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Bell Ends And Engine Inlets


We've been subjected to several weird and wonderful acts of awkwardness at the hands of those who run the Ruskin Museum. For example, they once decided that if they told us we could no longer have their original trailer wheels this would stop us taking the boat round to the yard to develop the engine installation and systems. People were enjoying us doing that, you see, so it had to stop, and if there were no wheels on our wagon we'd be stranded. It was a less than cunning plan because of two flaws. The first one is that we had more than enough wheels so giving back the two worst ones made no difference whatsoever. The second problem was that even if they had all of our wheels it would have been the work of moments to get the cradle onto some different wheels and carry on as normal. So we left the wheels with one of the units as none of us were about that day and some agent or another took them away and we put two different ones on the cradle.

Big problem - the scheme had failed. So they puzzled further and came up with something else that would stop us from doing our development work. They wanted the original inlet from the engine. Yes, that would do it! there was definitely only one of those and without that how could we run the engine?

So a particularly bizarre letter arrived at BBP HQ demanding not only that we take it off and sent it to them but also that we must insure it for a whopping seventeen thousand pounds! Just so we're clear here, this is the item in question.



This is the original in its unrestored condition lying in the hull as we found it, the aluminium bullet with the four vanes and it was always affectionately known as 'Donald's Bell End'. In fact when asked by Vicky at the museum (come back Vicky, the lunatics have taken over the asylum!) what it was called that's what we told her and for four years it hung on the wall in there thus described. Goodness only knows what the visiting public made of that.



So we fettled and welded and repaired it until it could be used again. We had to keep a very close eye on it in service because it's riddled with rot and regularly began to crack or rattle with loosened corrosion product in the voids whereupon we'd have to take it off and give it a good going over before using it again but we stayed ahead of its foibles. It looked pretty good in its final form and held on through the high power requirements of the Bute exercise so we were rather proud of it but how in the name of all that's sensible did someone came up with a figure of £17K? It's beyond imagination. Were it to appear at auction it might fetch a grand.



But no matter, the museum had decided to demand its return. We told them it had had so much work done on it it was more ours than theirs, the 17k idea was just a joke and left it bolted to the engine but the whole ridiculous episode triggered something we'd had in mind for a while. We really ought to build a new one.

It's a far from simple fabrication, though, so first thing was to make up a full set of paper templates in order to replicate its various panels and sections


You can see how the bullet has been made up in segments with a domed cap welded on the end. That was the obvious way to go so the next part of the process was to measure and draw out the profile of the bullet on a piece of MDF then make up a vane that fitted perfectly into the original inlet to check that everything matched and married up.


Above is the inlet sporting an extra vane. If we made four of those to fit our bullet we'd know we were right shape-wise. Below is the same vane tested against the profile drawing of the bullet ahead of making up some tooling. Notice on the original the mass of repairs and patching to keep it alive.

The diameter of the dome was checked every inch along its length to get the profile exact and as a reference for where it all fits to the engine we used an inlet from an Orpheus 101, which is identical in that regard but instead of the bullet seen above it has a removable nose so the starter, that lives inside, can be serviced without having to take the front of the engine off.

Above is the 101 inlet viewed from behind. We knew if we worked to this as a template our new inlet would fit perfectly. We used that to create a tool to build our new inlet from. Notice the open front end so you can get at the starter. The bullet type is a real pain when it comes to servicing the starter because the whole thing has to come off and that involves removing oil pipes and breathers and all manner of gubbins to get on the fixing bolts. It's little wonder that it was quickly superseded.

The tool for the engine end is a simple affair. Just some carefully cut sections of MDF such that the various sections have no choice but to fall in the correct places.

Then the metal cutting began with the first segments of the bullet.

They were easy enough to bash to a rough approximation of the required shape so we did that then washed them out to smooth panels on the English Wheel.

The wheeled sections came out well. Just need to assemble them into the lower portion of the bullet then make a domed end.

Getting there now. You can see the left hand edge has been through the shrinker to taper it in a little further. Get it close then fettle it.

But that would only take the job so far. It's easy to have something look right but to make sure it was right we raided the scrap bin at the wood yard over the road for a pile of thick MDF and made a hammer-form.



A hammer-form is basically a 3D shape you hammer the metal over until it's the same shape as the tool and in this instance we invoked the old Doddy philosophy. Get the bullet close-ish then heat shrink it onto the form later. That tightened it onto the form very nicely and allowed us to get the shape spot on.

Above, the bullet in its final form having been heat-shrunk to the hammer-form and its dished nose formed and almost ready to be welded in. Once that was welded it went back onto the tool to have a final shrink into its finished shape and then it went onto the assembly tool so the vanes could be made.

The vanes are made in two halves with a weld down the centre of the leading edge. We built the whole thing of a slightly heavier and stronger alloy than the original as weight isn't a consideration and we want it to be reliable.

We built a second hammer-form for the vanes and shrunk the metal over them. That way we can be sure they're all identical.

The outer shell comprises three parts, the shell itself, which is just a strip of aluminium rolled into a circle, the flange that bolts to the front of the engine and a channel that runs around the front of the shell that holds a rubber seal. They were all straightforward to build, though the 2mm thick mounting flange was hard work in the shrinker/stretcher and maintaining a perfect circle of that size is challenging too but with a little patience...


There it is. But at this point the museum realised the lack of an inlet wasn't going to slow us down for an instant so the demands stopped and we continued to develop the engine installation and on-board start system. But, as everyone knows, after a year of trying to negotiate the deal we were always promised the time finally arrived when we realised that, even with a deal, certain individuals would throw obstacles in our way until the end of time so we walked away.

It was supremely frustrating because essentially what the RM did was tell a lawyer that they'd loaned us a boat for a spruce up and a coat of paint and now we wouldn't give it back. The lawyer would duly write to our lawyers on this basis only to have the reality explained then, once their lawyer had his or her head around the situation, they'd explain to the museum just what an excellent offer and a super opportunity was available to them. At that point the museum would find a new lawyer and explain how they'd loaned us their boat for a spruce up and a coat of paint and now we wouldn't give it back. I think we were onto the fourth different barrister by the end of it and our legal eagles were shaking their heads in wonderment but the upshot of this is that we united as a team to get Bluebird out of her workshop and in lifting the rear end to fit the trailer wheels we crushed the new inlet under the font of the boat. Oops!


It was one of the dragons teeth on the underside of the bow that got shoved through it. That was annoying but we've mended worse so it was put to one side.

Another thing the museum did was demand the fuel system off the engine, which was completely pointless. It's of no use to them unless they plan a lengthy development of another engine using it and more than half of it is ours anyway because not all of the old parts could be saved so we had our donor units overhauled to make it a complete system, one module is from a hovercraft! But there was no point in trying to explain this because we have plenty of spares, but what this meant was that Bluebird's engine was as dead as the rest of her. We weren't going to leave it that way, however. Even if the boat is stuffed and mounted some of her will live on and the engine that powered her over water in 2018 will roar again, so, having sorted out a pile of jobs in the aftermath of Bluebird leaving her workshop, we got on with the task of getting the engine back together. Its fuel system is on now except for one rigid pipe that we're having to make up so now we need the inlet on because it not only deals with the aerodynamics at the sucky end, it also contains the pipework that delivers air to the starter then exhausts it away from the inlet. Time to mend it and get it finished off.


Fortunately it wasn't a difficult repair but only because we hadn't skipped the hammer-form. Lots of tooling and scrap parts were disposed of but that one remained under the bench so the damaged section was cut out, the whole thing put back on the tool and the section welded back in.

It'll all get a good polish in due course but there's not much point while it's still being bashed about. Next it had to be fitted with an internal strengthening ring as per the original but we fitted it a little further inside the bullet because the original needed a small modification to clear some bolts and after that our new inlet became mostly just a welding exercise.

The stiffening ring was a tricky thing to make as it has to be a perfect circle, a top hat profile and a different diameter from one side to the other so as to fit into a tapering hole but once it was perfect it was welded in with plug welds and it makes the bullet extremely strong.

The channel that holds the rubber seal is two L-shaped sections. It was much easier to make that way rather then trying to form a channel into a ring so it was pinned and welded. Just a few stitches to begin with but you get the idea.

And then we had our first trial fit to the engine. Our aircraft engineer, Al, set about back-marking and drilling the bolt holes around the mounting flange. To give an idea of the accuracy here, the holes take a 1/4UNF bolt so the holes are 6.4mm diameter so you're looking at only a couple of tenths of a mm of slack to play with on each hole and there's a whole ring of them around the flange. How accurate does your work have to be such that you can run in all the bolts without having to dress the holes oversize?

Of late, certain individuals have attempted to portray us as amateurs in the sense that our work is less than perfect but the simple fact of the matter is that it's as good as anyone can do it. It's just spite and jealousy.

Al back marked each hole then drilled 3/32" holes then upsized them to 1/8" and finally to 1/4" before deburring them. The tricky part was making sure it fitted precisely to the engine inlet before committing to a single hole but he got it exact.

So, with the mounting flange drilled, we had our first dry build of the inlet on the engine and it's looking very smart indeed.

Still a way to go with it but the good news is that all the parts fit nicely and flow perfectly onto the front of the engine. We'll build it in situ with a few tack welds then put it on the bench to finish the welding. Of course it won't fit after that with all the welding distortion but that will only need a round of metal whispering and after that it can have its internal bits made and installed. A small pitot tube for the engine fuel controls and the delivery plumbing for the starter need to go in but once complete and the last few bits are back on the engine we'll get it running and part of K7 will be alive once more. We might even run it up for the coach parties later in the summer.















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I love these mails as they are a bit of a masterclass in bespoke engineering work. I've stopped even commenting (or giving valuable energy) on those inadequates who just exist in the world to push paper about or make trouble. I remember Richard Noble - when embarking on his record-breaking career - remarking at a talk how some skilled and really quite brilliant engineering talents were treated so shabbily by their household name employers. He was shocked and saddened by it - but (obviously) extremely grateful for their help. It's kind of a disease in this country. We have so much talent - and people who possess it (eccentric as they might be at times) are often so happy to share…

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