Posts From Category: Simulations

Rudder Movement Test

I have uploaded a video (to YouTube) of the rudder mechanism on Hull #1 working from end-stop to end-stop. (Excuse the video quality, I used my phone to record it.)

I connected an Arduino (Mega2560 if you want to know) and a SparkFun stepper controller to the stepper motor which drives the rudder. I wanted to exercise the tiller gears and the rudder shaft for a while. The video shows the rudder swinging from almost completely to Starboard, all the way back to Port.

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Impending Launch.

The view from the office.
Repurposing the #001 Hull for upcoming sea trials.

Hull #002 is coming along, slowly. The stations are mounted on the strongback and I’ve been double-checking the alignment. I have time to do this because 1/32nd inch balsa wood is hard to come by, and because I haven’t cut the keelson yet. Well, I have, but I need to re-cut it. The original was cut to the wrong profile.

While the work on hull 2 continues, I’ve been using Hull #001 to test out aspects of the boat design and control systems. In fact, I am very close to launching the hull…

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Mission 001 - Galway Bay

As the hull is now watertight, and we’re mere weeks away from having a sealed hull with keel and rig, it’s OK to start looking at actually getting this thing to sail.

Up until now, I’ve been somewhat obsessed with getting the physical aspects of the boat to a certain juncture. The reasoning is simple; until there’s a boat, all of this other stuff is just a waste of time. Well, now there’s a boat…

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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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The Timetable.

The official start to “hurricane season” is June 1st, this year. The traditional end to the season is the end of November, but the hurricanes seem to wane out by early to mid-November. A back-of-the-envelope calculation says that it’ll take up to sixty days to get from Start to Finish. That’s a worst-case estimation, but useful nonetheless. If we are to launch before the start of the season, we’d need Beoga Beag sailing to the start line by April 1st at the latest. Let’s face it, that’s not going to happen. We don’t have the pilot boat designed yet, we have extensive testing to do on the pilot boat, we need to then build the main boat, and start to fit her out. No way are we going to make that date.

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