The quality of the comments we get here at AAC never ceases to amaze me. And, perhaps even more importantly, I’m constantly learning from, and being inspired by, the dialogue.
A couple of weeks ago I was chatting with my son-in-law Ryan about this and he pointed out that we needed to do more to acknowledge the contribution from our readers who comment.
(Ryan is a smart guy with a lot of web development experience. In fact, at their wedding I was heard to say, in the father-of-the-bride speech, “I’m not so much losing a daughter as gaining a software support department”.)
So, on an irregular basis, we will be publishing a post highlighting comments—sometimes the full text, sometimes an excerpt—that have:
- Really jumped out at us.
- Taught us something we didn’t know.
- Corrected something we were wrong about.
One thing I do want to make clear is that the selection process is, by its very definition, pretty arbitrary. The point being that just about all the comments we get are of great value, whether or not we select them to appear here.
Also I have, where appropriate, emphasized the point(s) that really jumped out at me.
Here we go:
…staying comfortable (as possible) is just good seamanship when (as is usually the case offshore), the end is not in sight. You are in it for the long haul. We always try to move at 2/3rds speed offshore—rarely is there a true reason for hurry.
Along those lines, we try never to ask more than 75-80% of Alchemy’s [Dick and Ginger’s Valiant 42] sailing capacity. It is in that upper 20% where damage to the boat and injuries to person is most likely to occur.
…John gives two preconditions for transiting the passage: be prepared to winter over without assistance and be prepared to abort if conditions are adverse.
I would add another. If you would not attempt the passage without emergency beacons, radios and sat phones to call for help, stay down south because you should not regard the ability to ask for assistance as an essential part of your plan.
It might also be interesting to ask, ‘If you could never tell anyone about it, would you still go?’ If the answer is ‘no’, then probably you are out to tick the box, and should not be contemplating this…
I urge you to read Trevor’s full comment by clicking on the “comments” link above.
Erik de Jong comments on the Dangers of Storing Your Boat With The Mast Stepped:
Let me start by saying that I totally agree that a mast should be taken down when stored, and that the freeze/thaw cycles are indeed an underestimated issue by most.
Having said that, the cycle loading and fatigue you talk about for the standing rigging is actually not really an issue when you’re standing on land. Agreed, there is a huge amount of load cycles over the time span of a winter, but they are so minor in load, that it does not really influence the life span of the rigging itself.
The different kinds of materials have also different kinds of load cycle lifetimes. Most materials in the rigging only age from loads that are more than 50% of the safe working load. Any boat would fall off her cradle far before the load reaches 50% of the safe working load.
Think of the paperclip test. When you bend it 180 degrees for maybe 6 times, the paperclip will break: Fatigue has occurred. When you apply very little bending force, just enough to deflect the paperclip a little, you can repeat that movement till your fingers fall off without the paperclip breaking. This is a load cycle that is not loaded up far enough to cause fatigue.
As a short summary, fatigue will never occur in structures that do not reach certain amounts of stress, i.e. that amount of stress that causes problems depends on the material used.
Whoops, well I got that one wrong! I changed the post to reflect Erik’s correction…and to bury the evidence of my misconception!
More on Fatigue from Eric Klem
Engineer, frequent commenter, and all around smart guy, Eric Klem sent me the following email that really advanced my understanding of material fatigue, so I have reprinted it here with the points that really jumped out at me in bold:
…The best way to understand fatigue for me is to look at an S-N curve for the material (if you really want to bore yourself, a google image search will turn up plenty of examples). It is a plot of how many cycles you can do at a given stress before something breaks. Not surprisingly, the higher the stress, the lower the number of cycles.
It is worth noting that constant stress does not fatigue a part, only changes in stress. This is the reason that it is customary to torque a bolt to ~70% of its yield strength because in a properly loaded connection, the stress is constant even when the load changes (the reason for this is another subject). If this same bolt were loaded to 70% of yield and then unloaded and the cycle was repeated, it would take very few cycles for it to break.
When your boat is sitting there on a calm day, even though there is a significant load on the shrouds, they are not fatiguing because the load is constant. Fatigue is actually little micro cracks starting from the surface of a part and moving in which grow a tiny bit every time they are stressed.
One of the most important things that an S-N curve shows is whether there is a stress level below which no fatigue will occur. If this level exists, it is called the fatigue limit. Steel has a very good fatigue limit of approximately 50% of yield strength, stainless is worse and aluminum basically doesn’t have one.
With all of these materials, it is worth noting that these numbers occur at 10^6 cycles or higher so in most applications you can go over these stress values and not have problems as you won’t get to the number of cycles required to cause a failure.
For your stainless shrouds, the changes in loading when on the hard are likely very low compared to this fatigue limit so my intuition is to agree with Erik. I have never actually looked at the loads so I can’t say for sure. With aluminum masts, your mast will certainly be fatiguing but if you look at the S-N curve, you will see that the cycle’s axis is logarithmic so the number of cycles to cause any issues at those extremely low stresses is so high it isn’t worth thinking about…
The comments to this post are intentionally closed. But that doesn’t mean we don’t want to hear from you. If you click on the”comments” links above each comment, you will be taken to the original so you can add your thoughts. This way the discussion won’t get fragmented. And if you want to comment on Eric’s email please do so here.
If when you click the link you don’t see the original comment, it means that it was made to a chapter of an Online Book you need to be a logged-in member to read.