Hi Phil, welcome to Ribnet!
Hmmm, Sadler 29? Lovely yacht! You will be horrified to find that a quality RIB 8m+ will blow your budget - do you want to buy new?
Taking Charles out for lunch is a great idea, but he has expensive taste and will probably do his "throwing your wallet in the harbour" routine

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There are four reasons for bolting twin engines onto the transom of a RIB:
1. You intend to cruise, extensively, out of 'phone and VHF radio range or in dangerous water, on your own (ie no accompanying boat).
2. You can't get the horesepower from a single engine.
3. Your budget's unlimited.
4. You have free fuel, or live in the Channel Islands

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8m+ RIBs are great - every half metre gives you more comfort, and make those long passages more bearable. Your wife and young children will enjoy it rather than enduring it. And your one-year-old will only just be old enough by the summer to endure the impact of RIBing. You might find that you want to keep to yachting for a couple of years!
With a budget of £30k you've only one option - used. A new 8m+ with twin engines will set you back £60k+ from a quality manufacturer.
As Roy mentions, Ribcraft are a quality manufacturer with a good reputation. We happen to be very satisfied with our Scorpion, and I can honestly say that along with Tornado (and XS), and Osprey, these four makes set a benchmark that other manufacturers try to follow (unless you can stretch to a Pascoe!).