Catamaran Chain Plates

I am attempting to calculate the correct loading on chainplates for a large cruising catamaran. ISO standards use the righting moment for calculating loads on stays and chainplates, and this seems to be the standard practice, however in the ISO draft document 12215-9 the suggestion is given that this is too extreme a loading condition for large cruising cats that are not designed to "fly a hull" and that a more realistic loading condition would be the heeling moment calculated when the "first reef is taken". This leads to even more questions: what wind speed? what percentage of the sail area to be used? what sail configuration? .. .. .. My question, then, is this: Is there a standard method used for calculating loads on chainplates and stays for catamarans not designed to fly a hull?
Thanks in advance

 

Given the propensity for multihulls to stay inverted when capsized - which in almost all situations is a BAD THING - it is an easy leap to the position that it is better to lose the rig than to capsize. Therefore, one can calculate the moment needed to capsize the hull, then design the sailplan to exert less than that moment in various windspeeds and reefing arrangements. Now that you know the design pressure of the sails, engineer the rig to suit the stresses imparted by the sails. The component that acts as the weakest link in the arrangement is up to you - should it rip out a mast tang, collapse the mast, or rip out the chainplates?

Joseph Norwood Jr., in his book "High Speed Sailing - Design Factors" promulgates a formula to estimate the righting moment of a catamaran hull (without consideration of the dynamic keel and sail forces) at the angle at which the windward hull becomes airborne:

N(hull) = W(y cos theta - z sin theta) - bBo(1-theta/theta0)

where:

N(hull) = heeling moment
W = total weight of the vessel
y = horizontal distance from the heeling axis in the lee hull to the CofG of the vessel
theta = angle of heel at which the windward hull buoyancy is reduced to zero
z = vertical height from baseline to CofG
b = centreline-to-centreline beam between hulls
Bo = unheeled buoyancy of weather hull
theta0 = heel angle at which the weather hull lifts out of the water

Presuming that you want to determine the heeling moment at less than weather hull buoyancy = 0; say, at 75% of Bo, then apply this factor to the equation. With the design maximum heeling moment thus calculated, design the rig to this figure.

 

I would apply a healthy margin of safety to the chainplates. A weak chainplate is likely to do real damage to the hull if it fails, and it will be hard to repair in any event. The extra weight to make it bullet-proof is minor.

If you want a weak link as a mechanical fuse, the place to put it is between the shroud and the chain plate. It could be as simple as a lashing. That way it doesn't do any permanent damage to something vital when it lets go. Once you recover the rig, all you have to replace is the fuse.

Even though a cruising cat isn't supposed to lift a hull, I'd use the full righting moment for sizing the chainplates. The dynamic loads on the rig could easily exceed the static load needed to fly the hull or capsize the boat. The inertia of the boat is high enough that the weak link would give long before there'd be enough energy imparted to the boat to heel it to a dangerous level. You don't want to be wondering if the rig is going to come down every time you encounter rough water.

If wind-driven capsize is the concern, there are better systems to automatically release the mainsheet when the hull comes out of the water.

But wind-driven capsize when sailing on the wind is not the reason for most multihull capsizes. The real dangers are wave-induced capsize and the "downwind trap". The latter is when you're sailing off the wind with more sail than is prudent, but don't realize it because the apparent wind is lower than the true wind. Then you stuff the lee bow into a wave and get slowed down, which increases the apparent wind just when the boat is most vulnerable.

 

I agree almost totally with Mr Speer

I agree almost totally with Mr Speer, for example the race catamaran 22 m long Jet Services was capsized running downwind because it hit a wave and stopped. The apparent wind, which very low as the boat was running at a speed close to the true wind, became as strong as the true wind. The cat was capsized like a Hobie Cat 16...

The rigging "fuse" is a belief that runned in the sixties, maybe dreamed par Arthur Piver, and it's found in the older books.

But a boat dismastling gently with the rigging gently put over the deck without any mess is a view of mind. I saw 3 masts fall down, and the 3 had been broken and broke a lot of material on the boat. The mast can destroy everything on the deck and kill someone. After that if it's hanging on the side of the boat, it may destroy the hull thus sinking the whole boat. In a strong wind and sea you'll have to cut any cable or rope in minutes and to send the mast with sails to the bottom of the sea; pretty costly.

The rig has to be calculated with a healthy margin like 2 for a cruising boat as a static calculation do not account on dynamic loads and rigging vibrations (on race tris vertical accelerations of 2 g has been measured...)

A well designed multihull even capsized is safer than a rubber life boat... Good design is to organize the survival in this unlikely event.

Boats dismast generally at the worst time, like going upwind in a strong sea while scaping a hurricane... So the rigging must be absolutely reliable and must stay on the deck whatever the circumstances.

It's better to use dynamic devices like;
-a fully battened mainsail able to empty the wind in a gust (your sailmaker or your naval architect shall explain it to you ),
-someone dedicated to the mainsail sheet and releasing it in the gusts,
-a pendular device releasing the mainsail if the heeling is to high and many other things may be designed. Read some recent books on multihulls. The technics of multihulls have highly evolved and the older books may have false assumptions now like "fuse" rigging, low volume outriggers on tris, or slim sterns.

A multihull must be powerful and reliable to take advantage of its potential so it's the skipper who has to control this power like the pilot of a powerful car.

 

Absolutely true - been there, done that. I know how easy it is to fall into the downwind trap.

A key element is the escape hatch. It is not just to get out from inside the capsized boat immediately after the event. The hatch is perhaps even more important to allow the crew to get back in. Once capsized, a properly organized and designed boat will have a "grab bag" of emergency supplies - flares, beacon, VHF, etc. that is accessible whether the boat is upright or inverted. But you will still want to go back inside for shelter or to retrieve supplies such as food and water.

The hatch is also essential for venting the inside of the hull. The heaving motion of the hull going up and down in the waves causes large pressure surges inside the overturned hull that can render that space uninhabitable. The hatch vents the inside allowing the air to move in and out with the waves instead of changing the pressure.

After the hatch is an interior design that has flat spaces above the inverted waterline that can be used for sleeping. This may be the underside of the table or berths or the cockpit well.

Another thing to think about is closing off the companionway. You wouldn't believe how effective the wave action is in removing anything and everything from inside the cabin through the companionway.

 

Prout Quest Chain plates

On a similar topic ---- I have a Prout Quest 33CS cruising catamaran. This is a 10.05 mtr LOA, 14.4 mtr beam, cutter rigged sailing cat of approximately 4300 Kgs (4 tons) displacement carrying 34.4 sq mtr genoa, 14.9 main and 6.5 staysail on a 10.7 mtr mast.

The mast is stayed by a forestay and twin aft backstays which are secured by strip stainless steel chain plates bolted to the lower hull moulding; cap shrouds secured to strip stainless steel chain plates bolted (x 3) to the coach roof sides; a baby stay to chain plate at the coach roof front and finally, aft lower shrouds secured only by ubolts bolted through two inch wide side deck.

I have no confidence in the latter u-bolt fixings that have no reinforcement below the deck to spread the load of the small nuts and washers and in fact they are showing signs of cracking the decking. This means that I am loath to tension the rig as much as it should be. I am therefore considering relocating these aft lower shrouds to new strip chain plates, similar to those of the cap shrouds, to be bolted onto the coach roof sides.

Has anybody had similar experience with the Prout Quest ? Does anybody have any views or recommendations for this proposed configuration reconfiguration?

 

Put a backing plate of aluminum, stainless, plywood or resin and reinforcement under the "U" bolt locations, drill out for the holes, bed and re-mount.