Making it round (or at least oval)

I fibbed a little in the last post. I wrapped the very last stave in plastic when I glued everything else up, so I still have access to install halyards etc in the mast once it's completely shaped. Otherwise the bits around where the halyards and antenna exit would be very difficult to work on.

With the mast in a state where it holds together reasonably well, I started shaping. First step was to mow down the rear either side of the sail track to make it an elongated octagon:

I went straight to the motor mower power planer for this. Otherwise the effort would kill me. I also did the work on the patio, where all the mess can simply be swept into the compost heap.

At this point I knocked a couple of cradles up to hold it, as once it gets more round it won't sit on a flat surface nicely:

I next went for a hexadecagon (being an engineer I didn't even need to look that word up).

It's getting reasonably light at this point. I reckon 12.9kg for a 6.1m mast isn't too shabby.

I also flattened the sides for the bottom 28cm, to make it work nicely in the tabernacle:

Which necessitates a quick test fit in the tabernacle. It's a little on the tight side, but once I'm done with sanding I reckon it'll be just about right:

My god it's long:

Back to the planer to make it a triacontadigon (yes, I had to look that word up), plus a lot of fussing and finessing with 0.3mm deep cuts to get it as straight and even as possible:

And now the really hard work begins, using a 40 grit belt sander belt to make it round:

Closing up the mast

With the successful work on shrouds etc, I have the details that I need to now confidently close the mast up.

First I made a plug to reinforce the shroud and jib halyard exit box area.

This has a diagonal channel inside for halyards.

I knocked up a little piece of Jarrah to close up the top.

And finally I tapered the last three staves, and cut fishmouths into two of them, so now it all sits neatly.

The good news is that I can lift this by myself even now. From here it's only going to get lighter as I plane and sand the mast into an oval.

Now to mix up a heroic batch of epoxy.

Buoyancy chamber inspection.

I find smol cats are particularly well suited to inspection of the insides of buoyancy chambers. Here’s Sam, our smol cat, ready to go to work:

The inspection procedure commences by entering the chamber at the aft end.

Some time spent inside checking for leaks, and criticising the fillets:

And finally, job complete, we exit via the forward hatch:

Back to the mast

My mast components have been languishing in the back yard for a year, covered in a length of heavy plastic to protect them from the elements. This has been mostly successful, but on pulling them out there's some evidence of water. I presume being in the bag in the sun there's been some condensation.

In any case, now I'm back to the mast proper. For those who don't remember (I barely do myself) I'm doing a modified birdsmouth construction, with a sail track, in Jarrah and Tasmanian Oak. Neither of these are traditional spar making timbers, on account of their density, but I figure I can get good performance by pushing the boundaries of the traditional birdsmouth technique, and thinning out the walls some. Plus, Jarrah and Tasmanian Oak are pretty-much indestructible.

Here's a photo of my prototype piece, that I did a while ago. It's mainly condtructed from 30mm x 12mm staves.

Luckily, the 6.1m long mast fits neatly across the back of the garage, on the bench I made specifically for this work. I confess this was something that I measured when we were looking at places in Perth, and I rejected a few on the basis that I couldn't do this work in the garage. The garage in the place we're in could be bigger (it'd be nice to fit Elena in lengthways with her bowsprit, but it'll do.

I've opted to fill in the base of the mast with Jarrah up to the boom fitting. If the mast is going to break anywhere, I figure it'll go at the boom, as that's where the main unsupported sideways forces are. There's a slot down the leading edge of the mast for drainage, and I've tapered the staves just above the boom, to minimise stress risers. You can also see here a length of coax that I've put in for a mast-mounted VHF antenna. Also two of the three staves that will fill in the front of the mast are mostly prepared - I just need to taper the tops and route the birdsmouth into the front. However I want to get all the inner details right before sealing up the mast, as I'll kick myself if I forget something.

Moving up the mast to the halyard exits. I'm using 6mm spunport line, which is a modern polyseter double braid, but in traditional colour and finish. I've elected not to use double-sheave exits, and have routed in a long slot for the halyards to come out at a shallow angle. If this proves troublesome, in use, I can always add exit blocks. Note also here that I've started finishing out the interior of the mast with filled epoxy. This is simply to ensure the halyards don't snag on anything.

So now I'm on to some design work. One of the reasons I put this aside previously was that I was unsure what to do with shrouds. The hounds really should be strong. Traditional masts tend to use a band encircling the mast, but that's not going to work well on this mast due to the sail track. I could split a mast band at the sail track, but then I'd be paranoid about the shrouds bending the band apart at the front and splitting the mast.

I think I've come up with a workable solution. Davey do chainplates with the eye at 90 degrees, intended for mounting on the stem at the bottom of the bobstay, or else on the gunnel for bobsprit shrouds, shown below:

Initially I was thinking of simply bolting these to the sides of the mast, with backing plates inside, but there's still the splitting problem as with the cut mast band. So then I thought I could join the chainplates to one another, clamping the mast. I have a pile of bronze rod that I've been turning all sorts of things from. All I need is a couple of lengths of rod with threads tapped in the ends, then stick these in holes in the mast, and flogg the chainplates up to either side. Something like:

The only danger is clogging up the space that's meant for running my halyards, but if I keep the diameter down to 12mm (oodles strong enough) there's heaps of space to run the halyards past.

Editing after some good advice from the woodenboat forum not to put the holes in line on the mast. I've tipped the fitting over a few degrees, and shrunk the holes in the timber to 4.8mm from 12.7mm. The bronze plugs now don't go through the timber section. To ensure that when the bolts (now 10g) are flogged up the mast walls don't get cut into by the plugs, I've added backing plates as well, which essentially act as large washers.

And another edit after a complete rethink. I've decided to fill in the mast in this section, bore a hole for the main halyard through my plug, and go with a more traditional hound. Here is is bolted to my prototype mast section. I used 2.4mm thick x 25mm wide bronze for the jib stay straps, and 3.2,, thick x 25mm bronze for the shroud straps. Bending the jib straps around the mast was a real pain.

The whole assembly bolts through the mast with a single 5/16 bolt (actually I'll use 5/16 bronze threaded rod with acorn nuts on either end.

I'll do a separate cutout for the jib halyard exit box immediately below the stay attachment. I can use the stay attachment for a roller furler, or use the halyard exit box for a normal jib.

My fingers are a mess after beating on those straps.

Varnishing the transom

One of the problems with stopping building the boat for a few years is that some bits haven’t aged as well as I’d like. Principally the transom.

Essentially I built this by laminating thick Jarrah planks on to 6mm ply, then hacking at the Jarrah to thin it out to ~6mm. When working on the boat originally I threw a quick coat of unthickened epoxy on to protect it, and went on to other bits.

So that’s how it’s sat for rather a while. Now that the decks are painted out it’s time to varnish it. Unfortunately it’s gone and cracked while sitting. So before varnishing I fed some unthickened epoxy into the cracks to fill them, then sanded the whole lot back to timber before applying another couple of coats of epoxy with the roller.

This is what it looks like after a couple of coats of epoxy. I’m confident the cracks won’t be a problem (after all, they’re only a veneer, so it’s still completely watertight), and it looks okay, cracks and all.

I also bought a gudgeon and pintle for mounting the rudder, but alas the wrong one arrived, so it’ll be a little while before I go drilling holes into the transom to stick more bits on.

And here's what it looks like after sanding back the epoxy and adding two coats of lacquer, thinned out with Penetrol. I'll sand out the gloss before adding another couple of coats.

Bowsprit

I'm trying to figure out how to hold my bowsprit on. The standard bowsprit for navigator is a rectangular block of wood, which is bolted to the kingplank with 9mm (3/8") bolts.

I've made a 60mm dia round bowsprit (more pirate-like), which doesn't really lend itself to just bolting down. Here it is being held on Elena in the approximately correct position by my extremely patient husband:

The observant will also notice I've modified my winch mount a little to get it to work better. I flipped it over and cut down the pieces for the bow roller, making it a little more compact. Then I mounted it a little further up the post. Now I don't have to remove the winch and haul Elena forward on the trailer to fit her in the garage. The last little bit is to undo the front bolt of the hitch and rotate that out of the way, giving me an extra few cm. I've been looking online and have found drawbar pivots, so I could even conceivably cut the drawbar just forward of the winch post and extend it half a meter or more, then simply spin the front section of the drawbar out of the way when needed. Of course then the jockey wheel would need to go behind the winch post, but that's doable.

Back to the topic at hand though, I thought a heel socket and gammon iron were the way to go, something like:

However getting these in just the right size is a bit of an ask. These sort of things are really made for much larger boats. I could fab something from lots of bent and welded plate, but it just wouldn't look that pretty. Truth be told I wasn't entirely hapy with the way my fabricated mainsheet eye turned out.

So I thought perhaps combining some bronze straps over Jarrah bases was perhaps the right go. I could even angle things so the screws through the deck and into the king plank are angled, making them really hard to pull out...

Building that starts with cutting out the strap. I bent it in the vice by simply ruling nine lines across the piece, and bending each ten degrees.

I cut out the heel iron base from jarrah, and cut a flat spot into the deck to mount it. The deck looks reasonably flat, but there's actually quite a pronounced curve:

When mounting, I added a pair of 10g 1.5" screws under the bowsprit. This means the iron is held down wy a total of eight 10g screws into the king plank.

I further reduced the diameter of the heel end of the bowsprit to provide a more positive engagement to the heel iron. In use I expect the forces to be back and down on the bowsprit, due to the tension in the bobstay. Wind loading on the jib will be back and up. So there will be significant backward forces at the heel, meaning the bowsprit could slip back through the heel iron. The step in the sprit ensures this won't happen. The photo shows the sprit partially inserted, highlighting the step.

Finally a picture to show the whole thing.

I had entertained notions of fitting the bowsprit in the parking lot prior to launch, but really it's just not going to work like that. The front of the sprit comes well forward of the trailer, plus the winch gets in the way of the bobstay. So I think really the only way to go will be to insert the bowsprit and the bobstay once Elena is in the water.

Now onto the gammon iron, which can be a little simpler as it really only needs to ensure the sprit can't move from side to side, and doesn't have any upwards forces to contend with.

By leaving the top of the gammon iron open, installing the bowsprit should be fairly easy. Just drop it in the gammon iron and seat the heel into the heel iron, then tension the bobstay.

Paint for the decks

Over the weekend I gave Elena a thorough clean and painted the decks. After much agonising I chose Toplac in Cream colour. I didn’t want anything too bold, but wanted something a little darker than the white interior.

I think it looks rather nice now. I can bed the fittings now as well.

You can also see a little of what I had to do to get Elena to fit in the garage of the new place. She’s too long on her trailer by about 20cm. I removed the winch and pulled her a little further forward using some rope, then unbolted the tow-hitch and spun that around 90 degrees. Now she fits nicely.

Boat progress!

Rather a lot of progress, accomplished through the simple expediency of spending money. This was necessary because we're moving to Perth in the next couple of months, so the boat must now be mobile.

Looks rather nice on a trailer, I reckon.

Top on the priority list for the new place is somewhere I can store Elena, on her trailer, and finish her off. :)

Modifying the trailer a little to give me a means of supporting my mast:

And here she is on moving day, with her mast (or at least the components for her mast) supported nicely.

Achieving low THD at modest power level with jellybean parts

My power amps to date follow a reasonably simple recipe: Differential JFET front end, a couple more voltage gain stages, then power MOSFET followers, based on the Tilbrook AEM6000, with updates to more modern parts and some minor tweaking. They work pretty well. I'm able to achieve reasonably good distortion performance, hitting 1ppm THD at 1KHz, low input referred noise level, of 5 ish nV/√Hz, and plenty of output power, easily putting 100W into an 8Ω load.

But most of my speakers don't need 100W. Most of them don't last terribly long at such power levels, and I get a lot of complaints from Perry when I wind the wick up too much anyway.

Also, they're expensive. Each of those output transistors are >$10. The differential JFET on the front end is another $5. I'm probably up for $100 or so in parts just to build one amp, which is something of a waste if I use it with a little 4" bass driver or tweeter.

I'd like to play with line arrays - it would be cool if I could drive each speaker independently, as then I could play with multi-channel DSP. That's not going to happen when the amp for each driver is $100. I have a workmate who's doing similar things - his question each time I suggest he use a linear amp rather than a class-D eBay one is "how much?" This is perfectly valid.

So I got to wondering what was possible using really cheap transistors for driving a single 3.5" speaker. I set the following as my design goals:

• Absolutely no compromise in THD. Preferably 1ppm at 10KHz.
• 5-10W should be adequate.
• Better noise performance than my AEM6000 derived amps, as I'll be running them at lower levels where noise will dominate.
• For exactly the same reason, better PSRR. I really don't like listening to hum.
• Really, really cheap. Like $10 per amp in parts.

Given that each of my nice Lateral MOSFETs costs more than my budget, that totally rules them out. This means I'm stuck with jellybean bipolars.

My basic three stage amp design should work well with a bipolar input stage. Similarly I can whack a bipolar follower stage on the output. How about if I use a diamond configuration for the current buffer. Even better, how about if I cascade a pair of diamond stages, to minimise loading of the final voltage amp transistors. Rumour has it that this is sure to oscillate, but I'm game.

I was able to get this topology to work well in simulation, with BD139 & BD140 transistors as my final drivers. I could hit 120dB open loop DC gain, with better than 50dB available to correct output stage distortion at 10KHz. This led straight away to sub ppm THD at 10KHz with 20dB gain under simulation. However I wanted a bit more output current than the 1.5A max the BD139 was happy giving me, so I tried TIP41 & TIP42, which are good for 6A. They proved a little slow with stability issues, so I stepped back to a BD179 & BD180, which provides up to 3A in a TO-126 package and is reasonably fast, plus really cheap, at $0.40 each in quantity.

A few simulations later I had the following: A simple input differential pair using BC547's for reasonably low noise, followed by a folded cascode with differential current sink loads to a pair of differential amps, then finally a very conventional symmetrical voltage amp stage using 2N3904 & 2N3906 transistors, then the first current gain stage; a diamond composed of 2N3904 and 2N3906 transistors, and finally our output stages, a bunch of BD179 & BD180's in a diamond configuration. One nice thing about the diamond is that the quiescent current is set by a simple current source/sink pair, making them immune to thermal runaway. A bad thing is that I'm effectively doubling the number of output drivers. Not a big problem when they're $0.40 each.

I published the simulated design on DIYAudio, and received some really useful feedback, adding a cool "clipping control" stage, which sucks charge out of the compensation caps in the event of clipping, ensuring there's no sticking to the supplies.

I then went ahead and knocked out a PCB for the "5W" version, using two sets of output drivers. I used KiCAD, as it's completely open source and it was about time I broke away from closed PCB software (Protel) and used something where I could share the design work with others.

This worked reasonably well, though I initially missed my THD target by rather a lot - measuring 15 ppm at 1 KHz into 8Ω when I was expecting closer to 1 ppm.

After some measurements I found the source of the problem was simply due to where I was sampling the output voltage for the feedback network. By tapping the net away from the output connection, I wasn't getting an honest representation of the output, being influenced by currents between output drivers. Cutting a trace and adding a link from feedback resistor to the actual output connection of the amp fixed this, and I was able to achieve around 2.4 ppm at 1 KHz, up to 6W into 8Ω with gain of 10 (20 dB). This mod has been folded into the rev 2 PCB.

Noise performance is also pretty good - with 4.2nV/√Hz (input referred - the plot shows the output with a gain of 10) at 1 KHz with the input shorted.

The folded cascode gives me a pretty wide common mode range of ~10V with +/-18V supplies, and the current sink loads for the first stage mean the whole lot will keep on trucking with a supply as low as +/-5V or so. When it clips, it does so cleanly, with no rail sticking or oscillation.

It plays square waves neatly too - here's a 10KHz square wave. Slew rate is ~12V/µs.

There's also a 10W variant with twice as many output transistors, a dual 5W/15W version with active crossover for driving my two way transmission line "noiseUnit" speakers, and a preamp implementation with relay-based volume control.

So if you're keen on a no compromises driver for headphones or small speakers, give this one a shot - the full design details, including all my KiCAD design files and Gerbers are at diyAudio. It's cheap but good :)

A new Microbee disk controller, from original design films.

One of the guys at the Microbee Software Preservation Project got his hands on design films for a Bee disk controller, so he imported them into KiCAD as image files, cleaned them up, and generated beautiful Ger bets that look exactly like they were made with Bishop graphics.

He sent me one of the resulting PCBs, and I spent some time last night throwing it together, using bits from my junk box. Note the cool labelling on my TI TMS2793 floppy disk controller IC. The bit on the right comes off when scrubbed with isopropyl, revealing the writing on the left. It’s still a TMS2793, but with a HP part number. I wonder if TI was responsible for the relabelling way back when?