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

[Chris Samoisy]

this one falls in the group of the Islander range 55 and 65ft of the wharram cats. in a way a smaller design of the islander 55. I would recommend the same rig (a wingsail schooner rig) as on the islander 55 (88ft/ton). just need to add a second mast and the mast support. the mast support is a channel with a bolt fore and after the mast, so you can change the position between the beams freely without taking the mast down. The lateen (or crab claw rig) on the picture is only on the first building because of the local weather conditions where this boat will be sailed and for the fishermen who are using a very similar rig on their small fishing boats. all year round south easterly passat winds, most of the time 15 , 20 knots or more. waves 2 to 4m are often here. the sails will be produced locally with locals and so the wooden mast. I would like to put light carbon spars one day later and increase the sailsize but only if realy needed. 2 outboarders 25 hp midship between the hulls, similar arrangement as tiki 46. I have just seen that the islander 55 has close weight caracteristiques, 7 tons empty and 12 tons laden.

[Galway Bay]

Thanks for reply - you seem to have considered many of the alternatives available to you. 

For what it is worth I live in a high wind area and it is my experience that for passable windward work the rig should not be much less in height than LOA. Much less and you will mostly be motoring to windward. My experience is with European traditional rig and a Wharram cutter.

The design point here is that there is no point building more windward ability into the hull than the rig can deliver, as you will pay a price [ extra drag ]on all other points of sail - including motoring.

After 20 yrs. with my Wharram and some small experience of some others my opinion of deep "V" hulls is that the real drawback is not the often quoted windward ability but the poor steering. My Wharram is easily the most difficult boat in my harbour to berth. This is a drawback in a fishing boat which comes / goes daily. Boatsmith has previously posted that he eliminated the skeg on his 8M [ Tiki 26 ] used for day charter for this reason.

I still favour a shallower hull for the purposes you outline for instance in Wharram terms something based on the very shallow "V""Amatasi" rather than deep "V".

I hope the discussion has been helpful to you and that you go ahead and build your dream.

[Jeremy Walker]

What Galway Bay says is easy to gree with and I connot help thinking that that the use of aluminium should have more influence on design.......for example the bottom plate could be made  alot wider to create a shallower draught and lower wetted area shape. Welded ally plate would make addition of centre plate cases an easy  thing to construct as well as  low maintenance factors. Lateral resistance in addition to shallow draught is available as and when needed, with a thick bottom plate taking to the ground without problem.

Since you have seen Wakataitea's craft, the use of a similar longtail engine is surely a better option than the OB motors as well? 

[Chris Samoisy]

at the end it comes up as what someone prefers on his boat. what qualities his boat should have and for what it should be used mostly later. It becomes even more difficult if you want to use it as a multipurpose vessel. in reality this ones first priority is a real cruising catamaran, fishing is just an idea like < why not not use it as a fishing boat aswell? So we made the calcs. and a possible setup of the cat as a fisher version.  what material you use on a boat is your personal choice aswell, as long the construction and design is made for it. the hullshape is reflecting of what seakeeping abilities you want on your boat. it is always a weighting up. in this point of view (hullshape) I m definetly sold to wharram shapes and wharram ideas of what means seaworthiness. but I m also sold to aluminium boatbuilding. OB motors i think are good because you just buy them and put them and that s it. if broken you change them. here 2 horizontal cuts to see the hullshapes (plane horizontal).real canoeshape on waterline height at 9.2tons/at 90cm, 1/11ratio beam/ WL Lenght. more up you go more it becomes a normal wharram shape at the end closer and closer like the roofshape (last cut at 120cm from baseline). the last one is not draft! the max. draft is at 101cm/12 tons.

[Galway Bay]

All discussions run their course and at 30 replies this one must be near it's end ...

I do not like the role of "naysayer" please I hope you build a boat and I am trying only to be helpful by providing a sounding board for your ideas. I am from an older time of Wharram design my familiarity is more with the Pahi and Classic rather than Tiki.

Much of what follows is a re-cap of what I have already said.

As to material Alu. is obviously excellent especially at this [ large ] size.

These waterlines look much less full forward than I would expect on a Wharram or other cruiser. My Pahi for comparison is practically parallel sided for 50% of it's length and quite blunt at bow and stern. This is a good catamaran shape there are very good reasons for it. It is very well suited to a boat with a moderate rig.

Again I have nowhere seen that you have previous experience. 50ft is I think just too big for an experiment which is what a first design must be. I know of no successful designer whose first boat was this big.

I would love to see you try first at maybe 25 ft. At 1/8 the cost.