Aluminun prop twin 4.3

[Carl skoglund]

Hi

I am currently running my 1989 searay amberjack with 2002 4.3 merc. apox 180 hours, 190 hp at the prop, alpha one drives. with out of the box 14.5 X 18 merc 4 blades R and L. I have cupped the props trying to bring down my cruising rpm of 3800 at 24 mph, I dont need speed I would like to run at 3200-3400 rpm at cruise. If i can find 3 bladed or 4 bladed at 15 x 18 R and L it should do, is that size available out of the box.?

 

[H20Rat]

Re: Aluminun prop twin 4.3

what is your WOT rpm? You don't prop a boat for a specific rpm at cruise, it needs to be correct at WOT operation only. The trouble with doing it at cruising speed is that you are probably going to be lugging the engine, which will dramatically shorten its life.

(think of it this way, you are driving a manual transmission car, and instead of cruising around at 3000 rpm in 3rd gear at 40 mph, you want to run in 5th gear at 1200 rpm at 40 mph. The engine is far below its optimum speed and your bearings, rods, and head gasket are all taking a beating.)

 

[wbc1957]

Re: Aluminun prop twin 4.3

I always thought the 4.3 twin ran on 14 1/2 x 17's, in the four blade. There is a pair of these in SS on ebay...item # 120715346644. Food for thought.