Grade 7 and Grade 70 chain are the eventual evolution of Grade 4, 40, and 43 chain. The chain is subjected to a more profound heat treatment which produces a very strong and yet very light chain link. The links of the chain are formed from low carbon steel wire, and the size of the link is actually referring to the size of the wire that was used. For example, 5/16 G7 was formed from a 5/16" wire, while 1/2 G7 was formed from a 1/2" wire.
Common dimensions for G7 chain are 5/16", 3/8", and 1/2". Larger and smaller links are available, but these are the three sizes that you will encounter in most marine chandlery stores.
5/16 G7 has a working strength of 4,700 pounds; and a breaking strength of 14,100 pounds.
3/8 G7 has a working strength of 6,600 pounds; and a breaking strength of 19,800 pounds.
1/2 G7 has a working strength of 11,300 pounds; and a breaking strength of 33,900 pounds.
The interesting thing to note is that the breaking strength is 3x greater than the working load, giving this type of chain a significant safety margin. When the chain is manufactured, it is then subjected to a rigorous test of strength, followed by a close inspection of each individual link.
Aside from strength, the next important factor to consider is the weight of the chain, as this directly affects the holding power of your anchor that is desperately trying to hold onto the bottom!
5/16 G7 weighs 1.0 pounds per foot.
3/8 G7 weighs 1.45 pounds per foot.
1/2 G7 weighs 2.7 pounds per foot.
A standard recommended length for an all chain rode on a cruising yacht is 300 feet. This will give you the ability to anchor in 25 feet of water with a 5 foot freeboard and still maintain 10:1 scope for storm condition anchoring. The weight at these particular sizes are as follows:
5/16 G7 would weigh 300 pounds.
3/8 G7 would weigh 435 pounds.
1/2 G7 would weigh 810 pounds.
This weight will help cause the chain to sag and pull towards the bottom, creating a catenary curve. The curve will make the chain fall vertically from the bow, but curve away from the boat as it runs. The chain will lay on the sea floor because of its weight and cause the load on the anchor to be parallel to the sea floor. This will help the anchor set, and hold, as all the load will be sideways and not upwards.
Modern light displacement boats are very sensitive to how much weight is added and extremely sensitive to where the weight is added, as it will drastically affect the trim and handeling of the yacht. The high strength to weight ratio allows you to carry a chain that is strong enough to hold your yacht during a blow but not weigh your boat down excessively.
When looking at G7 chain, an important feature to evaluate is Strength to Weight Ratio.
5/16 G7 is 4,700:1 = 4,700:1 working load, and 14,100:1 breaking load.
3/8 G7 is 4,552:1 working load, and 13,655:1 breaking load.
1/2 G7 is 4,185:1 working load, and 12,556:1 breaking load.
Lastly, you need to evaluate strength to cost ratio, as this will directly affect your wallet;
5/16 G7 costs $6.49 per foot
3/8 G7 costs $10.99 per foot
1/2 G7 costs $6.75* per foot (not galvanized)
If you look at the pounds that can be held (of working load) per dollar, it starts to look like this:
5/16 G7 724.2 pounds per dollar
3/8 G7 600 pounds per dollar
1/2 G7 1,674 pounds per dollar (not galvanized)
As you can see, the weight, cost, and strength of the chain all increase with size, but your money goes farther at holding your yacht as the size of the link increases. These numbers are all well and good, but how do they relate to your actual setup? Lets look at what the actual values would be of actual 300 foot rodes.
300 feet of 5/16 G7
Weight: 300 pounds
Working Load: 4,700 pounds
Breaking Load: 14,100 pounds
Safety Margin: 3x
Bulk Discount Cost*: $1,170
300 feet of 3/8 G7
Weight: 435 pounds
Working Load: 6,600 pounds
Breaking Load: 19,800 pounds
Safety Margin: 3x
Bulk Discount Cost*: $1,935
300 feet of 1/2 G7 (Not Galvanized)
Weight: 810 pounds
Working Load: 11,300 pounds
Breaking Load: 33,900 pounds
Safety Margin: 3x
Bulk Discount Cost*: $1,359
When you buy large quantities of an item, most chandlery stores will give you a discount. Being how you are not buying 300 individual feet of chain, but instead a 300 foot length, this is a rough estimate of how much you should expect to pay if you can negotiate the price down, and thus the amount of money that you would save by negotiating.