The Offshore Voyaging Reference Site

Cycle Loading—8 Tips for Boat and Gear Purchases

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I really enjoy running this web site. In fact, more and more each month. And one of the things that I have enjoyed lately is the participation of trained engineers like Eric, Chris, John and AAC Technical Correspondent, Matt Marsh. (If I have missed anyone, please speak up.)

The cool thing about these engineers’ participation is that I am now understanding, as never before, many of the things I have observed in my 55 years of messing around in boats.

So I thought that it would be interesting to relate some of my own experience to Matt’s post on cycle loading, and then derive some tips for boat and gear buying from the result.

If you have not read Matt’s post, please do so now, otherwise this post won’t make a lot of sense to you. Don’t worry, I will wait right here while you do.

Back? Great. OK, let’s look at what we ocean voyagers can learn from Matt’s post.


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Matt Marsh

Nice summary, John, and I generally concur with your recommendations. A few further thoughts, though:

“Avoid the Seventies”.
A valid point, but there were some good boats from that era and there’s plenty of equally bad (or worse) stuff from the ’80s, ’90s and even today. As scary as it is, David Pascoe’s famous rant Are they fibreglass boats anymore? is still very much valid: there are many cases where we do not have the slightest flippin’ clue what the hull is really made of.

Fatigue, particularly in aluminum.
When designing in metal, you’re limited in some cases by the yield strength of the metal (the stress at which it deforms without springing back) and the ultimate strength (the point where it breaks). (In many other cases, you’re limited by stiffness, not by strength- an important distinction, but not necessary for this discussion). If you repeatedly load and unload the metal to a significant fraction of its yield strength, those strengths will gradually decrease.
For steel, they eventually stop decreasing, at about 50% of their original values, after about a million to ten million cycles. This is the “fatigue limit” or “endurance limit”. For aluminum, they never stop decreasing, but the decline slows down dramatically and is pretty small beyond ten million cycles. With either metal, you can work around the problem by designing the part to be strong enough in the fatigued condition- i.e. by designing around the fatigue strength, not the original yield strength.
Aluminum’s big problem is that its composition and microstructure change quite dramatically in the region around a weld. A weld is already a stress concentration, and now we’re weakening the material in that area. That’s a recipe for rapid fatigue and, eventually, the formation of cracks. You can work around this problem, but it requires a co-ordinated, professional effort:
– The designer must thoroughly understand the load paths and be careful to avoid anything that would lead to a stress concentration, excess flexibility or vibration.
– The material supplier has to have strict QC to ensure that the individual components are of the correct composition and temper.
– The welders have to be really, really good at their jobs, following the proper weld sequence and being very careful to ensure good weld penetration without causing heat damage to the surrounding material.
There are a lot of aluminum boats that last darn near forever, and there are a lot that fail in their first few years- the difference usually boils down to fatigue, which can be prevented with proper design, well-trained labour and good QC.

Matt Marsh

I can’t weld aluminum- heck, I have a hard enough time with flux-core on mild steel. I have a lot of respect for anyone who can do good clean work with a MIG or TIG torch.

Flaws in aluminum construction are, I think, likely to reveal themselves within the first year or two of hard service. You do occasionally hear about more insidious vibration and fatigue issues in big multihulls that turn up later, but those are becoming less and less common. And any surveyor who knows aluminum shouldn’t have too much trouble recognizing the early stages of failure, provided she can actually get to the components in question. (This latter bit is, of course, often more than a little tricky.)

Marc Dacey

I could tell you very similar horror stories involving freshwater mid-30s sized yachts from 2008 and 2011 respectively. Big boat-show brands both. Both had defects that put them out of the running, even for farting around in Lake Ontario, for a friend of mine who has both the desire to own a nearly new boat, and the experience to tell when they are insufficiently well-built.

I used to dislike most production boats of the last 20 years in a vague way. Lately, I can get appallingly specific on why so many of them are eggshells held together with snot.

Chris

I’m about to run out of my $10 for 24 hrs WiFi, so I’ll keep this short. When it comes to sizing gear, remember it has to function “en suite.” A properly sized block, needs a properly sized bail, needs a properly sized bolt, needs a properly sized backing plate, etc.

We see a lot of folks who have up-sized their gear and just moved the failure point to a new location. One glaring example last month was a boat that had gone from a Lofrans Windlass to a Lighthouse and they had just bored new holes in the old backing plate because they didn’t want to mount the windlass where there was adequate structure to transfer the loads to the hull. Quote “It would have been a tripper.”

Chris

My Dad was an engineer. His intuition was far more valuable than his education. He was a mechanical (refrigeration) engineer and became a systems integrator on Gemini, Apollo, Apollo-Soyuz, Skylab, and Shuttle. He could intuit when designs were inadequate. Much of that came from breaking things. He used to tell me that nothing was quite so instructive as testing something to destruction — a lesson learned in WWII.

In our area of endeavor, testing to destruction might happen with anchors and such, but the big ticket items don’t get that treatment — it’s just too expensive. He once told me early adopters of any technology must be willing to lose everything because time to market pressures always resulted in often deeply buried, possibly fatal flaws. The history of engineering is chocka-block with examples.

Chris

Indeed!

In design meetings, I used to chuckle-laugh-snort when someone said a field item had been rugged-ized, I asked them why had they created something that needed to be. The answer was usually blather.

Dick Stevenson

John, A very interesting article. And I very much agree with you in appreciating the flock of participants who contribute so greatly to AAC. The above makes one have second thoughts about the light weight boats being turned out nowadays. I have thought, in the higher end boats, that light weight and longevity could be accomplished through good engineering and design. Now I am not so sure. I wonder what the hull life is on some of the cored hulls which must flex some, even in the best designed boats. This is certainly an area that I have never seen addressed in any publication.
My best to all, Dick Stevenson, s/v Alchemy

Jacques Landry

Good post John.

Reading your comment : “you must, as Matt so clearly explains, engineer it to be at least twice that strong if it is going to survive the ravages of cycle loading for a good long time” makes me laugh a little! Not that I don’t agree, believe me I do! But I am an engineer as well, and it use to be “at least three times as strong” as a safety factor. That is what the American auto industry was doing (as an example) until the Japanese got onto their turf. Then, to be competitive, they started calculating more precisely what safety factors were required, and it became much smaller with the years, and “finite element analysis” became the preferred tool to reach that goal (since empirical experimentation is too expensive)!

So that “finite element analysis” is what is done for your car, and you’re driving it! But you are driving it in the conditions it was designed for, respecting the rules of the road, and not overloading it! So maybe the boat designers are actually doing a good job, for the average Jo who buys his Jeanneau to sail the local lake. It is the wakos like you (and me, and many others reading these posts) that are demanding too much of our boat maybe! Going out at sea in a storm !? What are you thinking !

Oh yeah, that’s true, they have the “oceanic” models, one would think it means more than “salt water tolerant” 😉

In short, what Matt is saying, is that after all this hard work studying to become an engineer he recommends a safety factor of 2 !? Glad to see that some young engineers still have some common sense, but that will be hard to sell to boat manufacturers !

And be careful with tip #7, “avoid the seventies” as a lot of your readers would not like to be avoided

Matt Marsh

Thanks for chiming in, Jacques. I certainly agree with your point about FEA- as we gain a better understanding of the forces involved, it’s possible to make more efficient use of material. (And if I’m going to experience a car crash, I’d much rather be in my FEAed-to-the-max Hyundai than in a ’70s vehicle of comparable weight and size.)

Just to be clear, I am emphatically not recommending that 2.0 is the correct safety factor to use.

My point was that if a part is intended to survive extensive cycle loading, the design calculations should be based on the fatigued strength of the material after 10^6 to 10^7 cycles, which for many common boatbuilding materials is roughly half of their new-condition strength.

The “safety factor” (I hate that term, really… “factor of ignorance” would be more apt) is supposed to account for all the unknowable factors. Things like overloading in an emergency situation, a few inches of sub-par welding, unexpected shock loads, etc. I’ve seen this figure vary from 0.9 (some mining operations) to nearly 10 (propeller shafts on heavy displacement motoryachts), depending mainly on how well the various forces are understood. But I don’t think it’s appropriate to include fatigue in the standard factor of ignorance, as fatigue is well understood and quantifiable.

Joe Casey

It would be interesting to hear comments about the use of epoxy to construct ‘grp boats’. I know a couple of builders (power and sail) have gone this route. Cost aside, how does epoxy cycling and fatigue limit compare to other building materials?