Is a small crashbox any use?

[Ric]

My boat has longitudinal stiffeners glassed just above the waterline from bow to stern.

In order to fit the under deck turnbuckle for an inner forestay, I am building a horizontal marine ply "bridge" between the stiffeners which will be glassed in from the bow to about 40cm sternwards. It occurs to me that if I also glass in a vertical wall on its rear edge, I will have created a small crashbox occupying the front 40cm or so of the boat.

What sort of size are purpose built crash boxes on GRP boats? Will my mini job offer any protection in (say) a crash with a floating container? Or does holing typically occur further rearwards where the hull is generally thinner?

 

[richardabeattie]

What sort of volume would it be - how much buoyancy would it add?

 

[Ric]

What sort of volume would it be - how much buoyancy would it add?

The idea of a bow crashbox isn't how much buoyancy it adds, it is how much flooding it lessens in case of a frontal crash. I haven't been able to find much information about where yachts get holed below water-line in case of open-water collisions.

 

[Seajet]

I haven't seen any figures but I suspect hitting semi submerged containers head on quite rare, by sod's law more likely to gash the forward bilge to one side ?

I suspect to have any real protection from such a menace one would need to make the whole fore cabin an airtight closeable compartment* - I know of a transat Anderson 22 which has this feature.

* But be careful there's ventilation for the occupants in normal use, I heard of Services Victoria 34's with this feature and people either nearly or did suffocate.

Ric, the crash box ( collision bulkhead ) you mention must be worth having though, if relatively easily done, for collisions with boats, harbour walls, etc.

 

[Concerto]

You have not mentioned what type of yacht and your sailing area. I suspect you are working on an older yacht with heavier displacement.

If you sail at some speed and the coast is rocky, then contact might cause some damage, but may not breach the hull. Sailing anywhere else, you are more likely to hit something with your keel, rather than the hull. Although lots of containers are lost in the sea, but very rarely in the coastal waters of the UK. Then many containers will not float, so the chance of hitting one is extremely rare.

So returning to the OP's question. I personally would not bother building a crash box.

 

[William_H]

I agree with Concerto the forward upper part of the bow especially on a smaller boat is very strong and resistant to impact damage compared to the mass (inertia) of the boat itself. Far more risk of opening up the lower bow and bilge areas witha sharp cornered container or similar. IMHO for improving resistance to sinking the whole under bunk areas fron to back should be sealed off to make crash bulkhead or buoyancy tanks.
However I think OP would be better off making a wire extension of the forestay under the deck down to a floor stringer or bulkhead of forward bunk or even to the bow itself to take the forestay loads. Fit a turnscrew or similar in the wire to make it easy to take the tension. good luck olewill

 

[ianabc]

28 foot fibreglass sailboat moored alongside of me is no more.

Last autumn the owner left the harbour and less than 5 nm out the boat sank in less than a minute.

It appears that the newly renovated and quite beautiful yacht was sunk by a deadhead.

Fortunatly the new inflatable was being towed and owner along with dog escaped.

 

[bbg]

I would say there is no point in fitting a crash box unless it extends some way below the waterline. If you do fit one, you will need also to fit a meaningful access port so you can sponge out the water that could seep in through deck fittings etc. you don't want to be sailing around with a bow full of water.

 

[Iliade]

Fit a cheap plastic stopcock to allow water out and into the bilge.

I suspect that if you hit the bow hard enough to hole it you would also cause it to bow out enough to detach any recently added bulkhead. It may even increase the risk of damage by preventing the bow from flexing.

My last boat had a watertight half-height bulkhead some 7' back from the bow, forming the edge of the forepeak berth.* I fear you would need to go almost that far back just to resolve these issues.

* I was forever finding an inch of water there as condensation and any leaks couldn't run away.

 

[lw395]

I know of a boat which hit floating timber in the Itchen.
Motoring flat out against the tide in the dark AFAIK.
It pierced the hull close to the waterline and about a foot back from the stem.
Fist sized hole.
The locker under the bunk acted as a crashbox.

This was pointed out to me when I chopped great holes in similar panels to install heating matrices.