Keel layoutThe centre lamination of the hull is 6 mm thick, and has a tapered fin-box to take a removable 6mm fin. I did not take any pictures of this part of the build on Presto – but here is one from the build of BlackWatch which illustrates the construction of the fin box from the keel and the two surrounding buttocks.
This is a picture of the keel box - very much the same as fitted into Presto. The hawk-eyed will have noticed that the keel box is open at the front - this is because BW will probably sport a MacRig and the mast position needs to be close to the front of the keel box - it got a closing slip of 1/32 (0.8mm) ply later in the build
Onwards and upwardsHull is fully glued up, and at least most of the inside surfaces are effectively in the final shape. The outside surface is “stepped” so the next task is to just remove the steps using a razor plane and sanding block, and in that order.
As I get older I get a little wiser – this time I did the plane work directly over a dustbin – this took about 15 to 20 minutes
Then it was the turn of the sanding block – I use an ancient foam block with removable Velcro pads – it gives a good combination of soft backing to deal with a rounded hull, and is flat enough to sand across the deck area (although I finish with a wooden sanding block to avoid rounding the edges)
I don’t care much about the coarseness/fineness of the sandpaper but I care a lot about the quality and sharpness. I deeply advise you to get suitable “sandpaper”. For wood it doesn’t matter if it is aluminium oxide or silicon carbide paper but it should be called something like “open coat” or “production paper” and it should feel open and sharp to the fingers. In modelling use we never wear it out – it is more likely to get clogged or torn or suffer accidents with smears of glue!
Plane from the hull centre to both ends – take small cuts, approach the final shape gradually, keep turning the hull over and viewing it from all angles. Using the coloured tracer means that you can compare the two sides directly.
You have reached the correct shape when:
• You have JUST removed all the steps so that only the coloured line is left
• The bow looks central (I run a line of permanent marker down the centreline of the keel to help keep things symmetrical)
• The stern/transom looks as you would like it to, and is the same on both sides of the hull
When planing you need the Sharpest blade – plane only with the grain, and from the middle only towards ends, plane with the blade at and angle to the grain (My plane has the blade set at an angle but feel free to hold you plane at 15 or 20 degrees as you plane)
Sanding
This will be in 2 stages – roughing and finishing, and the transom will need special treatment (because it will be all end-grain wood).
Rough sanding is to take out the plane marks and flats, and produce a completely level surface which is the hull shape you want – only not perfectly smooth for the next process
Finish sanding is to get the wood finish you want without changing the shape.
Remember that balsa is a series of cellulose “tubes” lying parallel to each other held together by a very small amount of glue (lignin). So wherever you have end grain you need to fill the tubes to prevent water getting in (Remember the Kon-tiki sank - as all balsa does if the end grain is not protected. Remember too that the word Balsa means raft!)
Never try planing end grain!
It can be done, but not on balsa, not with your razor plane and not by me or you.! I use a very sharp new snap-off blade to shape the transom – cutting always towards the centre of the transom and supporting the hull on a cutting mat.
I aim to get the transom smooth and finished mainly by sandpaper.
andrew