Displacement Catamarans

[JordieS]

Hello everyone,

I've recently taken an interest in displacement catamarans, and I am currently doing a bit of research as I may design a 6 metre displacement Cat.

I am interested in them because they can go quite fast with relatively little power. Somewhere on the internet someone said they could go 10 knots in a 20ft displacement Cat with 6hp, does this sound reasonable? Do you think it is possible to get a 6m displacement Cat to do 15 knots with 15hp? or just 10 knots with 15hp?

Thanks

[sabahcat]

yes, if you just want hulls and want to travel on the perfect day
But what else do you want it to do?
carry a house and a load, then probably no

[JordieS]

The boat will just be a small runabout, no cabins or anything just a few seats for 7 people or so.

[Ad Hoc]

Make sure you L/B ratio is above 10, preferably closer to 15~20 L/B ratio. Then your L/D (length displacement ratio) by default should also be good. Just don't have the 2 hulls too close together...too much interference effects.

[bertho]

Jordies,
be aware than cat have more or less two time the hull area as a monohull have (deck area + 2 hull area, and need to achieve around the same displacement as a monohull to perform, so the material need to be two time lighter than what you will use for a monohull... for identical or better strength as the platform will have more stress, so, be ready to spend more $$ on the hull construction to have a good result.
all the best
bertho

[JordieS]

Bertho, correct me if I am wrong but a displacement Cat is faster because it's hull's can be thinner than a monohulls and since it is a displacement boat weight is not as important for speed as the boat does not produce dynamic lift.

[Richard Woods]

Have you seen my Skoota powercats?? See my website for more

eg Skoota 20 : 20ft, 25hp, 2 berth, 15 knots, 10mpg. Trailable

I don't think you'll do 10 knots with 6hp, but you will with a 9.9

Remember that 7 people weigh about 500Kgs, so more than the weight of the empty boat

Also remember that the engine will weigh more than one hull (one hull say 150lbs, one 25hp outboard near 200lbs)

Richard Woods of Woods Designs

www.sailingcatamarans.com

[bertho]

jordies,
with a "displacement catamaran" hull, if you double the volume to move on the water, even with a better ration length /beam, will be like two "better shape" hulls to move instead of one, two time wave resistance + interaction/slimming pb between the hulls as said ad hoc, two time friction area, ...no miracle.. especially with only 6 meters waterline, look perhaps for a longer waterline with slim hull, or built your cat very light !
bertho

[gordanm]

I'm also interested in design similar to what JordieS describes. For long time I've been searching internet for such design and but had no luck so far. My preferrable deck layout would be something similar to Bayliner 197 SD deckboat () http://www.bayliner.com/_eumeaf/index.php?content_id=5367&boat_id=38 (http://www.bayliner.com/_eumeaf/index.php?content_id=5367&boat_id=38). I would also be happy with 10 HP engine and max load of 4 people. Basicaly something similar to Jarcat 6 but without the cabin. Does such design exist and if not, what would be the possible reason?