The Offshore Voyaging Reference Site

Impact Resistance—Two Collision Scenarios


In Part 1 we looked at the impact resistance of several common boat building materials. Now, in this post, we will examine what happens in a grounding or collision and what proper design can do to ameliorate the damage.

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Hi Matt. Interesting article. I understood the mechanics of grounding a fin keel but not the impact on the bow of the boat. I have an older Hallberg Rassy with a 3/4 full keel. The ballast is encapsulated and fills the front of the keel and the aft section of the keel contains a stainless fuel tank. What could we expect to happen to the structure of the hull in a full speed grounding of the keel?


For those of you who have not seen it yet, this crash test of a Dehler 31, shot in 1998, might be of interest (in german):


Cet article m’ intéresse particulièrement car j’ ai talonné 3 fois, une fois sur un Beneteau 32 et 2 autres fois sur un HANSE 400 suite a 2 pannes moteur par défaut d’ alimentation gas-oil ( réservoir obstrué)
Etant ingénieur de formation je recherchais les données de calcul de la jonction coque quille . l article de Morgan cloud tombe a point,
le calcul de résistance des matériaux note ment due au au cisaillement des tiges filetées ( boulonnerie) est intéressant et difficile a évaluer, l’ on peut toutefois prendre raisonnablement pour référence une vitesse de croisière de 7 nœuds
le nombre de boulons est important appliquée sur la surface de la semelle
Les pression et charges se repartissent proportionnellement sur le carre de la surface ce qui bien évidement modifie le calcul ainsi que l epaisseur et la qualite du materiaux
Ce calcul est complexe et spécifique a la construction du bateau, la résistance au choc sera donc variable
Je n’ ai malheureusement pas les données de construction de mon bateau ( HANSE ) et donc, savoir la capacité d ‘encaissement de la jonction quille coque
Je peux bien entendu le calculer mais qu ‘ approximativement,
Mon bateau sera prochainement expertisé dans le litige qui m’ oppose au constructeur HANSE aussi cet article technique de bon niveau est particulièrement précieux, a ce titre je remercie vivement Morgan Cloud de la qualité technologique de cet exposé, c’ est un point important que tout acquéreur de voilier doit connaitre pour naviguer en sécurité
Merci a tous
Cordialement D FAIVET

Dick Stevenson

Thank you, this is a very interesting and under-reported/under-considered topic.
When based in Mamaroneck, NY, 15+ years ago and hanging around my boatyard I had occasion to observe 2 boats brought in which had smacked one of Long Island Sound’s many rocks with the leading bottom edge of their keel. If memory serves, both came in under their own steam, one weeping a bit of water. They were both late modal 40 foot modestly priced boats that you see everywhere on the East Coast and both looked fine to casual outside observation. I was gobsmacked to hear that both ended up being considered total losses: insurance write offs. To do the repairs by a boatyard to the hulls/ modular interior exceeded the present value of the boats. Later discussions with surveyor friends indicated that this was not an uncommon outcome to a hard grounding/rocking.
I would add to your list of suggestions to survive a grounding an unusual design characteristic which my Valiant incorporates. That is to have the bearing surface, keel to hull, (this is for externally hung keels) not parallel to the waterline but raised on the forward end and lower on the stern. This would mitigate the shear stresses on the keel bolts and distribute the grounding loads into the hull much more evenly (less lever action from the keel). (also contributes to a better sump.) An engineer could describe this much better, I suspect, and determine the best angle for maximum protection. I also suspect that the engineering could determine how much difference this simple design shift actually makes. From my intuitive take, when visualizing a grounding when the boat is out of water, I am thinking that it would make a big difference.
As always, interested in your thoughts,
Dick Stevenson, s/v Alchemy

Dick Stevenson

Dear Matt,
Another thought this time pertaining to a collision with an object hitting the hull rather than the keel.
Below the waterline, when stripping the bottom paint years ago, I had the yard grind down the leading edge a bit (waterline to keel) and add a Kevlar leading edge. My boat is not ever going to have a watertight bulkhead, but that gave me a bit of added security. I worried about a collision above the waterline a bit until I went, DUH, I have a bobstay, which, if it hits a container or something, will certainly ruin my day, but will go a long way towards not ruining my hull. It was a relatively easy project when done at a time the bottom paint is stripped already.
Again, I am not sure what protection I have gained from an engineering standpoint, but casual research indicates that the Kevlar will resist impact better and distribute loads rather than crushing immediately.
Again, always interested in your thoughts,
Dick Stevenson, s/v Alchemy

Dick Stevenson

Matt, In my world, I have known many hundreds, perhaps thousands of cruising boats and I can think of very few who have water tight bulkheads and all have gone well out of reach of the Coast Guard. I suspect that the vast majority of sailboats crossing the oceans have no watertight bulkheads. Perhaps you are wise beyond my knowing, but watertight bulkheads would put too high a bar for me to adhere to for going wandering afield. Ice fields maybe, but not beyond CG range.
Dick Stevenson, s/v Alchemy

Colin Speedie

Hi Dick

In principle I’d agree with you. Watertight bulkheads, while a great idea, are not necessarily essential except for really serious cruising – perhaps in ice, as you suggest.

But I bet if they became easy and cheap to install at build (as Matt suggests), then there would be a steady acceptance that they were a good idea. Our Ovni has an extra watertight bulkhead forward and the extra cost was minuscule in aluminium, basically just filling in the hole in a ring bulkhead. As such it wasn’t difficult to specify the addition, and on a bad night, careering along we’re very glad to have it. We’ve seen enough stuff out there to know that one day it might come in useful.

But retro-fitting, or even installing from new on a series GRP production boat would be prohibitively complex and costly, and faced with that choice we might have thought differently.

Best wishes


Dick Stevenson

Colin, Agreed and well put. For me, on a bad night at sea, I find my anxiety level is a set amount and that I just fill in the blanks as to what content to be anxious about. Dick

Erik de Jong

Hi Matt,

Great article again!
I would like to add one important thing when it comes to grounding forces on the keel though, and that is the influence of the aspect ration of the keel itself.

When a shallow but longer keel hits the ground, just like you have sketched it, the vessel will have a shorter ride-out distance. This will result in a higher deceleration, and therefore a significantly higher force. This force will also be more a shear force along the hull, rather than an up and down force at the ends of the keel-hull attachment.

When looking at a deep and short keel, there is practically no shear force at the keel-hull joint, and, as you mentioned, a rather large force up and down at the ends. However, studies have shown that when one grounds a vessel that has a deep keel, the hull will travel a significant longer way before coming to a full stop, resulting in smaller deceleration forces.

Both loadcases are so different from each other, that structural keel design is a rather complicated part of the vessels design. All materials behave very differently in shear.

The ride-out distance of the hull (due to the bow down dive), is about the same as that the keel is deep under the hull. To get to a free-body-diagram, the travelled distance of the centre of gravity and the mass of the vessel in its entirety will determine what the total deceleration force will be. In general, boats with deeper keels are lighter, because they need less ballast. This also reduces the forces on the keel during a grounding.

And as per request:
We have run aground as well (unfortunately a bit too often I must admit).
Mainly due to badly or non-charted waters, sometimes for not paying enough attention.
One particular case remains very clear in my mind. It was the first unplanned grounding of our current boat. We were sailing along quite nicely somewhere in Norway, doing about 8-8,5 knots of boat speed, about a knot of current with us, and all of a sudden there was an incredibly loud bang, a shaking rig, the foredeck underwater, and I was glued against the wheel for a few seconds. We ran aground, and bounced right back to where we came from.
I designed and built the boat myself at a somewhat young age, so that was a scary moment of truth where the words: “what the heck were you thinking” shoot right through the back of my mind.
The boat weighs about 22 tons in “expedition” mode (@50′ hull length), and has a draft of 10′. It was one of our first shake down cruises with the boat, and we had barely any interior. What we had was not more than some furniture from the Ikea that was screwed down to the floorboards. All we owned was laying in big piles against the watertight bulkheads. I had never seen such a mess on a boat!
Of course, my first thought was the keel structure, but after lifting the floorboards, it became apparent pretty quickly that nothing serious had happened, because there was no water coming in, everything looked normal and all welds seemed intact.
Later the boat was taken out of the water for inspection, and besides an egg-sized dent, there was no damage to any part of the vessel.
A few years later, I made an FEA study of this grounding case, and that confirmed that nothing should have happened to the structure. This gives me a very comfortable feeling I must say.

Best Regards,
Erik de Jong

Nicolas Kats

The 20′ sole of the keel on my 39′ LOD Colin Archer type steel boat is extra long. Draft is 6′, wt 16 ton.
This sole is one piece of steel plate 2 cm thick.
The ‘garboards’, welded to each side of the sole are also 2 cm thick & they are 2′ high, full length, one piece each also.
This creates a formidably strong keel ‘box’.
Hitting & bouncing off a rock at 5 kts (at low tide in an enclosed bay) had zero effect. Small chip of antifouling paint came off, that’s all.
I have no worries about the keel & hull integrity when grounding at any speed. It is nice to be utterly indifferent to this particular aspect of grounding!

Dick Stevenson

You must indeed, have a very robust boat to survive that kind of collision. Your report brings up the consideration of collateral damage.
I believe many surveyors would condemn a rig that had endured the collision reported. Just from shock loading if not from actual inspection. Mitigating factors might be that you were sailing and the sails would absorb some of the impact and would have the rig in tension. Hitting mud or sand or something you rode over could help as opposed to stopping solid on a rock.
Any element of the boat that depends on alignment would also be suspect. Steering cables, engine to shaft etc. The list could go on.
Speaking of shock loading, part of my hurricane prep list includes tightening down the rig in all reasonable ways. Good experienced friends think the rig is always suspect after the sustained herky-jerk slamming of a major hurricane. Even in a storm, the shaking of the rig as the winds whirl around is impressive and very unsettling.
Like a lightning strike, I suspect damage from a cruising speed grounding/rocking will not all be immediately apparent and will be found months to come.
My best, Dick Stevenson, s/v Alchemy

Erik de Jong

Hi Dick,

The rigging or rudder were not really my biggest concern. I was more worried about the engine mounts and propeller shaft alignment, we have a flexible mounted 1500lbs engine.

With a metal boat, it is fairly easy to determine any kind of damage. You can aid yourself with all kinds of tests, from dye pen. and magnetic particle tests to non destructive load tests.

I had the runners and aft-lowers tested at 130% SWL after the grounding, and some other critical components dye tested. No concerns remained after these tests. We’re about 4 years and 20.000 miles further now, including a few off-season north Atlantic trips and crossing, no additional damage has shown up during this time.

But to be honest, I don’t think that the rig is pressed harder during a collision or grounding, than that it is during a Chinese gybe or some serious slamming while riding out a gale.

Erik de Jong

Paul Mills

Hi Matt,

Thanks for yet another well written and accessible article. Although I feel I have little to bring to the exchanges and comments that follow, I always gain new perspective and understanding.

keep it up!


Eric Klem

Hi Matt,

Sorry for the late comment, I just got back into the world of internet after a few weeks hiding in more remote places.

I think that understanding the geometry of the keel and its relationship to impact loads is quite important and I thought that I would add my 2 cents to what you and Erik have said on this. In a static loading case with no deflection, a shallow and long keel will be far superior to a deep and short one. The distance over which the moment will be reacted is much greater so the forces will be much lower. In addition, the shear force will be the same but likely spread over a much greater area meaning that the stress will be lower (this is not entirely fair as a deeper, shorter keel will likely require a much stronger hull attachment to deal with normal bending loads anyways).

When dynamics are considered, things get a bit different. The energy of the boat needs to be dissipated. If the keel strikes a rock, the boat will pitch which dissipates a significant amount of energy. The greater the vertical distance between the point of contact and the center of mass, the greater the pitch will be making the loads lower. Having a deeper keel can help this as it may move the point of contact lower but with a properly shaped rock, it won’t make any difference as the rock will determine the point of contact. Another place that energy is dissipated is in deflection. A short keel will definitely deflect more but I suspect (no numbers to back it up), that most keels are quite rigid when compared to the supporting structure in a fiberglass boat so it will be negligible. The supporting structure will definitely deflect a lot but I don’t have a good feeling about a short structure deflecting a lot.

And it is worth noting that a boat that draws less water is less likely to hit stuff. I used to sail a boat that drew 11.5′ and we found all sorts of things that were not properly charted because everyone else had less draft so they would go safely over the top. Note, I am not advocating for a shallow draft boat, just one that isn’t super deep.

Thinking out loud here, could you design a keel hull joint that was much more robust but not too heavy, difficult to manufacture or expensive? I am thinking of a joint which is significantly longer than the keel is, almost like a giant tapered flange. Also, if you could appropriately taper this flange, you might be able to get the edge of the flange to deflect significantly so that the load is more evenly distributed over the entire joint when there is a big bending moment. It would take a little analysis to make sure that this was possible and that it wouldn’t result in silly thin sections. The idea mentioned above of tapering the Valiant’s joint is clever although it would be interesting to compare that angle to the typical angle a boat pitches to when running aground as the greater the boat pitches, the less effective it is.


John Harries

Hi Eric,

Really interesting comment, thank you. As a lay person without engineering training, the thing that jumps out at me when reading this discussion between you, Matt, and Erik, that do have said training, is that the keel to hull interface is an area that simply does not get enough good engineering applied to to it, particularly in production boats.

This is backs up my own, admittedly completely unscientific, observation in boatyards that most fin keel production boats that suffer a grounding at over 5 knots suffer substantial damage in the keel to hull area. A common practice situation that is simply not good enough for voyaging boats.

Definitely something to think about for the Adventure 40.

Dick Stevenson

Eric, Your comments remind me of an observation an acquaintance made to me 10+ yrs ago. He had sailed the Bahamas (and lots of other places) for years and had rarely gone aground. When older and restricting his sailing grounds to the FL coast and Bahamas, he replaced his 6+ foot draft vessel for a shoal draft vessel. You guessed it, he went aground regularly as he attempted to get into nooks and crannies . Design matters but sometimes it is just the skipper.
My best, Dick Stevenson, s/v Alchemy

Hans van der Sloot

Hi Matt, would you consider plywood-composite constructions like RM yachts (RM 1370 / 1380) as solid enough?

John Harries

Hi Hans,

It’s unlikely Matt will answer see (Part 5):

That said, plywood composite can be a very strong construction method. Like most materials how much impact it can survive is a lot about how the boat was built.

So without a full on engineering study there is no way for Matt, or anyone else, to reliably opine on a given boat’s impact resistance.

In the next chapter Matt goes into the impact properties of hull materials: