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

[Raf]

Thanks Chris for the very valuable specs. Some points to consider: 1) Why choosing U section for the beams?  Full round section gives higher strength for the same material and therefore, lighter and cheaper. 2) Lifting the stern, is possible only by increasing the rocker, this will increase maneuverability but will also increase pitching motion, not a desirable property. To get a maneuverable and stable boat, pitching wise, transom on a wide flat stern on the waterline, is the answer.    

[Chris Samoisy]

[Chris Samoisy]

the beams are 380mm high and 280 mm wide, in a kind of box beam, very strong built. why did I not take existing extrusions? these beams you can fabricate on place if you have a "bending bench" which can bend 4m long pieces. available aluminium extrusions are much less good for welding than aluminium plate in marine grade (5086 or 5083). the same is for corrosion resistance. for a mast aluminium extrusions are okay. these beams have welded on brackets for the attachments of the beams to the hull, so good welding is very important.

[Olivier R.]

First sorry to reply in French, I have just one comment and maybe the google translation is not bad. This is a very nice boat but the weight is a problem for small details. This boat will be much heavier than a plywood boat so it will take the chain to the anchor and more stronger pulleys, a bigger and more powerful heavy mat, and therefore more complicated arrangements for heavier must be so insulated.

En premier désolé de répondre en français, j'ai juste une remarque et peut-être la traduction google sera pas mauvaise. C'est un très joli bateau mais le poids est un problème pour de petit détails. Ce bateau sera beaucoup plus lourd que un bateau en contreplaqué donc il lui faudra de la chaine pour l'ancre en plus, des poulies plus fortes, un mat plus grand et plus lourd un moteur plus puissant, des aménagements plus compliqué donc plus lourd car il faut faire l'isolation etc. 

[boatsmith]

these chines add minimal volume to the hull while adding to the build cost and weight.. I would think that the chines are indeed a way to provide additional buoyancy  but would be more effective with a  more pronounced chine, as in less deadrise.

[Chris Samoisy]

a few of you are saying this boat is heavy. as comparison a tiki 46 is 1.4m shorter in WLL. an Islander 55 is about 1.2m longer in Loa than this alucat. can anybody tell me the max. total weight of an Islander 55? This design is in the middle of tiki46 and the islander 55 in view of weight and size, even closer to the islander 55. I think actually the weight is not to bad as a workboat configuration. imagine 6mm underwater hull skin and a 20mm keelplate! that is crazy strong built. you can fall dry on sharply pointed rocks with this design, which is important for local fishermen on remote islands. You can built this boat from 4mm alu which would take at least 15 % of weight away. what is the max. weight of a 63 ft gaia or the 65 ft islander? than you can make comparison to bigger cats. This design is at a max. weight of 12 tons. I estimate the boats weight between 7 and 8 tons. so you will get 4 to 5 tons of extra loading which is pretty much compared to similar "plastic cats". However it is everyones choice of how strong he wants to have his boat built and he has to manage afterwards the heavy seas with his choice. a few of the wharram builders sometimes add more stringers or stronger beams or insulation or other stuff. at the end if the hull design is made for it it is okay in my view.

[Galway Bay]

Chris -

   Tehini is 51 ft . Construction ply on stringers [ not stitch ++]. Weight in my catalogue 4t empty 6.5t cruising with stores. What it actually weighs - well there have been several comments on JWD estimates of weight - and time !!]

   Weight is not a problem when designed for. It is a concern when you change build material etc. and overload an existing design. You have made this point yourself but it is worth repeating.

   I also am with boatsmith - if you are going to the work / expense of a chine it makes sense to make a softer "V" bottom for speed / steering /loading.

[Chris Samoisy]

a chine can have a few reasons. one uses a chine to have more space inside for example. in this alucat design we have put a chine because of having the possibility to give the underwater hull shape a better form and the aluminium construction needs it for stability. if you have one flat sheet on the side you are pretty much fixed with it. you can change the wide of the keel and the shape ofe the roof and that gives you the underwater shape. so that minimizes your design pretty much (hobby-horsing and other minus). I was very surprised what is possible with a chine on the right height and with the variable distance you can give between the chines and the curveture on the sides. by keeping enough draft (important for lateral surface as keel which wharram cats need for windward ability) you can make a very nice aproachment to modern hullshapes underwater and still keep the seawaorthiness of the V hull shape. If you look at the Amatasi it is the best example for it. take a similar hull design on abig cat you will have nice hulls. There Wharram uses the advantage of a chine to give exactly these changes underwater.  now another strong point of aluminium boatbuilding: if you look at the size of an entire side sheet it would be to big and not handy to put on place. so you are happy to have the entire side cut in two long sheets (one chine). all pieces are anyway cnc cut and will sit perfect on place. so the sides come together nicely where you put the chine welding, a kind of reference line. a chine is always a good line from where you cane work from. in aluminium a chine doesn t make a significant price difference, where in plywood i can imagine the hassle a chine brings with it. even the ringframes are two or three pieces which you weld togehter very quikly. the entire boat it s a big puzzle of pieces to weld together. this construction is meant to build upside down, put the roof down on a flat surface and than you tig the longitudinals and the ringframes on it a.s.o. don t compare plywood boatbuilding construction time with aluminium. aluminium boatbuilding with cnc cutting files  are so much faster to built! you must have seen it that you can belive it. ask Nz and australian alu-boatbuilders. i like also that you can weld all kind of small pieces on the roof or wherever. no holes to drilling in the hull with aluminium. the pahi 52 comes in my mind also which is similar in size.

[Galway Bay]

I know nothing about scantlings in alu. but I am happy to hear the beams are 380mm high not  200mm which is the sort of size I might expect on a 30 ft. boat.

 Reading your posts again I get the idea this is maybe a commercial proposition for you ? You speak of "two versions being available ?" So what question really should we try to answer for you ? Yes you can build this boat and it will "work"  i.e. it will float, sail somewhat, and proceed under power and be long lived and safe.

But could you build this on spec. as a cruiser in the hope of recovering your money +...well look at the sale offers on this site compared to build costs. Even a pro. builder would not launch a new build without the first deposit...

Fishing boats are even more difficult. Long lining is mostly done in small [cheap] boats. This boat is hugely more expensive. No one will buy it unless it guarantees them to catch a correspondingly huge catch.

50 ft is a very big platform to try out new ideas on, or for a first design. Unless you are seriously wealthy I would love to see you try the concept at perhaps 30 ft. Nobody gets it perfect first time.

J.W. tried this concept [ and won a design competition ] at 25? 27? ft. Interesting that he did not use the deep "V".

Personally I think that if he had developed this shape 30 yr. ago we would all be sailing better boats now.

 

[Raf]

Skegs aren't there? The stern is higher and the bow goes in deeper, this is a recipe for a boat that will broach easily and will demand a lot of rudder work to keep in coarse. (broaching is the tendency of a boat to slide away from the wanted coarse and put it self across the waves. This is especially so when going downhill) The sailplane will have to be further ford which it means smaller main and bigger jib or adding a bowsprit. To change hull shape and boat weight might results in very undesirable behaviours and possible disastrous losses of time and money. 

[Chris Samoisy]

you are right, the skegs and the hang on stern rudders like a tiki 46 are not visible on the pictures. This design will of course have them, pretty much the same as wharrams have. I find the setup of the rudder still the best and most simple one. calculations have be done plenty during the last 3 years of development on this design. These days there are many pc programms who calculate all kind of stuff. LCF, LCB, Cp,Cb, Cm, Cwp and many more of indicators which need to be understood well and changed well if needed to get a nice hull shape. especially LCF, LCB and CLR are very important that they are positioned on the right place and right relation to each other and not to forget the rig which plays also into it. thatfore good PC programms are a  "must". A few years ago I was a kind saying that these people just sitting on a chair the whole day long are not able to design better than a many years long ecperinced sailor and boatbuilder. PC programms cannot do it better. During the planing stage (which has taken at least 3 years) it has shown me, and I did not like to agree with, that the  " seat warmers" in the office are right with their calculations and much more precise. However, it does not mean that long known approved hull designs are not fullfilling these criterias. often the calculations are just showing  that the design is seaworthy and always will be. the changes you make on newer designs are anyway just slightly different of existing approved designs. designing a good hull is always a compromise.

[Galway Bay]

Chris - you seem to be an experienced and competent alu. fabricator. I hope you go ahead and build this boat or some other.

A couple of last suggestions on design and meant in a positive and helpful way -

I have fished from my cat [ who has not ?] and over the side is better than over the stern as from this position drift / fish pulling will easily result in lines getting caught in skegs / steering gear. So central cabin would leave the sides clear for fishing gear.

Work boats used typically 100 sq ft sail/ ton. They were not renowned for windward work until leisure sailors increased their sail areas.  51 ft  Tehini sets 100 - 150 /ton. On my 31 ft I set 200 / ton. Now I only have your thumbnail sketch but it looks like about 50 / ton.

If this is true this is a motorboat with auxiliary sail. As such it will perform better with a wider shallower hull.