The general RYA advice is against prop guards for safety boats.
Obviously you need to risk assess it yourself, in your specific situation - but will it:
- lead to complacency about prop safety?
- discourage safe practice, e.g. use of kill cords / engine off when alongside people in the water?
- reduce speed at recovering casualties ashore if required?
- increase the risk that someone gets struck by a boat at speed (because it is 'safer')
I do understand your problem but if you aren't confident about safety boat drivers ability to keep props away from people in the water can you really trust them to:
- know how to deal with an unconscious casualty in the water?
- respond correctly to an entrapment?
or any other critical issues.
From a prop-life prospect then whilst you might go through fewer props I think you might find people take bigger risks with the "bottom" and when you do hit something it will be hard and could be the whole leg not just a prop that needs replacing.
Perhaps if buying a new safety boat and really worried about this I would consider the jet drive leg options on the market. Personally though I would be ensuring that I was confident that all my safety boat crew were proficient at handling the boats and if not:
- providing additional training (many clubs consider PB2 to be the minimum, others insist on full safety boat qualification in addition, that is quite a big time demand for training a large number of people, but you could probably produce a shortened course which focusses on the priorities, e.g. I would want to see first aid and probably VHF protocols covered in more detail from what I've seen at typical clubs, as well as a good understanding on the local emergency procedures).
- insisting on regular competence reviews and assessments (e.g. once every 3 yrs having to do a 1/2 day refresher / workshop to ensure up-to-date). This would also be a good opportunity to carry out an "exercise" to test all procedures from recovering casualties to resus on board, recovery to shore etc.
- having procedures in place to remove incompetent operators, and a "management" process in place so you can spot those who need coaching before a problem happens.
If getting volunteers to man boats who can demonstrate competence is a problem:
- are you supporting them adequately?
- do you want them if they don't see the importance of these skills?
- could you get a smaller team of more dedicated volunteers who take it seriously and train well? Are there people who would like to do it regularly in return for good training? Could you recruit volunteers (possibly from outside the club) just to do this? How do you reward your volunteers? e.g. are those who do their fair share given any discount on membership? if you had non sailing volunteers would you expect them to pay for membership?
- if you need "professional staff" it may be time to consider employing people. I know of one club whos entire safety boat team is paid (albeit minimum wage).