If you can afford a new boat you can afford to fuel your exisitng one may be pertinent, in the way that Rolls Royce traditionally are held to advise that if you need to know the cost of ownership you can't afford one. The older and cheaper end of the market, who might be more critically interested in lower fuel costs, are not likely to scrape up the Ł60K or so that a new MotorCat or similar will cost.

Now if they want to give me one I'll extol it to the heights because, like many river boaters, I do want low wash at 4 knots and achievable 15+ knots when downstream of Tilbury. But I would get shafted by the EA on licence fees, although I could claim to be rafting up with myself (pretty kinky eh?)