Impact Resistance—Two Collision Scenarios
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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?
Thanks
Stan
Hi Stan,
To visualize the bow impact in another way: Place three playing cards on edge, forming an “A” shape when looking down at them. Push on the apex of the “A” as if it were the bow of a boat hitting something. The sides of the “A” will splay apart. The cross-bar of the “A” is the forward bulkhead. If it’s joined securely to the sides, the apex will buckle but the area behind the bulkhead will stay more-or-less watertight. If the bulkhead is just tacked on lightly, the sides will rip free of it and the bulkhead will be useless.
What we’re seeing in this scenario is that there are some conditions (such as hitting a certain sharp chunk of rock, or being rammed by a drunken speedboater) where there will be a hull breach, no matter how over-built the boat is. We can either design the boat so that this damage is contained to one area (such as the forepeak), or we can allow the boat to sink.
It’s hard to say exactly what will happen in any given accident with a particular boat. With a traditional full or 3/4 full keel design, the keel structure is often just a continuation of the hull structure (i.e. there is no keel-to-hull joint). So there is no sharp aft corner to be driven up through the hull bottom, but the front of the keel can crush and buckle in the same way as the forefoot of a hull. In my opinion, one big advantage of this type of keel is that the ballast (which lines the inside of the most vulnerable part of the keel) takes the brunt of the impact; damage to the leading edge of the keel might expose the ballast but often won’t let water into the boat.
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): http://www.youtube.com/watch?v=YIglL5vks4g
Interesting video, Marcus. Very interesting.
I think that really drives home the point that is is quite possible to design and build a boat that can take a high-speed grounding in stride with no major damage.
Bonjour
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
Matt,
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
Interesting idea about sloping the keel root bearing surface. This would also let you use a much deeper transverse frame at the trailing edge, right where that extra strength is most needed.
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
Hi Dick,
I’ve used a Kevlar collision layer once before, in the carbon belly pan of a solar car. Carbon is a bit brittle and doesn’t handle abrasion very well. Our theory was that, if the suspension failed and the car slid on its belly, the tougher Kevlar would protect the driver if the carbon was shredded up by the pavement. (Thankfully, we never had to rely on it.)
Kevlar is really, really hard to cut or abrade, and so makes a pretty good protective outer layer on keels, stems, etc.
It is, however, generally a bad idea to mix two different composites in parallel in the same load path (more here) as the stiffer material will take a disproportionately large share of the load.
In short: Using Kevlar for an outer protective layer on a fibreglass part: Good. Making the whole part out of Kevlar: Good. Mixing Kevlar and fibreglass in a single part: Not good.