Day 37
Days Remaining 197

The Moth Factory

Polystyrene blanks for gantry and compression struts

ready for gluing to an MDF base for mould making.

It's official. My house has become a moth factory... It's not as clinical as the workshop, or as well equipped, but it is warm and dry and there is tea. Lots of tea.

As I type the kettle is boiling and the following items are glued up and curing off.

The Bow Fairing.  Location: Dining Room Table.
The bow on the Blackbird is very blunt and full - This looks odd compared to a water cleaving points that we are used to, but water cleaving pointyness stalls very quickly and causes a lot of drag in the air, and the most usual mode for the boat will be airborne, heeling to windward, with an apparent wind speed of 10 to 35mph at an angle of somewhere between 22 and 12 degrees, and - a sharp bow will be very stalled after about 5 or 6 degrees.  A rounded bow, will have a higher stall angle and should be a significant factor in keeping the platform drag numbers down.

How will it perform in water? - ie low riding or in transition? Depends how it is set up I guess. If we set up with a bow down trim it will push a fair bit of water out of the way. Bow up a smidge and it should be fine. That's what testing is for.

Oh there's something else about the bow you might like. It is asymmetric.  To allow the wand to be closer to the centreline, the bow is kind of rotated anticlockwise when looking from the bow to stern. It affects the flow over it not one jot, but the wand is in a better place and puts less strain on the wand mech to stay linear, which means it works better across the range.

The Wing Bar Sockets.  Location: Kitchen Floor.
Finally I got my head around how to do this - I want the wing bar sockets to be bonded together and laminated to form an elbow that is completely watertight before they get bonded into place as they pass right through the boat - if they leak, the boat will leak... So they need to be at the right angle when bonded together. I got the geometry of the racks from studying a lot of pictures, and figuring out rig loads and such. The front bars are at a compound angle (ie they rake backwards AND upwards) and the total included angle came out at 122 degrees.  The rear wing bars come outwards perpendicular to the centreline, so are only angled upwards (at a similar angle to the front bars) and that makes the rear bar elbow a shallower angle of 139 degrees.  Once these were known it was relatively easy to set up a bit of geometry on the kitchen floor (floor tiles can be surprisingly accurate to use as guidelines!).  They are curing off now.

The Gantry Moulding Blanks.  Location: Utility Room Work Top.
The Mk1 Gantry I want to try is pretty punchy. It is made up of aero section mouldings (and no tubes) to form a rigid structure in a sort of H section.  The individual aero sections are NACA 0008 and the longest is 580mm in chord. I'm currently preparing the CNC machined polystyrene surfaces to make moulds from them and the parts will come from those moulds. Fitting the gantry is a while away, but these things TAKE a while!

The Compression Strut Mould blanks.  Location: Utility Room Work Top.
The all important compression struts that link the chain plate to the mast post are highly loaded. The Mk1 rack arrangement for the boat uses round wing bars with fairings, but the comp struts need to be all carbon, so I am making a mould that will produce an ellipse of the right sort of proportion and size (about 22 x 50mm) An aero section would be better of course, but that would have a sharp and fragile trailing edge. I don't like compromises, but in this instance, an ellipse is ideal - and I can always make a stick on trailing edge fairing for it for race days......And you guessed it, the Mk 2 front wing bar assembly uses the same tube for the comp strut as it does for the wing bar, with the inboard end transitioning into a tube to slide into the same wing bar socket.

D