Big houseboat ?

Heres my art at work... oh oh, i think she looks too fat and not long enough for 22 x 70

I should at least find some graph paper to draw to scale.

 

If you are in Canada leaving the boat in the water wille mean you are iced in.

Thin pontoons may crush easily , so the house will need to be dragged up on the beach every winter.

A steel barge might not have this hassle.

Since its not going to move very much the difference between a pontoon boat and a barge would be just a different method of moving the unit.

 

Summer cottage

 

Some lakes in BC dont freeze

 

You need to write a SOR. It is important to define exactly what you want to do with the boat. If you don't you will be distracted by every potential "better" option and never get anywhere.

I know because I am in the exact same situation and it can be very wasteful to go down the wrong path and then change directions because you got another idea.

http://www.boatdesign.net/forums/boat-design/design-spiral-where-start-building-boat-28580.html

 

Parkland, you are being offered some very gentle and polite hints above that your proposal might Not Be A Good Idea.

I was thinking that if the hulls were marine grade ally, then they should last..... but then I saw your sketch above with the double decker superstructure and I got worried.
If you really want to go down this route, despite the good advice to the contrary, please do a weight calculation before you even start to buy materials.
I think you might get a surprise.
Spreadsheets are good for weight calculations - then you can also put in the longitudinal centre of gravity of each item as well, to see if she will trim reasonably level when finished and launched.
You should really also be concerned with the vertical centre of gravity of each component as well - put these in to the spreadsheet, and see where the overall VCG is.
Again, you might get a surprise.

For a weight calculation, you need to calculate the surface area of all the hull surfaces and multiply this by the shell thickness and the density to get the aluminium weight.
Do the same for all the connecting cross beams, structural members, etc.
And all of the decking (plywood?), superstructure framework and cladding, and additional decking / roofing above.
Then tally up all the outfit equipment, furniture, engines, fuel and water tanks, other bits and bobs - try to think of absolutely EVERYTHING that might end up on the boat.
And then add a margin of at least 10% on top.

How does this compare with the immersed displacement of your three hulls?
If it is such that the hulls are half immersed, ie draft is equal to tube radius, then you have not got much extra deadweight available in reserve for additional stores, people (parties are nice, with all that deck space....), dinghies, toys, a generator, air conditioning......

Once you go past the half way mark, you are rapidly losing the amount of buoyancy for each inch of immersion that you had at the half way mark.

Some years ago, a gentleman designed and built himself a power cat party boat on the beach here in Barbados.
He was very much a know it all, and nobody could offer any advice to him - he had it all covered.
He was especially proud of his very sleek go fast hulls.
Took him about 18 months to build.
Launch date was a few days before Christmas, and he proudly announced how he had bookings for a full house (50 pax) for a coastal cruise on Christmas Day.
The boat was pushed down the beach into the water with great fanfare - and floated in lightship mode (no fuel, water or passengers on board) with the main deck awash.......
He had never done even a basic weight calculation on the boat.
Needless to say, he didnt carry passengers cruising on Christmas Day......
(He hauled the boat out, widened the hulls significantly at more huge expense, and relaunched her 6 months later).

Moral of this story - be careful. Especially re your weights.
(You can worry about things like transverse strength a bit later, in the unlikely event that you do have masses of buoyancy available).

 

While you are at it, maybe you should also compare the costs with a monohull 'barge'

For the same price, you may get a lot more room, without all the expense and trouble of 'attachments'. You actaully end up with a 'floor' as part of the structure in a barge, and dont have to add all the 'walking room' after you have spent so much on the 'floating gear'

Pontoons seem like a simple idea until you factor in all the crossbeams, floor and engine supports - all of which have to be capable of resisting the marine environment, as they dont have the protection of a hull around them.