It's often said that offshore sailing is much harder on the boat than lake or inshore sailing. Surely, we can quantify that statement and determine whether or not it's true.
Spoiler alert: It's true. Oh man, is it ever true.
Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
by Matt Marsh
Reading Time: 5 minutes
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- Offshore Sailboat Winches, Selection and Positioning
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- Pitfalls to Avoid When Buying a New Voyaging Boat
- Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
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- US$30,000 Starter Cruiser—Part 1, How We Shopped For Our First Cruising Sailboat
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- At What Age should You Stop Sailing And Buy a Motorboat?
- A Motorsailer For Offshore Voyaging?
Matt, Engineering Correspondent, is a Professional Engineer and true renaissance man, with a wide range of expertise including photography and all things boat design. He has a unique ability to make complex subjects easy to understand and he keeps an eye on the rest of us to make sure that we don’t make any technical mistakes. Working as M. B. Marsh Marine Design, Matt designs innovative powerboats of all shapes and sizes.
Good analysis Matt.
Being an engineer I have to agree with your approach and conclusion, but I know some out there will comment about a few points you did not mention!
To start with, the “abuse” experienced by the club racer might induce higher stresses than that from some ocean crossing conditions (ok, with reefed down sail and common sense), as the boat will be sailed over-canvassed and to the limit most of the time. You will also have several heavy sailors running on the deck at all time and that is quite dramatic for most balsa/honeycomb/other laminates as well as for the whole rigging. This combination must have an effect as the boat will be driven close to breaking all the time. But I have to agree that this would not be apparent in a simple “hours of usage” summation. Some will say that an ocean going vessel will carry more “cargo”, but 8-10 250lbs sailors on a light 30 footer would be about equivalent!
The Great Lakes will also present a few other challenges. The wave period being way shorter combined with too much sail will be tough on the rig. The engine not being used often might not be a plus, as we know that diesel engines like to run, or at least get to their operating temperature before being shut down. And finally, winter! I am sure you have looked at the effect of freezing temperatures on all parts of a sailing vessel, with different thermal expansion coefficients for different parts of the vessel while most will lay them down fully rigged over winter. And all that water that expand when freezing in all crevasses, holes, and engine “voids”.
The final conclusion with all of this taken into account will be just the same, but might not be as black and white!
I personally would still rather buy an ocean going vessel that has been well kept (John, call me when yours’ for sale !) than a club racer that has been abused by “racers” who don’t really care about how long this thing is going to hold up together!
Matt, Lovely essay and such fun for me to read your analysis where you bring your engineering skills and knowledge to a subject that all of us are aware of on largely an experiential and subjective manner. Although it makes no difference to your theme, I find open ocean waves/swell periods in the 10-15 second area rather than 2-3 seconds but perhaps we are referring to different things. I also agree with much of what Jacques writes. Competitive pursuits often lead inevitably to abuse as Jacques well describes which seems different from the inevitability of the extra-ordinary challenges a boat faces on ocean passages. Having just finished a 2 1/2 day crossing of the North Sea, I am also reminded of how hard work I generally find open water passages are for people as well, even relatively benign passages as ours was.
Dick Stevenson, s/v Alchemy
Re. wave periods.
There was a fair bit of discussion on this going into the post… it is hard to find good statistics on average wave periods, although there’s plenty of data on height and spectrum.
The numbers I ended up using are a very (very!) rough approximation to what one might expect on the Newport-Bermuda passage, with a bit of messy stuff in the Gulf Stream and with some weather nearby. A real wave system is a superposition of several components, including (and sometimes dominated by) the long-period swell.
I think the important take-away point from this is that if you know a boat is going to see cyclical loads numbering in the millions, you have to design everything around the fatigued strength of the materials, which is often about half of the new-condition strength you find in material property tables. Any factor of safety / factor of ignorance, allowance for dynamic or impact loading, etc. starts from this point, not from the new-condition strength. If you know that the part will be rusted, outdated or broken due to abuse before it’s seen enough load cycles to become fatigued, this particular failure mode may not be a concern.
Hi Dick, regarding your North Sea passage……. from where to where did you do it and generally speaking how was it?
My future plan, from Galedonia Canal to Norway.
Txs, Ben (Freedom 32 “Christel” of Lunenburg, NS)
Nice write-up Matt. A good reminder of courses I’ve had during my instruction. For those who love books that send you to sleep, Lloyd’s “Seakeeping” gives great information on determining the effect that various sea states have on vessels. Rather technical, but comprehensive. I think it’s available as a Google book as well…
The issue is certainly a long-view one; I just spent two days on the Chesapeake that undoubtedly loaded the boat in question great than five weeks of Pacific Ocean sailing with several squalls did. But in the long run, a skipper’s choices add up. Additional cyclic stresses that a boat experiences: wind loading (obvious, but there & a factor in sail life), thermal (depending on your location & seasons), and rudder systems.
I just thought of another “cycle loading” aspect : In a weekend club race, there might be more tacks and gybes (with the inevitable mistakes) than during a 2-3 weeks ocean crossing !
But don’t read me wrong : this is a very nice analysis and I agree with it, it just you are still young so us old salts need to put our 2 cents in 😉
Jacques, you are certainly correct that it’s not a “black and white” comparison, and that a club racer is subject to some loads that an ocean cruiser might not see (e.g. lots of winch gorillas running around on deck at every tack).
The point about running a racer over-canvassed and near the limit, with rough tacks and the occasional crash gybe, is an important one worthy of further emphasis.
If we are racing, pushing a boat right to its limit and sometimes doing something stupid, we expect that our actions will produce loads near or in excess of what the boat was designed to handle, and that we may break things as a result.
If we are on a long cruise, trying to make sure that we go easy on the boat and don’t overload anything, we do not expect that we will be loading any critical hardware to its limits. But, because of the fatigue that results from loading and unloading each component a million or more times, some of that critical hardware could be only half as strong as we think it is.
It is this insidious nature of material fatigue that makes it a risk. If a part wasn’t designed with fatigue in mind, repeated use- even at loads well below its intended strength- can gradually weaken it. On an inshore boat, other factors (corrosion, overloading, damage, wear, etc.) are likely to take effect before fatigue becomes an issue, but in continuous offshore duty, it’s possible (even likely) for a part to become fatigued before it’s old enough to show obvious signs of deterioration.
Matt, my experiences with a 40 year old lake racer and my “never seen salt water” pilothouse cutter would support your theses. The racer, despite being sailed hard and overcanvassed (by myself for the last 14 years..I’m the fourth owner) had its original standing rigging for 39 years (1/4 inch 7 x 19 with Merriman terminals/turnbuckles).
I’ve removed it and bought new standing rigging as a 40th-birthday present “just because”. While it still looks fine, it’s cheap insurance. The fact that I’ve had to retab bulkheads and furniture make it clear that the boat has and will continue to flex, especially as I like to make it go fast. The rigging is hardly exempt.
The steel cutter, by contrast, has 11 5/16ths with Staylok terminals on a similarly sized mast as the old racer: The whole rig is “overdone” by comparision and has, like the 39-year-old stuff, no visible wear. But it too is original to 1988 when the boat was splashed and so before we leave for the ocean, I will “roll back the odometer” with all-new standing (and running) rigging, due to concerns exactly as you’ve expressed. Many of the items I am replacing or upgrading are getting a better and larger set of fasteners based on the same logic. A related habit for the ocean-voyager is to examine the deck each morning for evidence of popped pins or bits of line or metal where they shouldn’t be. They say rust never sleeps, and they are right. Less obvious is the invisble (at first) damage of what you are callling “cycle loading”.
A very good reminder to both lake and ocean sailors alike.
Anybody want an half interest in an (Adventure) Swan 48? All you need to do is charter a helio to fly me out to her. I’ll bring some line for temporary forestay and backstay, a spare hank on jib and a tool box and sail her back.
http://www.wavetrain.net/news-a-views/463-come-and-get-it-free-swan-48-available
re our current discussion: Amazing testimony to the strength of a carbon mast. Still standing after months at sea with no forestay or backstay.
Now, if I was not still on a cane…
Hey John,
Do you need a cane to sit in the helio as a spotter?
Richard
Don’t get me started. I would very much enjoy a salvaged Swan between, say, 48 and 53 feet.
Ben, Happy to share my passage notes for the North Sea: this year across to Germany, last year up the coast to the Caledonia Canal, your departure point. Not sure its so pertinent to this stream so happy to take it off site unless John et al want me to post here. Feel free to send me your address to Alchemy128(at)aol.com.
Dick Stevenson, s/v Alchemy
Matt, Your lesson is very important: Millions of small vibrations can harm or destroy a good ship. It is important to reduce the vibrations to a minimum or take them away if possible. In my practical life at sea I have seen many examples that confirm your opinion. A small adjustment can do great improvement, but it is also fantastic how much beatings some particles can take before they brake down. Modern laser measurement can contribute to great improvement, but also an observant, listening sailor can be valuable. Be aware of the small vibrations, they can be important in the long run!
are not erik’s comments as shown in today’s ‘comments of the month’ posting saying the same as the above e-book excerpt except erik’s terminology is much more technical ? i just want to be sure i am not missing something in erik’s comments that is not covered by this excerpt, which i believe i do understand although it too is somewhat technical but is also logical and less technical…cheers
richard in tampa bay (but w/n about six weeks of returning to the virgin isles area for a while)
Hi Richard,
Absolutely, same thing said a different way. Having said that, I found Eric’s and Erik’s comments helped my understanding along several notches.
One of the key ah ha moments that I got from Eric Klem is that these curves are logarithmic, which explains why boats built of aluminium last for so long and stay so stiff, even though the material has no fatigue limit—this seeming contradiction had confused me, now I get it.
Hi Richard,
My apologies if my other comment was too technical. I’ll try again in different wording:
All metals are different in their characteristics, but react in a very similar way when it comes to fatigue (with the exception of Carbon Fiber, which is technically speaking a metal as well). Lets take stainless as an example here.
We have a piece of rod, lets say a cap shroud. It is designed to have a 15,000 lbs breaking strength. The load that the cap shroud will feel is determined by the stability of the boat. Waves will make the boat roll, technically speaking changing the heel angle all the time, and therefore the righting moment of the boat with it. So basically with every wave passing, but also with every wind gust passing, there is a load increase and a load decrease on the cap shroud. If the loads are in the vicinity of the designed breaking load of 15,000 lbs, the shroud will most likely fail within the first few minutes out at sea. When the load is let’s say around 10,000 lbs, the shroud might survive a trip to Bermuda and when you’re lucky back as well. Now when the load cycles do not reach about 50% of the breaking strength, so 7,500 lbs in this instance, fatigue is unlikely to occur at all, regardless of the mileage one puts on his boat.
Load-unload cycles are weakening metals when the loads are too high. This is the reason why there are always Breaking Load Limit (BLL), as well as Save Working Load (SWL). Depending on the application, SWL could be half of BLL, but when the piece of equipment is subject to shock loads (like an anchor chain for example) the difference between SWL and BLL can be as much as factor 5 or 6. The main reason for indicating a SWL on equipment is to prevent it failing prematurely due to fatigue.
This is the reason that you can easily over load equipment when doing club racing like Matt describes, but the same equipment on the same boat offshore would fail potentially on the first trip, and failure can even happen on a bright sunny day.
So when selecting equipment for an offshore going boat, SWL is almost a holy number for me, but when selecting equipment for a boat that will never go offshore, often a size smaller can be safely chosen to reduce cost of building/refitting.
You can easily test this yourself with a paperclip. When bending it, you’re pushing it past its limit as it has changed shape. You can only do that a hand full of times before it breaks. But when you deflect it without bending, it can take an almost infinite amount of cycles and you will be tired before the paperclip gives way.
Hi Erik,
Great explanation, thank you. It might be my aging brain, but I do find that reading explanations about this subject from the three of you, over time, even though there is some repetition and overlap, really helps to advance, and then lock in, my understanding of this vital subject.
Now you can go back to running finite element analysis on the A40 keel, he says cracking the whip! 🙂
thanks erik and john…i just hope all those who build sailboats, particularly the production line boats, understand these dynamics and allow for them…my gut feel is for the most part they give these dynamics short shrift, but i would think any knowledgeable buyer could specify ‘oversized’ rigging if he or she wanted that although most production-boat buyers are probably not that knowledgeable…for instance, really need a magnifying glass to properly assess condition of rigging…it’s the hairline cracks that give away the wearing but that are easily overlooked by the naked eye…cheers
richard in tampa bay
If I could continue along the thread of Eric’s excellent description of fatigue characteristics of materials by pointing out that fiberglass is not a monolithic material like aluminum or other metals, but a composite. Without the resin component to stabilize the individual fibers, fiberglass cloth, or for that matter kevlar or carbon fabric has no stiffness. Thus in common fiberglass boat construction you find a material— polyester resin— with strain to failure characteristics that in layman’s terms could be described as brittle— combined with fibers of glass with very different strain to failure characteristics. The result is that when a fiberglass panel is repeatedly flexed to the point where it experiences fatigue it progressively weakens through micro-fracture of individual glass fibers that are unequally loaded due to differential adhesion to the resin, differential load orientation, or micro-fracture to the brittle resin. Add a core to the structure and you introduce a whole additional set of fatigue variables.
Bottom line is that in designing fiberglass structures one needs to engineer to avoid loading to the point of fatigue but also allow for progressive deterioration of the composite material itself if it does experience cyclical loading.
As a footnote, wood is one of the best materials on the planet for surviving millions of cycles of loading. Engineered by nature to bend with the wind, with millions of cellulose honeycomb structures. Of course it is nowhere as near as stiff as metal or glass—.
Which brings us to another observation on poor composite engineering. How many times have you seen a boat described in glossy ad copy as being “Kevlar reinforced” for greater impact resistance? Brings to mind bulletproof body armor, movie heroes overcoming the bad guys while plucking bullets out of their chests and the like.
Kevlar has very low elongation to failure characteristics (high stiffness) and is several times as strong as glass in tension but actually weaker in compression. So what is the effect of adding 10% Kevlar to the outer skin of your new 50′ Yuppie Magnum? When the hull flexes to the point of loading the entire load strain is taken by the Kevlar with the other 90% of glass fibers merely along for the ride. Since you saved money to buy the Kevlar by using polyester resin the Kevlar soon breaks or loses adhesion to the resin because it is very slippery stuff. So you are left with a 80-90% laminate—– but the ad guys are happy.
Adding to the discussion re: engines, I think there is more to be said. It looks like you have calculated fatigue on the basis of one cycle per engine revolution. I am not familiar with propeller drivetrain loading, but with a 2:1 drive ratio and a 2 blade prop, I presume that is blade pass frequency, which is a thing, but I would think VERY low load fluctuation relative to the nominal driving load. Blade pass frequency is important if the blade passes a fixed obstruction or wall on every rev (maybe the propeller support/skeg provides this?). But the obstruction normally needs to be very close to the blade (a few mm) to make this loading significant. The other occasion where this frequency/loading is important is in driveline imbalance. In summary, on a balanced drivetrain, I don’t see this as a critical loading scenario. Open to correction here.
On an engine, the fatigue loading is primarily of two types, denoted as high cycle fatigue and low cycle fatiue (HCF, LCF). HCF is based on the firing event which loads the piston, cylinder head, rod, crank throw, etc. and occurs once every 2 revs (per cylinder) on a 4-stroke engine. In your scenario, this loading accrues 1M cycles in 21 hours so… you’re getting there very quickly and everything exposed to that loading is (should be) designed for infinite life (i.e. loads are below the fatigue limit which as you note is typically ~50% of ultimate strength) regardless of application. Even the freshwater racer is reaching 1M cycles in half a year! More than 1M cycles is pretty much irrelevant, so in terms of HCF, the two applications are basically equivalent. (Some will argue for 10M cycles as infinite life but this is largely an academic discussion since, as you noted, it’s a logarithmic relationship, so the stress limit to reach 10M cycles is only a few percent below the stress limit to reach 1M cycles–a smaller difference than the error stack up of calculating most of these loads.)
The other cycle of interest is the one that probably breaks the most engine parts: LCF. These are the thermal cycles from a cold engine to a hot engine and back. They apply not only to each cycle from a stopped engine to an engine running full power, but to a lesser extent, from an engine running low load to high load. For a diesel engine, this lesser cycle is accompanied by a change in exhaust temperature on the order of 200 to 700 deg C. This loading affects exhaust manifolds, cylinder heads, pistons, turbos (if so equipped) and other parts. These cycles are primarily responsible for thermal fatigue but speed-sensitive equipment may also be affected. My rough calcs indicate that for either freshwater boat or ocean cruiser, it would take over 1000 years to get to a million thermal cycles. Therefore, it is not necessary or desirable (and sometimes not possible) to design to infinite life with regards to LCF. Meaning that parts are NOT designed to infinite life in this regard, making the life of the components much more sensitive to the frequency and depth of cyclic loading.
Regarding LCF, the two applications are much closer, depending on usage. A true offshore boat may go a week at a time without starting the engine and then (when offshore) would tend to run the engine for a longer period of time (a day or two) at relatively constant load. By comparison, the freshwater racing boat likely uses the engine twice on any day that it sees use, likely accomplishing a full cycle each time (off to full power). With your example of the freshwater boat operating 40 days/year (20 races + 10 weekends), I get 120 LCF cycles per year. For an ocean cruiser, I assumed 120 days/year usage and a cycle every 3 days on average which comes to 40 LCF cycles per year or one third the amount of the freshwater boat. Of course, YMMV (your mileage may vary) depending on your actual value for those parameters, but I think it’s clear that the ocean-going boat is not necessarily a lot worse on a LCF basis.
Not to mention those of us who don’t live in the frozen tundra and can use their boats all year! 🙂
Anecdotally, this reflects what I noticed when I was in a race-positive boat club…the smaller boats with turbo-diesels that blasted off with big crews for weeknight racing all summer (often redlining the engine to get to the line) would have audible (as in swearing) mechanical issues after a few seasons. I usually wait until our cruiser’s diesel is showing about 60-70C on the temp gauge before applying load, and I also tend to leave dock or mooring leisurely until getting up to 75% WOT (about 1800-1900 RPM in our case). Then I run for at least one hour. I rarely alter the throttle as if we are motoring, we’ve factored in a cushion for arrival that allows for current. Of course, if the wind’s good, we just sail.