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scape · 17-01-2012, 11:59

Hi all,

I am a recent signup but have been reading the forums for a little over a year now, and while my sailing experience is fresh I am quickly sucking up as much information as I can get.. I seem to have been bitten by the bug

My first time out was on a Hobie 18 (very tall mast, maybe 30ft, for it's length/width I thought) with a jib and even a spinnaker (we never used it though) and I had a blast. Instantly I fell in love with sailing, funny how all it takes is an actual ride and some participation and you can really get in to what sailing is all about.

Needless to say I then immediately tried out a basic dingy with a small lateen rig and got some basic steps down. Next up was hopping on board a Conser 47, which to say the least is a complete hot rod of a cat. I don't remember the wind speed that day but it was mild, with 4 adults and just the jib we sailed at about 7 knots with ease around the bay, the monohull owners had to have been jealous. The Conser had maybe a 65ft mast?

So to get to my question and stop jabbering, I'm curious how important sail area is to boat size (other than the somewhat obvious, 'yes very'.) I really like the idea of building an affordable bare bones cat (Woods design seems rather affordable) some day in the future (don't worry I'm in no rush to pretend that I know what I am doing) and am curious as to why many catamaran has such different sail area versus hull sizes. Is there some magic formula to determine this?

An example is Rhea from Wood: it is 37ft LWL (11.7m) and a 23ft beam (7.1m) with a shallow draft and yet has a 915sq/ft sail area (85m^2). Seems like it would be a fast boat with that sail size, yet the Conser 47 is about 1065sq/ft (99 m^2) of sail area and considerably larger, heavier hull which sits a bit lower in the water with its LAR keels.

Any insight would be great

If there is another post that I missed that goes into detail on this, please point me in the right direction!

Intentional Drifter · 17-01-2012, 12:35

Hi, Scape, and welcome to CF!

You might want to check out Calvin Markwood's Multihull Dynamics, Inc. - Home

Calvin goes into a lot of the variables involved related to performance. It costs some to get the specific performance analyses, but he's got some good articles that don't cost to get them.

I'd also suggest: Richard Woods' site: Woods Designs Sailing Catamarans Chris White's Chris White Designs Multihull Sailboats High Performance Cruising Yachts - Catamaran and Trimaran

ID

singlespeed · 17-01-2012, 13:02

Hey Scape,
I've been sailing cats, big and small, since '68 and your question is a lot more complex than you think. The mast height is only one variable to consider, you must also consider hull form, total weight, boat beam, sail plan aspect ratio, sail area to weight, number of crew, etc. Knowing what type of sailing you will do (racing, cruising, daysailing) and the condions you expect to encounter (off shore, close to shore, lake, wind strength) in the area you will be sailing should serve as a starting point in design selection.

djmarchand · 17-01-2012, 13:10

The yacht design parameter- SA/Disp or sail area divided by displacement (well not exactly, google the term for its exact definition) is the simplest predictor of sailboat performance. Monohulls with a SA/Disp of about 20 or greater are fast for their size. Some cats get up to 30 and are screamers.

David

scape · 17-01-2012, 15:00

Thanks for the replies everyone. I assumed it was quite complex, just did not know what variables to look for.

I'll check out that first link for sure, the second two I know of quite well as I drool over many of their boats (White and Schionning especially, Woods has quite a few winners as well, and probably more affordable for me to potentially build.)

The SA/D for Rhea (Woods design) is 30+, which made me think that there must be much more involved, as I don't think that boat is considered a screamer.

I'll keep reading up! Thanks

Sand crab · 17-01-2012, 15:20

Keep in mind that just about all the displacement stats are very optimistic. Some are outright fabrications while others don't allow any weight for all the extras you might want. It's not uncommon for boats to come in 25% to 40% heavier. And yes, SA/D of 30 is a screamer. Now if you can keep the weight down that would be a very fast boat.

Intentional Drifter · 17-01-2012, 15:36

A little word on the displacement issue. Sand crab brings up a good point, how do you know how much your cat really weighs? I thought I was going to get an answer the other days when I hauled her. Asked the Travel lift operator, "you got scales on that?" "Yup" "Let me know how much she weighs, OK?"

Boat comes out. Factory displacement loaded to the waterline is supposed to be around 16,500 lbs. "So, what's she weigh?" "31,000" "No way!"

Being a cat, we had to use the 300 ton lift for the beam and the boat barely budged the needles. So, the supervisor is walking by and over hears the conversation. He says, "Don't believe it! This lift is made for much heavier boats than yours. The manufacturer told us it is no where close to accurate with boats that aren't at least 50 tons."

So, there you are. Even if you ask at the lift, you can't necessarily get a correct answer.

ID

Sand crab · 17-01-2012, 15:58

I think the Rhea might be in metric tons so thats 10% heavier if you were figuring standard tons. It would be worth a check. Also check out Kelsall, Schionning and others for plans. Fusion has a 40 footer kit that will fit in a shipping container and they will "mail" it to you. Hulls, cabin everything is already built. You just assemble.
Rhea 40 cruiser with central cuddy by Woods Designs

Rapanui · 18-01-2012, 02:22

Sail area / displacement gives you a good idea of the light weather performance. In my experience I had a FP Maldives at 200 (150) sq ft/ton unladen (laden) which sailed exceptionally well and have a FP Athena at about 160 (120) sq ft/ ton which needs a bit more wind to get going.
For some idea of speed given enough wind, displacement vs length is a good starter. There is a useful graph in Gregor Tarjan's book 'Catamarans' which plots a number of modern cruising cats on this basis. You can plot your candidate on this graph and have some idea of its likely performance although of course there are a lot more factors such as hull shape and rig efficiency. I think you will not go far wrong if you benchmark metrics for your candidate and against other production boats for which thanks to this forum people are prepared to share masses of info. Thats what I did when I moved onto the Athena and it worked out well.
I apologise to my naval architecture friends at this naive and simplistic approach, but it works for me

44'cruisingcat · 18-01-2012, 11:30

Quote:

Originally Posted by djmarchand

The yacht design parameter- SA/Disp or sail area divided by displacement (well not exactly, google the term for its exact definition) is the simplest predictor of sailboat performance. Monohulls with a SA/Disp of about 20 or greater are fast for their size. Some cats get up to 30 and are screamers.

David

20 or 30 whats? What units are you quoting here?

Sand crab · 18-01-2012, 11:40

It's not in units of measurement. It's actually a ratio. If you correctly enter the numbers whether metric or standard you would get the same number.

SA/D Ratio is the sail area/displacement ratio. This ratio indicates how fast the boat is in light wind. The higher the number the faster the boat.

44'cruisingcat · 18-01-2012, 12:32

Yes, but there must be units of sail area and displacement. Is it m2 per tonne or what?

Sand crab · 18-01-2012, 12:43

It goes both ways. This one uses feet and pounds, or tons or cubic feet. Convert to metric and it will give you the same number.
SA / D Ratio
Remember it's only a ratio of sail area to displacement. You can view it or look at it as either metric or standard and it's still the same number.