have a look at the Yamaha 80 hp 4 stroke .
Normally 50 to 60 hp would be enough for a 5 meter RIB but given that you will be carrying divers you will need that extra bit of power to get on the plane .
This engine will give you reliability , power , range and economy . I'm not sure if it will come in at under stg£5,000 .
Our own training boat Ally Cat which can be seen on our website www.powerboat.org
is a 6.8 M Lencraft Lynx ( Mil Spec hull ) She is powered by a 100 hp Yamaha 4 stroke and uses only about stg£6 per day in fuel during a one day Powerboat Coxswains course . Her top end speed is 35 knots .( petrol is about stg£0.55 per litre here in Ireland .
Best wishes ,
National Motor Boat and Power Boat Trainer
ISA / IWAI