Posts From Category: Navigation

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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Sailboat Guidance System

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A quick look at the almost-complete Sailboat Guidance System (SGS). The board on the left is Igor. Otto is on the right, and the main CPU (Mother) is hidden underneath. The ribbon cable brings all the I/O to and from the boards. The red insulating tape is to remind me of some of the last remaining wires which need to be connected. You can also just about make out the DC/DC converter which is just underneath the ribbon cable at the bottom of the picture.

REDIS

After a lunchtime conversation with a friend of mine, I ditched some of the earlier design considerations around message-passing and MQ-based systems in favour of the NoSQL database Redis.

Redis is an open source, BSD licensed, advanced key-value store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets and sorted sets.

That’s according to the official blurb on the web site, at least. It has several interesting components which are useful for Beoga Beag. It has a rudimentary pub/sub architecture pattern, it is extremely lightweight, it stores its database in memory rather than on a disk (so it’s less likely to burn through the Compact Flash file system), and it has an “append-only file” (AOF) archive mechanism.

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Rudder box with gears

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The above photograph is of the new rudder box with a NEMA17 stepper (underneath) and the two gears. The smaller gear is on the stepper and the larger, quarter gear will be clamped to the rudder shaft. It’s a 4:1 ratio and the rudder gear is 90 degrees so one complete turn of the stepper will bring the rudder from end to end. That’s plus or minus 100 steps.

This box will be mounted above the deck, with the rudder shaft disappearing down through the hull.

Rudder angles greater than around 40 degrees either side are counter-productive, as the rudder stock starts to act more as a brake than a steering surface. So a 90 degree swing is plenty.

First signs of weathering.

Rust on the Wind Direction Indicator bearing, after a couple of weeks of weather testing.
Rust on the Wind Direction Indicator bearing, after a couple of weeks of weather testing.

In order to test the spiffy, new Wind Direction Indicator mentioned in a previous post, I assembled the mechanical components (without the potentiometers) and mounted it outside. As we live in a wind-swept area, with a high concentration of sea salt in the air, it was a useful test of how the system would perform. What I couldn’t test was the effect of high temperature/humidity in a salt-water environment. To do that, I’d have to move to the Caribbean (which seems like a good idea, given the Irish summer we’ve had so far!).

Anyway, I digress. After a few weeks of spinning freely in the breeze, I disassembled the unit to examine the condition of the internal parts, and the news is not good.

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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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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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Battery and Solar Design

Sealed Lead-Acid Battery
Sealed Lead-Acid Battery.

Looking at the system power design, the majority of the circuits will run off a +5 volt rail. Those elements which need a different voltage, such as the main processor board, will derive their own requirements from the main Vcc rail.

There will be at least two Vcc busses on board. Labeled, oddly enough, as Vcc1 and Vcc2. The difference between them is that Vcc1 is always on, at all times, and Vcc2 (through VccN) are selectable by Igor.

The main processor runs off Vcc2, but Igor (and Otto) both run off Vcc1. In situations where voltage levels are critical, Vcc2 will be switched off and the boat will continue on whatever course had previously been set, until either voltage levels are healthy, the specified “wake-up” time has elapsed, or there are critical issues which require Mother to get involved. A critical situation could be something like a dramatic wind shift, or an error such as a mis-reading from a sensor.

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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 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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Why Antigua...?

Looking down on English Harbour from Shirley Heights in Antigua
Looking down on English Harbour from Shirley Heights in Antigua.

A first pass over the route and the finish line suggests Antigua could be a good destination. We last visited in 2009 and this is as good an excuse as any, to go back.

The finish line is from latitude 10 degrees to latitude 25 degrees North, which is all of 900nm long. However, according to the rules, we must declare a 50km (27nm) segment of that finish line. Although the finish line is pointing True North, when you consider the race is from Northern Europe, that means that the line is heavily biased towards the starboard end. The Northern tip, in other words. Finishing on this end of the line shortens the final leg.

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