Don't be fooled by the 30HP rating of the A4. At the most common operating speeds of about 1,800 rpm
, they only put out about 15-17 hp at best.
Actually, the A4 experts over at Moyer Marine Atomic 4 Engine Rebuilding and Parts
seem to rate the horsepower even lower than this. Here's parts
of a prop selection article.
A good prop selection for the Atomic 4 is one which will allow the engine to develop a maximum of at least 2500 RPM, while retaining enough efficiency to allow for a cruising power setting of approximately 2000 RPM (at which point the engine produces 16 horse power).
The problem in selecting props for direct drive Atomic 4 engines is that there is a very small range of props which are small enough to allow the engine to turn at such a high RPM and still retain the efficiency necessary to move the boat well at less than maximum RPM. For whatever reason, many boat builders throughout the years tended to install propellers that were too large to allow for the factory recommendation of a cruising RPM in the range of 2000 RPM. In fact, many people report maximum available RPM of 1500 or less, which limits cruising horsepower to 10 horse power or less.
So, the apparently underpowered
may be very comparable in actual power output to the A$.