orkney1
New member
Hi, I have recently re-engined a 7m rib which was running a mercruiser 1.7 120hp diesel. We are now running a ford 2.5 turbo diesel estimated 130-140hp. This engine is turning 1000rpm slower for a similar power. I have changed the prop for more pitch and now get 30knts at 3850rpm flat out. The boat is a bit sluggish to get up on the plane, she only makes boost at around 2800rpm onwards but she is already almost planing at these revs. Once on the plane and making boost it absolutely flies, acceleration from 20 - 30knt is very quick. She'll boost to 15psi whilst accelerating to top speed. Once she's at 30 boost falls away which suggests the engine is not having to work too hard to maintain 30kts. It feels like it has the power for more speed but getting up on the plane would become a problem I think. I am wondering if changing the drive ratio to turn the prop faster would be a solution? Then run a slightly smaller diameter prop that has less grip but a similar pitch. When getting on the plane the smaller prop would 'slip' allowing the engine to rev harder and make boost. As the boat starts to plane the 'slip' will decrease and I should achieve a higher top speed? Have i got the physics of this correct or is this not how it works at all?
