Multihull Structure Thoughts

I have done some reverse engineering on information I had about OMRA 60 racing tri’s. OMRA 60 tris are built with 0.6 mm carbon fibre skins either side of a 20 mm PVC cores. In high load areas or areas needing more flexibility some Kevlar or S glass is used. To check this out I also verified a statement that the tri’s had 600 gsm carbon fibre skins.

Carbon fibre comes in many fabric types (unidirectional, 45/45 biax, woven cloth etc) and in several strengths of carbon filaments that can have from moderate to high tensile and or high modulus characteristics. So when your designing a carbon structure, depending on its purpose, depends on the strength of the carbon and the type of fabric chosen. For high performance boats the carbon for a mast is a different type from the carbon needed for a foam carbon hull.

Most unidirectional and biax carbon fibre is about 0.1 mm thick per 100 gsm of weight. Which means with vacuum bagging or resin infusion the OMRA 60 tri have a 600 gsm skin that would be 0.6 mm thick assuming a 65% carbon fibre to 35% epoxy resin ratio. I imagine that there would be internal bulkheads, ribs, stringers backing up the skin structure in high stress area’s. Why is this interesting? Because these boats sail at speeds up to 40 knots across oceans and can average 24 knots for a day. An OMRA tri is 60 x 60 foot displaces 11000 lbs and carries 3000 plus square feet of sail upwind.

If you are a EG Spirit 380 cat builder with 38 x 21 x 11,000 lbs displacement and 900 square foot of sail and it says to use 600 gsm 45/45 biax either side of a 12 mm foam or balsa core don’t add an extra layer of glass just to be sure. It is strong enough as designed. Your Spirit will never be stressed anywhere near an OMRA 60 tri.

If you need anymore evidence of the capability of a carbon fibre foam sandwich hull, take a look at Orange 2 racing cat. The boat is 121 x 59 foot, displaces 67000 lbs and carries 8600 square feet upwind. It has 0.8 mm carbon fibre skins (equals about 800 gsm carbon fibre skins) of 4700 mPa (681,677 psi) carbon fibre. This is either side of a Nomex core in the rear of the hulls and PVC linear foam in forward sections of the hull. This boat went around the world in 50 days topping out at 48 knots, averaging 25 knots and had a best days run of over 700 miles.
The PDF gives a small table of types of Carbon Fibre and there tensile and modulus strengths.

 

Slingshot. Yes a flat table vacuum bag or infusion approach is very good as a technique. But choose your design well as fully infused flat panels can only bend so far. Kelsall approach requires cuts in panels to bend to shape which then requires additional glassing to repair the cuts. This is the reason why Rob Denney does simple full molds to infuse a hull etc.

 

Re; post #271

Not only did Orange 2 'survive' her racing days, but she was found to be in great shape by her new owners. Enough to add a central saloon and turn her into a go-fast private catamaran. The following pictures are of her in her new configuration (Vitalia II) that I took in early 2017 in Sint Maarten. Needless to say, when they went out day sailing, they made all the other boat envious. You could watch her skipping across the water from Simpson Bay to Saba on a clear day. I hear you can now charter this boat, should anyone have the need for speed.

 

I was more thinking of your Kurt Hughes flat panel example (shown a few pages back) with strip foam added later to make the curve of the hull

 

OldMulti, I want to complement you on this thread. You have contributed a wealth of information to us all. I'm the BigCat guy, but the money for the project fell through. I don't have a web address for yachts any more. I'm busy with earthnurture.com . My old webpage is still on the Wayback machine, of course. BigCatCatamarans.com on archive.org - from 2010. Someone else bought the web name later.

 

For those interested in resin infusion and building of a real cat please view the following web site and associated facebook entries. The boat is a Schoining GForce 1500 50 x 26 x 18000 lbs displacement with up to 1300 sq foot of sail. The plan was 6000 hours of build the reality was 10,000 hours of build. A good operator. http://precat.nl/en/progress

 

From designing my boat, I found in a smallish 40' to 45' catamaran, if you want it to have narrow enough hulls to be fast, there is no way you can get a decent side bed in back without them unless you move the bed high enough into the bridge deck.

One other thought I had, but not sure if it's true. I would think it it would cause ever so slightly less pounding as the water rises in to pound the bridge deck it would lift the hull a little first, downside is it must create a little more pitching movement in large waves.

 

Great point, but if you are cruising and not racing, it may not be strong enough when you hit a reef or a half sunk cargo container.

 

Sailhawaii. Many cats have double bunks on the bridgedeck aft. Early Crowther cats had this as a common approach. I can assure you that if they are beside an aft cockpit they are the most comfortable position to sleep on cat in a seaway. The down side is one person has to crawl over the other to get out occasionally. I have sailed on 40 foot plus cats that have 4 double bunks on the wingdeck with 14:1 hulls. The boats go fast and you can sleep comfortably. French roomerans try to emulate homes for the ladies and as a result 8:1 hulls that have flairs to allow double beds in hulls with side walkways.

You have 2 options in boat building make it near bulletproof or make it repairable. Near bulletproof requires a lot of weight (even if made of light weight materials). To give you a feel as to what a relatively light built boat can survive, there was a 42 foot cat called Bagatelle in a Brisbane Gladstone race that sailed onto a reef at over 10 knots outside Gladstone in Queensland . The crew pulled it of the reef and sailed the last 50 miles to Gladstone. Minor repairs required beside the bent rudders and broken daggerboards. Bagatelle was built with 2 layers of 330 gsm cloth and 1 layer of 18 oz (600 gsm) woven rovings in polyester on the outside of the hull. Thats not bulletproof just slightly over strength. Following this reef hit Bagatelle sailed for a decade before it hit another reef and wiped out the bottom of 1 hull. A guy salvaged the boat rebuilt the bottom and the boat is still sailing today. It depends on what you hit, how hard you hit and if your lucky and can repair it.

 

OM,

I'm a big fan of the Schionning Designs, thanks for mentioning them. However, I have a couple of questions about the materials they currently recommend (really for everyone).

1. Thoughts on the durability, reliability and longevity of the duFlex panels from ATL? Seems there have been some anecdotal discussion and issues with moisture (often discovered during second hand pre-purchase surveys). Of course this is not a Schionning issue per se.
1.b. And they are still discussing paper honeycomb. Why not a synthetic honeycomb to completely alleviate the moisture concern?

2. Any thoughts on the size limit for this panel construction methodology? The 51-77 ft designs are becoming a realization of a fantasy for many folks, but is there a perceived limit where hull dynamics would force you into other approach?

 

Just a short one on comfort. The most comfortable position on a boat is near the pitch centre, about 60% aft of the bow waterline for most boats. Also the nearer the centre of the boat's beam the better. So if you can locate sea bunks and a galley in this area it helps a lot for comfort. If you want a real sea boat do not have all your bunks or galley forward of the centre line. Yes there are some people who can sleep and eat anywhere on a boat but for most people they need minimal motion. Also unless you are a daysailor or short term sailor plan a protected covered area in the cockpit. Sunburn, rain and wave action spray are very real and over a long term very tiring. Finally there are real differences in the way V shaped hull tri's, round bilge tri's, fat hull cats, thin hull cats move through seaways. Get a real ride in a seaway on each type if you can before you choose a design. Some people prefer the motion of one type over another. Choose the design you can live on, not the design that fits the most bunks.

 

Coastal Ogre your questions are good. I will deal with Duflex first. I have heard similar rumors and have seen the underwing of a 50 foot Schoining cat being repaired due to failure of the Duflex core. But please refer to the following web addresses for a more detailed explanation of some of the problems. In short like all materials there can be problems but you hope its not at the manufacture stage. Read down each page.

building a catamaran.. outgassing with epoxy https://buildacat.com/bblog6b.html

Building a Catamaran.. Outgassing? https://buildacat.com/bbloggas.html

Building a cat, The ATL nightmare https://buildacat.com/bblog32.html

Paper, aluminium, synthetic honeycomb all have the same characteristics. They are strong and stiff. The stiffness is the problem. Great in aircraft that doesn’t have wave impacts. Boats in most instances need some flexibility in their cores to absorb an impact hitting a thin skin. The alternative is to have a solid light structure like ply that bends as a total unit.

Panel methodology can be built in any size boat as long as you have the equipment to create and move the panels. The hull dynamics of a chine or round bilge is relatively unimportant for cruising boats of any size compared to the ease of build for larger boats.

 

While we are on some problems of construction. The following article will give a view on some common mistakes in core construction approaches. Its a bit old but it has all the failings I have seen EG insufficient rounding or tapering of foam prior to glassing, incorrect drilling and filling of thru hull fittings etc. Professional BoatBuilder - 70 - Apr-May 2001 https://pbbackissues.advanced-pub.com/?issueID=70&pageID=95
The PDF is a theoretical study on the delamination of Carbon fiber in EG Masts. Its for understanding, not a criticism of carbon fibre. Mast are a very highly stressed item. If designed and built well a mast, a hull or anything else built in carbon will work well.

 

Thank you BigCat. I read a lot of your initial design thoughts and found them interesting. The big cat could have taught us a lot and may have been real fun to sail.

 

Another interesting cat is a 55 x 33 foot 25000 lbs displacement wing deck cat that has 1500 square foot of sail. The boat was designed as a cruiser racer with 14 :1 hulls. This boat was fast under original owner and is now used as a very fast “trading boat” between Lord Howe Island and Australia. The new owner makes his money by getting things people on Lord Howe want but cannot afford the freight to the island. He also carries a few people to and from Lord Howe. Nice way to make a living. Back to the boat. The hulls are 600 gsm 45/45 biax fiberglass 17 mm WRC 600 gsm 45/45 biax in epoxy. Sounds light, but it also has ring frames every meter of hull length. It now sounds strong. The deck layup is similar but has different reinforcing beams as required for cabin layout, tracks, hatches etc. The main cabin roof is mainly 600 gsm 45/45 biax 25 mm corecell foam 600 gsm 45/45 biax. But the roof also has 4 longitudinal channels cut into the roof foam and filled with solid glass to act as track supports and reinforce the roof structure. The main cross beam is a box structure 900 mm high 500 mm wide across the boat. The top flange is 23 x 500 mm wide 1200 gsm glass unidirectionals, the bottom flange is 16 x 500 mm wide 1200 gsm glass unidirectionals. There are bulkheads every 500 mm in the crossbeam. The rear beam is the same concept with less unidirectional top and bottom. This boat has done thousands of reliable sea miles.