Why Surface Piercing Propulsion?  

Surface Drive Comparison to other propulsion systems

The specialized “Surface Drive” propeller operates half in and half out of the water in the “cut water” or “planing wake” region that occurs immediately behind a boats transom when planing. This is due to the propeller shafts flat, almost horizontal exiting through the boats transom, not the bottom, as conventional 12º – 18º inclined shaft, inboard powered boats do.

Contrary to conventional wisdom, the incline of the traditional inboards propeller shaft causes it’s propeller to have an angle of attack such that, the majority of thrust is produced with the lower blades during the propellers rotation. Therefore throughout the top half of its rotation the blades only generate minimal thrust, and cause resistance. Further resistance is created by pushing through the water the propeller shaft, its support strut and the entire rudder. Additionally the top blades receive disturbed water caused by the spinning shaft and strut ahead of it creating further inefficiencies.  As well inefficiencies occur with the inboards 12º - 18º propeller shaft angle expending a significant portion of thrust down into the water. The objective is to simply push the boat horizontally. In the case of a twin inboard boat, multiply the resistance and inefficiencies by two.

The result for the Surface Drive is an approximate 20% gain in efficiency over inboards due to the improved thrust angle and the eliminated resistance of the underwater appendages. With the Surface Drive, only the thrust producing bottom propeller blades and rudder tip are immersed which also provides the added benefit of reduced draft.

 Efficiency of Other Drive Systems

     Jet Drives

Are basically a water pump and operate on a "Differential of momentum principle" They are inherently less efficient than Surface Drives because:

  1. Energy provided by the engine is required to lift the water up through the bottom of the boat, change its direction and process it through the jet.
  2. A downwards force is caused by lifting the mass of water up into the hull to feed the impeller, thus reducing the hulls lift and ability to plane.
  3. Resistance is caused by the water intake opening under the hull.
  4. Further resistance is caused by the significant wetted area within the jet’s ducts, and impellers.
  5. The small, fast spinning impellers are less efficient than a larger, slower turning propeller.


Out-drives are less efficient that a Surface Drive because:

  1. The nose cone of the lower housing creates turbulent water ahead of the propeller decreasing efficiency of the prop.
  2. The resistance of pushing the lower housing through the water.
  3. Power losses through the resistance of turning the associated U-joints, gears, bearings, shafts, lube oil, etc.
  4. Other negatives with an Out-Drive include sensitive water seals and oil filled housings susceptible to water contamination, shafts, bearings, gears, U-joints prone to wear and breakage. Hydraulic hoses and fittings, aluminum housings, susceptible to corrosion and damage. In general this design is complex, sensitive and expensive to repair.

What About Damage from Floating Debris

Inboards and out-drives are more sensitive to damage from floating debris because:

  1. With the surface drive’s flat shaft angle there is inherently less of the drive protruding below the boat, which reduces draft and the exposure of the drive.
  2. Designers of inboard and outboard style drives are conscious of the water resistance caused by the shaft, strut and rudder or the leg so they tend to build these components to be light and slim, making them more prone to damage.
  3. The surface drives shaft, strut and most of the rudder are unaffected by water resistance being out of the water at planing speeds. Therefore these components can be built strong. Surfacing props are typically of Stainless Steel and very robust.

The bottom line is that a surface drive surpasses the efficiency of any other high speed drive system available. Typically an efficiency gain of 10 - 20% is recognized over Out-drives, Inboards, or Water Jets.