiot simplicity and the vast complex calculus of getting to the windward mark, and we haven't even talked about sail trim yet!

[theuselessone]

Thanks for your replies. I think that what I heard was a combination of your responses. I think this may be something dinghies use more often than big boats. Apparently, some sailors have their cockpits marked up with crayon (or some other writing material) writing down angles.

[poltergeist]

Don't know what your search criteria were, but try "using a tactical compass."

You'll find all kinds of good info. Now, if I could just train myself to use one effectively, I'd probably have better results.

Kurt

[Hudsonian]

I think the issue that you're referring to is "staying on the lifted tack on the windward leg". When within the laylines it wouldn't matter whether we are on port or starboard tack if the wind didn't shift. But frequently the wind oscillates back and forth to the left and right. When the wind shifts to the left boats on starboard tack are lifted and those on port tack are headed; therefore, boats on port should tack to starboard (the lifted tack). Conversely, when the wind shifts to the right boats on port tack are lifted and those on starboard tack are headed; therefore, boats on starboard should tack to port (the lifted tack).

To determine the median wind direction you record a series of obervations of wind direction by sailing close hauled before the race and recording the bearing after each lift or header.

[theuselessone]

Thanks, "Tactical Compass" does seem to be what I was looking for. According to Basic Downwind Performance, Part Two by Zack Leonard:

In an oscillating breeze with steady velocity, the decision-making is easy—it generally pays to sail close to rhumb line, jibing onto each new header as it comes. So, how do you know when your boat gets lifted and it's time to jibe while sailing downwind? It's a little trickier than upwind, but if you have a VMG chart (velocity made good) and you are sailing by your numbers, then the compass will tell you when this occurs (see sidebar). You'll have to head up to keep your speed the same, but just be sure not to confuse this with heading up because you're in a lull!

Say you're sailing in a small, non- spinnaker dinghy like a Snipe, then you only need to head up until the jib won't stay set. When you get to that point, note the compass angle and check that angle occasionally. If you have to head up much higher for the jib to break, then you have been lifted and should consider a jibe. As little as a 10-degree shift can make a huge difference. If your boat sails fastest when it's eight degrees above dead downwind, then you will only deviate from the course to the mark by three degrees by staying in phase on the headed jibe. If you get out of phase, you can deviate from the direct course to the mark by 13 degrees. I'm not much of a mathematician, but the geometry looks a whole lot better when you sail in phase, sailing the headers down the course the whole time.

And from Advanced Sailing Lessons : Using the Compass: Advanced Sailing Video Lessons, I'm guessing you would use a similar strategy for going upwind as well.

[zz4gta]

First the OP was asking when to tack during a race, not how to find the favored end of the starting line. The favored end is the one which is farthest upwind. It has almost nothing to do with the position of the mark itself.

Deciding when to tack depends on a lot of factors. But lets say everything is equal except the wind direction. Find the wind direction, try not to do it by flogging your sails, it's the absolute worst thing to do to your sails. Motor into it as slow as you can, or better yet, record your upwind tacking angles while you're out an hour before the start practicing. You are an hour early to the line right? Lets say your on stb tack and your compass bearing is 315. You tack over to port and the compass now reads 45. Well you just tacked through 90*. The wind is coming from 0* or N.

If it was coming from the east (90*), a stb tack would be 45* and port would be 135*. Simple enough right? Now if you're cruising along and notice you can come up to 53* on stb, you are no "Lifted" 8*. If you're on stb and you come down to 39*, the you're down or "Knocked" 6* and should probably tack over to port which you will notice is now lifted 6* to a bearing of 129*.

I know this is a bit much to digest, but just write down your bearings on each tack every 3-5 minutes and then review them after the race. It's best to put the snactician in charge of this task.

By writing this information down, you'll be able to see what the wind was doing at different times in the day. Was it a persistent shift or oscillating? If it was persistent, is it going left to right? How will that effect race #2 of the day? Were the waves or wind more or less on different sides of the course? What were the tides/current doing? Who won the last race and why? Trust me, it wasn't the boat...

[Hudsonian]

You try to stay on the lifted tack on the windward leg and stay on the headed tack when going downwind. It is much easier to determine which is the lifted tack when going to windward than it is to determine which is the headed tack when going downwind.

[theuselessone]

Thanks for all the replies, and zz4gta for the explanation. It seems that this is simpler than I thought this was (well, different at least). So the key point seems to be that the compass was used to find when a tack was lifted or headed, and to tack when it was headed.