We have heard nothing but rave reviews from folk who have bought the Indigo prop. It has been described as having a “turbine-like” smoothness, and it is sized to allow a direct drive Atomic 4 (in reasonably good condition) to operate at RPM ranges of 2,200 to 2,400 RPM, which provides good power when sea conditions deteriorate. – Updated: November 4, 2003
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.
Except for certain boats with an aperture type of keel/rudder combination, the standard prop recommendation was usually a compromise between a 12 x 6 and a 12 x 7 two-blade. For aperture configurations, props as large as 13 x 8 have been recommended by the builders. The Pearson Triton and Vanguard are examples of boats equipped with such large props.
In recent years, several props have been designed especially for direct drive Atomic 4’s which (according to customer feedback) show great promise! First on the scene was a plastic prop designed by the CDI company. This prop is a 12 x 7 with rather squared off tips. CDI reports that in dynamic tests, the plastic construction allows the prop to flex down to a 12 x 6 configuration under high power settings which has the benefit of allowing the engine to reach a higher point on its power curve. The tip design produces good performance in reverse. In the three-blade category, Michigan Wheel produces a 10 x 6 three-blade that is receiving good reviews. Its strong point is in allowing exceptionally high maximum RPM while retaining good efficiency down to around 2200 RPM. Last, but by no means least, is a stainless steel three-blade developed by Indigo Electronics. This prop is a 10 x 7 (10 x 7.4 to be exact). Like everything else that Tom Stevens brings to the market place, this prop has been thoroughly tested on his Tartan 34. Excellent quality control during production (insuring good balance and consistency in pitch between blades) produces extremely smooth performance, and the additional pitch gives good performance in reverse. This prop handily meets the desired perimeters of 2500 maximum RPM (plus) and cruising capability in the 2000 RPM range.
With continued good reports on all three of these new props, it is becoming easier to get out of the low RPM blues that have plagued the direct drive fleet for decades! So, if you feel that you may be over-propped and under-powered (like when sea conditions deteriorate), here are some suggestions:
The most conservative suggestion: You could simply resize or re-pitch your existing prop. As a rule of thumb, decreasing the diameter or pitch by one inch will increase your RPM by approximately 200 RPM. Most prop shops are reluctant to decrease a prop more than one inch in diameter and one inch in pitch. However, such a combined change could get you as much as 400 additional RPM which could translate into 3 or 4 more horsepower.
If you feel the need to start over again: Try to get recommendations from the owner of a boat just like yours. The Internet is full of chat rooms and support groups representing practically all of the production boats powered by the Atomic 4. While each of the current props mentioned above are reported to work well on a large variety of boats, it is also true that as reports continue to come in, some will probably work better on one model of boat than another. As a case in point, the small diameter three-blade props are reported to be working well on the Alberg 30, which contradicts what we thought we knew about what would work on an aperture type of keel/rudder configuration.
Be sure to consider your own boating style: Some of our customers prefer the low RPM feel of larger props, some are avid racers and are more concerned over an ounce or two of drag while sailing, and still others have strong preferences for two-blade or three-blade designs. In this connection, I am pleased to report that my own historic concerns over harming your engine by loading it down with too much prop have been somewhat overstated. It turns out that the updraft carburetor, along with a general over-design of its mechanical parts, creates a sort of self protective shell around the Atomic 4 that even I underestimated in my earlier writing (newsletters circa 1993).
If you were one of our subscribers during that period of time, you’ll recall that I related my experience in an AI-E Skyraider that I flew over Vietnam (powered by a large supercharged radial recip engine) with the Atomic 4. While it was extremely easy to overstress that engine,the Atomic 4 is (by comparison) almost impossible to overstress. – Updated: November 4, 2003