Posts Tagged “SOG”

The New Winged Sail

The wing-sail is in two parts, a leading-edge and a hinged 'tail' or trailing edge.
The wing-sail is in two parts, a leading-edge and a hinged "tail" or trailing edge. This is a CAD drawing of the leading edge.

My original plan was to use a traditional mast and mainsail, with Yannick Lemonnier of West Sails volunteering to produce the sail. Yannick is no stranger to mad schemes himself, having competed in far too many Figaro races. These days, he spends his time sailing his Moth winged-beast, or racing his International 14. That is, when he’s not making sails for everyone from Beoga Beag to the Volvo Open 70s.

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Bon Voyage, Snoopy!

The 'Team Joker' entry in the Microtransat race, with Snoopy on the foredeck, keeping the boat safe from marauders.
The "Team Joker" entry in the Microtransat race, with Snoopy on the foredeck, keeping the boat safe from marauders.

Today, March 23rd, Team Joker are planning to launch their ninth boat, Snoopy Sloop. This has been an educational (and obviously fun!) experience for Robin Lovelock and his fleet of robotic warrior boats. Here in Beoga Beag land, we wish them well.

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Of Laylines and Beats

A Google Earth track of the virtual boat attempting an 'Olympic' course.
A Google Earth track of the virtual boat attempting an "Olympic" course. The course is to go from the leeward mark upwind to the windward (or weather) mark, sail across to the gybe mark, gybe (obviously!) and sail down to the leeward mark. After that, sail up to the windward mark again and then straight downwind to the leeward mark.

On the left, you can see the track left by simulating the Beoga Beag navigation software. It’s a short, olympic course suitable for dinghies and smaller boats. For a dinghy race, the whole thing should take less than an hour so the legs are quite short.

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Five Degrees of Wrong.

So, as I mentioned, the virtual boat was too eager to tack. If you’re dead downwind of the mark, and you set off on a starboard tack, within a metre of being on the left-hand side of the course, the other tack is favoured.

I added code that essentially stated “unless the other tack is at least five degrees better than the existing one, ignore it.” So, if I’m at 44 degrees TWA and the other tack is better by a degree (-43 say), stay where you are. This works quite nicely. If you look at the plotted course, it shows the boat sailing nice upwind legs, to the waypoint. As Henry would say, “it’s sailing up the ladder.”

Without that little extra piece of code, it would tack repeatedly, attempting to sail directly upwind by constantly tacking. A strategy that’s doomed to fail because tacking slows the boat down, and isn’t something you should do too often.

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Polar Curves

Polar curve for a Figaro, courtesy of SailOnline.
Polar curve for a Figaro, courtesy of SailOnline.

Even before a boat is built, the designers can predict how fast it will go at various sail angles. Using this information, they can make modifications to the hull to suit the type of sailing. For example, if an around-the-world race looks like it will see a lot of downwind sailing, it’s possible to optimise the downwind performance, and run test simulations with the boat, before ever committing to fibreglass.

The standard mechanism for displaying this information is a polar curve. Because the boat should sail at the same speed on either tack, only one side is shown. Essentially, a polar curve allows the designer (and the boat owner) to predict the hull speed for a particular true wind angle and strength. In the example above (courtesy of SailOnline.org), you’ll notice that the boats fastest speed is at a true wind angle of about 120 degrees. In the case of a 30 knot breeze (the red line), the boat should get over nine knots through the water. At TWA’s of twenty degrees and less, the boat will stop, regardless of the wind speed.

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To tack, or not to tack, that is the question...

Looking at the upper-level navigation software introduces some particularly interesting questions. The low-level software will keep the boat on a TWA, or true wind angle. Technically, it’s an apparent wind angle, but that’s ok.

The upper level has to decide what is the best TWA. To do this, it has the current position of the boat and the position of the next waypoint. It also knows the current TWA and the compass heading. Without bogging down in the maths, it can compute the distance and bearing to the next waypoint using something called a Haversin algorithm. Given the current TWA and the heading, it can determine the wind direction. We can compute the VMG or “velocity made good” for each new heading possibility, based on the predicted Polar (more on that anon). So, we can see that a particular heading is the best course to get us as fast as possible to the next mark. All of this is standard stuff, and is used on sailboat race courses every day.

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The Main Systems Design

In terms of the system architecture, we’re planning on using two separate computer control planes to manage and steer the boat from start to finish.

At the lowest level, a custom Atmel (ATmega8) board will act as a basic “autohelm”, driving the boat to a specific True Wind Angle or TWA. As the breeze shifts, so too will the boat, to maintain that TWA. This is a basic PID algorithm for controlling the rudder and mainsail in relation to the specified TWA and is very similar in design and implementation to your average sailboat autohelm. This is the control board, also known as Otto or Otto von Helm, to give him his full title. Otto will most likely be assisted by Igor, in charge of power management and the boat environment. It is Igor who will turn on and off the other systems on board, including the main system (Mother).

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