Broaching

[Steve Rust]

Del
I guess I left out one critical item in my last post. The beam to lenght ratio applies to the individual hull at the waterline, not overall beam to lenght.

[delmarrey]

I don't remember where but I believe I read somewhere, I think one of my multihull mags, that the ratio of length was 4:1. I'll have to go threw the mags again and see if I can find it.

But even at the water line that seems a bit off. Most cats now a days have a square nose so the water line is pretty much the same as the deck.

I believe I found the ratio in here some where but I don't have time to read it all over again................._-/)

Performance Charactoristics of Catamaran Hull Types

[Steve Rust]

My Searunner has a mainhull waterline beam of about 5'6" that flairs out to about 6' at the wing. That is just a little more than 7 to 1 but it is an older design and they tended to have wider main hulls than the newer designs. I think those ratios are a good starting point for a good blend of performance and load carrying ability. 4 to 1 would give you a hull beam of 10' on a 40' foot cat so that ratio is to low. I suspect that the hulls on some of the newer cats are getting fatter than 10 to 1 ratio.

[delmarrey]

I believe I found the artical I was thinking about. The #'s 1 & 4 are there but it was 1.4 not 4:1. And you must be talking individual hulls and not the overall beam, a little confusion there.

Originally Posted by MULTIHULL MAG 2002

The above critique is based upon the proven science that the hull-speed limitation factor is a result of the ratio of hull waterline length (WL) to hull waterline beam (HWB). The customary hull speed limitation formula being VMAX=1.4 WL. In reality, the 1.4 is a variable, not a constant. That number increases based upon the WL/HWB ratio. Catamarans are theoretically proportionately capable of higher speeds than monohulls simply because their hulls are narrower, therefore, they generate bow waves at much higher speeds. Those theoretical speeds are generally higher than the vessel can go anyway for other limitations, so hull speed limitation factor becomes a moot point in large catamarans.
A few paragraphs down:

Since Polynesian times, certain fundamental ratios have been observed, the single most important one being overall length/beam ratio (BMAX) That ratio is beam equals one half the waterline length. (BMAX=WL/2). This is occasionally stretched to overall length (LOA) in some cases but any further and you begin to get into difficulty. A fifty foot boat with a thirty-two foot beam may be close to unmanageable. On one such boat, a good client of mine who is an experienced cruising catamaran sailor, brought such a boat back from South Africa to Florida. On the trip, they stuffed the bows so badly it threw everyone forward; Fortunately, with only minor injuries. He complained that the boat pounded so badly it actually hurt his feet. There were numerous other complaints. The boat now sits unused tied to the dock. Proof of this can be found in the saga of Play Station which was originally designed at 100 feet but nose dived so badly it was considered dangerous. Lengthened to 125 feet (which narrowed the BMAX/WL ratio) it became the record breaker.

Here is a link to the artical

http://www.sailcopress.com/elusive_c...aran_perfo.htm

But the ratios seem to be all over the place. Here's another artical w/ 8:1

http://www.cruisingworld.com/article...ID=419&catID=0

And another one w/ 12:1 and 16:1

http://www.southwindssailing.com/art...iseMulti.shtml

I would speculate that it all comes down to the purpose of the cat, just like mono's
...................................._-/)