How many is it safe to rescue?

davidaprice

davidaprice

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I've been daydreaming about a scenario: I'm sailing singlehanded on my Maxi 999 (33' AWB, displacement 4200kg) in the sheltered waters of the Finnish archipelago, and a passing harbour cruise ship hits a rock and starts to sink.

As I take passengers on boat my boat, how many is it safe to take? She's registered for 7, but that's just because she has berths for 7.

Rough back-of-an-envelope calculations tell me that the boat's area at water-line is about 11 square metres; if she sinks 1cm she displaces another 0.11 cubic metres of water (about 110kg, or 1.5 average people). So for every 3 people I take on board, she sinks 2cm. And her inspection papers show that her lowest water entry point (the companionway) is about 120cm above the water-line, so she can take 120 * 1.5 = 180 people before she sinks! So weight doesn't seem to be the limiting factor.

There's seating for about 11 sitting below, and five (plus me) in the cockpit. Motoring in comfortable conditions, maybe another eight standing below and eight sitting on the coachroof. And in extremis, maybe 6 each side on the decks with their legs dangling over the edge. So simple space limits us to about 44.

How about stability? it would take 21 70kg people to match the weight of my 1500kg keel. Then we're relying on form stability to stop a capsize from the wake of a passing mobo. Maybe this is the limiting factor?
 
Keep away from Italy & its unlikely to ever happen to you


davidaprice said:
I've been daydreaming about a scenario: I'm sailing singlehanded on my Maxi 999 (33' AWB, displacement 4200kg) in the sheltered waters of the Finnish archipelago, and a passing harbour cruise ship hits a rock and starts to sink.

As I take passengers on boat my boat, how many is it safe to take? She's registered for 7, but that's just because she has berths for 7.

Rough back-of-an-envelope calculations tell me that the boat's area at water-line is about 11 square metres; if she sinks 1cm she displaces another 0.11 cubic metres of water (about 110kg, or 1.5 average people). So for every 3 people I take on board, she sinks 2cm. And her inspection papers show that her lowest water entry point (the companionway) is about 120cm above the water-line, so she can take 120 * 1.5 = 180 people before she sinks! So weight doesn't seem to be the limiting factor.

There's seating for about 11 sitting below, and five (plus me) in the cockpit. Motoring in comfortable conditions, maybe another eight standing below and eight sitting on the coachroof. And in extremis, maybe 6 each side on the decks with their legs dangling over the edge. So simple space limits us to about 44.

How about stability? it would take 21 70kg people to match the weight of my 1500kg keel. Then we're relying on form stability to stop a capsize from the wake of a passing mobo. Maybe this is the limiting factor?
Click to expand...
 
sailorman said:
Keep away from Italy & its unlikely to ever happen to you
Click to expand...

I'm not talking about Costa Concordia here, but the many 50- to 100-person harbour ferries and harbour cruise boats we have around here. It's not unknown for them to go aground (lots of shallow rocks), though usually they aren't holed. You may remember that a year or two ago one ferry's captain accidentally got locked in the heads while under way.
 
Sounds like a logical approach david but remember that what matters is not just the weight but where the c of g of all these people are in relation to the centre of buoyancy and the latter will of course rise as more poeople push the boat down in the water. Mind you you could always help the situation by throwing your mast overboard. maybe judiciously opening the seacocks to let the boat sink a bit nearer its lowest water entry point.

Given when it was made, isnt the 999 a MAB rather than an AWB? :cool:
 
davidaprice said:
I've been daydreaming about a scenario: I'm sailing singlehanded on my Maxi 999 (33' AWB, displacement 4200kg) in the sheltered waters of the Finnish archipelago, and a passing harbour cruise ship hits a rock and starts to sink.

As I take passengers on boat my boat, how many is it safe to take? She's registered for 7, but that's just because she has berths for 7.

Rough back-of-an-envelope calculations tell me that the boat's area at water-line is about 11 square metres; if she sinks 1cm she displaces another 0.11 cubic metres of water (about 110kg, or 1.5 average people). So for every 3 people I take on board, she sinks 2cm. And her inspection papers show that her lowest water entry point (the companionway) is about 120cm above the water-line, so she can take 120 * 1.5 = 180 people before she sinks! So weight doesn't seem to be the limiting factor.

There's seating for about 11 sitting below, and five (plus me) in the cockpit. Motoring in comfortable conditions, maybe another eight standing below and eight sitting on the coachroof. And in extremis, maybe 6 each side on the decks with their legs dangling over the edge. So simple space limits us to about 44.

How about stability? it would take 21 70kg people to match the weight of my 1500kg keel. Then we're relying on form stability to stop a capsize from the wake of a passing mobo. Maybe this is the limiting factor?
Click to expand...

44 may be ok, provided they are not Americans. At 70kg average thats 3050kg total. The CE plate on my HR34 lists 1800kg total load over the boat weight, so provided the people are well distributed and as many below as possible, your total should be ok in calm conditions.
 
davidaprice said:
She was built in 1987, but was one of the first boats built in the 'modern' AWB style, layout indistinguishable from many modern boats. I like to think of her as a 'MAAWB' (Manky Auld Average White Boat) ;)
Click to expand...

Or My auld average white boat. She isnt average really being a Maxi.
 
Just remember if you go near people in the water in need of rescue you might not have a choice about how many you have onboard :eek:.

You will also loose the use of your engine for fear of cutting people up...
 
If you cut them up you can start using the smaller places like under the cabin sole,the anchor locker etc infact if you liquidized them the tanks are also an option.......IS THIS WHEN THE DAYDREAM BECOMES A NIGHTMARE.....sleep tight.
 
Had some net around the prop on a 1980s First 35. Invited about twenty people to add to a crew of ten for pre-lunch drinkies. Got them all on the foredeck and I went under the stern in a dinghy and removed the net with ease.

Having proposed the plan in Les Routieres the night before, we went back to the boat for debate and more drinks. Most of us were engineery types. Drunken commonsense dictated that we rigged a pair of linse from the masthead to adjacent pontoons.

Totally superfluous.

Of course you will have to inform them that it's last on first off, which will happen automatically if you've overdone it.

So I'd reckon on as many as can sardine downstairs and another 40 or so on deck.
 
Its a non-issue. Commercial ferries should have sufficient liferaft capacity for their entire complement of passengers and crew. Your role will be limited to making everyone cups of tea.
 
Babylon said:
Its a non-issue. Commercial ferries should have sufficient liferaft capacity for their entire complement of passengers and crew. Your role will be limited to making everyone cups of tea.
Click to expand...

Well... Here's a pic of that harbour ferry mentioned earlier. Captain went to a quick pee when underway, got locked into the loo and couldn't get out until ferry was already on the rocks. 54 passengers were on board. Liferaft capacity is compliant to solas regulations for inshore ferry but have a look, these rafts are the type that keep you afloat when passengers are in the water and hang into the raft with their arm. Probably coming onboard any boat would be preferred than staying in the water.

showphoto.aspx
 
This would make a nice little project for a first or second year naval architecture student - to calculate the changes in metacentric height, load line, CoG etc.

In my day, it would have taken ages, but now with modern technology, it should be a doddle to do the hydrostatics with steadily increasing bodies on board and whether they are standing or sitting on the coachroof - assuming, of course, that the Maxi's roof can stand the weight of ten or more standing people without buckling. I think the average weight for humans has gone up from 75kg to 80kg now.
Shorn
 
as suggested before the problem i can see is how to limit the numbers, your calculations look well thought out but when you try to leave the remaining passengers may disagree. i don't know what the answer to this might be - perhaps only picking up people from the water/floats so you can get away? frightened people aren't noted for there logic or generally compassion for others!
 
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