Designing a fast open deck catamaran

I have thought about twin diesels like most big catamarans have, but they are heavy, expensive, take up space inside hulls, often have parts that are hard to get. When inevitably there is some diesel spills in bilge you have permanent diesel smell in your living area. If I go diesel route then it most likely would be some kind of DIY contraption using deck mounted car diesel powering long tail propeller shaft like plenty of boats in East Asia have.
Most big Wharram catamarans manage with outboards. IIRC some Australian performance catamaran designers also have outboards as an option. For a home builder outboards have very big advantage - they are plug&play. No shafts or troublesome sail drives, just simple raisable sled structure where outboard bolts on. Drawback is they have lower thrust and easily come out of water in steep waves making noise and aerated water not thrust. My 9.5m cat have 6 hp outboard with long leg and high thrust propeller. Maximum wind I can push against in flat water is about 30 knots. In open water with waves about 20 knots. You just have to be careful, think twice where you are going, never go in dangerous spots where you can't sail out. I have managed with this weak outboard sailing in West coast of Norway and tidal waters of Wadden sea without trouble. On the other hand when winds are favorable I can easily make good progress against tidal currents that would stall or push most regular small monohull sailboats backwards with sails and engine going full power.

40 hp would be kinda under powered, but my initial intention with next boat is to build solid basic boat then test it extensively in Baltic sea and upgrade whatever feels inadequate. Good thing with outboards are they are easy to swap for more powerful model if required. If I decide on twin outboard setup that also is possible to upgrade without too much trouble.

 

How much did your 9.5m weigh approximately?
I once drove a 4cyl diesel longtail ,they are not very comfortable and another problem will be higher engine location , longer shaft and more difficult control .
Space and smell inside hulls with your cat arrangment is really important.
Good to hear you've navigated such difficult waters with a 6hp .

 

With all the stuff for about month long cruise and 3 - 5 people onboard I estimate total weight is about 1.8 - 2 tons. Empty boat with minimum stuff for weekend sail maybe about 1.2 - 1.3 tons. Exact weight is unknown since it has never been lifted with crane with accurate scale. What I have noticed is that when motoring into wind weight has little impact, aero drag is what kills speed.

Diesel longtail drive would only be backup solution if outboards don't work satisfactory. With many examples of similar boats powered by outboards that really shouldn't be the case. It would also be quite a lot of work to put such system together compared to bolt on outboard. Performance catamarans generally sail really well so engines are much less needed. If wind angle allows to deploy my 50m2 genaker I can sail in as little as 3 - 4 knot wind at my motor cruise speed. In 20+ knot winds and steep waves my boat is essentially without motor because of difficulties of keeping outboard leg in the water. An even longer leg than standard long leg to submerge propeller deeper would help more than any extra horsepower.

 

Thanks for the answers . I once saw a quite big cat on the net that had 2x90hp outboards. I wondered why not diesel. If they relies mainly on sails that's ok.
And again more ideas to consider.

 

Hi Mulkari,

Your designs looks nice. One thing: I noticed the centerboard and rudder look to be too small. According to woods the centerboard should be about 4% of the sail area. Not sure about the rudder.

Sailing Catamarans - Keels or Daggerboards, the pros and cons https://www.sailingcatamarans.com/index.php/articles/12-to-be-published-mainly-technical/53-keels-or-daggerboards-the-pros-and-cons

 

I would go with dual outboards for all the reasons you mention, plus they offer redundancy/backup, can be easily removed and serviced off and away from the vessel, will provide better maneuverability, and of course don’t require a hole below the waterline and a seal, or the need for underwater/haul out cleaning.

And I would mount them along the rear crossbeam to reduce porpoising and to keep the weight off the sterns and clutter free.

 

That would definitely go fast in calm water. However unless they have some sort of super long legs in rough water all that extra horsepower would just make a lot of noise and aerated water.

Does it hold true for larger boats? Currently as drawn my centerboard total area is around 2 m2. Upwind sail area would be around 100 - 120 m2. It would indicate I need almost 2.5m2 per board. I have never seen boards that big on similar size boats. Assuming board about 50 - 60 cm wide it would have to go 4 m deep which clearly is not the case in real boats. They generally go about 2 - 2.5m deep. My 9.5m cat have upwind sail area of 47 m2 and single surface piercing fairly low aspect ratio centerboard with about 0.9m2 area and it works acceptably despite being much less efficient than if mounted below hull.

That's most likely what I will do although before going with 2 outboards I would like to experiment with one centrally mounted outboard that steer together with rudders. If that gives enough maneuverability then I save about 100 kg of weight and it cost less. Outboard would be located about 3 - 4 m from sterns below rear deck on swiveling mount to give it best chance to maintain thrust in choppy water.

 

Yeah, I don't know the answer to this. Certainly what you drew is in line with a lot of cats out there. You say the boards on your current boat are fine, but you can't really know unless you race against the same boat with bigger boards, or have a lot of experience racing with different size boards. You could have also, for example, had a boat that is twice as heavy and found the performance acceptable. I'm guessing some cats have smaller boards just because it's more practical: they take up less space, cost less, are easier to handle, etc.

Kurt Hughes seems to use about 1.5% per board for larger cats, although one of his designs is more like 2.5%. that would equate to 1.8m^2 on your boat or 0.77m x 2.3m with a 3:1 aspect ratio.

The 7:1 or 8:1 ratio 4m long board you mention is what you see on AC72 catamarans.

 

With the same sail area, a lighter boat with low windage will be faster than a heavy one with full bridgedeck cabin - so less lateral area is needed to absorb the side force.

 

I have sailed my 9.5 m cat against minikeel cats and I go upwind much better. My design goal was not to equal daggerboard boat, but to be somewhere in between minikeel boat and daggerboard boat while having only 0.3 m draft and rudders and centerboard safely kick up in case of grounding. It works really well for my intended use case often sailing in shallow water and frequently parking on beach.

That's good point that boards on most cats may be a bit undersized to make them cheaper and more easy handling although in case of conventional daggerboards there is only little weight and cost penalty for having longer board since case is the same size regardless. Board would just stick out of case a bit higher when raised. In my design with board cases horizontal and pivoting centerboards larger board means longer case that has to be integrated into interior layout. I want my centerboard cases tall enough that top are always above waterline to allow access to boards without lifting the boat. A 4 m long and 0.8 m tall case would really screw up with interrior so some kind of compromise has to be found.
Racing cats obviously would optimize for best speed upwind and have daggerboards that have best compromise between lift and wet surface area.

That makes sense, a wind pressure on tall bridge deck cabin would cause extra sideways drift if board size is the same compared to similar, but lighter and faster open deck cat.

 

Yes too ... but more impotant - the windpressure on a tall bridgedeck cabin slowes the boat upwind - and a slower boat causes a bigger angel of attack (what means more drift) if side force and and board area are given - or the board area must be increased, until the accepted AOA is reached on the lower speed.

Its the same as with se sails - on lower wind speeds you need larger sails - on lower water speeds you need larger boards.

 

What if you make offset centerboard to save space inside ?

 

That's the plan, put the board box as close to the chine as possible, and use the space behind for fresh water tank and possibly waste water tank too. On top of the board box would be galley table with stove and sink. However 2.2 meters is plenty for this. If I stretch the board box longer it starts to intrude into other spaces. Besides board box should be supported really well which means it should terminate at major bulkheads for best possible support. Front would terminate at mast beam bulkhead, rear at bulkhead separating galley from toilet/shower. If I make board box 3m or longer I destroy space intended for toilet/shower. I'm really not sure if boards that long are required. For exapmle
Gunboat 48 max draft 2.25 m
Outremer 4x 2.16 m
Dazcat 1495 2.75
Orc 50 2.7 m

No one would say these boats are slow upwind yet none have boards 4 m deep and most likely boards are fairly high aspect ratio so total area of both boards likely are closer to 2 m2 than 4m2.

 

I'm not very familiar with boards but evaluation of examples is good idea. I think there are professionals here who can help you better . The board box shift will be towards the outside the hull ?

 

Yes the board boxes would be offset towards outside of hulls.

I looked other performance cat Mumby 48 and it has only 1.8 m draft with boards down. That is only 1.33m of board sticking out the bottom. If board is 0,5 m wide that is only about 0,67 m2 of area per board. That is smaller board than my little 9.5 m cat have. They have 2 boards so combined area is bigger, but not by much while having almost 3x more upwind sail area. Yet I couldn't find any info that Mumbys would struggle upwind.