+1 re Ad Hoc's comments above.
Whereabouts in the Caribbean is this floating bar going to be moored? I presume that it will be static, or will it be self propelled, and able to take the punters on coastal cruises?
Re the 40' x 8' x 4' pontoons, what is your proposed construction material?
Have you calculated (or do you know) the weight of a pontoon, and the total buoyancy available at the load waterline with 200 punters on board?
Re if you go for a catamaran or a trimaran configuration, this will also depend on the estimated weight of the vessel - will there be enough buoyancy supplied by two hulls, or will you need the third hull as well?
If you have drawn a general arrangement plan or sketch, can you post a copy please?
If you have, you need to do a rough initial weights estimate - be aware that it is very easy for weight to add up on a boat, and you need to make sure that you have enough buoyancy to support it all, as well as having a good reserve of stability.
Remember to include all the outfit items, including furniture (even the loos...), fresh water tanks, and stores (a good supply of booze is heavy stuff, especially when you need enough on board to keep 200 thirsty punters happy).
Re the 30' x 30' upper deck, that is 900 sq ft in area - if people are in party mode, and like being close to each other, the 200 punters could easily all fit on this deck, unless you have crew members limiting the number that can go 'up top' - and the stability analysis will help to determine the maximum number that the top deck can accommodate (you also have to make sure that it is strong enough to cope with this load).
We have hurricanes occasionally, but we would beach it then.
4 feet maximum wave height under normal circumstances, mainly 1 to 2.
I am wondering how you are going to haul a catamaran or trimaran of this size up the beach?
If you simply run it aground, even in a sheltered area, then the sea level will rise significantly as the hurricane approaches, and the air pressure drops, and you also have to consider the effect of fetch and wind speed on wave height.
I am thinking that a cat or trimaran as per the sketch in your original post is not going to be very happy in 4' waves.....
As TANSL has noted above, you are going to need some VERY strong cross beams connecting the hulls, to cope with the racking / twisting loads of an 80' x 50' multihull (especially if it has to cope with 4' waves).
It would be prudent to try to design the whole superstructure to withstand these loads, rather than just relying on the cross beams.
Re if a 20' span between the pontoons will affect the stability - yes it will, quite significantly, as the overall beam will then be 36' (assuming 8' wide hulls) for a catamaran, rather than 50' as proposed in your original post.
But if you are still looking at a trimaran with 20' between the hulls, then the overall beam will be 64', and all else being equal, the stability will then be dramatically improved (however the racking loads on the structure will also increase significantly).
