The first person who mentioned CNC Milling of keels to me was Charles. I had to go and look it up!! “Computer Numerical Control” is what it stands for – and CNC milling is I guess a vertical milling machine that can reproduce a 3D object from a digitised image….
(Some reader is bound to correct me – but here is the wikipedia entry )
We’ve all heard stories in the bar about the old mould for our class producing twisted and overweight keels – and how they had to be worked by the craftsman’s hand to make them true. Then we will have all heard something like the mould for Aussie keel was imported here so that we could cast ‘true’ keels and get them to minimum weight easily. In fact, I think I might have had the first of these on 3934 in the autumn of 2008. Then have you seen the beautiful sculpted shapes and winglets that have been appearing too?? All this is basically an art form by artisans – so when is science going to arrive, you may wonder?? Well, I think the answer maybe “any moment now….”
As you will all know, we can now simulate drag, windage and all that stuff on a computer now, way before having to build anything in real life. The keels are still cast in sand, like the Romans did, and there is a tremendous labour content in casting and finishing our keels – did you know a new one costs about £1,500 by the time you get it under a hull? Well, the Blog has heard that P&B have two experimental CNC keels under wraps. They have digitised the shapes and are starting to try and figure out more systematically what the optimum might be. And we might actually get cheaper keels maybe?! In this case I think the CNC milling machines are used to create an acrylic plug, from which the lead keels would then be cast.
What might we expect?? Well nobody has ever been definitive about the best tapering bulb shape – and there must surely be an answer that’s not “forty two”!! The thing I recall Charles pointing out is that the flow line of the keel must bear some kind of relationship to the waterline of whatever hull it sits beneath. You would think for example that the central fore and aft line through the keel needs to be parallel to the waterline for example – well, who have we ever heard being definitive about that? Especially when we went from Mk 9 to Mk 10 hulls which float a bit differently. Hmmmmm …. well, hopefully, we are about to find out. And I bet it’s not forty two…..
I would point out (as a measurer) there is only a certain amount you can do with the shape as the keel still needs to fit in the profile templates with a minimum clearance. I will take some pictures next time to show you how it’s done so no winglets!
If you want a proper CNC milled keel, you actually cast the keel oversize by typically 2-3mm, and machine the casting (SG iron typically machines really well). This eliminates changes in shape from the casting process and requires absolutely minimal finishing, but given the shape of a F15 keel would be very expensive to do. I suspect GBP1500 wouldn’t pay for the programming let alone the machining on what would most likely need to be a 5-6 axis machine costing upwards of at least GBP150 an hour and requiring the best part of a day to complete. Typically on what have been IOR, IMS, CHS, IRC and ORC boats, milled keels are typically 2-4 times that of conventionally produced items. The Flying15 keel will quite likely be no different from a cost perspective.