Need help with designing a concept catamaran.....

OK, this is my bouyancy force per hull: 5,334,495.14 lbs

So now I just try and not build anything above the waterline that will outweigh this?

 

I am not very good in imperial units but I think 10 million lb total displacement means you have a decimal point in the wrong place.
As always, don't focus on one factor when developing a design. Spiral in and always stick to your SOR
And don't expect useful replies unless you show your SOR and complete design to date

Richard Woods of Woods Designs
www.sailingcatamarans.com

 

To get to the correct boyancy number, I need to first know the over-all weight of the entire vessel. Is this correct?

 

OK, I think this number is closer........533,0877.75 lbs
or rather, 0.4535924 kg
or, 23712925.74 Newtons

 

You need the weight and the volume, which gives the density of the vessel. Also, don't use mixed units. Stick to metric or standard.

 

And for what do you need the density of the vessel?
533,0877.75 lbs is a lot for a 66' LOA boat, isn´t it?

 

Yes, the vessel is 66 feet LOA. I must not be calculating the volume correctly.

 

You still have a decimal point problem .453Kg is about 1lb

you don't use newtons for displacement/buoyancy/weight

Seems like you have a steep learning curve ahead of you, so I'm not sure how you know your idea "has merit"

Richard Woods

 

This post/reply has been edited only for clarity of communication...

I beleive the four mast/sail "concept" has merritt based on a few things:

1. potentially larger sail area
2. lower bridge clearance when sailing coastal waters
3. lower center of gravity

• the vessel remains level -- the hull and keel stays in a more consistently plane at the water level
  
• less hull pitching, rocking, diving from wind impacting/pushing a tall mast and sail

4. more forward/lateral propulsion
5. all the advantages of a long, fast sailing catamaran

The primary reason most do not want to attempt designing with more than 1 mast is simplicity. 1 mast w/ mainsail, and genoa can be handled quite easily by one person, when the lines are all rigged to a central point.

Introducing The Catamaran 4 Sail System

• a soft wing sail at each corner of the vessel
• all four sails are connected by heavy duty rigging for synchronous tacking on all four sails
• tacking, hoisting, lowering, and trimming is synchronous, and powered by motorized or mechanical winches
• all rigging is lead, and controlled from simgle, central, common point (at the helm)
  
• a rudder is also employed on each catamaran hull to help in traditional tacking

This is an idea. Not a plan, and not a design. It has been originally posted here as a "concept", but in my error, I have presented it as a plan, or an effort to move towards a plan.
Of course I would like to see it through to construction, and launching, and that is why I have presented the "idea" here -- to get help from this community.

 

1- Why soft wingsails? As leading wingsail and aerodynamics experts have been kind enough to tell us here on BDF, the popular belief that wings are more efficient per se is not true.

Soft wingsails have been hyped for years, and to date there does not appear to be a single instance of anyone actually proving the claims about their speed. It is of course extremely simple to prove these claims - all the promoters have to do is to turn up to a few races against comparable boats and give a link to the results. What possible good reason could they have for not doing so, particularly when (for example) one promised to do it about ten years ago? If people are selling a wonder product and they refuse to actually prove the claims about its performance year after year, what's the logical conclusion? The most logical one seems to be that the product does not live up to their claims and they refuse to admit it, perhaps even to themselves.

The only (apparently) unbiased guys who tried boat-on-boat trials of a much-hyped soft wingsail were the builders of Seascape boats, who said it didn't work as claimed.

2- I'm not an aerodynamics expert, but I think you'll find that if you add extra masts, you are effectively lowering the aspect ratio of your sails. You don't count the aspect ratio of each rig in isolation, unless the rigs are so separated from each other by a sufficient distance that the airflow does not interact. Of course, this means that positive interaction does not occur.

Anyone can put more sail area on a rig of the same height. All you'd have to do is to put on a bowsprit and a longer boom, or a ketch, schooner or yawl rig, and that would be much cheaper than putting on four masts and have lower centre of gravity, all else being comparable. Of course, there's a reason that these days sloops predominate in the sailing world - they are generally more efficient.

3- A rudder on each hull is standard in catamarans.

 

Aren't wing sails more areodynamic?

Adding a longer boom can definitely increase your sail aspect ratio......and can be accomplished on most any catamaran

The Sails dont have to be soft wing. They could go traditional, they could go solid winged, or they could go with even longer booms, whatever makes the boat faster....they all work

The main idea I wanted to push is "synchronous" tacking, trimming, hoisting, lowering etc., keeping the craft as level as possible, and keeping the keels in the water.

Does a higher wetted surface area equal more water friction? Would less pitching, diving, rolling, yawing keep the hulls in the water, and keep them traveling on a more linear path, especially under speed?

Why aren't more catamarans rigged with a ketch rig, a schooner rig, or a yawl rig?

 

Define "aerodynamic". Do you mean the L/D ratio of the rig? That's not all that relevant. Do you mean maximum lift? If so, "maximum" by what measure? Area? Weight? Heeling moment?

Try reading posts on this site by Tom Speer, Prof Mark Drela and Mikko Brummer, among others. They are true experts and they say, for example, that thick wings are not more efficient aerodynamically, per se. They work brilliantly for some applications but not for others.

Whether synchronous sail hoisting/trimming etc is a good thing is an open question. One "mechanical" issue is that many of us like to monitor each sail as it goes up and down, or is rolled in or out. Otherwise, if something as simple as a rope getting jammed occurs (and that can be caused by gear failure, a fender line or a guest's towel getting caught in a pulley) then the sail can be torn apart. If the desire is for simplicity then why four masts?

Why should cats be rigged as a schooner, ketch or yawl? It's often heavier, more complicated and most expensive and has generally inferior aerodynamics AFAIK. You need twice as many mast bases, twice as many halyards, twice as many sheets, twice as many blocks, twice as many winches. It was fine when old technology made sails and rigs harder to handle, but modern gear is much stronger, lighter and lower in friction.

Cruising cats rarely have "keels" out of the water even with sloop rigs. Less pitching is good, but where's the evidence that putting on more masts will reduce pitching? They may well increase topweight and therefore pitching.

I'm not an aerodynamics expert or a designer - I'm a sailor/historian. One thing I've noticed in researching the history of technological development in racing sailboats and in interviewing most of the world's top small boat designers is that effective innovations are almost always created by people who are extremely familiar with, and very respectful of, the state of the art. They will have practical experience with many different designs, and will have deep (although not blind) respect for others they have only read about or heard of. They will normally create breakthroughs by using new technology and by taking ideas from one area and applying them to another, but that requires deep understanding of the existing state of the art. I can't recall a truly "left field" idea that has really worked, apart from the Windsurfer which is arguably a special case since it was intended by its creators to be much more modest than it ended up being.

 

Try the basic. Displacement (@freshwater)= Length waterline x breadth waterline x Depth x Cb coefficient. Cb for a double pointy end, round bilge hull will be around 0.38 to 0.43.

 

Thanks, RX, for pointing me to the Cb coefficient! I found various other hull calculations here Hull (watercraft) - Wikipedia https://en.wikipedia.org/wiki/Hull_(watercraft). Very useful info. Thanks!