John,
Anchoring is more of a religious issue than an absolute engineering question - although I am, by
trade, an engineer, most of what I learned was gained by asking the skippers in the anchorage what they did - if they were among those that did NOT drag during a blow.
Absolute strength per unit weight is higher for G4 galvanized similar weight stainless. But, that is only part of the equation. I sized my ground tackle for the highest expected load I would expect to "live through" on my 43' Voyage catamaran. That got me 5/16 HT chain (G4), a 25 kg Bruce and - the critical part - a 12 kg
Fortress. I had sufficient chain, 50 meters, for my usual anchoring
depth of 3 to 4 meters.
The critical item is the Bruce was shackled directly to the chain and 2 meters of chain were shackled to the "trip point", the hole in the front of the anchor intended to remove the anchor when set very deeply. The other end of the short chain was shackled to the
Fortress - two anchors in a line with the Fortress being deployed first then the Bruce being set on a 10:1
scope for heavy
weather.
Now, the boat end of the chain was shackled to a twin 6 meter
bridle - in the form of a V with the chain effectively shackled to each catamaran bow just above the water line. The line was 5/8" (sorry, I don't know the metric equivalent offhand) and connected to the chain with a KONG chain grabber.
In heavy
weather, a second 3/4"
bridle was "bent" onto the chain with a rolling hitch, just above the KONG. both the KONG and the rolling hitch were IN THE WATER directly in front of the boat under normal conditions - up to about 20 knots.
Everything worked for several years - we never dragged, not even once. Our normal anchoring procedure was to sail into our chosen anchorage - well away from other boats, depoy the Fortress, depoy the chain and Bruce, count to 10 and hold on. The boat stopped instantly, turned into the
wind and we released the brake on the main. We usually furled the
jib as we entered the anchorage.
The storm,
Hurricane Wilma - my 5th storm since I bought the boat - was predicted so severe that I left the boat and went to shelter inland. the near eye went over the anchorage, breaking nearly every other boat loose and putting them aground. My boat ran down the chain, over the anchor setup, and stopped SUDDENLY when it got to the other end of the setup.
Both bridles snapped. The anchor pulled out of its platform, OEM plus a second layer of reenforcing I added when I down sized the chain. One of my previous encounters had a man, much more experienced than I was, tell me to be sure and tie a large overhand
knot in the chain - that
knot jammed in the hawse pipe - clearly had it of not jammed, the chain attached to a factory provided hard point with a shackle would have broken the hard point and the boat would have joined the others on the beach - probably a total loss.
In addition to the minor damage of lines and windless shelves, the CHAIN STRETCHED since it was stressed well above its safe working load -- but not to the breaking point.
Stainless chain would have snapped - the safe working load is, in some chains, similar than that of galvanized - but the breaking load is not as high. Galvanized stretches - and becomes useless while saving the boat - where stainless just snaps.
My experience indicates that Bahamian anchoring is not of any value in an ultimate storm - only tandem anchors help more than one anchor - in my case, the Fortress was twisted to a potato chip as it held in place for the time it took the Bruce to reset. When I went out to the boat, I dived the ground tackle and the wire tie I put 2 meters from the anchor (so I could know when the setup was about to
lift out of the water so I could go and retrieve the Fortress) was buried. The Bruce was 2 meters into the mud - it was undamaged. Fortress replaced the damaged flukes at no charge.
Don't buy excessive size chain - 5/16" is sufficient for a 43' catamaran if you don't mind replacing it after a 100 year storm. Don't use stainless steel chain or shackles unless the shackles are drop forged by a reliable supplier (Wichard or Kong)
There isn't any way to absorb the force required to break a 5/16" chain on most production boats - and larger chains are just more weight without more benefit.
Most of all, if you doubt your 5/16" chain is strong enough for the expected storm, seek shelter on land!
Good luck.