Showing all 2 results

Low Resistance Propulsion Systems


The RPS (Retractable Propulsion System) and SPS (Saildrive Propulsion System) provide minimal resistance when under sail and during races. The systems have the following features: - The systems are light weight - CP Propeller - Require minimum space inside the hull and engine room - Can be posisitoned near the center of the vessel, for optimum weight distribution - Highly efficient when motoring. - Replace a normal stern propeller The RPS system can swing up into the wet box in the hull. When the doors are closed there is no resistance when sailing. When down the doors close around the leg so the hull looks smooth. It is only the leg that protrudes from the hull. The wet box takes up app. 2m x 1m x 1m inside the hull. The SPS system stays down but the propeller can go into feathering posisiton. This will give a little resistance when sailing but the resistance is far less than from a normal shaft, P bracket and propeller in sailing position. The SPS system takes up a lot less space in the engine room since there is no need for a wet box.

Power supply and layout

Both the RPS and SPS can be driven by an electrical motor or a combustion engine. The RPS will always have the power in at a horisontal direction. The power unit can be aft of the RPS or on the side depending on how it is most convenient. If a combustion engine is used a clutch unit must be mounted on the engine. In the clutch unit it is possible to have 2 PTO/PTI. The PTI can used for regeneration. The SPS can have an electrical motor mounted directly on top of the leg. This will take up as little space as possible in the engine room. Both of the systems exsist in three different sizes. The sizes are called 286, 335 and 386. The numbers correspond to the diameter of the propeller hub and that correspond to the power. - 286 goes up to approx. 400kW - 335 goes up to approx. 540kW - 386 goes up to approx. 660kW The lower leg are similar on the RPS and the SPS. The RPS can adjust the propeller pitch forward to astern. The SPS can adjust the propeller pitch fethering to astern.

Propeller orientation

When designing the system for the boat it is important to decide where the focus lies. Is the focus on motoring or on regenerating. The propeller on a RPS/SPS can be positioned forward or astern. That is just a question of turning the leg 180 degress. If the focus is on having the best possible efficiency when motoring the propeller should be pointing forward when in motoring. If the focus is on regenerating the propeller should be pointing astern when motoring. The difference in efficiency is causted by the shadow from the propulsion leg. When regenerating the propeller should always turn opposite of the motoring direction.


Regeneration is posssible on both of the systems. If a RPS is used for regeneration a hybrid clutch must be mounted on the input shaft of the clutch unit. A generator must be mounted on the PTO/PTI. The power will then come from the propeller into the clutch unit and out through the PTO/PTI. The size of the generator is limited due to the size of the PTO/PTI. If a SPS is used for regenation the electrical motor/generator on the SPS is mounted directly on  top of the leg it is then the size of the generator or the electronis parts connected to the generator that is the limit. On a conventionel propulsion system about 20% of the designed power take out can be regenerated. On a RPS/SPS about 27% can be regenerated due to the fact that the leg can be turned 180 degress. Turning the propeller blades the opposite way makes them about 30% more efficient than a standard positioned propeller when regenerating.


When motoring the systems work the same way. The leg can be used with the propeller pointing forward or astern. The leg is not intended to be used as a steering system. The leg should be in a fixed position when motoring.


When maneuvering the engine is in a fixed medium-range RPM and the lever only adjusts astern – ahead by changing the pitch of the propeller. The leg can now be turned sideways to work as a thruster. The thruster will be with the max. power from the engine/motor. The leg turns from ahead to sideways in less than 3 seconds. The leg can give propulsion in any direction when maneuvering. The boat will often be maneuvered without the use of a bow thruster. The boat will be able to go sideways, sideways and astern, sideways and ahead or whater ever is needed.