MAIB Reoprt on Hooligan V keel loss

[misterg]

[ QUOTE ]
The one thing I will say without prejudice is that having seen many yacht keel roots is there are few that don't show some signs of rust and cracked filler ie at least some movement.

[/ QUOTE ]

Would you mind clarifying what you mean by 'keel roots' in the context of your comment, please?

(I would normally think of this as the place where metal meets GRP, etc, but the comments in the report refer to what is effectively a change in section of the steel keel - from a position of near ignorance, I would expect 'cracking' / movement in the former, but not the latter.)

No connections / involvement, etc.

Andy

 

[alant]

"As for Bavarias I would keep quiet after counting how many made it to the Fastnet. Very disappointing entry number and success rate."

Can you expand on these statements please.
I'm still trying to work out if you'r knocking or applauding them.

How many entered?
How many retired & where did they get to down course?
How many completed?
How many keels dropped off or any other major gear failure? /forums/images/graemlins/tongue.gif
etc.

 

[Eygthene]

Andy (Misterg) has got much closer to a realistic analysis of the circumstances surrounding this failure.

At the time of this design, (and still at present as far as I can tell), the ISO standard for keels is in draft form, so the designer was able to adopt the American guidelines, which require only that the keel should be capable of supporting its own weight when heeled at 90 degrees, with a safety factor of 2:1 over the yield strength of the material.

As Andy pointed out, the yield strength figure used in the report was not actually appropriate to the material involved and the true factor of safety worked out to less than 1 as a result. The keel was bound to bend at high angles of heel and fatigue failure would be expected to occur sooner rather than later.

The draft ISO standard follows a similar line, though there is an allowance for dynamic loads included.

However, the yield strength of the material quoted in the standards is for rolled steel and when there is a weld in the highly stressed region near the root of the keel, this introduces material of a reduced yield strength as well as concentrating the stresses along the weld.

For this reason, the design of the welded joint and its effect on the dynamic properties of the structure will further reduce the safety factor and ought be taken into account in the design. There is no indication that this aspect has been considered in the draft that I have read.

Until a satisfactory standard for a fabricated fin is agreed, I think the designers should be constrained to using a cast steel fin. How many more keel failures will it take before this is recognised?

 

[misterg]

[ QUOTE ]
However, the yield strength of the material quoted in the standards is for rolled steel and when there is a weld in the highly stressed region near the root of the keel, this introduces material of a reduced yield strength as well as concentrating the stresses along the weld.

[/ QUOTE ]

John,

Does this comment apply to the 'as designed' keel as well as the 'as made' version? (I think it does, but I'm not clear whether you mean it to)

[ QUOTE ]

For this reason, the design of the welded joint and its effect on the dynamic properties of the structure will further reduce the safety factor and ought be taken into account in the design. There is no indication that this aspect has been considered in the draft that I have read.

[/ QUOTE ]

I was surprised how simplistic the American design code was (as quoted in the report). I am also surprised that there seems to be an expectation of a common standard for lightly build racing machines, and for production 'cruisers'. This is akin to a common set of rules for Formula1 racing cars and granny's little run-about. Either racing development is stifled by conservative design rules, or mass production drifts towards the same narrow margins of safety (weaker is cheaper) while claiming that the boat "meets the standards" - We have already heard of manufacturers of production boats claiming that their construction "meets the draft ISO standard" when discussing structural failures.

I think when we take out all the experienced supervising surveyors required to verify a boat is built 'to code' (from whichever country), it will open the floodgates to such minimum standards engineering.

Andy

 

[Eygthene]

Andy,

The drawing of the as designed keel shown in the MAIB report (Fig 14) does not show where the welds are, nor what is welded to what (such welds must be shown on any proper design drawing), but the schematic (Fig18) does show a couple of small fillet welds where the keel enters the bottom of the taper box.

I would be very surprised if the as-designed keel would be manufactured without these welds. As soon as welds are placed there, they would have a deleterious effects on the fatigue strength of the structure. Also, whether welded there or not, the point where the keel enters the bottom of the taper box would be a point of high stress concentration which also needs to be factored in to the design calculation, as this further significantly reduces the "safety margin". You have already shown that in fact there was no safety margin in this design, even without adding these particular considerations.

The sooner the Draft ISO standard is agreed and put into effect the better, but there were still a lot of items awaiting clarification/drafting, in the 2004 draft version that I was able to download off the web.

John

 

[misterg]

Thanks John.

I have emailed the MAIB about my concerns, and have included the point about taking fatigue into consideration, too.

Andy

 

[Eygthene]

Well done Andy. I am trying to trace an ISO standard that I referred to a few years ago, concerning welds and the appropriate design factors. I think that standard was related to factory chimneys, though we were using it in relation to wind turbine towers. I should have a reference to it somewhere. When I do find it, I will put together something to send to the people working on this ISO standard about keels.

John

 

[Duffer]

I was very saddened by Mr Butcher's death. The other crew members did remarkably well to raise the alarm and get themselves rescued at night at that time of year.

 

[Sailfree]

As a non practising structural engineer I find these discussions very interesting.

When I did practise designing bridges it was shortly after the Westgate and Milford Haven bridge collapses. I remember new additional checks being bought in very quickly by the Morrison committee. I also remember that you always tried to avoid welding to a tension face as it induced fatigue. With stress reversals on a keel I would have thought this a given for a boat and that welding on stressed parts should be avoided unless a fatigue analysis was completed.

In previous threads I have expressed concern that yacht design still appears to be based on "rules of thumb" and empirical design that is revised only after something fails. I can understand this better for racing boats (in F1 Colin Chapman was not beyond beefing a component up until it did not fail!) With boat costs being driven down by sales volume competition I believe its only a matter of time before some serious underdesigns occur on production boats (I believe the Bavaria match boats are the only ones so far to have an "alleged" underdesign that required post build modification).