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[ooyah/2]

Sorry chaps,
I posted on Mayhem while the video was still uploading. It took nearly two hours!

Should be OK now; I've just viewed it.

Tim,
Just viewed it, that's what I call an " OUCH " moment, put it down to pilot error.

Your K7 sits on top of the water and not like mine bury it's self when full power is applied from stop before coming up onto the plain, must be something to do with the bigger scale.

It's the rough water that's causing the bounce, if I had to apply full power on water like that it would flip over.

I can't see anything wrong with the boat's set up, it's running well but cut down the rudder throw.

Is the damage to the sponson repairable ?

George.

[Tim_M]

Anything is repairable! This models looking pretty beat up but not to worry. It's the test bed for MkII.
I'll put some splints inside the sponson  and with a bit of luck it'll hold up.

BTW, I hadn't ignored your earlier advice ref. the rudder. I had already shortened the throw mechanically. I can afford to cut it down more and I realised I have a dual rate switch for that channel of the Tx. I can keep a bit in reserve for parking, just like the car!

Driver error? Got that by the bucket load, old chum!

[klz]

Hi Tim your model is looking good, I have now added one to my very long list of models I want to build keep up the good work.

KLZ

[ooyah/2]

Anything is repairable! This models looking pretty beat up but not to worry. It's the test bed for MkII.
I'll put some splints inside the sponson  and with a bit of luck it'll hold up.

BTW, I hadn't ignored your earlier advice ref. the rudder. I had already shortened the throw mechanically. I can afford to cut it down more and I realised I have a dual rate switch for that channel of the Tx. I can keep a bit in reserve for parking, just like the car!

Driver error? Got that by the bucket load, old chum!

Tim,
Forgot to tell you that I whacked the far end of our lake which has a concrete edge all around when U started out on my tests.
At least I have the age advantage at 75 although still good eyesight,still if I was nearer I could dip into your bucket.

K7 M1 has turned out great so I look forwards to Mk 11.

George.

[Tim_M]

Round about now, I had planned to unleash the MkII build project on an unsuspecting Mayhem. My PC, however, had other ideas. Down side is I am considerably poorer than I was a couple of weeks ago. Up side is that the hard disk was OK and I managed to recover all my files and here is my new work bench. Not quite as powerful as the laptop but that screen is sooooooo much nicer to work on!

As for Windows 8? The jury's still out. Does anybody know how to get rid of the 'trovi' hijack that comes built in??

[Tim_M]

So, in the meantime there has been a small amount of progress on MkI. She is recovering nicely from major surgery. The sponson repair was quite determined but is now looking reasonable. Smooth as a baby's bottom. Baby elephant, maybe. Even 'Donald' looks surprised! I was thinking of leaving it red (and painting the other on green) to help me with my (lack of) steering ability but decided later to have a go with my new airbrush. And the result? Let's just say I have a lot to learn before I go anywhere near a real model.......

[Tim_M]

I had a look at the cooling. There was a lot of back pressure in the plumbing! I managed to open up the feed in the rudder from about 2.5mm diameter to 4mm. That's almost three times the area! Carrying on, I looked at the speed controller. The holes in the nipples are a rather disappointing 2.5mm diameter. I was going to drill them out but fortunately I took one out (rather than drill in situ). The thread is small and there's not enough material left to drill out the holes. Luckily I didn't plough ahead without looking. Nothing that can be done there. The motor cooling jacket is very restrictive. It's nicely made but the gap between the motor and the shell is really too tight. Again, nothing to do there. I've decided to plumb everything in parallel rather than in series and I'm waiting for some little Y shaped unions to arrive from Hong Kong. Couldn't find anything locally but they were dirt cheap, even with the postage.

[Tim_M]

 ( I'm making multiple posts to get the pictures and words to stay together - if that's not the right thing to do, can somebody tell me?)

Finally for today, just to prove that I have been thoroughly bitten by the modelling bug, here is a sneak preview of next year's 'big build'. It's another subject that I've wanted to tackle for ever and I have finally managed to get a reasonable start on the CAD model. It's going to be a major challenge!

Extra points for anyone (else) who knows what it is 

[Mad Scientist]

Why, it's a Westland SRN2 hovercraft, of course!

Tom

[Tim_M]

Wow, a reply from Canada on the mystery model. Of course, the SRN2 underwent trials on the St Lawrence river back in the day. For me, it's still the most elegant hovercraft ever built and I spent many a childhood hour on the jetty at the ferry terminal staring across at the Westland slipway waiting for it to lift off. You really could see all the way underneath before they started adding skirts. Elegant, impractical, unstable, mega difficult to model - but it just has to be done   

[Ramon]

That's one awesome project ahead Tim but could I respectfully suggest you create another thread on it and not allow it to encroach and possibly deviate this superb thread on your Bluebird 


I guess I'm not the only one awaiting further progress on that and more on the Mk2 version as that develops. Every time I stand at our pond which is just over 400ft long - but walled - I wonder about whether it would be a viable project to take on


Posted with the best intention in mind


Regards - Ramon

[Tim_M]

Yes Ramon, The SRN2 will be a completely separate thread in due course -  I will put the build on Mayhem (eventually) because although it's not a boat it is the 'Saunders Roe Nautical 2". I only posted the glimpse of the future because I was chuffed with the CAD model. That's the furthest I've ever got after many attempts!

K7 Mk II will be a new thread too. Just have to get the plans finished before I open it up to the public.

Appreciate your interest in the K7 build!

[ooyah/2]

That's one awesome project ahead Tim but could I respectfully suggest you create another thread on it and not allow it to encroach and possibly deviate this superb thread on your Bluebird 


I guess I'm not the only one awaiting further progress on that and more on the Mk2 version as that develops. Every time I stand at our pond which is just over 400ft long - but walled - I wonder about whether it would be a viable project to take on


Posted with the best intention in mind


Regards - Ramon

Ramon,

I get about an 8 sec run in my club pond and at 40 mph that's about 150 yds, if my maths are correct.
You do need a bit of space, Loch Lomond is my favorite on a calm day but at 26" long and a brushless motor with 3S Lipo's if I run flat out it will take off, so just a touch below full throttle.

Very exciting little boat and what a crowd puller, everybody knows  Bluebird K7.

George

[Ramon]

Tim, been following it from your first post - keep it coming 


George - been thinking about doing this for so many years it's untrue - hence the interest in Tim's and others models.

Haven't got there yet but Tim's accounts of his endeavours are having a distinct influence 

I need to keep focused on the 'Wide a Wake' first though


Ramon

[Tim_M]

I've got a question.
Reading Graham's log on his Krik Lisa M reminded me that I needed to ask about motor mounts etc.

Modern motors use ball bearings. It's generally not a good idea to put an axial load on a ball bearing (unless they are specifically designed for it - the races have a slightly different profile - and my guess is that Chinese motors don't use top-notch bearings). If the motor is fixed and the shaft is a fixed length, there will be an axial load on the bearings - no getting away from it. 'Thrust washers!', I hear you cry. Nope, that's not going to work. You would have to build the driveline so that the balls were centred in the races (impossible in practice) and the slightest wear on the washer or change in temperature  results in the load being carried by the bearings, not the washer. We are talking microns here - bearings are made with incredible precision.

In 'real' drives, it's normal practice to arrange for a proper thrust bearing and include some sort of sliding or compressible coupling (typically a splined 'telescope') to keep the axial load away from the motor shaft. I've looked around and there's nothing on the market that would fit the bill for a model (size, price, rpm capability). My approach with Mk1 was to make a sliding motor mount - there is a picture way back in the thread. I was surprised that nobody challenged that. I'm planning to do the same with MkII. Apart from anything, it makes setting up the drive line so easy!

I've added a couple of picture of what I plan to do. Yes, I intend to hang the motor under the mount. That gives me scope to get the minimum angle on the prop shaft. (Still need to find the optimum angle from Mk1 trials). The motor mount is actually simple but I will probably have to compromise a bit on the fancy shapes as my 'machine tools' are a hacksaw and file. 

All this leaves me with three options:
1) I've got the basic engineering wrong. (I doubt that - it's my job to know this stuff!)
2) I am the first person to have thought about his. (I seriously doubt that - boats have been around (even) longer than me).
3) It doesn't matter. I'm being a pedantic old ***. (I'm renowned for it). The bearings will outlast the rest of the boat  - especially with my driving. What matters at work doesn't matter for hobby use. I know that my motor has some end float in the shaft anyway - perhaps it's worn out! (I still think there are people making money out of selling PTFE washers that are a complete waste of time though  )

I joined the forum mainly to get advice from those who know. Go ahead  please - but try to keep the abuse minimal 

[Ramon]

Tim - check your messages - PM sent


Ramon

[Tim_M]

Quick vote chaps - I'm almost ready to start logging the Mk II design and build. Shall I start a new thread or carry on with this one??
(It's getting a bit long!)

[Ramon]

I'm up for continuing Tim - a natural progression.


As I see it the length of the thread is not such an issue for anyone interested in the subject. That said others may have other views and prefer a fresh start. Should you refer back though it will mean having two threads open.


Whatever you decide I shall still be following - looking forwards to it 


Ramon

[ooyah/2]

Tim,

I'm with Ramon.
Stick with the same thread, sometimes another thread on the same K7 subject can get lost.
This way you can always read back without too much hassle.

George.

[Tim_M]

This is where it will go then 

I will try to upload some pictures of the (design) progress later. (After I've changed a set of brake pads and done the shopping!)

[Tim_M]

Mk I is in the garage convalescing and waiting for new plumbing.

It's time to launch the Mk II log 

Let me preface this by saying that I still know very little about model boats. I'm indebted to those who contributed to getting Mk I off the shelf and into the water and everything that has been learned so far has led to the starting point of Mk II. I'm not trying to teach anybody how to build a model here, just sharing what I'm doing and there is an unashamed engineering bias to my approach. I've decided to post early in the project on the grounds that "Why don't you...?" is far more useful than "You should have..."

Are you sitting comfortably? Then we'll begin.

The first was to build a 3D solid model of the outside of the boat at 1/8 scale using the original drawings kindly sent to me by a fellow Mayhemer. I've got it pretty accurate to the prototype but there is no rear wedge and the sponson shoes are wider - this time, less of a guess and more based on what works on Mk I. It looks like this:

[Tim_M]

The next stage is to create the framework of the model by cutting the frames from the solid. In CAD, the best way I can describe it is a 'reverse saw cut'. I slice down through the model where I want the frame and get the program to give me only the slice where the saw cut would be. Make sense? (Actually, each frame still has a copy of the whole model attached to it. If I want to move a frame, I just edit the cut position and the shape of the frame updates to suit. Very cool! This approach means I have a lot of data in the final model but my modest PC can cope with it just fine)

After a LOT of chopping and trimming I have something like this:

[Tim_M]

Coming on nicely. What you can't see, unless you have amazing eyesight, is that I made a bit of a cock-up around the air intakes. The cowling is circular cross section until it approaches the front, where it changes to elliptical. I got the solid model right but managed to cut the frames out with the cowl upside down  . Not a major issue, I just have to re-model a couple of frames and the stringers.

The basic design is similar to Mk I; two closed boxes running down the outside, which makes the model very stiff. I have decided to make one major change though. The cowling was very difficult to build on Mk I, it made the model a nightmare to handle during the build and the scale (ish) access hatch made the inside inaccessible! This part of the boat serves no great structural purpose in the model so I am going to split the model in two right along the cowling.  It gives me another advantage in that I can seal the hull with a simple, watertight internal hatch that is invisible when running. Not scale, but then neither is sticky tape to seal the scale hatch. I can do it on this scale of model because there is enough room for all the hardware under the hatch. It probably wouldn't work at a smaller scale.

I will reduce the size of the hatch and make it simpler when I know just where things will be inside but for now I've put the biggest one in that I can fit, just to see what's possible

[Tim_M]

You can see that I have already put the motor and battery into the CAD model. There's two reasons. Obviously, I'm designing the structure around the hardware but also, at a later stage, I will get the CAD system to calculate the CG position. The weight of the motor and battery is included in their models.

I spent a fair bit of time considering where to put things inside the boat. One thing that lets down many models is that they look too 'twitchy'. That's because you can't scale time. (Or you can, using video. I've seen some amazing videos on UTube where they are slowed down just a tad and the models look totally convincing). What I wanted to do was increase the mass moment of inertia of the model as much as possible. "Ooo - er," I hear you say. " He's gone off on one....." If you are not familiar with moments of inertia, he's a little demonstration you can do in the workshop or just in your head. Take two hammers - come on, we all have at least two hammers  -. Hold the heads together in one hand , with the handles sticking out. Now try to spin them like an aircraft propeller. OK, turn the hammers round so you are gripping the ends of the handles and the heads are outboard. Now try to spin them. A lot more difficult, isn't it? You still have the same mass (weight) and the CG is in the same place (in your hand) BUT it's a lot harder to get those hammers moving.

I wanted to spread the weight out as far as possible in the boat. The heavy bits are the motor and the battery. The CG on Bluebird is just behind the back of the sponsons. I thought about putting the motor way up front, almost under the forward boom. That would allow me to put the battery a long way back. However, that would require another hatch between the booms (difficult to seal), there's not much room that far up front to offset the motor and I would need a very long (expensive) prop shaft. The last straw was that all the wiring would lie in completely the wrong directions. I decided to stick with what works on Mk I. The battery is up between the booms but this time it will live in a drawer, designed in front the start. It goes in under the aft boom nicely and this allows me to use my simple hatch. If I can move the motor back a bit (once the CG calcs have been done) I will but for the moment I'm sticking with my existing hardware. I'm back to where I was but at least I'm satisfied that I have explored the possibilities rather than just copy what I did before.

You may notice that in the 'exploded' picture, the cowl frames are solid but in the assembled view, the centres have been cut away. They are two different models of the cowl that are part of the cunning build plan. I'll cover how I plan to build the cowling in another post and let you all rip it to shreds.

Just in case you are thinking I rely too much on CAD I would point out that I could (just about) still generate the design the old fashioned way - drawing it out and projecting all the sections. However, that wouldn't give me a fraction of the information I can get from 3D CAD - and it's much harder! Besides which, I'm not sure quite how well Mrs M would take the installation of an A0 drawing board in the dining room  .

For now, I need to carry on trimming and fitting the parts together. The solid parts will eventually be used to create 2D drawings as templates for the real wood. (Yes, a set of full scale, printable plans will be posted in due course for anybody that wants to have a go). This thread will grow slowly but I'll try to keep you posted on where I've got to and why I'm going that way.

[ooyah/2]

Tim,
Can I suggest that you consider .

1--Moving the battery position to the sides of K7, if you look back at my K7 you will see that I have  2- x 3s Lipo's to
     give a longer run time, you could then put the E.S.C. in that forward position as it doesn't need to come back out.
     If you position them as your drawing you will find it difficult to get them out and in.
 
2--Consider beefing up the skins of the bow where the sponson supports come thro' to prevent the hull cracks that       have in mine due to a lot of use.
  Say epoxy some 1/4" thk ply plates to the inside of the hull

Just some thoughts.

George.