Originally Posted by jonp
An idea on what prop is currently on it and what max rpm the engine is reaching with that prop would be handy. Raising the outboard by each hole will often increase rpm 50-100rpm due to less drag. The difference between aluminium and stainless is marginal but the price difference is quite considerable 6/1, also aluminium is easier on the gearbox if you do a lot of in out gear changing.
Even though ribs are light in comparison to other vessels, the tubes can cause considerable drag depending on the manufacturer.
An example is my 2014 zodiac 550 pro tour is the lightest in the 550 models at 315 kg, yet I've had to drop to a 13x13 prop to reach the Yam f70 max rpm. The boat before was also relatively light with the latest Suzuki 90 which I needed to drop from the 21 pitch down to a 17 to reach max rpm.