Performance from Different Propellers

Devilray

Recruit
Joined
Oct 20, 2009
Messages
2
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.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Performance from Different Propellers

Running that far out of the reccomended range is bad for the engines.
Could cause premature engine failure.Your motors will be much better off allowed to rev within their range.4 blade props should get the hull up easier and stay up with less power.Getting the motor within its range should improve performance and economy.While I'm not an expert in prop technology
Its my feeling the larger diameter 13" pitch will do better.Others may be along with more specific suggestions.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: Performance from Different Propellers

The issue with incorrect props is one of engine stress which relates directly to fuel economy. For example, if one installs a prop with too much pitch, as yours apparently is, the engine is lugging (working hard) which means the throttles are open wider than would otherwise be necessary to achieve the same speed with the proper prop. Now don't confuse throttle opening as a direct correlation with rpm because again, the prop affects that correlation. With the proper prop, boat speed can be better controlled with the throttle, and you will find you need less throttle opening to make the same speed. Although engine rpm may be a bit higher at that speed, it will be achieved with less throttle opening because the engine is operating under less stress. This of course is less noticeable when prop selection is just a little bit incorrect.
 

Devilray

Recruit
Joined
Oct 20, 2009
Messages
2
Re: Performance from Different Propellers

Thanks for the advice Silvertip and Steelespike. I have to get this right as I don't have the luxury of a prop test/swap service, and my pockets aren't that deep!?! I wish those manufacturers of interchangeable blades would come up with something for the Bravo 2x.
 
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