material thicknesses for catamaran formers and cross beams

[Farmer0101]

I am custom building a 48ft catamaran constructed of aluminum. I am using Mumby catamaran for a guide. I cannot seem to find any information on the formers and cross beams thicknesses. Any help or information would be greatly appreciated.

 

[bajansailor]

Welcome to the Forum Farmer.

Can you supply a bit more information about this 48' cat that you are building please?
Is she your own design?
How far advanced is the build so far - have you started building the hulls yet?
If you have, can you post some photos please?

I had not heard of Mumby catamarans before, but Google quickly found a few for sale - here is one of them. They do look like a very nice design.
Mumby 48 Aluminium Catamaran For Sale, 14.63m, 2003 https://phuket.boatshed.com/mumby_48-boat-217130.html

Re the formers and cross beam thicknesses, what do you mean by formers?
And re cross beam, do you mean that deep box beam across the aft end of the cockpit (re the photos in the linked advert above)?

 

[Farmer0101]

Thank you for the reply. I am starting this fall. Welder by trade, worked for a welding shop. Still working on blueprints. Formers would be for the hull shape and beam is the box across the aft

 

[Farmer0101]

New to this - hope you have patience. Thanks again

 

[Rumars]

Let me see if I understand you correctly, you are designing a 48ft Al sailing cat and you have no ideea how to dimension the bulkheads and beams. "Using the Mumby for guide" simply means you read somewhere the hulls are 4mm and the superstructure is 3mm, and that's your entire knowledge about scantlings.
If what I wrote is correct you are well on your way to failure. Best way forward is to find a designer for Al cats and buy a set of plans. Mumby himself does this (or used to anyway), as do others.

If my description is faulty, I apologize for misunderstanding you.

 

[bajansailor]

@Rumars I think that you are spot on, as usual, with your assessment above.

@Farmer0101 please do take heed of Rumars advice and buy a set of plans for a vessel that you like - there is a lot more to designing a vessel than just coming up with the lines plan.
The last entry on the Mumby Facebook page was at the end of December last year - have you tried to contact them to see if you can buy a set of plans from Mr Mumby?
Log into Facebook https://www.facebook.com/MumbyCatamarans

 

[Farmer0101]

Thank you for the info. I have study plans from bruce roberts, jaz shipyards, and hoping to hear from mumby soon. The info I'm gathering is going to the drafting department at nor arc manufacturing - pet project with friends. Shoud be in good hands. New to sailing but have been building plans for thirty years or more.

 

[bajansailor]

The info I'm gathering is going to the drafting department at nor arc manufacturing - pet project with friends. Should be in good hands.

Have any of your friends at nor arc manufacturing had any prior experience of boat design?

If you do hear from Mr Mumby, and he still has construction plans for sale, and you do really like this boat (I can see why you like it, it is an impressive design), then simply buy the plans.
Even by building the boat yourself, the cost of the bought plans will still only be a tiny fraction of the cost of building the boat, and it will be a worthwhile investment to purchase plans for a boat that is proven to work.

 

[Farmer0101]

Thanks, nor arc is into mining and offshore drilling barges and rigs. The cat is my retirement project. The reason I was asking about thickness in aft and formers is to address cracking problems cats are having inline with the mast and rear beam sections. We are toying with the idea of doing retractable pods for propulsion. Weight is a concern with performance hulls. It will be sailing mostly rivers, channels, and the Great Lakes. Draft has to be at a minimum. The final print will be ours and a one off build.

 

[Rumars]

Wich cats have problems with cracking, please do tell.
Just buy a pair of outboards, that's instant pod drives for you.
What do you want to actually build, a sailing cat or a pontoon boat?

 

[Farmer0101]

OK, I better explain. Prints are from our architect with my wish list. Not drawn yet. Still gathering info. Wish list as of now is mumby hull style with gunboat stern, bow style from the bali 4.8, solar cat roof stretching all the way back to stern, walk thru cabin, steering from the front and cabin, smooth hull for beaching but extra thick thinking of electric outboards either in pods or swing down. No clew on sails yet; toying with the idea of folding mast or a fram style for self lowering. Dont know yet. Meeting with some designers soon for advice. Funding is not a problem but I only get one crack at this. Picking as many brains as I can.

 

[rnlock]

"Architect"? If that means naval architect, maybe you're ok if he knows what he's doing.

If you know what the loads are, and "cross beam" means something like aka, then it should be relatively easy to figure out. For an engineer, anyway. But defining the loads isn't so easy. If your project is very much like the "Mumby" cat, then maybe you can use their scantlings, but if it's not real close, some sizes may need to be different.

 

[rxcomposite]

Crossbeam. One of the most difficult thing to design. There is an an excellent article on alloy crossbeam sizing published in PBB. Search for it as the title skips me for a moment.