Via email to Mayhem...This is an article about the design and build of a model of Donald Campbell's Bluebird K7. The project is still very much a work in progress, but if I don't submit the 'story so far', it will never happen!
Let me begin by saying that what I know about model boats could be written on the back of a postage stamp, in block capitals, and still leave room for your shopping list so I'm receptive to comments, suggestions and criticism where I deserve it.
As a child of the fifties, I had a fascination with Campbell's Bluebird boat, K7. It never really left me and I eventually set about building a semi-scale model in 2001. I acquired the Nexus 'plan' (notoriously inaccurate I'm told, but the general shape is reasonably right) and converted that into a 3D CAD model. The model was designed as a solid lump and then sliced up to create the lateral frames. A bit more modelling and jiggery pokery and I came up with a reasonably efficient structure; two box sections running the length of the boat with structural links between the three planing surfaces.
As a solid lump
Sliced up and turned into a structure
I got the parts printed out full size and traced them onto lite ply. The main structure turned out quite light and very stiff. Topsides was planked in balsa and the access hatch cut afterwards to keep the curves smooth. I dread to think how many hours I spent sandpapering!
Converted from CAD to Wood.
About half sanded
If only the top coat had gone as well.........
To make the hull waterproof and stronger I applied Ronseal wood hardener inside and out before final sanding and then applied a two-part epoxy finish to the outside before painting. I learned quickly that the two finishes interact and it made life a bit tricky - the epoxy sets too fast.
I had already bought a motor, speed controller, driveshaft and propeller. What I hadn't bought was a battery. I kept looking at the boat, looking at the prop and having serious doubts as to whether the thing would actually move, let alone 'fly'. And so there it sat, on its own shelf, gathering dust and dings for more than ten years while other things took priority over batteries in the wallet department.
Finally, the price of LiPo batteries and chargers dropped to a point where I was prepared to take the plunge, just to satisfy my curiosity. 12th August 2014, the glorious 12th, I set off to the local boating lake at 8 am, dodging the showers (and the parking fees). I had already mentally written my 'hull for sale' advert as I dropped her into the water. One click on the throttle and she's away in a remarkably convincing taxi. Another couple of clicks and to my amazement, she's climbing up onto the sponsons. This has potential. With a big grin I opened the throttle a little more and it was clear that the mighty vessel is capable of planing. Ah, but all is not well. The steering, sluggish in taxi, is non-existent as she starts to climb. By this time, the boat is on the other side of the lake. For some reason that I really don't understand, I could just about manage a left turn (only) and brought the boat back.
Ready for the first trial
Original rudder arrangement
So, the first modification was to put the rudder in line with the prop. Didn't take long as I cannibalised a model car that someone had given me. One of the swing arms was perfect for the job! (I had been given me the car with an assurance that it was a petrol engine. I intended to fit the engine to Bluebird but it turned out to be a glow engine. Glow fuel is unobtainable on the Isle of Wight. You can't get it by post and, strictly, you can't transport it on the ferries. Back to the drawing board on that one. I didn't worry too much about stealing the suspension. One thing I will never do is run model cars!)
Second rudder arrangement
Second trial. Now that's more like it. The steering has plenty of authority at low speed. As she climbs, the steering starts to lose its grip a bit. OK, head to open water and see what happens. A touch more power, lifting nicely. Crikey! The back end visibly lifts and she's away - in a right hand circle and there's nothing I can do about it. OK, that one I do understand. Surface piercing prop, no fixed fin yet and a rudder that is probably completely out of the water by now. I have to say, I never planned for the boat to be that fast and I am somewhat taken aback at how quick it is once it gets up on its toes. It also sounds awesome!
Looking cool in taxi
Far from full speed but planing (the photographer couldn't keep up when she took off!)
So, we really need to do something about the steering. As I write this, Bluebird is on the operating table waiting for glue to dry. I have repositioned the prop shaft about 15mm to the right and slightly lower. I will be lowering the rudder and fitting fixed fins to the sponsons and the rear planing wedge. With anything like luck she will be back on the water for another trial at the weekend.
Vital statistics
The model is just over 1m long. I wanted a big model; if there is one thing I do know about model boats, it's that they shrink remarkably on contact with water. I scaled the design so that I could cut the main frames from a 1m sheet of ply.
The motor is a
Feigao 5408407XL (Hacker clone) with a Barracuda 80 amp speed controller. The battery is a 3S 5000mAh LiPo and that set up should be spinning the Octura 442 at around 25 000 rpm.
Construction is 3mm lite ply and balsa, reinforced with polyester resin ('wood hardener') and the empty spaces are filled with polystyrene blocks. Somewhere along the line I got careless and although the bare main frame structure weighed in at around 400 grams initially, the final running weight is just over 3 kg. Far too heavy, really, but it does make the model look very realistic in the water. The canopy, by the way, was vacuum formed on the wooden plug you can see in the photos.
Design philosophy
I wanted a large, imposing that would look convincing on the water, not hop about all over the place. I wasn't too concerned about speed just as long as it got up on the plane. I went pretty much as big as I dared. I gave some thought to the scaling process. Hydrodynamics is a complex science and I'm not inclined to do maths unless I'm getting paid for it. Feel free to correct me if I have got this all wrong! In (very) rough terms, the speed should scale linearly (that is to say 300 mph = 30 mph for a 1/10th scale model) but the weight is a whole new ball game. For a displacement model, the weight needs to be reduced by the cube of the scale. For a planing model, things are a bit different. The lift forces generated by the water depend (roughly) on the square of the speed. And there's a sting in the tail too. The area of the surfaces that the lift force acts on decreases as the square of the scale. So, for a 1/10th scale model, we have 100th of the lift force and 100th of the lift area. The weight should be 1/10 000th of the real thing!
Bluebird was 8m long and weighed 2 1/2 tons. My model is 1m long so that's 1/8 scale. It should run at 37.5 mph and weigh - oh - 3kg! So I'm not a million miles out of that. Thinking about it, the model does sit well when static, albeit a bit on the shallow side, but I still think it could be lighter (less wear and tear on the power train). My feeling is that I got carried away with the waterproofing, filler and paint. It all adds up!
I cheated a bit by setting the sponson planing surfaces slightly deeper than scale on the basis that ripples on the lake would be like huge waves on the real thing and the flat bottomed hull would never run clear. I also made all the planing surfaces a little wider to help with the scale lift forces.
The motor mount is perhaps unusual. I wanted the thrust to be applied at the prop, not the motor, for two reasons. First, I didn't want to load the motor bearings and, second, I wanted to keep the thrust on the waterline to prevent forcing the bow down. I had read somewhere that flexible prop shafts wind up and get shorter under load (makes sense to me) so I came up with a sliding motor mount. It's constructed from brass tubes and un-etched printed circuit board material, which made it possible to solder the whole thing together before reinforcing it with fibreglass. It works jolly well! There is no lateral play but the motor is free to slide forwards and backwards to find its own position with no end thrust. I would be interested to hear how others tackle this aspect of design.
And for those who would like to complain about the colour, yes, I know it's too dark. However, I remember the real Bluebird from the days of black and white TV and photographs. Blue always came out darker in black and white. I'll put it right in the end!
Where next?
The current model lacks a lot when it comes to finish. I was fairly confident that it would never get up to speed and I lost heart a little in the final stages. Now that I've seen that the power train is more than capable of driving a model this size I have got the bug again. I shall carry on developing the model until it runs nicely. At that point, I will either re-finish it and add some detail or build a new hull from scratch. I'm learning a lot as I go and I reckon I could design a lighter structure, improve the performance and get a better appearance easier that way. After all, I have all the expensive bits to hand now!
Roll on the weekend!
I'll update this article as things progress.