Open source 12-15m high performance/semi-cruising catamaran design

The engine data is for a pair of 20hp outboard motors. There is room for diesels and saildrives under the midship floor but i dont want that, im happy with outboard motors.
I havnt done the budget on structure yet, just the basic shell, unpainted or gelcoated etc around $60k. The rig and sails will be another $50k minimum - probably more like $75k with all the hardware. Then everything on the sundries list and many more things i havnt got in there yet...

I have another spreadsheet which calculates the strucutural panel weights and costs very accurately.

All up project cost excluding any labour likely to be around $150k

 

updated the spreadsheet with the Center of Gravity calculation in there. Remember this was deliberately located 1.3m Aft of midships so that the original C foils used for lift only - if we indeed go that way- and the resulting moment calculation from the rig driving force. So then remember we needed the CoG at this location to acheive the result of 1 degree pitch up when the foil is lifting 50% boat weight with the foil at this forward location?
Well we can move stuff around now to acheive different goals should we decide to go another way. The main consideration now is whether to ditch the complexity of 3 or 4 boards and just use a single pair of C boards for leeway and lift - ALA Beiker Fujin. If this were the desired configuration, then the C boards would be located further aft for ideal leeway positioning and we would have to reconsider the CoG and foil lift with it and we may have to move some of the items in the boat to arrive at a desired CoG. Many of the items are easily moved as they are located below floor level, tankage, batteries, watermaker etc, many heavy items can be shifted along under floor level if need be.

 

difficult to know precisely but the C-boards on Fujin do appear to have overly large chord. Might be to provide the lee resistance area. If you design for only 50% lift and keep both boards down the windward board with its concave profile to the slip is going to provide maximum lee resistance and considerably lower lift. Might even help to reduce heel? Lee foil develops more lift than windward foil.
Setting the c-foils on the COG axis may make the boat too nervous and pitch happy. Setting them slightly forward of COG and apportioning more lift at the rudders would seem to yield greater stability.

 

Fujins Boards;

I would suggest that Fujins boards appear to have a rather large chord for multiple reasons. Having a large chord allows a greater thickness and thus the structural problem is easier to solve. Also a large chord means a lower aspect ratio and thus the boards will not stall as easily or abruptly during a tack or other slow speed maneuver which can be a problem on big cats. And yes, the increased area of a large chord means the board will not have to be as long to get the job done. Insanely long boards would be more difficult to structurally design and also a PITA to deal with in reality, deep draft when down and huge above deck when raised etc

I cant see any benefit to running with 2 boards down in this configuration like Fujin uses. Their sailing videos show they use 1 board only at a time. Using asymmetric boards of this size, there is enough lift available with 1 board down and with a powerful rig, your flying a hull anyway. Putting the second board down reduces righting moment and increases drag when reaching or beating. The downwind case would be interesting and perhaps the only reason consider 2 boards down. Its probably going to be slower then having both boards up - dont know for sure and depends on the % lift vs leeway defined by the shape and curvature of the C boards. Conundrums like this are solved by having seperate leeway and forward lifting foils. Imaging being able to completely retract the leeway boards, and extending the horizontal C boards up forward to pitch the boat up when running fast downwind in heavy air. The boat wouldnt bury at all and would be very fast running with the trades covering huge distances per day...

To my reckoning, this is going to end up going 1 of 2 ways.
The first way is the configuration just like Fujin.
The second way is basically the way of the Ultime / Mod 70 trimarans etc which is with 1 leeway board in a center nacelle and 2 very much lifting C boards located at approx 58% LWL. The assymetrical C boards act like the ama foils on the big trimarans and a single symmetrical dagger in the center.
Ill do some renderings of this next...

 

All good points if you are ready to fly a hull for long periods. I am an old fart so absolute speed takes a back seat to "speed in comfort".
So your 50% lift is 50% of total boat wt per foil? That appears to be the Fujin philosophy. My vision was 50% from both foils in total, so 25% per foil.
you are right, drag would high with both foils down together. Good for downwind running.
Your original design brief emphasized heave attenuation. Best to have the foils more forward then.
Do I have it right? Convention: Bring the COE aft so the bows can be fine without excessive bury and thus shift the lee boards aft to match.
Yours: introduce c-boards fwd to minimize heave, prevent bury, and reduce wetted surface drag. So, can you shift COE fwd to match the foils?

 

i dont know what you're asking?

Think of it like this - Fujin is a foil assisted catamaran using mostly vertical C boards for leeway and a portion of vertical lift. They have rudder stabilizers which help reduce pitching also.

The other arrangment im thinking of is packaging the foils and daggerboard arrangement from a large racing trimaran into a cat. The C boards are located a bit further forward and are angled more horizontal in the hull to provide a large portion of vertical lift and a small portion of leeway lift.

The horizontal foils allows the boat to be pressed hard without burying and attenuates heave improving speed, comfort and safety. This is how those big trimarans are able to cross the indian ocean in 4 days flat. They can maintain 38 kts boat speed 24 hours a day for 5500NM in the southern ocean in strong winds because the boat is settled thanks to the foils and the bows are elevated with the entire boat enjoying a slight bow up trim in the following seas. If the boat didnt trim like that i doubt they would have the confidence to maintain those speeds all through the night without excessive risk of capsize or pitchpole like the older designs used to... Without those foils, remember the older designs use to trim bow down and creates a more dangerous condition in those fast down wind conditions - you never see that anymore. To me it shows enormous merit in the benefits of the more horizontal C foils not just in speed, but also improved seakeeping characteristic and you don't get so much of that using fujins arrangement...

 

here is a good view of the windward c-foil of Fujin retracted. The question is, where is the bottom end of the foil? If it is flush with the lower hull (seems reasonable) then given the curvature and the amount above deck, the C-foil could have significant horizontal area.
Trying to nail down exactly how much lift is expected.
edit. design images appear to show mainly very little horizontal c-board area on Fujin.
So, 2 small chord strong lift c-boards plus one centerboard in nacelle.

 

Thats exactly the way im leaning

With significant aft rake on the centerboard i dont think the lack of an end plate at free surface will be too much of a negative performance issue. Additionally - we can have an end plate on the bottom like maserati mod 70 which you cant do with trunks in the hull !
And no lost space from trunks in the most useful/voluminous part of the hull

 

renders of central dagger arrangement;

I think the mast will have to be moved aft a bit, shorter foot on the main and bigger solent - this will allow the dagger to come aft a bit too.
Have to do some structural calculations soon - the foils and dagger board will be a challenge - nothing a shed load of carbon wont solve...

One more thing- i was thinking about the boarding problem for swimmers. Then it dawned on me that the tender is being raised through the cockpit sole using the main boom. The tender could be sitting on a slatted platform made from teak or similar which is lowered into the water like a bit of a dive platform then suspended by lanyards. Once the tender parks on it can be raised up and then locked into position with glass dowels into the main bridedeck. so theres a neat little dive/swim platform available

 

That daggerboard seems to really mess with the fore sails. Any conflict?

 

In the previous rendering, the board is shown fully raised clear of the water - for when parked in the marina! when tacking upwind, the board will be all the way down! When running downwind, board would be partially lowered and allow the foresail clew to pass over the top... This is an identical arrangement to the big trimarans remember - its been tried and tested the world over...



The canting keel boats have it even worse... the boards are very close to where the foresail clew ends up...

 

OK, hopefully the water is always deep enough. Sure is easier to design and build than the swinging centerboard, and more reliable.

 

If the board is left at 400mm immersed depth- same as hull draft- then it should only protrude approx 2m above deck level. If you drop it further to say 1m draft and allow protection for the rudders then it will only protrude 1.4m above deck and the clew will either clear it entirely or just brush over it as you tack... so long as there is no sharp edges it wont foul the sail...

If the water is less than 1m deep - id suggest tacking the sail would be the least of your worries- if your even still sailing at all... most likely youll be slowly motoring in water that skinny

 

Yes, until you collide with a container and pitchpole or severely injure yourself. At the speeds this boat is going to be traveling it's not going to be pretty, hope that dagger will break as planned, that main mast beam being only 50 lbs and all.