Adventure 40 Keel—Strength and Grounding Resistance

If you go cruising for long enough, sooner or later this will happen. Luckily we were moving very slowly when I pulled this stunt in the Bahamas, but that's not always the way.

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?

Difficult

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.

Expensive

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

Typical Beneteau First 40.7 keel bolt arrangement. Image from the report on the investigation of the loss of the yacht Cheeki Rafiki—UK Crown copyright.

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.

Filling this "keel smile" with goop changes nothing. This keel is almost certainly unsafe, or at least heading that way with further use, particularly offshore. A major repair is required.

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?

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