I have an issue with my current propellers and what will happen when I replace them.
My boat is a Catamaran Hull, locally built, of what they term the Australian Form, i.e both hulls are symmetrical. It is fitted with twin Mercruiser 4.3L MPI, coupled to Bravo 2X outdrives, 2:1 gear ratio, with 18.75 x 19 aluminium props. Counter rotating. I cruise mainly at 3000rpm and 20kts and very rarely hit WOT.
My current performance is:
Maximum RPM achieved 3300RPM (way short of the 4400 ~ 4800 specified)
Speed by GPS 22kts
Weight approx. 6t including 800 litres fuel and 400 litres water.
Waterline length 35ft, beam 16ft.
Fuel consumption (by Floscan) 40litres/hr each engine or 4litres/nm combined running at 3000rpm and 20kts.
The Mercruiser Prop selector tells me I need to replace the current props with 19.5 x 13. (this was using the Deep V Offshore Fishing category that does not really apply to my boat).
Castle Marine Prop calc tells me I need 18 x 15
Another calculation tells me I need 17.75 x 17.
As any of the above props will bring my WOT speed within the Mercruiser range, what will the effects of these propellors have on my Fuel Consumption and my cruising speed. What would happen if I just left them as is and accepted the max RPM as 3300? Thanks for any help.
My boat is a Catamaran Hull, locally built, of what they term the Australian Form, i.e both hulls are symmetrical. It is fitted with twin Mercruiser 4.3L MPI, coupled to Bravo 2X outdrives, 2:1 gear ratio, with 18.75 x 19 aluminium props. Counter rotating. I cruise mainly at 3000rpm and 20kts and very rarely hit WOT.
My current performance is:
Maximum RPM achieved 3300RPM (way short of the 4400 ~ 4800 specified)
Speed by GPS 22kts
Weight approx. 6t including 800 litres fuel and 400 litres water.
Waterline length 35ft, beam 16ft.
Fuel consumption (by Floscan) 40litres/hr each engine or 4litres/nm combined running at 3000rpm and 20kts.
The Mercruiser Prop selector tells me I need to replace the current props with 19.5 x 13. (this was using the Deep V Offshore Fishing category that does not really apply to my boat).
Castle Marine Prop calc tells me I need 18 x 15
Another calculation tells me I need 17.75 x 17.
As any of the above props will bring my WOT speed within the Mercruiser range, what will the effects of these propellors have on my Fuel Consumption and my cruising speed. What would happen if I just left them as is and accepted the max RPM as 3300? Thanks for any help.