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Author Topic: Donald Campbell's Bluebird K7  (Read 53515 times)

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Donald Campbell's Bluebird K7
« on: August 19, 2014, 07:13:23 PM »

 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.
 
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Footski

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Re: Donald Campbell's Bluebird K7
« Reply #1 on: August 20, 2014, 08:03:06 AM »

Not one for fast electrics, but that I like. Beautifully built. :-))
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Re: Donald Campbell's Bluebird K7
« Reply #2 on: August 20, 2014, 10:18:56 AM »

An old friend of mine, Alf Blake, has built a superb model of K7, complete with a scratch built, fully operational turbine. The paint is exactly the same shade as the original, everything is perfectly to scale...only thing missing is Mr Woppit, although by now I guess Alf has found one.


The thing is absolutely frightening when he fires it up, he bolted it to his bench for the initial test, and as the revs increased, the bench moved across the workshop....gawd knows what the rev ceiling is.


I've got some photos of it somewhere, I'll get them posted up here.


Rich
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Re: Donald Campbell's Bluebird K7
« Reply #3 on: August 20, 2014, 11:08:36 AM »

I believe they idle at about 80,000 rpm and are governed to 120,000 :o .  The most frightening K7  I`ve read  about was pulse jet powered, South African I think.   The more elegant K4 was my favourite Bluebird.
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Perkasaman2

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Re: Donald Campbell's Bluebird K7
« Reply #4 on: August 20, 2014, 03:08:49 PM »

Well done! Your model is superb.  :-))
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Vintage

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Re: Donald Campbell's Bluebird K7
« Reply #5 on: August 20, 2014, 03:29:14 PM »

Can only echo the above, your model looks superb and it's fantastic to see someone who's so new to model boats taking on such a challenging project.

I imagine you've read "The Bluebird Years" - if you haven't then it's a fascinating insight into the Campbell story...

Keep us updated  :-))

Mark

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Re: Donald Campbell's Bluebird K7
« Reply #6 on: August 20, 2014, 04:14:00 PM »

When I was a kid I made up a kit of bluebird all balsa and powered by a jetex motor, went well but only lasted for about  30 seconds, love to see you model on the water as I have seen another one for years,    Ray.
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Re: Donald Campbell's Bluebird K7
« Reply #7 on: August 22, 2014, 02:31:24 PM »

Can only echo the above, your model looks superb and it's fantastic to see someone who's so new to model boats taking on such a challenging project.


I can only echo the above sentiment too. Excellent approach and a very well written informative post to boot.


Good luck with the next phase(s)  :-))


Ramon
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ooyah/2

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Re: Donald Campbell's Bluebird K7
« Reply #8 on: August 22, 2014, 09:07:32 PM »


  Via email to Mayhem...

Hi
Some time ago I posted a thread on the build and running of a ( TOUCHWOOD   K7 )  Bluebird,  http://www.modelboatmayhem.co.uk/forum/index.php/topic,26255.0.html at 1/12th scale

Originally powered with 2 xNimh batteries in parallel with a brushless motor and eventually changed to 2 x 3S Lipo's in parallel and it is a flyer  40 mph +.
A short video on you-tube    https://www.youtube.com/watch?v=fGPMBKTA1Wo shows it at top speed.

If I can suggest that you do not try to turn K7 at speed as it will flip over and if you could remove the rear planing wedge it will allow a greater flow of water to the prop.
As you have discovered you need a lot of water to get the full benefit of it's speed.
I originally wanted to build at 1/6 scale and install a KJ66 jet engine that I had built but gave up the idea as my club lake was too small.

Good to see a scratch built hull, well done, nice job.

George.
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tugs62

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Re: Donald Campbell's Bluebird K7
« Reply #9 on: August 22, 2014, 09:22:59 PM »

I want one now just have to find some plans :-)) :-))
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Neil

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Re: Donald Campbell's Bluebird K7
« Reply #10 on: August 22, 2014, 10:10:52 PM »

well...for someone proclaiming to know zilch about model boat building..................you coyuld certainly teach me a thing ot two............it sure is beautiful.
superb and well done. neil.
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Re: Donald Campbell's Bluebird K7
« Reply #11 on: August 22, 2014, 10:12:15 PM »

I want one now just have to find some plans :-)) :-))

no good you building one Tugs.........you'd only be able to sail it on Fleetwood lake once every five years or so...............it's never calm enough %% %% %% %% %%
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Tim_M

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Re: Donald Campbell's Bluebird K7
« Reply #12 on: August 26, 2014, 08:32:01 PM »

First and foremost, thank you gents for your encouraging compliments. My model is not quite as beautiful as the photos suggest, but I'll keep practicing.

There's more to this model boat malarkey than meets the eye!
Run 3 was a bit of a damp squib after run 2 but a great learning experience. Please indulge me while I share what I saw and what I concluded. Where I'm wrong, put me right! Just don't laugh too loud. {-)

I had moved the prop about 10 mm starboard and lowered it about 5mm, thinking that it was too far clear of the water and causing excessive 'prop walk'. I had also fitted fins to the sponsons and one on the port side of the rear planing wedge. I lowered the rudder about 5mm.

One click of throttle and taxiing is fine. Good steering. A bit more power and the nose comes up. Not planing yet, just forcing her way through the water (displacement mode); the water is 'back filling' after the steps on the sponsons. From the off, she looks stern heavy, sluggish, though I haven't moved the CG. More power and the sponsons just about start to plane - the water is coming off the back of the steps cleanly. She's struggling and still pulling hard to starboard. Now, that has me completely confused. I decided that the rear fin might be affecting the flow into the prop and removed it. Sure enough, the performance improves, but not much. I head for open water and pile on the power. Full port rudder and she is still turning to starboard. Then suddenly the stern lifts and she starts jumping around like crazy, up and down and side to side and throwing up a huge 'rooster tail'. I did this a couple of times but I really couldn't see much through the spray. Time to head home and have a think before I do some serious damage. The sponson fins had fallen off, by the way. Wrong glue!

What I think I learned.
1) The difference in performance when a surface piercing is running right and when it's fully submerged is not subtle. It's HUGE!
2) I realised that lowering the prop was about the worst thing I could have done. I was never going to get the prop up the speed and was probably about to inflict severe pain on the motor and ESC.
3) Having realised the prop was fully submerged, I was totally at a loss to explain the starboard turn. After all, the sideways forces on the top and bottom of the prop should cancel out. I was pondering (like you do...) and thinking I should check the alignment; maybe I had built in a twist. Then it came to me. Torque roll. The torque of the prop (considerable when submerged!) was forcing the starboard sponson down. A quick trawl round the net and I think I've got that right.

So, where next? I have moved the prop up and further starboard, about 20mm off centre now. I will need to fit some fins but this time I shall keep the rear one well away from the prop.

Here are the problems. What I'm aiming for is realism:
Speed about 35 mph - I think I have ample power available for this.
Controlled acceleration
Good straight line (only) stability. The lake has a quite narrow corridor. Long ago it was great but some bright spark put a ruddy great island in the middle as a bird sanctuary - and a fountain in the middle of one of the open areas to boot. Now the water is full of feathers and the whole area is encrusted with bird droppings and infested with rats. Don't get me wrong. I like ducks. Paxo, anybody?  Anyway, I can't afford to have the model go off in a great curve before I gain control.

These requirements are not the ones normally applied to hydros (massive acceleration off the line, maximum speed and turning on a sixpence) as far as I can gather.

Somehow I need to tame that torque roll and get the prop up out of the water quicker. That's got me stumped. The prop centre line has to be about 30 - 40 mm below the static waterline to get the stern clear of the surface when she's planing. The design is  a bit reluctant to get up onto the rear wedge. The real boat was too. Campbell's method was to momentarily apply full left rudder and the water brake, slamming on the brakes as it were, causing the boat to pitch and lift the stern. But that was a completely different beast.

At the moment I am running with a planing wedge at the stern. Of course, the real thing had to have that because there was no prop but I'm wondering if the wedge and the prop are getting into an argument as to who's going to take control. Maybe I would be better setting her up as a prop rider? (I plan to try that at some stage anyway).

More than enough from me. I told you I knew nothing about model boats and now I've proved it! Any and all help gratefully received!!
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Tim_M

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Calling all 3 point hydro experts - help needed!
« Reply #13 on: August 26, 2014, 08:40:30 PM »

Gents,
Could you spare the time to run through my post about Bluebird K7 (stared by Martin last week)?
It's a long thread but I could use all the advice I can get on setting up my model!
Thanks!
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Re: Calling all 3 point hydro experts - help needed!
« Reply #14 on: August 27, 2014, 08:50:11 PM »

Gents,
Could you spare the time to run through my post about Bluebird K7 (stared by Martin last week)?
It's a long thread but I could use all the advice I can get on setting up my model!
Thanks!
Tim,
If I can try and explain how I built my 1/12 scale K7 and had it running with no problems, mine is a fibre glass hull and sponsons which I bought the parts only from Touchwood models and after reading Ernie Lazenby's article in Astec models where( https://astecmodels.co.uk/ernies-set-ups-3.php) he describes his build of the 1/24 th scale K7. I scaled up the main dimensions in mine.
K7 is powered with a 2850 kv brushless motor with 2 x 3S lipo's in parallel and the all up weight is 1.80 kg.

PicNo 1
The sponsons are 8" apart inside and each are 1" wide getting larger to 1.25" at the rear, the tail of the sponsons are 3/4" from the underside of the hull and the prop hub is at 3/4" inline with the sponson with no rear wedge to effect the flow to the prop.so these 3 points make the contact points of the hydro, C.G. is approx 2" from the rear heal of the sponson towards the stern.O/A length of K7 is 26"

Pic No2
The motor is offset 6 mm from the center line towards starboard and the shaft is 3/16" dia stainless in a brass tube and set at a slight downward angle and the rudder is on the port side of the prop and the blade in line with the hub of the prop, prop hub is 2" from stern as is the center line of the rudder.

Pic No 3
You can see the 1.25" width at the sponson tail

Pic No4
The 3/16" dia Stainless steel shaft with a 26mm carbon fibre surface prop from Cornwall models.
 
I think that your turning to starboard is caused  by your sponsons not being wide enough and the prop is too far out from the rudder which doesn't give the prop wash anything to bite on, and the center lines must all be as my first pic.
I am no expert on fast electric models but after following Ernies build mine will run straight at 40 mph with hands off the transmitter but I dare not try to turn it at speed as it just flips over.
 
I do hope that this is of help, I know of at least another2 K7's at 1/12  scale that run just as well as mine after the builders followed my guide.

Also the you-tube link to my K 7 at speed.  (https://www.youtube.com/watch?v=fGPMBKTA1Wo )

George.
 
 
 
 
 
 
 
 
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Re: Donald Campbell's Bluebird K7
« Reply #15 on: August 28, 2014, 12:19:45 PM »

Thanks for the advice George! That's a lovely looking model you have there. I'm starting to realise that a prop-driven model is quite different to a jet driven prototype.

I'm pretty sure that my CG is way too far back. I also see the planing shoes on the sponsons that are not there in the Nexus plan I used. I was wondering about those as they do seem to show up in photos. I shall add your mods to the list of experiments. I know I'll get the boat up and running soon. The main thing is I'm enjoying discovering how!

Rgds,
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Re: Donald Campbell's Bluebird K7
« Reply #16 on: August 28, 2014, 07:06:37 PM »

That is brilliant acceleration and aero lift of your K7 George.......but too fast for me........Derek

https://www.youtube.com/watch?v=fGPMBKTA1Wo
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Re: Donald Campbell's Bluebird K7
« Reply #17 on: August 28, 2014, 08:03:40 PM »

Hi George,
I've just been out to the workshop. My model is about 1 1/2 times the size of yours so using your suggestions, the sponson steps should be about 1 7/8" wide. Mine are currently about 1 1/2" (still wider than shown in the Nexus plan). They are growing right now with a target of 2". The CG should be 3" back. Mine is 6" back. That could be something I have to live with as I can't get the battery any further forward. As a last resort, I have some sheet  lead {-). My prop is completely horizontal. I spent ages getting it that way! There's enough flex shaft after the tube to get a little bit of adjustment. I shouldn't really change so many things at once but I've seen the proof of your set up.

BTW I'm going to do another couple of runs with the wedge in place as I have an experiment I want to run on the prop. (Not so much to make the boat go fast, just something I'm curious about). Then, it will be wedge off. Follow the experts (for once).

Rgds,
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Re: Donald Campbell's Bluebird K7
« Reply #18 on: August 28, 2014, 09:10:31 PM »

Hi George,
I've just been out to the workshop. My model is about 1 1/2 times the size of yours so using your suggestions, the sponson steps should be about 1 7/8" wide. Mine are currently about 1 1/2" (still wider than shown in the Nexus plan). They are growing right now with a target of 2". The CG should be 3" back. Mine is 6" back. That could be something I have to live with as I can't get the battery any further forward. As a last resort, I have some sheet  lead {-) . My prop is completely horizontal. I spent ages getting it that way! There's enough flex shaft after the tube to get a little bit of adjustment. I shouldn't really change so many things at once but I've seen the proof of your set up.

BTW I'm going to do another couple of runs with the wedge in place as I have an experiment I want to run on the prop. (Not so much to make the boat go fast, just something I'm curious about). Then, it will be wedge off. Follow the experts (for once).

Rgds,

Tim,
I have a drawing from Nexus which is to a very peculiar scale but makes a model about a meter long which as you say is 1.5  x the size of mine.

Drg No 1462 x Andrew McCoy.
I intended to build this and install a jet engine into it that I made but on discussions with somebody down south who had one I decided that my lake just was not large enough for a jet powered K7. plus the fact that I didn't understand the electronics required to control a 120,000 rpm.  jet engine.

Now assuming that the drawing that I have is the one that you have all dimensions should be 1/2 size again.

On the drawing from the underside of the flat hull it's 1/2" to the bottom edge of the sponsons.
On mine it's 3/4" so on the  +1/2 again yours should be 1.125" but at least 1"

On the drawing the sponson width is 1.25" and mine is 1.25", so yours should be 1.875"

I have already suggested the removal of the rear planing wedge and the prop to be angled slightly down and the C.G.  on mine is approx 2" from the sponson heal, not the wedge rear so yours should be about 3", C.G. is very important and can be found by experiment in moving a small weight.

I would suggest that in your experiments you keep a diary and move or alter one thing at a time, it all will be worth while in the end when  K7 pops onto the plane and away she goes in a straight line..

Now I know these are a lot of suggestions but I have never known anybody to build one to your size , so it's all conjecture on my part but the drawing is very good and you have made an excellent job on building your K7.

I don't see why it wont work on the principle that I scaled up mine from a very small 1/24 K7 built by Ernie Lazenby, one of the finest exponents of fast electric scale models in the U.K..

Here is the link to Ernies Jet powered K7, it's worth a look. https://www.youtube.com/watch?v=JNBRa3oKit8

Keep experimenting and keep us all posted and if I can be of help just give a yell, just wait until you open the throttle and K7 takes off then all of your work will be worth it.

George.
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Re: Donald Campbell's Bluebird K7
« Reply #19 on: August 28, 2014, 09:23:13 PM »

That is brilliant acceleration and aero lift of your K7 George.......but too fast for me........Derek

https://www.youtube.com/watch?v=fGPMBKTA1Wo



 It needs a lot of water and acts just like the full size.
 I live about 25 mls from Loch Lomond and on one occasion when the loch was flat calm I set it out about 100 yds from me and opened it up , it just accelerated faster and faster and eventualy took off, fortunately I ws able to snap shut the throttle and it dropped down onto the Starboard sponson which saved me a swim in very cold February day.

On my club lake I can only get about  6-7 sec power run, which is quite frightening for an old guy like me.

George.
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Tim_M

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Re: Donald Campbell's Bluebird K7
« Reply #20 on: August 29, 2014, 08:15:36 AM »

Thanks George. I value the input from someone with practical experience of this boat. We agree on the measurements! I think you have realised that my interest lies in design and development more than running. I didn't expect this model to run at all and that's why it sat on the shelf so long. It turns out that I'm very close to having a working model now and I shall keep going, stage by stage. I can't believe the amount of power these brushless motors pump out! Eventually, I'm going to design a new (lighter) model incorporating everything I've learned and that should be a thing of beauty O0. My local lake gives me a straight run of 200m. If everything dies, I can just wait for the boat to drift in. There is a much larger natural lake a couple of miles away but I'm not sure how to get to the water's edge. I will have to be very confident in the model before I set it loose there! Busy weekend coming up but there should still be time to get a few bits sorted.

Rgds,
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ooyah/2

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Re: Donald Campbell's Bluebird K7
« Reply #21 on: August 29, 2014, 09:59:30 PM »

Thanks George. I value the input from someone with practical experience of this boat. We agree on the measurements! I think you have realised that my interest lies in design and development more than running. I didn't expect this model to run at all and that's why it sat on the shelf so long. It turns out that I'm very close to having a working model now and I shall keep going, stage by stage. I can't believe the amount of power these brushless motors pump out! Eventually, I'm going to design a new (lighter) model incorporating everything I've learned and that should be a thing of beauty O0 . My local lake gives me a straight run of 200m. If everything dies, I can just wait for the boat to drift in. There is a much larger natural lake a couple of miles away but I'm not sure how to get to the water's edge. I will have to be very confident in the model before I set it loose there! Busy weekend coming up but there should still be time to get a few bits sorted.

Rgds,

Tim ,
I can understand your interest in the development side of K7, I had the same when I was building my Flashsteamer
( OOYAH ) which was a 4 year journey.

I would point out that although you wish to build to scale there are some points that have to be made out of scale especially for a prop driven K7, which is basically the sponsons being wider apart and the planing wedges wider and the rear wedge dispensed with to allow free flow of water to the prop.

So jot these points in your diary, it's very important to keep a build and performance diary,  to keep you in mind if you ever design and build another K7 and keep it as light as possible.

I think you have made a great job of the K7 build and you will get there in the end, frustrating tho' it may be.

George.

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Tim_M

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Re: Donald Campbell's Bluebird K7
« Reply #22 on: August 31, 2014, 08:22:26 PM »

Hi George,
Not much progress this weekend but Bluebird is sporting a pair of nice new running shoes at the front - very sharp! Not so much luck with the CG. I trimmed out a couple of bulkheads under the cockpit and now have a space for the battery as far forward as I can get it. I've also removed the 'engine' (a spray can lid with an old hairdryer fan inside). It looked nice but surprisingly weighed 85 g - right where I didn't want it! However, the CG is still way back. I had to add 350 g to the nose to get the boat to balance at 3". That's over 10% weight increase so a non-starter. The solution is to move the motor forward and that's going to need a new prop shaft. Not a problem; the hardware will all be transferred into MkII eventually. (My thinking when I designed the boat was to put the CG at the centre of lift- i.e. 2/3 of the distance between the forward and aft shoes, assuming each gave the same lift. Twit! I forgot that, just like an aircraft, the CG has to be in front of the centre of lift otherwise the craft is inherently unstable. That one slipped under the wire %))

I have some questions for you:
1) I assume you used a rigid shaft since you mentioned stainless steel. Logically, I need to have the inner end of the shaft tube above the static waterline or provide some sort of low-resistance seal (such as a grease filled stuffing box). What was your configuration? What are your thoughts on flexible shafts? By the way, any seal I use could be 'disposable'. I have to take the shaft out after every run anyway and oil it as I'm running in salt water.

2) Can you remember what paint you used? When I built my model initially, I went to a friend who ran an automotive paint supply shop. It turned out that 'Monza Blue' covered a whole multitude of sins, depending on the car manufacturer. I settled for a blue I could get at the local hardware shop. Sadly, they still have it in stock and the only other choice is 'Sky Blue'.

I'm sure I will come up with more questions to keep you occupied. {-) Thanks for the help!
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Flundle (Speedline Models)

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Re: Donald Campbell's Bluebird K7
« Reply #23 on: August 31, 2014, 10:53:06 PM »

Original colour was Himmelblau, a 1954 Mercedes car colour. Still available as a mix from car paint suppliers.
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Tim_M

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Re: Donald Campbell's Bluebird K7
« Reply #24 on: September 01, 2014, 07:36:29 PM »

Thanks for that Mr Speedline. Sky blue it is, then!
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