For more detailed information on the concept please read the text below. Compared to earlier attempts I have scaled back the design, make it more affordable, more achievable. Please note that although 45ft may seem long, the very long ends means that it is much less of a boat than the 45ft length would at first indicate. Unloaded weights, guesstimate is 2 tonnes. Loaded weight guestimate 2.7 tonnes.
It may be simpler just to build in a deep Vee section, as frames on hull side can be straight sawn timber, instead of plywood or laminated frames. This would save time and result in a deeper draught, thus more lateral resistance, this may increase upwind ability a fraction too. Please note that I tried two masts, but could not really get it to look right. Crab claw sail is shown as it is a very cheap rig to make
It may well be cheaper just to buy a used catamaran (like a Wharram), especially where a sailor has half completed their voyage, and their boat has been sitting in a lagoon for awhile with few prospective buyers about. Not sure if this will continue to be the case as economy slowly improves
Roller furling jib is only high tech rigging gear. Mast could be on centerline, however this would require greater mast loads or alternatively extending the overhang on port side. Crossbeams are strip planked. Now this may seem a lot of work, however since they are above the waterline, they do not need to be long boarded or faired close, thus a rough surface would suffice. A nice rounded surface gives lower windage. Mainsail can be brailed in strong winds, also there is option of taking down the main rig and raising a smaller sail in strong breezes. Minimal electrics for navigation lights, cabin lights, bilge pump, cooling fans, radio.
A small amount of weight can be kept in the smaller hull for extra righting moment, drinking water, spare food, spare anchor etc
Resale value would probably not be fantastic as it is an unusual craft, not a trimaran, not a catamaran. I think it would be wise to keep initial money spent in a craft like this to a minimum as the boat selling price would probably be similar or a bit lower than cost to build. More information on costings is below
Some rough specifications
Weight unladen: 2t
Weight laden: 2.7t (5940lb)
Sail area mainsail: 300 sqr ft
Sail area foresail: 114 sqr ft
Length overall (LOA): 45ft
Length waterline: 35ft
Waterline beam: 3.75ft (approximate)
Main hull fineness ratio: 9.3:1
Bruce Number: 1.12
SA/D: 20.2 (middle of medium cruiser range)
Draught : 50cm
Attempt 3 - Modification of 52ft Wharram Pahi
Overall Concept and thinking
Some key characteristics of what I am looking a
Moderate performance is acceptable
Low center of gravity
Good seakindly hulls
Lower end technology where possible
Moderate weight on a long hull length
Obviously some solutions are out there, build a Wharram, go for a smaller boat, build 'low tech' in africa, buy second hand etc. My thinking is in this concept just to build one large hull, the second hull can be much smaller, simple in design, have no moving parts. This to me seems more economical use of materials than builing say two hulls at 40ft each.
There are no deckhouses. Overhangs are reduced to the minimum. These add weight, cost, windage and complexity. By putting more materials in just one large hull, it should be possible to get more value for the money. An exception is one simple, large box on the starboard side. This is for light storage, the overhang here should be fairly light, additionally as it is below gunnel level therefore windage can be kept to a minimum. The overhang can be placed inbetween the two mast stays providing a more intergrated structure.
This is not easy. My best estimate is $75K in 2013 US dollars. It could be a bit more.
One way is to make a list of all the components, materials, labour etc, and add all the prices up. Another way, which I feel is quite legitimate is to look at an existing build, and apply a factor. Using the latter logic, Alex N. built a 12m proa 'Nixie' in Thailand during 2012/2013. He says that it cost him $25K for materials, plus another $25K for labour, rent, incidentals etc. This gives a total cost of $50K. Now if we assume that cost is roughly proportional to weight, and we know that Nixie weighs in at 2t, thus a 3t boat might cost roundabouts $75K. This figure has a degree of uncertainty to it, however as a first number it should suffice. Also note that a brand new 38ft Wharram ethnic design was for sale in 2010 from a commercial boat yard in the Phillipines for $40K. However this boat has hulls that are maybe too small.
Looking at boats, it can be seen that many of the large prices are due to fittings. Pumps, goosenecks, roller furlers, electrics etc. By keeping these to a minimum costs can be kept down. Of course it is possible to go for a 'less refined' boat such as Hans Klaar and his Ontong Java, in short copy what he did though on a smaller scale. The downside is that such a boat lacks many, many modern fittings. It is an option to keep in the back of the mind though.
Its hard to be exact, but rough figures are 3 tonnes unladen. I base this figure that the 51ft Wharram Tehini design is quoted at 4 tonnes, and Alex's Nixie comes in at 2 tonnes. Thus a rough figure of 3 tonnes seems reasonable.
A daggerboard of centerboard would be nice, however that increases cost. Since money is always tight, doing away with these lowers cost and weight, albeit with the downside of poor upwind ability
Overloading and Construction Method
By eliminating deckhouses, weight can be kept to a minimum. A large hull has better load carrying capacity than a smaller hull. Construction is fiberglass over plywood. Living in Australia, costs of living are high, renting a shed to complete your build costs more. Yes its an option, however building in Thailand or the Phillipines seems to make more sense. The boat can be built much faster with two or three assistants. Note that Ontong Java was built in Gambia in under four months. Yes its not a boat for everyone. Labour costs in Thailand are around 20 dollars a day. Labour costs in Africa are much less, however skills and materials are much more limited. The 12m proa Nixie was built in Phuket, Thailand in seven months
Because there is the potential of the boat being pushed too hard and the entire boats weight being supported by the outrigger, crossbeams will have to be as strong as found in the Wharram Tehini catamarans
Masts and Rigging
I will start with stays. A wider stay angle gives lower compression loads on the mast. For this reason the crossbeams are extended to starboard by several feet to reduce mast compression loads. Additionally placing the masts on the crossbeams immediately adjacent to the main hull lowers bending stresses on the crossbeams.
My thinking is only one fixed mast. Wharram plans call for two masts. From this single mast a low aspect crab claw sail and a jib can be set. My guess is that the addition of a jib will allow it to point a little higher than two crab claw sails as per Wharram plans. Crab claw sails dont point as high, however they have the benefit of low cost, an example being Ontong Java's mainsail which cost $200. Self steering can be achieved via jib to rudder bungee line.
Building masts out of wood seems the most cost effective way of building in countries with low labour costs
Here is another way of progressing. I did these skeches slightly earlier, however comparing the two, I feel I prefer the concept of modifying a Wharram design. Please note that although Tahiti Nui Freedom has a 50ft hull, it has zero overhangs, one mast, and a very simple outrigger. Thus complexity, weight and cost would be a small fraction of a 50ft cat.
Please note that there are higher resolution copies of the same diagrams done, but they wont fit onto a typical computer screen
My start point was the midpoint cross - section. I based this shape on an inverted Gothic Arch. The Gothic Arch has the proportions of an equilateral triangle. From the midpoint cross-section I was able to determine what main hull beam would be, also draught, and height from keel to gunnel. However I am not sure if this gothic arch section is superior to a simple deep Vee, upside is better load carrying, downside is greater complexity
I put in a very similar hullshape into computer software, and it came up with a draught at 61cm (2ft) at 4500kg load. This shape was not perfect, however it does give a rough estimate of what draught should be.
Attempt 1 - Modification of 50ft Nui Tahiti Freedom