In the last chapter we looked at non-destructive testing. But what if you can't find anyone to do that for you, or, as is quite likely, the results are inconclusive?
To Torque or Not To Torque
One idea I had was to torque the bolts and see if anything bad happens. Of course, if we have not yet bought the boat, it's unlikely that the owner will allow that. But let's assume we have bought the boat (or the seller is desperate).
So would I try torquing the bolts as a way to give myself confidence before going to sea?
A definite maybe on that. Here's what I think:
- If the boat had bronze keelboats into lead, probably not. As I explain in Part 1, I already have a high level of confidence in that construction method.
- If the boat had stainless steel into lead or mild steel into cast iron, and ultrasonic testing did not work out or was inconclusive, probably yes.
But I would also go into it with my eyes open, since there is a real risk of breaking a bolt and/or crushing the fibreglass laminate and/or structural members the bolts pass through.
Sounds terrible but, then again, if we don't trust the bolts in the first place, how much worse off are we if we try this and it goes badly? I would argue not a lot.
Torque To Use
The next problem is that as soon as we mention this idea to any sort of marine expert (Bill from Mars Metals included), their first words are:
Check with the builder or designer for the correct torque settings.
Eminently sensible...and totally impractical, at least in many cases, because:
- Many of the builders of the boats we are likely to be refitting are no longer in business.
- The designer might still be around but, even if they are, they may not have done engineering on the boat since many builders were not willing to pay for it.
- Even if the designer has the correct number to hand, how can we know that the builder put everything together as specified so it can withstand the specified torque?
In summary, great if you can get a torque number, but don't bet on it and don't overly rely on any numbers you do get.
Now what? Well, the good news is that we can calculate the correct torque with a handy dandy calculator that I used for several projects last year, including upgrading the series drogue chain plates on Morgan's Cloud.
Not That Simple
Great, all set? Err, no. It's not that simple:
- While the calculator can come up with the correct bolt tension not to shear the bolt, crush the laminate, or strip the threads in an iron keel, that assumes we know the strength of everything, which may be tricky for the bolt and keel and near-impossible for the laminate.
- Torquing a bolt is actually a very imprecise process with a margin of error when converting to bolt tension—what really matters and will govern whether or not bad shit happens—of plus or minus 25% and that assumes the wrench is properly calibrated and used.
- Just a bit of corrosion on the nut-to-bolt interface can result in far less bolt tension than desired, at a given torque, and lubricating the nut can result in far more.
- We also must choose a preload to use, normally defined as a percentage of the bolt yield strength.
Here is how I would go about it:
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