Biplane Rigs

mike,
yeah, the sails I'm still wondering about. Initially I'll try out a Hobie Tandem Island sail, just one is all that's around that I could borrow. i could also use windsurfing gear, so that's got the wishbone boom. The problem with the windsurfing gear is that is has to be reefed for a certain adventure race I want to enter, and it would be a good thing to have anyway for some of my big trips I got planned.

 

I would be very interested to see some performance numbers off of the more extreme cats pictured, and compare them to traditional maxi-cats.

 

Not really a maxi, but Parlier`s is probably the best example...

 

That looks so cool! Thanks!

Eric

 

For some reason you can't view Kohler's site if you use Internet Explorer. Works okay with Firefox, Chrome, etc.

 

as of last night a few people have been having problems with that site, as discussed on the associated forum.

 

Not exactly sure what this is, but he's gone thru a few different sail types. So far the Hobie TI sail still has my interest but I'm certainly sill looking around.

 

oh man does that just heat up the blood or what?!
my little adventure racing cat will never be that .....
but it sure wants to make you build one close to that!

 

designing a cat is fine. Tri's are harder because of the stresses the ama's must take. I had a design for a small flat bottomed cat that could plane.
it was 24 ft long. had 3.5 ft wide hulls. stepped about one third back from the stern...parallel rigged...
building a model is a great way to see how it works...
remember there is NO perfect hull--believe me ive looked!!

 

I agree on this- a wishbone rig would be the way to go..cant see anything more simple- you could use simple bearings too...if i was building one this is how i would approach it...trouble is the best way would be carbon masts but that gets expensive for two...i thought a composite mast might work well--i.e. use thick core cell laminates--would cut down on the carbon fiber and you could use poly resin although it would be better to go with epoxy as thats a given...

 

A few more dual-mast catamarans:

Dragon Wings
http://www.duckworksmagazine.com/04/s/vintage/multihulls/index.cfm

China Moon
http://groups.yahoo.com/group/junkrig/photos/album/1410219041/pic/list

PHA
http://wharrambuilders.ning.com/forum/topics/tiki30-n119-pha-for-sale

 

I know, everything is a compromise, sort of, but built to a requirement and it meets that, then everything else aside it's perfect.

So I'm debating mast placement .....
on the inside , middle or outside section of the hull?

Because of my folding system I have even more considerations.

In the video you'll see me move the mast around a bit
just so the mast is out of the way during the folding operation.

As for hull types, if asymmetric I can place the mast on the outside against the flat part of the hull.
that would also allow me to leave the mast where it is If I change up the folding system just a bit.

If symmetric then I could use the middle,
and I'd not know how to get it on the inside but that would give me a bit of room for sitting out on the hull, no trap wires so I can't hang over.

 

hausmuzik,
thanks for the links and welcome to the Boatdesign forums. You seem pretty much informed on the subject of biplane rigged cats, which is sorta an obscure subject for most. Anything else you can scare up and post about would be great!

 

Ok, I'll look into the wishbone idea, which is really a windsurfing boom? and those are really stout! does it need to be so? also it isn't the lightest thing x2 and that adds up. can I somehow reef with it? roller furling? I'm having trouble getting past the windsurfing gear in my head, as I did a lot of that.

did y'all see that quad hulled beach cat? and the mast system?
I know I got a better pic of it somewhere .....





ok, a bit of zooming in on the original shows it better:

 

A couple of things here which are not so: First, for a free-standing wishbone rig, cored laminates will not let you reduce the amount of carbon fiber. Such masts work by bending, and you need a certain minimum wall thickness to carry the loads, tension on one side of the mast, compression on the other, the stresses acting along the plane of the laminate (very high shear, by the way, on the leading and trailing edges.) If you core this type of laminate, you still need that basic thickness of carbon material because of the way the stresses are oriented. Cored construction in this regard does not buy you anything--the laminates have to be solid. Cored laminates in basic round-section masts will only add cost, complication, and weight.

Also, when you design such masts, one finds that if you keep increasing diameter, you will tend to reduce mast weight--i.e. you gain strength and stiffness quickly by increasing diameter and reducing wall thickness. But at the same time, you increase the chances that the mast wall will buckle under load before the fibers reach their inherent maximum strength and stiffness. In addition, aerodynamically, bigger sections are higher drag than smaller sections. So the key is to have the smallest diameter and the right amount of wall thickness consistent with strength and stiffness. That said, for any given boat, there comes to be a very narrow range of mast diameter and wall thickness that is pretty much ideal. The engineering trick is to figure out what that is. In my experience, for any given boat design and once you determine the loads on the mast, the wall thickness should be a minimum of 3% of the diameter--that gets you in the right ballpark. After you get there, then you have to figure out the laminate and what fabrics are going to go where--it cannot be all unidirectional; you have to have 0/90 and +/-45 deg fabrics in there.

Most carbon fiber is sized (i.e. coated) to accept epoxy resins, or epoxy-based vinylesters. Carbon can be sized to accept polyester resin, but these fabrics are much harder to come by, particularly in the limited quantities required to build a mast.

As for bearings, simple bearings (by which you probably mean journal bearings--without any rolling elements) only work on the smallest of mast designs. Anything above LOA = 30' really should have roller bearings made of hard materials--preferrably 300-level stainless steels (304, 316). Plastic rolling elements will deform easily under load, and that greatly increases rolling friction, really. Aluminum and bronze bearing bodies are OK.

Just some thougths from one who has been there, done that.

By the way, some of the boat designs shown so far look to have masts that are really spindly, and they are probably NOT designed and engineered to operate under very high loads--something to keep in mind. I also saw a picture of Team Phillips up there--you may recall that she had a catastrophic mast failure. I wasn't there nor was I asked to analyze it, but from the pictures and reports that I read, her masts looked to be very poorly designed and built. Nice concept, but it needed a lot of refinement for a better execution.

Eric