Long Ears

In the morning, there was a small brown scrap on our lawn where none had been before. Turning over, as I thought, the corpse of a headless bat, I was startled by squeaks of fear or rage. The mouse-sized animal had curled its wings in tight around its body, and tucked its head into its chest; even its ears had curled up, looking something like a ram's horns. The face was relatively pretty, as bats go, without the weird pressed-up nose that some species have; the massive ears are the feature you'll remember, though.

Brown long-eared bat

According to the excellent "Exploring Irish Mammals", Plecotus auritus, described by Linaeus in 1758, is found across most of Europe north of the Pyreness, and also in the far east, in regions within Russia, China, and Japan. This bat likes to "foliage glean", plucking its prey from leaves or even the ground as it flys.

~

My tiny specimen was soaked in dew, and motionless - this last must be what saved it from our cat, who loves to chase, but tires of prey which can no longer flee. I tucked my patient away for the day in an empty toolbox, with a few crumbs of cat food for sustenance.

~

After sunset, I took my patient for a walk, to a wooded and cat-free place. Out of the toolbox, the ears inflated and the head looked up; then, the tiny creatures crawled from my gloved hand onto the rough bark of a large tree, hooking on tight with claws on the leading edge of the wing, while black and hand-like hind paws sought purchase.

Making a new rudder, part 1

Following on from the previous posts on the design of rudder foils, our hero now begins the tricky fabrication stage.

As discussed previously, my new rudder will have a fibreglass skin (6 layers) wrapped around a foam core. Not just any old foam - Corecell, a light, stiff material that is absorbs little resin (or water) and will contain any damage to a very localised area. It won't rot, delaminate or crack, and the sheet I bought from the helpful folk at MID has a density of just 60kg / cubic metre.

I've never used this material before, so wasn't sure how easy it would be to work with. First, I trimmed the lengths I needed for the rudder itself from the original 4ft by 8ft x 25mm sheet using a jig saw. No problems, and surprisingly little dust - most of the material from the kerf seemed to stick together, hinting at one potential difficultly: since this foam is a very good insulator, heat from cutting tools disspates slowly - so cutting at higher speeds can cause melting.

Next, I used an off-cut to practice my foam-shaping skills (never shaped foam before, didn't want to start by destroying my proto-rudder). First, I clamped a straight-edged piece of wood atop the foam as a cutting guide (straightness verified against a handy glass window - float glass is very flat). Then, I set the cutting depth on my hand-held circular saw and cut a series of trenches in the foam, the depths calculated from my spreadsheet describing the NACA 0012 foil I want to make.


Next, I sprayed the yellow foam with blue paint, making certain it penetrated to the bottom of the trenches.
Once the blue paint was on, it was now "safe" to start cutting away excess foam; I tried a random orbital sander, but, especially at high revs, discs clogged too quickly. Tried a sharp penknife: not too useful. A block plane worked reasonably well, and a rasp/file came in very handy for tidying up small irregularities. The paint worked a treat - you can see below how the blue lines allow me to be sure I've not removed too much material.


Shaping the leading edge of the foil was quite easy; shaping the trailing edge - much more material to remove, but accuracy is less crucial - turned out to be more difficult. I actually tried chiseling, which worked up to a point, but was slow and difficult. Not sure about the best way to do this yet - I hope to try an angle grinder soon to slice the bulk off, then follow that with plane, disc sander and rasp for the final finish.

Conclusions so far: Corecell is very easy to cut. Shaping it is much easier than shaping wood, but the same techniques won't work.

This post is part of a series on making a fibreglass rudder with a foam core:
Designing a rudder, part 1
Designing a rudder, part 2
Making a rudder, part 1

Astronavigation / Celestial Navigation, Part 2

Last night, a (briefly) clear sky gave me a chance to try a key element of practical celestial navigation. Following the example of Marvin Creamer and countless forgotten sailors of the pre-sextant era, I attempted to measure the altitude of some celestial bodies using no instruments whatsoever (partly because, aside from my telescope, I haven't made or bought any yet).

Why no instruments? Obviously, this decreases the accuracy of the observations that you can make; not so obviously, it is still possible to be accurate enough to make measurements that are accurate enough to be useful. Marvin Creamer made fairly accurate landfalls all around the world using this method, generally maintaining his latitude to within half a degree of the intended value , and any experienced navigation venturing deep into desert or out of sight of land before the coming of "modern" instruments like the sextant almost certainly relied on such techniques. So, it is possible, but is it useful?

Well, sometimes; today, people like myself who like to use the "road" less travelled have the benefit of tools like GPS (I recommend the excellent, cheap, tough Garmin Etrex); however, the very nature of independent travel in remote places means that equipment may well be lost, broken, or simply out-of-charge. By learning how to navigate without any instruments which aren't permanent (I hope!) parts of my body, I'll be able to travel with a permanent built-in backup for the GPS and the compass (you'ld be surprised how many sailors run into GPS trouble). Apart from hypothetical equipment failures, navigation is a subject that I enjoy for its own sake; there is something deeply magical about looking up into the night sky, knowing the stars by ancient names from foreign tongues, and, simply by looking, to find yourself.

Back to practicalities. At this latitude, the July sky isn't really dark at 23:00, so there weren't very many bodies to measure - simple identification was tricky, because only the very brightest bodies were visible. Even worse, not being at sea, I didn't have a clear horizon either. Nevertheless, I extended an arm and spread thumb and forefinger as far apart as they would go; for the average person, the span between thumb and forefinger will cover about 15 degrees of sky. I levelled my arm at the invisible horizon, and began to measure.

Waving my arm around the summer sky, I measured the altitude of a rising moon (itself covering only 0.5 degrees of sky, a useful checking-fact) at about 10 degrees and the bright orange of Arcturus (Alpha Bootes) at about 38 degrees. I sighted Dubhe (part of the Plough Constellation) too, although I forget the number; then, checked the observed altitudes against what Mobile StarChart app on my phone said they should be. I got the moon nearly dead-on - its real altitude being about 10.5 degrees - and Arcturus turned out to be just over 40 degrees above the horizon. The error for Dubhe was higher, about 3 degrees.

For a first attempt, with no clear horizon, these measurements strike me as acceptable; if I had been using them to find my latitude, I would've had a pretty fair chance of finding my home country. Marvin Creamer did a lot better, though, so the next time I've got a clear horizon after dark, I'll give the procedure another try. In the meantime, I'm now wondering what simple instrument I might be able to construct to get those errors a little smaller. Ideally, these will be simple instruments, such as a lost sailor on a small boat might plausibly fabricate. Also, wouldn't it be nice to measure latitude and longitude?

Coming soon...

Designing a new rudder, part 2

By measuring photographs and doing a little maths, courtesy of the NACA 0012 formula, I was able to discover what size and shape my new rudder (transom-hung, constant foil cross section) should be. Next problem: what materials?

My old rudder was a single piece, almost certainly a tropical hardwood, dense and strong. Marine plywood was an obvious option, except that no local suppliers had real marine-grade plywood (non-marine grade, which I have used for cabin furniture, can have voids and gaps within interior laminates, hidden weaknesses which could be fatal in a load-bearing structure). My old rudder was a single piece of tropical hardwood, which I also tried to source: a local supplier had lots of beautiful teak and iroko, but no planks wide enough to make a rudder in a single piece - they simply aren't there to be had (they've all been made into rudders already?). It might be just as well: a solid piece of timber can be sundered by a single stress-grown crack, not such a problem with laminates.

Abandoning nature, I talked to the very helpful Liam Phelan of mid.ie, and began to investigate the possibility of a synthetic foam core (Corecell) wrapped in fibreglass. To get an accurate spec on which foam and how much glass, Liam suggested I talk to Martin Armstrong, chief technologist at Gurit, a firm which supplies composite materials to pretty much everybody who builds composite structures - submarines, wind farms, huge racing yachts, aircraft, etc. Martin is a busy guy, but he spent half an hour talking an amateur sailor and novice builder through the materials and techniques necessary to fabricate a composite rudder.

First, the core: A550 foam (Corecell) for the rudder core; a single 8ft x 4ft x 25mm sheet would suffice. I wasn't sure how easy this would be to shape, but Martin reassured me that it is far less dense than wood, while also having no grain; normal wood working tools would suffice, it could even be sanded into shape; a surf form might be handy. Only one problem to watch: being an excellent insulator, it is really bad at dissipating heat, so power tools should have fresh, sharp blades to minimize friction.

Layers making up my composite rudder

Then, the exterior, from which will come much of the strength; Martin specified six layers of glass cloth:

Layer 1: 290g 4-harness satin, laid at a 45 degree angle, and with a 100mm overlap both sides at the leading edge, and a similar tail at the trailing edge
Layer 2,3,4: uni-directional 500g fabric running top to bottom (no overlap)
Layer 5: 290g 4-harness satin, 45 degrees again
Layer 6: 290g 4-harness satin, 0 degrees

This post is part of a series on making a fibreglass rudder with a foam core:
Designing a rudder, part 1
Designing a rudder, part 2
Making a rudder, part 1

Car cleaning tip

One of our cars has a light-grey plastic trim in the interior all around the ceiling. Looks nice and bright, but not so easy to keep clean. Today, we found the solution: all the black grubbiness around areas that hands touch a lot (e.g. the sunroof controls) was removed instantly by the simple application of a popular brand of baby wipe. Looks good as new now. Excellent stuff.

Astronavigation (Celestial Navigation) for Beginners

Knowing one's way around the night sky is a useful thing, if, like me, you have a telescope and want to know where to point it, or if, like me, you have ambitions to learn astronavigation. Until last week, sunset came early enough that I could get a few minutes of practice on every clear evening, standing in my garden and counting out the stars. This time of year, the orangey-red light Arcturus is usually the first that I see; the distinctive blue blaze of Vega is to the east, and when the sun's glow has faded a little more, Pollux, Castor, and Capella (actually 4 stars, an exotic double-binary) show up nicely.

The stars that I am really watching for, though, are Polaris (the north star) and Etamin; obviously, Polaris is very useful, in that it gives a navigator a course to steer anywhere in the northern hemisphere above maybe 10 degrees of latitude (ish) - but why my interest in Etamin (gamma Draconis)? Well, it so happens that my home port on the eastern seaboard of the north Atlantic is just a smidgen north of Etamin's declination (celestial latitude), which is 51 degrees, 29 minutes, 20 seconds. Now, Polaris has the useful feature of always (where always = "several hundred lifetimes") being 51 degrees and X minutes above my local horizon; Etamin, by contrast, whirls around the sky, never dipping below the horizon, but once per day passing through the zenith - what you might call "Etamin-noon".

In practical terms, this means that were I some day to be lost in the blue vastness of the North Atlantic, no GPS, compass, sextant or chrometer to guide me home, I could use Etamin to find the latitude of home, sailing north if Etamin passed north-of-zenith, and sailing south if it passed south-of-zenith. Once at the right latitude, I would need only to keep an easterly course, and a sharp look-out for pointy rocks. Of course, measuring the fixed, non-whirling altitude of Polaris is more convenient - it can be done whenever Polaris is visible - but that would require an instrument, ideally a sextant. Marvin Creamer, an American amateur sailor and retired professor of Geography, once sailed around the world on Globestar using techniques like this and no instruments whatsoever, making surprisingly accurate landfalls.

Unfortunately, during part of the year, Etamin-noon would fall during daylight hours - but even then, other bright stars at similar latitudes could give useful hints. Which bright stars pass directly over your home port / next port? Just follow the linked query at Wolfram Alpha to see a table listing the hundred brightest stars by declination, and you'll soon be on your way. A useful tool to help you practice is the (totally free) Mobile StarChart, a java applet you can install on your mobile phone - it only has about thirty star names, but is open source, so you could add more.

Living a long way from the sea? Astronavigation can also be pretty useful in the desert, and was much practiced by people like Popski. Must learn how to use a sun-compass one of these days.

Designing a new rudder

So, our beloved Briongloid, a 6.6M fin-keeled sailing yacht went adrift from her mooring, and spent an uncomfortable day bouncing on pointy rocks. The pounding reduced her wooden rudder to matchwood - so it's time to make a new one.

How big, and what shape? From a profile scale illustration of a Pandora International (our boat's model) I figured out the height and width - about 1.65 metres * 0.37 metres. Now, I just needed the cross-section's shape.

It turns out that the best shape for a rudder is a foil - like the shape of a bird or aircraft wing, the magic of the foil shape is that it generates lift (unlike, say, a flat surface, which only creates drag). Back in the 1930's, the boffins at NACA, the forerunner of NASA, investigated different foil types to find the best shapes for different aeronautical (and incidentally marine) applications.



For relatively slow-moving displacement craft like our yacht, their "NACA 0012" foil is the best fit; by creating a Google Calc document based on the NACA 0012 formula, I generated the cross-section above (y and x axes are not in proportion). Note the very round leading edge and thin trailing end.

Many fins and rudders taper from one end to the other, and give the leading edge a crescent profile; this tapering reduces drag by about 4% - for me, not worth the much-increased difficulty of shaping the foil.

This post is part of a series on making a fibreglass rudder with a foam core:
Designing a rudder, part 1
Designing a rudder, part 2
Making a rudder, part 1