John Cobb- Crusader/Railton Special

[malcolm uk]

The BBP operations team were at the Cairn when wreaths were laid to remember John Cobb at the time of the accident 60 years ago. We were able later in the day to gather useful data regarding wakes and washes from moving craft. Inspite of their experience with World Record attempts the role of the operations team for K6 and their actions that day meant they were probably culpable in the accident which took place.

[KW Mitchell]

It is important to understand that Crusader fell foul of the little-known effects of dynamic instability. This was elegantly enunciated by the Norris brothers following analysis of cinematographic recordings of Crusader's demise (The Engineer, 1957).

They showed that the craft suffered violent pitching oscillations of 5-6Hz, initiated by swell, which led to the collapse of the front wedge. Such instability was shown to be related to the height of the CG above the wedges and their inherently large moments therein, plus factors relating to the 'spring-rate' of the wedges. The issues are discussed in further detail - together with the lessons learnt in relation to K7's design - in Neil's book 'Donald Campbell Bluebird And The The Final Record Attempt' pp's 198/9.

It is also possible to argue that even if Crusader's front wedge had been strengthened, such extreme forces on a single front wedge would have led to failure - and - even if Cobb had been able to run on perfect water that fateful day, a speed would have eventually been reached whereby the oscillation was inevitable.

[f1steveuk]

Evening!

The lines shown in the two stills are supposed to be there. I'll go through my picture file later for a close up.

Other than that, the front shoe wasn't a weak spot as such, but became problematical, and quickly worsened. I have copies of the original factory drawings, and quite a few unpublished pictures, of the area, plus there was one other factor that contributed other than the structure, the waves and the mix of construction materials.

[Stuart Baker]

It is important to understand that Crusader fell foul of the little-known effects of dynamic instability. This was elegantly enunciated by the Norris brothers following analysis of cinematographic recordings of Crusader's demise (The Engineer, 1957).

They showed that the craft suffered violent pitching oscillations of 5-6Hz, initiated by swell, which led to the collapse of the front wedge. Such instability was shown to be related to the height of the CG above the wedges and their inherently large moments therein, plus factors relating to the 'spring-rate' of the wedges.

It is also possible to argue that even if Crusader's front wedge had been strengthened, such extreme forces on a single front wedge would have led to failure - and - even if Cobb had been able to run on perfect water that fateful day, a speed would have eventually been reached whereby the oscillation was inevitable.

Isn't this like having an out of balance wheel on your car? If you can quickly accelerate through the natural frequency of the oscillation, then you should be able to return to stability?

We use this technique to reduce machining cycle times. Traditional logic says that you increase cutter speeds until hitting a resonance and then back off. However, if you characterise the natural frquency of a tool / component / fixture combination, then it is possible to identify a much higher cutter speed that sits between harmonics of the natural frequency. The technique was pioneered by the Advanced Manufacturing Research Centre in Sheffield (who incidentally are also supporting the Bloodhound project http://www.amrc.co.uk/featuredstudy/bloodhound/).

S

[KW Mitchell]

The problem with the two analogies you give here, is that it is only part of the system that suffers the resonance. That means that the amplitude of oscillation is constrained by the wheel and its mountings within the car, or, the cutter/chuck etc. within the overall lathe structure/housing. The more apt analogy is to compare what would happen if the whole car oscillated or the whole lathe on it's mountings. If the amplitude at the resonance point was so large then the consequences can be well imagined.

Resonances of relatively low amplitude in hydroplanes can be 'punched' through by accelerating. There is no better example of this than K7 itself; DMC refers to it at 150mph on the first '67 run (see Neil's book, p246) when classical tramping was experienced and the natural damping of the boat limited this to controllable limits as the speed was approached and then exceeded.

Cobb experienced a longitudinal pitching oscillation of such magnitude - and at some 50mph more than K7 - which could not be constrained by the ability of the overall craft to absorb it (due to its design limitations), and, the strength of the most vulnerable part of the design i.e. the single front wedge, to withstand the loads.

As to speed, Cobb would be accelerating hard at that point hence there was no option to accelerate through it when the craft was pitching so violently and a key part catastrophically failed. If it hadn't failed, Crusader's limited dynamic stability would have eventually manifested itself and the craft would have become uncontrollable in pitch - albeit at a higher speed - but with the same disastrous consequences.

[polo]

At a speed record week I spoke to Frank Lyall who was one of the observers / timekeepers at the Cobb/ Crusader trials. He told me that work had been carried out on the front planing shoe including the drilling of holes inside the bulkhead where the planing shoe was. He did not give a reason for it, he just said thats what they did.
If you look at the position of the rudder and its construction and attachment to the rear of the front shoe. it doesn't take a great deal of imagination to think that due to the oscillations that this could have been ripped off the boat due to the continuous resistance changes due to its depth in the water and the hammering the shoe was taking at the same time.

[f1steveuk]

Just to let you know. Manuscript 1st draft at the publishers

[ron54]

Well done, looking forward to it.

[Karl Ludvigsen]

New to this forum as I am, just a quick note to say that I'm the fellow working on the biography of Reid Railton, in which the Crusader naturally occupies a significant role. I'm closely cooperating with Steve in this endeavour, giving due acknowledgement to his profound knowledge of this and other maritime matters.

My story of course also includes the Malcolm Campbell boats, which are addressed elsewhere in the forum.

Thanks a lot for the many interesting comments made above. Now all I have to do is figure out why and how it happened!

[conistoncollie]

Hello Karl,
Very much looking forward to your forthcoming book and wish you good luck.
I remember the Railton Special in the old Science Museum in Birmingham (before it all got moved to the trendy Think Tank).
The Railton Special was always a fascinating exhibit. If I recall correctly it even had a push-button which played some original (rather scratchy) audio tape.