biplane rig on a C cat

Hi all,

I recently bought a pair of old C class cat hulls (25' long) that I'm trying to come up with a rig for.

I'm thinking of using two Hobie 18 rigs in a biplane configuration, as used Hobie 18s are cheap and plentifull. The masts are 29.5' long. If I make the beam 14' wide and step the masts 8' apart, then I can have close to the stock rigging configuration (stock width on a h18 is 8').

What I'm unsure of is how the sails will interact with each other. The boom is about 7' long, which would put the two masts about one boom length apart. That seems a bit close compared to other biplane rigs I've seen.

Right now I'm thinking of using the main sails only - 166 ft^2 each, for 333 ft^2 total. That's a bit smaller than the 350 ft^2 class maximum, but I figure if the concept works than I can spend the real money on a custom rig.

Anybody have any thoughts on this?

 

If you step the masts on the beam, you are going to have to beef the sucker way up! The joy of the biplane configuration is that the compression can be taken on the hulls, needing just diagonals from P head to S heel, and a strut head to head. Plus maybe a couple of diamonds to keep them in column
Also, more separation means less interaction.
Steve "whose C-class hulls?"

 

Well, the masts will be stepped 2' inboard of the hulls, which will put bending stress on the main beam, but much less than a single large mast 6' inboard of the hulls. So I think I could get away with less in the way of a main beam than it originally had. I haven't run those numbers yet, though.

If I step the masts directly over the hulls, then the rigging gets funky. I figure it would need a beefy compression member between the mast tips.

Do you have any idea how these sails will interact? Will that interaction be beneficial?

I'm not sure who built the hulls. They came out of the LA area and were built in the mid 70s.

 

No. (short answer to both)

But, if you spend enough time, and if you have the patience and enough coffee, to search through Marchaj's books, I'm sure he has something in there.
Interactions are rarely beneficial, however, so be prepared to be disappointed. The main good thing about the biplane is getting more sail lower down than with a "conventional" rig. Also, when calculating sail areas and such, make sure you can fly a hull with the rig. Kinda sticky if you can't.

Steve

 

Hi,
I remember reading about the biplane rig of Parliers new hydroplaneur, the rigs separation affected the efficiency but the relationship wasn't at all linear. Maybe you could scale down the rigs/separation from the Parlier cat with froude numbers. That cat isn't as far as I understand a windward machine.
Also higher aspect rigs should lessen the interference between the rigs. This goes hand in hand with the above mentioned wish to be able to fly a hull.


Patrik

 

This figure from Kroo's Applied Aerodynamics text, shows the interference between biplane demiwings.



For more details, see http://www.desktopaero.com/appliedaero/configuration/multiplesurfaces.html

 

It also depends how quick your boat is going to be, if like Parlier's cat you are going to be acheiving 1.5 - 2 times windspeed the apparent wind is always going to be around 45 degrees off the bow and so there is never likely to be much interference or blanketing from the upwind sail.

So if you are going to have a light fast catamaran then perhaps the biplane will work well. Also I would consider stepping the masts as far apart as possible and use the "funky" beefy compression member instead.

Let us now how it turns out

Gareth

 

If your boat came out of the Long Beach area and was built in the 70s it is possible that it is one of the Aquarius clones. Could be a very nice boat. My guess would be that the hulls are some combination of foam and Kevlar and weigh about 130 pounds each.
I'm a little suspect of the dual beach cat rig solution for a number of reasons.
The masts may not be up to it because of the increased loads driven by increased stability. You would want to run numbers to make sure of this.
Second, it would seem that the weather rig poaches all the space on the trampoline and is right in your face as well. Not very habitable for daysailing.
I don't know how you would sheet hard enough to get the leeches to stand without some pretty significant traveler structure.
On the flip side, it's a lot easier to stand up a Hobie mast than a 38' tall single stick, but how do you get that spreader in there?
I think it might be easier to build up a wing mast out of plywood and composites.
SHC

 

blanketing

I've heard that on a beam reach the windward sail will blanket the leeward, cutting effiency. However, the apparent wind moves so far forward that it's not as big a problem as one might expect.

Just what I've heard; no personal experience.

Charlie

 

Team Phillips was fast with a pseudo-" biplaner rig" - well, until the boat broke!
The trick I think/recall is the interplane distance - space between the wings - and then the aspect ratio. Have some friends in NZ planning on a dual mast rig on a 54+ft Catamaran.

Just signed in/registered - thanks to DuluthBoat for pointing the way!
Cheers - Jim

 

c_spray,
Did you use your twin hulls? I need to find one for a School Project, here in Central Texas. Building a trimaran, twin Hobie 16 rigs. Just need a central hull now....
Lane Thomas,
Teacher of sailors in public education......

 

You really ought to calculate aspect ratio as the luff length squared divided by the total area, instead of dividing by the area of one side.

If you want to maintain the same performance as the single mast rig, you need to to keep the height about the same and only cut the chord in half. Of course, nobody does it that way, because one of the reasons they go to biplane rigs is to lower the center of effort. If you keep the rigs the same height then as they come together they converge to the same induced drag as the single rig. On this basis, the farther you separate the rigs, the lower the induced drag compared to the single rig.

If you keep the geometric aspect ratio of each rig the same as for the single rig, then the biplane rig ends up being 30% shorter. It would require the two masts to be located very far apart to have the same induced drag as the single rig. On this basis, the interference between the rigs is always adverse.

So if you want a biplane rig that breaks even on the basis of windward performance, you have to give each rig a somewhat higher aspect ratio than the equivalent single rig. There will be some reduction in the height of the center of effort, but it won't be the full 30%.