A-4 in Newport 27

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Posted by John ( on November 21, 2004 at 21:42:24:

In Reply to: A-4 temp. and RPM range. posted by Adam on 20:45:10 11/21/04:

If the propeller is properly sized for the boat the engine speed should be somewhere around 2000 rpm at crusing speed as the maximum engine speed (red line) is about 3200 if I remember correctly). 1800-2000 will give you plenty of power and fuel economy. If it is very much higher or lower your prop may need changing.

The water temperature depends on whether your engine is raw water cooler or has a closed circuit freshwater system. The raw water system should have a 140 degree thermostat to prevent salt deposits. A fresh water cooled engine will be happiest at 180 degrees. 200 isn't boiling yet but is getting close.

With a 27' boat the waterline length is going to limit your hull speed to around 5 knots. That is the point where the power needed increases quickly with increasing speed and you run out of engine power or start cavitating your prop. If you want more, you need to get a longer boat or a power boat with a planing hull.

I don't think the "pop-out-of=-gear" thing is a safety feature. If the prop is sized properly for the boat it is probably impossible to over rev the engine in gear. With a displacement hull you run out of power first. It sounds more like a misadjusted reversing gear or (according to Don Moyers manual) a cavitating prop. Check his website (www.moyermarine.com) and get his manual. Try adjusting the reversing gear clutch pre-load as he describes (it is not that hard) and see if that helps. If it is a cavitating prop, that could account for low boat speed and you need either a larger diameter or higher pitch.

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