Multihull Collision Survivability

[redreuben]

Couch, television, vcr, fridge, 1000 litres cotton wool, insurance policy.
Problem irrelevant.
FFS, **** HAPPENS !
RR

[marshmat]

Re. capsize survival:

Originally Posted by truecougarblue

A 10cm deep layer of foam within a bridge deck 15m X 15m on a 20 m cat would give 22.5 metric tons of displacement at very little relative additional cost to build. That would raise your inverted waterline a bit. Make it spray foam or epoxy it in and it would even enhance the structure. It also makes one think differently about any foam insulation applied to undersole both in the hulls and the deck house roof.

Today's multihulls are often built with cored structures anyway. Using an extremely thick (10 cm?) core for the deck and cabin top is a potentially appealing option. It's a fairly straightforward matter to design such a thing so that there's enough flotation in the right places to keep her up and flat after a capsize. Getting appropriate structural properties with several sheets of core glued together might be a bit trickier, but no more so than engineering a composite cat in the first place....

I'm very much in favour of painting the underside of any multihull's bridgedeck in bright orange non-skid, and including plenty of tie-off points and grab lines under there. Just in case.

[buzzman]

The type of foam used as buoyancy foam is important too.

Many small boat (amateur) builders advocate mixed, poured polyurethane foam - mainly I suspect because it is simple and easy to use and easily and completely fills compartments before enclosing them....

...however, my research and forums elsewhere have indicated that PU foam eventually absorbs much of its volume in water, so using this is questionable, especially for vessels which are 'wet'. [NB: I found the PU foam inside the thwarts of my small 'glass speedboat were mostly water after I ground them out to repair failed ply inside. They weighed significantly less after drying out than whenthey were removed. Didn't actually weight them but at least half the weight after drying out].

The consensus appears to be, from what I've read, the order of preference for buoyancy and waterproofness (my word ) is as follows:
1. Extruded PolyStyrene (XPS)
2. Expanded PolyStyrene (EPS)
3. Extruded PolyPropylene (EPP)
4. Expanded PolyUrethane (EPU)
5. Foamed (liquid) polyurethane (PU)

One thing to note is that EPP is flexible whereas the styrenes are not. I had already been planning to incorporate XPS inside closed compartments so that even if watertight sections were ruptured the amount of water able to 'leak in' would not significantly affect the vessels floatation.

I also remember seeing a 'quick and dirty' calc somewhere as to the lift (buoyancy) of XPS but cannot now find it.

Do any of the engineers or experts on here know of such a rudimentary 'back of the envelope' method of calculating the desired volume of foam to keep a known weight/mass of hull above water???

Keep It Simple - I need a calculator to balance my chequebook - and often get that wrong!

[oldsailor7]

Standard density EPS varies from 1.3 to 1.9 lbs/cu ft.
Fresh water weighs 63 lbs/cu ft and seawater 64.
Work it out.

Collapsible plastic acid bottles are cheap. Inflated in place make excellent flotation and don't absorb water. In place of brittle styrofoam they don't crumble on impact and so are less likely to be washed out of a hull after a hull fracture, which also can happen with smaller plastic milk or juice bottles.

[Silver Raven]

Originally Posted by marshmat

Re. capsize survival:

Today's multihulls are often built with cored structures anyway. Using an extremely thick (10 cm?) core for the deck and cabin top is a potentially appealing option. It's a fairly straightforward matter to design such a thing so that there's enough flotation in the right places to keep her up and flat after a capsize. Getting appropriate structural properties with several sheets of core glued together might be a bit trickier, but no more so than engineering a composite cat in the first place....

I'm very much in favour of painting the underside of any multihull's bridgedeck in bright orange non-skid, and including plenty of tie-off points and grab lines under there. Just in case.

Gooday 'marsh' - Why do you want "bright orange non-skid, etc etc" don't you think it might be better to design & build a safer multihulls & learn to sail it properly. How many ocean sailing - cruising or racing - miles have you done? O.K. - so "Just in case" - of what? Crossed a few bits of water over the last few years, seen some big seas, some even bigger winds, a few - very scary - water-spouts (now they do put the fear of Gawd into your britches) but didn't at anytime feel we were in any bother. Well reefed down, short crew rotation, maximum attention, etc. Maybe - better designed boats, better sailing proceedures, more training might be a consideration. ie - if you think you should be reefing - you should have done it 20 min's ago. What you think? Yes/no ??? Y'all have a great week - eh. Ciao, james

[cavalier mk2]

I went with the Air Sea rescue orange non skid underwing some time ago. It didn't cost anything extra or add weight so why not? Years ago I read how the orange underwing Hedley Nicol insisted on for Clipper 1 saved Tom Corkhill when he screwed up which emphasizes a point. You may not make a mistake but if the other watch does everything helps.....

[cavalier mk2]

Its true, The non-skid can cover up imperfections the fish might find upsetting....

[Silver Raven]

Originally Posted by cavalier mk2

Its true, The non-skid can cover up imperfections the fish might find upsetting....

Gooday 'cav' - good comment - we wouldn't want to upset the fish - they might taste sour when cooked.

Why not - if you want to protect your vessel - mono - 2 hulls - 3 hulls - - put more (thicker) laminate in the front - most easily damaged areas.

If it's wood - then more laninates of wood or carbon or better yet kevlar. Especially if it's attached over an extension filler piece of sacrificial light foam & then wrapped in carbon & kevlar.

Better protection (for your life's support systems - ie - the baot - floating) than a lot of weight in some false belief that a bunch of floatation will be good to hang on to as you float around the ocean trying to keep pace with the tsunami floatsam from Japan?????? Food for thought, Yes/no Ciao, james

[CatBuilder]

Originally Posted by Silver Raven

Gooday 'cav' - good comment - we wouldn't want to upset the fish - they might taste sour when cooked.

Why not - if you want to protect your vessel - mono - 2 hulls - 3 hulls - - put more (thicker) laminate in the front - most easily damaged areas.

If it's wood - then more laninates of wood or carbon or better yet kevlar. Especially if it's attached over an extension filler piece of sacrificial light foam & then wrapped in carbon & kevlar.

Better protection (for your life's support systems - ie - the baot - floating) than a lot of weight in some false belief that a bunch of floatation will be good to hang on to as you float around the ocean trying to keep pace with the tsunami floatsam from Japan?????? Food for thought, Yes/no Ciao, james

Or just put in several watertight bulkheads...

[MikeJohns]

Perhaps we can move posts 111 onward into a new thread titled "Multihull Capsize"

Jeff?

[Boat Design Net Moderator]

This seems reasonable.

The posts dealing with capsize more than collision survival have now been spit to their own thread at Multihull Capsize Prevention <split>

This thread here can then continue to discuss collision survivability.

[HASYB]

Originally Posted by Boat Design Net Moderator

This seems reasonable.

The posts dealing with capsize more than collision survival have been now been spit to their own thread at http://www.boatdesign.net/forums/mul...lit-43251.html

This thread here can then continue to discuss collision survivability.

Hi Mod, intentional even more appropriate would be: Multihull Capsize Prevention.

Cheers,

[Boat Design Net Moderator]

Now updated. Thanks.