You often hear people suggest that you should oversize the stays of your yacht if you want to go ocean voyaging. They make it sound like if the stays are the weak point and making them larger will turn any sailboat into a blue water yacht.
The reasoning behind this is in the ocean, you will encounter storms with no place to hide. You will be forced to sail through weather you would only encounter in a nightmare, and your rigging will need to hold all of this abuse. By increasing the size of your stays, you are also increasing the strength of the wire, making the entire system stronger! Or so the common thought would lead you to believe.
The problem with increasing the size of your steel standing rigging is two fold. First, the added wire size directly translates into added weight aloft. This will make your yacht much more tender and life during a storm will be far less than deplorable. The second reason is the wire size of your rigging is not the weak link in your standing rigging. Your entire rig is a calculated design where everything shares the responsibility. Increasing the wire size but not increasing the size of the clevis pins that hold the terminals is pointless. Now you have heavier rigging of the same strength! Remember, a chain is only as strong as its weakest link, so increasing the size of one link will not make the chain any stronger. True upsizing of your rigging would entail increasing the size of everything involved in your standing rigging.
With steel rigging, there is a severe weight penalty for increasing the size of the wire. Synthetic standing rigging doesn't carry such a weight penalty; instead it carries a financial penalty. Increasing the size of the line used will increase the cost per foot dramatically! For example, 6mm New England Ropes STS-HSR costs $2.79 per foot. 7mm New England Ropes STS-HSR costs 3.49 per foot. That is a $0.70 increase for each foot of line you need to buy! If you take it a step further and go to 9mm New England Ropes STS-HSR, you are now looking at $6.09 per foot! Now you are looking at an increase of $3.30 for every foot just so that you can upsize your rigging by 3mm!
Windage is another concern with increasing the size of your synthetic standing rigging, as it is suddenly a larger stay to pass through the wind. This is only a real concern for racers, as the average cruiser has enough junk on the deck to nullify any penalty from larger stays.
After the financial burden, increasing the size of your synthetic standing rigging does offer one major advantage, it decreases the amount of creep you will experience. Having larger stays means that each stay will be loaded a lower percentage of its maximum. If you apply a static load to synthetic standing rigging, it will creep. If the load is greater than 10% of its maximum breaking load, you will experience significant creep. If the load is less than 10%, you will experience less creep. As you increase in size, the strength of the line increases dramatically, and so would the decrease in creep.
Increasing the size of your steel rigging is pointless, as this will simply add weight aloft and cause you to heel over more while sailing. Increasing the size of your synthetic standing rigging will cost a lot more but it will also give you less creep.
Ideally, you should try to keep your yacht's rigging at the designed size. When the rigging was designed, the designer factored in the heeling forces of the wind and the ballast in the keel. Altering from this would mean deviating away from an expertly calculated state into an experimental state.