Safe collision

How strong should the boat's bow need to be on a 30ft hull?

 

Real strong

 

It depends. A 30ft inflatable can absorb more energy than a comparable steel hull. However, the puncture resistance is a lot lower.

 

I'm building a 30ft plywood cat. What I'm trying to understand is the meaning of "crash bulkhead",,, In larger vessels it used to keep hull watertight in the case of front collision. But does this also mean that stem should be allowed to crash in order to absorb impact energy ultimately minimizing other hull's structure deformation?

 

You don't need to "allow" the stem to crash. Collision bulkheads are a safety measure for when everything else fails.

 

If it helps, I think he meant 'crush', like in a car where the front end is made to collapse to absorb energy.

 

In that case, I have seen an interesting approach. The space between the stem and bulkhead is foam filled. It acts like a crumple zone in a car frame and absorbs the energy of the impact.

 

A collision bulkhead is as the name suggest - a stop point when the crap hits the fan and you've stuffed the bow into something, hard enough to breach it. It simply keeps the rest of the boat from flooding.

 

Generally speaking the bow of a boat is the strongest part. That's why if you take a course in ship or boat handling they always tell you, if a collision is unavoidable turn toward the other vessel and hit it with your bow. What PAR said is right. That bulkhead is supposed to be watertight and take the force of the collision.

(of course you should have taken every action available to avoid the collision up to that point, until you are "in extremis". )

 

The Vendee boats (now IMOCA 60s') had/have these bulkheads partly to ensure not sinking in case they hit an iceberg or other debris especially at night. I personally know of a 35'er sunk by a discarded domestic fridge off Portsmouth UK.....

 

...crumple zones to reduce the impact rather than be too rigid as they used to be....
this is exactly the point I'm trying to understand.
Does the nose of the BOAT is designed the same way, I mean in the case of direct front collision should the bow constructed to be a maximum rigid or the stem is allowed to break? In comparison for example with the dagger board and board case/box, where the board should break first saving the box and ultimately hull's structure

 

There are daggerboard cases with a foam piece that will collapse instead of breaking the board. There is no universal answer. It depends on the design. A weak bow that absorbs the impact will also let go of the tension in the forestay and make the mast fall.

 

I'm just guessing here, but I'd say in a mono hull a large impact on a 'maximum rigid' bow may not effect the rest of the hull very much. On a catamaran such as you're building, with 'maximum rigid' bows, a large impact on one hull might put a lot of torsion or racking stress on the connections of the beams to the catamaran hulls. If those connections broke or were loosened up much, that would seem to be a major problem.

If that was so, I'd guess that a crumple zone would be more useful on a catamaran compared to a mono hull. But whether or not they are needed at all, on either type of hull, I don't actually know.

Are the connections between hulls and beams a particular weak spot in catamarans?

 

Of course they are points of stress concentration, typical efforts for multihulls (no mono hulls), which must be designed with special attention.

 

Thanks to everyone participated in this discussion!
Seems like I have general idea now of how the catamaran's bow should be constructed.
Below is the extract of my conversation with the designer.-

-..."make sure the below WL stem is good and strong. The deck/gunwale area will be by default because you have to fit the deck and also, maybe, a netting beam. In between it doesn't matter so much"