Floating island - Trimaran or Catamaran?

We are looking into building a floating island to use as an offshore bar in the Caribbean.
Ideal dimensions, 50 feet by 80 feet.
We have access to 4 by 8 by 40 pontoons.
The discussion we are having is whether it’s better to go with a trimaran configuration for greater stability.
Typical wave height is 1 to 2 feet.
If we use a cat configuration the pontoons would be 20 feet apart.

 

Stability says you need pontoons on the outer edges. Structural stresses says they need to be evenly distributed under the load.. how's the sea state in the location? Like worst case scenario once in 10 to 20 years? Typical is not what matters..

 

We have hurricanes occasionally, but we would beach it then.
4 feet maximum wave height under normal circumstances, mainly 1 to 2.
My question I guess is, would a 20 foot span between pontoons affect stability?

 

As for stability, it is difficult to know without having more data, but what is certain is that you will need some fairly strong transverse beams, between pontoons.

 

What it's used for? Like what loads are expected? Any structures on deck? As Tansl said, more data needed..

 

This will be used for hosting events.
200 persons maximum.
It will have an upper level. 30 by 30 feet.

 

Until you draw up a general arrangement of how you want this to be, once can only offer hints/suggestions.
Since the stability will be dictated on how the decks in the vessel are arranged.

 

+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).

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).

 

Does it really make sense to make one big rigid structure in this application? I can see why you'd want the decking to be rigid (people in a bar may already have trouble walking straight) but I think you could have a rigid truss deck and then a bunch of floats with some degree of freedom underneath so only the vertical motion is transmitted to the decking. Imagine a pontoon bridge but with rows of boats to make the bridge deck more than half as wide as it is long.

 

Thank you for all this information.

 

George, a trimaran is a catamaran that pivots in the middle what matters is displacement in which case three pontoons are better than two. Is 20ft separation between pontoons enough ?
Not if it’s 60 ft high and everyone runs to one side to look at the dolphins.
As mentioned above you need to come up with a more detailed statement of requirements and a rough plan.

 

Please see attached rough sketch.
Based on the discussion, a trimaran is preferred.
8 feet wide, 40 feet long pontoons, made with 3/8 steel bottom, 1/4 inch steel sides and tops.
The distance between the pontoons is 12 feet.

 

Georgehype. Please go to a professional designer with your specs. There are 2 naval architects who have responded on this tread, Tansal and Adhoc who would be able to assist. The proposal may appear to be simple but the design is a bit more complex especially when you are proposing up to 200 people onboard.

 

Thanks, that’s what I came here hoping to to find. I will be reaching out to them individually.

 

@georgehype re your sketch above, this gives you a total deck area of 4,000 square feet.

You could get the same amount of main deck area with a barge shaped like the M/V Harbour Master which is 100' x 40' - and she has two additional decks above.
Harbour Master | Port of Spain https://www.facebook.com/HarbourMasterTT/



She was built by Washburn & Doughty in Maine, hull #50 in this list -
https://www.washburndoughty.com/vessel-list

Alternatively, if you spread this 4,000 square feet over 3 decks, you could perhaps use three 40' long pontoons with a spacing of 5' between the pontoons, giving an overall beam of 34' - and if there are 3 decks, then the total deck surface area is 4,080 square feet.
Although you would have to first do a detailed estimate of all the weights, to see if you will have enough buoyancy, and also assess the stability.

A smaller 40' x 34' 'trimaran' with three decks would be much easier and cheaper to build than an 80' x 50' trimaran, and much easier to dry dock as well for maintenance.

Edit - re an 80' long trimaran, built with 40' long steel pontoons, how are you proposing to join the two 40' pontoons together to create an 80' long hull?