Adventure 40 Keel—Strength and Grounding Resistance

A few days ago we covered the shape and type of keel that the Adventure 40 will have. Now let’s move on to the strength and grounding resistance of the keel.

The Grounding Problem

Why does this matter so much?

As some wag said:

Cruisers are divided into two groups, those who have run aground, and those who lie about it.

The point being that most all of us will run aground sooner or later. But that shouldn’t be a cruise ender. Sadly, though, in many, probably most modern production cruising boats, unless we are going very slowly, it will be.

How slowly?

The 3-Knot Rule

Maxime, who is in charge of making the Adventure 40 real, has an extensive background and contacts in the French sail-training world where boats get run aground all the time.

The experts in that world tell him, based on bitter experience, that most modern production sailboats are likely to sustain some damage, requiring repairs, in any grounding on a hard object at over about 3 knots.

And in many cases, particularly if the speed of impact is over three knots, the damage will be extensive.

Not good, given that most any decent design will be doing at least that speed with the engine idling in forward gear and probably at least 5 knots under sail, and are we really going to run at 3 knots or less every time we’re near the hard stuff? Not likely.

Difficult and Expensive

So what do these “extensive repairs” look like?


With modern construction techniques that often consist of dropping a reinforcing grid into the boat on top of putty, and then not even passing the keel bolts through said structure, but only through the skin, the damage modalities are very difficult to repair.

For example, a really good repair will usually require removing much of the interior cabinetry to get at the grid, and then days of grinding and expert glassing, followed by reassembling the boat.


It’s hard to see how any yard, no matter how efficient, could take this on for less than US$20,000, and I have seen a full repair of this type that went over US$100,000.

Not Acceptable

Our cruising dreams dashed through a simple mistake that many of us, maybe most, will make sooner or later?

A cruising boat should not be this fragile.

And Dangerous, Too

However, the results of a grounding could be a lot worse than broken cruising dreams. To understand why and how bad this is, let’s look at the Cheeki Rafiki tragedy.

Work done by the investigators, assisted by the Wolfson Unit of Southampton University, in response to the Cheekie Rafiki tragedy, showed it was almost impossible to reliably assess the damage after the groundings the boat experienced, and that this difficulty is intrinsic to the construction methods used on many of today’s cruising boats.

Was that just an isolated incident? No, the danger is built into many production boats:

Wait, it gets worse:

An engineering study commissioned by World Sailing shows that the standards required for keel strength for offshore certification under ISO are inadequate to the point that as little as 5 years of ocean racing or 25 years of moderate cruising can result in fatigue to the point that failure becomes likely, even if the boat has never been grounded—see Further Reading.

Yup, you got it, many modern cruisers are built inadequately from the start, even if certified to go offshore, and are impossible to survey properly for damage.

This situation is so bad that World Sailing recently added regular keel inspection requirements to the newest version (2022-23) of the Offshore Special Regulations. Although, given the difficulty of actually assessing keel integrity in the real world, that’s a band-aid over a gaping wound.

World Sailing are also lobbying ISO to significantly upgrade the keel strength requirements; however, after several years of representations to ISO, it’s still not clear when, or even if, there will be any improvement.

Not Just Theory

This is not just theory. Walk around any boatyard and look at the keels on boats over 10 years old. You will find, as I did, that many, often a majority, are showing signs of movement in the keel-to-hull joint. And once there is movement, fatigue, which is probably already occurring, will be accelerated.

The Adventure 40 Ain’t Waiting

This all sucks, big time, but the Adventure 40 team is not waiting for the regulators to fix this mess.

Maxime has been working on this issue since last year. Here’s a summary of his plan to make the Adventure 40 keel and attachment as close to grounding proof as is practical and strong enough to withstand decades of offshore sailing.

Much Worse With Speed

To understand Maxime’s approach, the first thing we need to grasp is that the impact energy a given structure must survive increases far more than linearly with the speed.

Setting The Goal

That’s scary enough, but Maxime started out with the goal I set of building the Adventure 40 strong enough to withstand a grounding at hull speed without significant damage, and quantified that at 8 knots.

Yes, that’s way faster than any prudent mariner would be traveling around the hard stuff, if for no other reason than that a grounding at that speed could maim the crew, but by using such a high number, in concert with good engineering, we will end up with a structure easily able to forgive more likely mistakes, and also one that will be able to withstand the normal strains of offshore sailing for decades.

In short, setting a high-impact speed goal, combined with good engineering, will yield a keel design with good safety margins regardless of whether or not she is ever run aground.

It’s a Big Challenge

Just how big a challenge has Maxime set himself? Huge. An 8-knot grounding is seven times worse than a 3-knot grounding, all other things being equal.

But just making the boat seven times stronger than most built today is not practical. Way too costly and way too heavy. And, besides, there’s a better way.

A Metaphor That Helps

Think about a glass that drops off a table onto a concrete floor. It breaks. How could we fix that?

One option would be to make the glass really, really thick, but then we would end up with a glass that was impractical to use, and, anyway, the more rigid we make a structure the more vulnerable it becomes to impact forces.

A fundamental here is that to withstand impact force without damage, a structure must stretch, compress and/or bend. Very stiff structures do not absorb shock loads well. Hit a stiff cored hull panel with a hammer to test this…assuming you like expensive experiments.

So a better option is to make the glass a bit thicker and then coat it with a thin covering of rubber. Problem solved.

We Need a Bumper

Same with the Adventure 40. We need a shock absorber as well as building the structure strong.

With that concept firmly in mind, Maxime took the problem to a group of engineering students at CentraleSupélec University, to see if all this could be analyzed, rather than just iterating empirically (a nice way of saying “guessing”) as is often done around boats.

But what material should be used for the shock absorber in the model?

Doing The Numbers

Well, we all know that lead keels deform quite a bit when hitting a rock. Just walk around any boatyard and you’ll see evidence of that.

So let’s see what Maxime and the students found when they ran a lead-keeled Adventure 40 onto an immovable rock at 8 knots in the computer:

Yikes, the front of the lead keel deforms a bunch.

But that’s good news, because in so doing the peak forces the whole structure, including the critical keel-to-hull joint where most damage normally occurs, are subjected to, are much reduced because the deceleration from 8 knots to stopped is spread over a longer period of time, as we can see here:

And, better yet, there are no nasty high peaks of deceleration in the above graph. The deceleration of hitting the rock very quickly produces about 6g and then peaks at 8g.

Not trivial, but a heck of a lot less than if the keel was, say, made of cast iron.

The Breakthrough

And this in turn has allowed Maxime and his team to take a very complex dynamic problem and simplify it to arrive at the maximum impact that the entire keel and its attachment to the hull must be engineered to withstand.

Wait, read that again. That last paragraph contains the gold nugget of knowledge that will make the Adventure 40 better.

Here’s why:

Now that Maxime’s team have calculated peak deceleration from modelling, they can derive the forces the entire structure must be engineered to withstand.

So the remaining work is to design the keel bolts and structure to absorb the resulting forces without damage, an engineering problem that the engineers Vincent and his team will work with are eminently qualified through both experience and training to do.

Heck, we are getting this done in the country that engineers and builds Ultime Multihulls and IMOCA 60s, so getting the engineering right to achieve this goal for the A40 should be doable!

That said, that simplification should not lead any of us to undervalue the work that Maxime and the students have already done. None of the presently used construction standards make even an attempt to quantify and plan for the dynamic forces in a grounding.

And, better yet, a keel engineered to withstand these extreme grounding forces will be many times stronger than that presently required by ISO and/or than required to withstand normal sailing loads.

Not Perfect

So does this mean that engineering the Adventure 40 keel and attachment to withstand an 8-knot grounding onto a hard rock is a solved problem? No, not completely.

First off, to arrive at optimal shock absorber characteristics:

  • hard enough not to be destroyed by the impact before slowing the boat, but
  • soft enough to extend the time it takes for the boat to stop;

the students modelled with pure lead, rather than a lead and antimony alloy, as is usual in the manufacturing of keels.

So the question now becomes, is it possible to build a keel out of hardened lead but with a pure lead bumper at the leading edge? Yes, it’s a challenge, but I for one am betting it’s solvable.

How Much Certainty?

The other thing we need to think about is the accuracy limitations of the modelling done to date, as well as changes in the assumed scenario that will produce higher forces; for example, changing the shape and/or impact point of the rock—there are a lot of variables here.

Realistically, all of this means that the builder will never be able to honestly say, “Run an Adventure 40 aground on a hard rock at 8 knots and everything will be fine every time” (except for bruising of the keel and probable injuries to the crew).

In fact, the 8-knot no-damage target speed may have to be reduced if we find that building the structure to withstand the forces, even with the lead bumper, is impractical. Rest assured, however, that said safe speed will be way more than 3 knots.

But what the builder will be able to say is, “The Adventure 40 will forgive the typical grounding mistakes made by a seamanlike owner and will survive decades of offshore voyaging without dangerous strength deterioration”.

Way Better

That’s a huge step forward when compared to most of the production sailboats available out there today, about which most-everyone just sticks their heads up their…in the sand…rather than even thinking about the likely outcome of a grounding or what condition the keel-to-hull joint will be in after even just 10 years of hard offshore voyaging.

Fix In The Field

Of course, all that leads to another obvious issue: what happens after we have hit the rock and mashed up the front of the keel as shown in the first graphic?

Clearly, we need to think about the way that owners can fix or replace the soft lead bumper in the field.

But let’s not forget that being in, say, Newfoundland with a huge bruise on the front of the keel after a grounding, beats hell out of being in a normal production boat in the same situation with the reinforcing web that all the interior furniture is attached to separated from the hull, and the aft end of the keel driven up into the hull causing a crack that’s letting water gush into the boat.

We can easily sail the Adventure 40 with a mushed forward part of the keel to a yard to haul. And, better yet, the repair is all outside the boat and should be doable while we live aboard—you are not a real cruiser until you have lived aboard for a couple of weeks on the hard.

Worst case we can chisel off the deformed lead—no, it won’t be fun, but it’s doable with normal tools—and fill the divot with epoxy putty. Good enough to continue the cruise or sail the boat to a well-equipped yard for a better repair.

An infinitely better situation than that on a typical modern production boat where our cruise is over and we are faced with a huge repair that will take a well-equipped high-end boatyard with highly-skilled composite technicians months to complete at a cost that could exceed the value of the boat…assuming the boat didn’t sink under us.

The Cherry On Top

And here’s the cherry on top of this analysis. So far it looks like we will end up with a lead keel, which is by far the best option for a cruiser, since lead does not rust and is heavier than iron or steel for a given volume.

Isn’t it ironic that after all this high-tech analysis we end up with the ballast material that we cruisers have known intuitive for decades was best?

This also makes me wonder how much of the increase in grounding damage and failed keels should be laid at the door of the cost-driven change to cast iron and steel keels, that started in the eighties, since these materials are far less ductile and, therefore, impose far higher forces on the boat in a grounding.

All that said, to get the strength and longevity we want it may be necessary to use some steel and/or cast iron in the structure. The point being that we don’t want to tie Maxime and Vincent’s hands while they are engineering the best possible solution.

One More Thing

Given that it now seems likely that the Adventure 40 will have a lead keel, I will—I have not mentioned this to Maxime yet…surprise, surprise, surprise—lobby hard for bronze keel bolts.

My reasoning being that a properly engineered and built lead keel with bronze keel bolts is pretty much a forever-keel (see Further Reading), which is in keeping with a core Adventure 40 principle:

We are building a boat that, with reasonable maintenance, will safely take people voyaging for decades to come.

And, yes, using lead and bronze will undoubtably make the boat more expensive than a steel, or iron, keel with steel bolts, but the benefits in cost of ownership over time, as well as resale value, will far outweigh that.


So after two articles, here’s what the Adventure 40 keel looks like:

  • External bolted-on fin keel.
  • Modern shape with a bulb for performance and better load-carrying capability.
  • 6.5′ (2 m) draft.
  • Likely made of lead.
  • But possibly with some steel.
  • With bronze keel bolts if all lead…if I get my way.
  • Capable of withstanding the grounding impacts that a seamanlike owner may subject the boat to, with only damage that’s external and relatively easy to repair when out cruising.
  • With a keel-to-hull joint many times stronger than modern practice or current offshore standards require.

Or, to put it more simply, a real offshore cruising boat keel designed, engineered and built to be fit for task in the real world. It’s the Adventure 40 way.

Maxime’s Paper

In the interests of brevity I have summarized the living hell out of Maxime’s work, leaving out a lot of really interesting stuff. And I could easily have made a mistake. So if something is wrong in the above, it’s almost certainly my fault, not his.

Anyway, I highly recommend reading his original paper. See Further Reading. I found it fascinating and much easier to understand, even though I have no engineering training, than I expected.

Maxime’s ability to explain complex engineering in simple terms is amazing. Doubly amazing when we remember that English is not his first language—bodes well for the future of the Adventure 40 with him in charge.

Further Reading

On keels:

Supporting papers:

²The study focuses on high-performance race boat keels built of welded steel structures, so some would argue that it’s not relevant to the boats we sail. I don’t agree. The ISO rules are not scantling rules that specify how a boat is to be put together, but rather rules that specify what forces and usage the boat must be able to withstand. So if said rules allow a boat to be built and certified as fit to go offshore with a keel that can drop off due to fatigue alone, with no groundings taken into account, after just five years of hard racing, or 25 years of cruising, surely we should not be trusting our loved ones to the same regulation regardless of how the keel is built? Fibreglass structures are subject to fatigue, too.


If you have questions, please leave a comment. The answer may be way past my pay grade, but Maxime will be here.

If you see an error in my interpretation of Maxime’s paper, let me know, but read Maxime’s paper first to make sure it’s not just my effort to simplify that’s bothering you.

Also, if any of you engineers who comment here at AAC have corrections or suggestions for improvement, Maxime and I are all ears, although do keep in mind that it’s early days yet and much more engineering work will be done by Maxime, Vincent, and the team in the weeks and months ahead.

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