Hello again. I have a recently aquired 1984 Eastern 22' lobster style boat with a 1986 Merc 150 BlackMax OB. The hull design is a V, which is relatively flat in the stern. The hull is not stepped.<br /><br />I've been working bugs out of this engine; first the oil pump alarm (which was partially solved by replacing the pump shaft sensor, but it still alarms at low rpm), then overheating, which was solved by doing the bi-annual water pump/thermostat/diaphram maintenance that was obviously ignored by the previous owner.<br /><br />So, the last big problem is that I can only get 3700 rpm under ideal tilt conditions. The spec for the engine is 5000 to 5600 rpm. The current prop is a cupped Michigan 14.5" diameter, 17" pitch. I am able to get at least 6000 RPM in neutral, although the engine doesn't sound perfectly smooth at that rpm.<br /><br />I have checked the calibration of the boat tach by simultaneously taching the power head with a calibrated contact tach, and it is dead nuts. The boat hull weighs 2200 lb, max fuel is 70 gallons for 560 lb, engine is 350 lb, optional equipment is estimated at 500 lb, 2 people/gear is 400 lb, for a total estimate of around 4000. I have tested with four people and various amounts of fuel, and with 2 people and almost no fuel, and the max rpm is always 3700, so I don't seem to be on the edge of any weight/performance ratio.<br /><br />I have adjusted the position of the carb linkage, and the butterflys are wide open. I have rebuilt the carbuerators and the fuel pump, replaced the fuel filter and burned through all old gas. I am running 87 octane.<br /><br />I'm not really concerned about top speed or hole shot. This is just a lobster/fishing boat used in the atlantic ocean. But I am concerned about over straining the engine and getting stranded in the big blue sea!<br /><br />The 14.25/17 prop I have seems like it should be about right, but based on the prop rules of thumb, it seems like I would need to reduce diameter and/or pitch to unrealistic (and unavailable) values to gain 1300+ rpm. I can get a 14.25D/15P prop in stainless, but I just want aluminum (cost) and the 300-400 rpm that will give is just not enough.<br /><br />So, I'm leaning more towards having a power head problem that is causing a loss of HP. I have not checked engine timing yet, and plan to do that tomorrow. Could timing cause a large decrease in max RPM? I do not own a compression tester, but I finally need one in by backyard mechanic career, so that is coming.<br /><br />Any other suggestions are much appreciated.<br /><br />Thanks,<br />Greg Calder
