serious damage

• a hole in a hull is always serious damage

Agreed

• a deep scratch that weakens the laminate under the gelcoat is serious damage, as it will require work to permanently prevent further deterioration

• a small scratch to the gelcoat is not serious damage,

•  but damage over a large area may be considered serious as the work required to restore the boats original appearance may be considerable;

• broken stanchions and lifelines constitute serious damage when the boat no longer meets the appropriate safety regulations;

• damage to a sail that cannot be repaired using tape is serious damage, especially in classes that impose restrictions on the number of sails that may be carried on board and on the replacement of sails during a regatta or season;

• is it made less functional?
• if so, was the performance of the boat seriously impaired?
• was the value of the sail diminished or the cost of repairs such that, overall the market value of the boat was significantly diminished.

This does raise an interesting issue.  Consider damage caused by a collision to a sail lashed on deck that was squished and cut in the contact.  The sail was never needed in that race and never needed for the rest of the regatta.

• JM M3 asks 'was the performance of the boat or crew seriously impaired'.  Note the use of the past tense.  We are talking about serious damage here, so the only time and place where we will be considering whether damage was serious or not will be in a protest hearing after the race.
• JM M3 appears to be referring to damage that impairs a boat's performance in in the relevant race, and does so seriously, not merely hypothetically or potentially.
• Bear in mind that 'serious damage' is only relevant to rule 44 which concerns whether a boat may take an on-water penalty in one race:  the race where the incident occurred.  Any future disadvantage to the other boat will be dealt with by redress under rule 62.1( b ).  However, if the question of whether damage was serious or not arises in a protest hearing, it will almost invariably be the case that the protest committee will have found that physical damage has occurred, and have considered the liklihood of the effect of the damage on the boat's future performance.

• damage to standing or running rigging when such damage significantly increases the risk of rig failure. Guidance may be required from a specialist to establish the extent of the increased risk.

To resume: a prudent owner can accept that in the course of racing his boat will suffer minor damage and will depreciate in value.

However, no prudent owner can accept that breaking boats to the extent that considerable time and expense is required to make good damage is a normal part of racing.

I disagree.  The whole reason we have rules like rule 44.1( b ) and 62.1( b ) is that contact resulting in serious damage is a wholly foreseeable, albeit hopefully infrequent, incidence of racing sailboats.

The Racing Rules of Sailing have been written, and should be applied, so that sailing remains a non-contact sport, and that the penalty for causing serious damage, as defined above, is to retire from the race.

Can't fault that.