They can, but they also may not . like most things in life -- it depends.

There are several reasons for buying a 3B prop.

1) It's cool & you like it.

2) Scale appearance

3) Noise suppression -- some clubs & competitions have stringent noise limits

4) Aero-braking -- this is the common application to trainers

5) Geometric limitations prevent the use of a 2-blade prop that can handle the full output of the engine -- eg. insufficient ground clearance, insufficient airframe clearance.


The use of multi-blade props incurs performance penalties -- such a loss of performance is the price for meeting the other relevant criteria.

Regarding efficiency --- staying with the same manufacturer, the same material of construction, the same prop blade geometry (aspect ratio, shape, etc), a 3 blade prop for the same power load will always be less efficient that the 2 blade -- it isn't debatable -- it is simple fact & physics. There is more mass, more wetted area, more frontal area, one more root-transition zone, and for a prop of the same pitch -- less diameter. All of which simply means more parasitic power losses from drag, plus the loss of disc area in the case of smaller diameter. The engine makes the same HP (same load), but it just produces less usefull work.

However, as soon as you start mixing & matching the prop performance factors noted above, you can have some 3B props that actually "outperform" some 2B props -- but you are comparing apples to oranges in those cases & this is where much of the confusion & endless debate arrises.

The more efficient the 2B prop design, the smaller the performance differences will be between the particular manufacturer's 2B & 3B props. Conversely, the poorer a given manufacturer's 2B's perform, the greater will be the disparity in performance (poorer) of the equivalent 3B.


Guidelines for 2B - 3B conversion are simply that -- guidelines. In the prop size-ranges that are appropriate for 40 - 90 engines, for a particular prop manufacturer, using the same material of construction & with props of the same blade geometry (shape, aspect ratio, etc) --- the approximate equivalency is: going from 2B to 3B, reduce the diameter 1" for the same pitch, -- or, reduce the pitch 2" for the same diameter.

In the case of a typical BB 46 engine, an 11-5 2B, 11-6 2B, or 10-7 2B is about right for general flying. All of those props will let the engine pull acceptable RPM's & generate reasonable thrust. Staying with the same prop manufacturer etc, an appropriate 3B would be a 10-5, 10-6, or 9-7. If you could find them, you could also use an 11-3 3B, or an 11-4 3B, but airspeed may begin to suffer unacceptably.

If you mix manufacturers, materials, blade shapes & aspect ratios, it becomes very difficult to use anything other than suck-it-and-see testing to determine which 3B prop is equivalent to your favorite 2B, or vice-versa. The guidelines become usefull only for ball-park guessing.