aluminium "wharram" catamaran

[wakataitea]

hey guys,

on the way across the Indian ocean, we meet this young man who has the idea to build a fishing / pleasure catamaran "a la wharram style"

he is really into it and we spend many days talking about it. especial the material...

question to you guy's... what do you think about it...? size, deck layout, material....?

the plans are finished and the file is ready to send to a CNC machine in SA to cut out the parts. he wants to weld the boat in Rodigues island together.

this navel architect made the final drawings...

http://www.yacht-design.ch/projekte/segelyachtenboote/125-alu-cat

my question? are there many alu wharrams out there? haven't seen any on the way...

cheers hans

[Shaun]

Hey there Hans,
For me it looks like a great workboat, acres of working deck space and simple rigging.

How close in lines and design is it to a Wharram?

Cheers,
Shaun

[Raf]

I like the cabin. No nonsense, however, to my feelings is missing the simmetry. The boat is definitely a Wharram looking, why not sticking with a certified design? Custom design tend to have lesser resale value and the structural strength remains always some what dubious. Also the outside rudders are easier to build and maintain.

[boatsmith]

how much does the aluminum weigh?

[Galway Bay]

I have never seen an alu. wharram, but there seem to have been some built in Australia there are some discussions on the Wharram site. Some of these owners [ all ? ] seem to be happy.

David are you perhaps being a little mischievous ? You of all people on this site must know that Alu. is much heavier than ply ...

But really on the basis of this sketch I can only add a further list of questions -

Fishing boats carry a lot of heavy gear. Deck winches / hydraulics + the generator / hydraulic pump to drive them + ice + cooler + big diesels to get to port reliably while the catch is still saleable. Ooops I forgot the fish ...

What figure has been allowed for this ? Will the boat carry it ? Perhaps so ...but we cannot tell because we see nothing of the most important part of the boat .. the part under the water.

Questions like this I could ask forever. Perhaps they have been answered ? But how ?  What is the waterplane area ? Max hull beam at waterline ? Draught ? Rocker ?

Hopefully either he or his architect have considered all these + 1000 other questions and he will not be throwing his time and money away.

[Omar M. Rashash]

I seem to recall that aluminum boats have their very own specific issues with galvanic corrosion, more so than most other construction methods.  IIRC, this has more to do with stray currents, than anything else.

[Bradley [Brad] Warne]

Hi folks,

I have read several good questions but to me David's is where I'd start too. First, to the point of knowing the weight of Aluminum, it's not that I can't find the weight of Aluminum but more what is the design weight of this Aluminum boat? Then next I'd want to compare that to a similar hull with a similar water line length to get an idea of how closely it will float to the desired waterline. (I'm guessing that the Naval Architect probably has accounted for this) Once I understood how she floats empty then the question of how much more weight will be added in equipment (and where would it need to be located) is what I'd look at to see if I had any capacity to store the catch of fish. (I'd also want a little room for beer.)

That said I do find the idea and the lines kind of uniquely attractive.

[boatsmith]

Not mischievous. From looking at the drawing it would appear to be a very Wharram shape to the hull. And looking at the strucural drawing it would appear to have a lot of framing.  If there are cutting files developed then someone can tell us what the aluminum specified weighs. I for one am interested in this number. I like the look and concept as well. 

[Omar M. Rashash]

 I'm not a naval architect, nor do I play one on TV.  However, I do have a copy of " Boat Strength for Builders, Designers, and Owners" by Dave Gerr.  It would appear the information is there to convert wooden scantling information to aluminum.  JWD has stated that aluminum is an acceptable material for their cats.  If i wanted a 50' aluminum cat, I'd look at converting an existing, documented design, with known sailing characteristics, before I had one designed and built from scratch.

 

Additionally, how about this link- Aluminum Wharrams

[Chris Samoisy]

only alu weights 5.7 tons

[Chris Samoisy]

here some specs: Loa 15.5m, Boa 8.0m, max. beam hull 2.5m, WLL 13.0m, draft 90cm/9.2tons (max beam at WLL 116cm). single chine construction with sometimes little inside curcetures on the sides. bow going deeper down and keeping slimline until mid section. after midsection until stern keeping chine larger but keel going more up so the hull turns better. an aproachment to more "modern" hull forms but still in a "flat" plate single chine building. max. weight 12 tons/101cm draft. two loadcases: as cruiser or local fishing boat. it is not a commercial fishing boat with no commercial fishing equipment on it. for local fishermen to go longline fishing in the open sea in safety. as fisher version each hull will have a insulated compartment, max weight incl. fish and ice is 1.5 tons each hull. Material thickness: keel 20mm, sides up to chine 6mm, sides above and deck 5mm, ringframes 6mm each 700mm distance, stringers 45/6mm flat. as longer a boat as better becomes aluminium in view of strength to weight. this boat is even overbuilt, normally australian cats are from 4mm skin material (mumby or easton cats.) there is "sanderling" an oro 46ft 30 years old, 3.9mm aluminium. no corrosion still going strong, and no paint at all even underwater. another is for sale in raiatea, an narai mk4 built in SA 1997, of 4.5mm alu, round beams dia 200mm/6mm. our beams will be of u form alu 6mm and welded together, also pretty much overbuilt. seaworthiness and seakeeping was most important in this design. dont try to turn ply boat plans into aluminium boat plans. aluminium is different stuff and needs another way of boat construction. you can take a boat design and make another construction out of it in a similar design. but is not less interesting!

[boatsmith]

Shore ain't no clorox bottle

[Galway Bay]

Thanks Chris for a very valuable contribution. Weight is of course behind every decision in boatbuilding. Interesting that for comparison the Tehini at similar dimensions is given as 4 tons dry 6.5 tons loaded for long distance. So the Alu. seems to be working out heavier. I very much agree that if changes in materials /use  etc. result in major changes in weight then this needs re-design perhaps wider hulls etc. Sure you can just load up the boat and it will still "work" i.e. it will not sink but with a new shape you will achieve much more.

I am only an amateur but you did ask for comment - About Alu construction I know little but are those beams at 200mm deep enough ? If you have not done so already perhaps ask your arch. to calculate the deflection under load from the down thrust of the mast ? About shape - keeping a deep hull forward and tapering up to the w/l aft is a description of the basic hull shape used in racing kayaks - obviously fast - it is a low wetted surface shape. However I would be cautious about keeping "slimline" forward cats need pretty blunt ends to counter pitching.

Do keep us posted if you go ahead. Especially on anything that might apply to Wharram cats.