Propeller Lore - The Duel

The test ship
The ship, a Bavaria 37 Lagoon in owner's version built 1992/93, is equipped with a Volvo Penta 2003 Turbo (with turbocharger) with saildrive. With an engine power of 31.6 kW (43 hp) and a displacement of 8000 kg, the power-to-weight ratio is 3.95 kW/ton (5.37 hp/ton), which is actually sufficient for this size of ship. Some experience with the ship nevertheless led to considerations about changes to the propulsion system: The berth makes considerable demands on the maneuverability of the ship in unfavorable wind directions. The distance between the saildrive and the rudder causes a delayed response of the ship when the rudder is given, which has a negative effect in the present box situation.

However, this conversion entails considerable costs, plus you have to reckon with a leaking stuffing box, a high level of noise and vibration, and the risk of getting a rope in the propeller. The installation of a bow propeller is also ruled out for space reasons. The only solution is to optimize the propeller.

The new propeller
When choosing the right propeller, we take advice, but based on the accessories catalog for a given Volvo Penta engine, the right propeller can be easily selected. A propeller with a diameter of 18 inches and a pitch of 12 inches comes into question: We finally decide on a new patented three-blade folding propeller from Volvo Penta, offered for the Saildrive. The comparison curves made us optimistic about achieving the goal with a comparatively inexpensive (though not cheap) solution. One possible improvement would be to convert to a shaft system: the propeller would move closer to the rudder blade.

Hull speeds
We are excited to see the result of our investment after the ship is floating in the water again. We quickly realize: the action was worth it. Speed measurements show a higher speed over the entire speed range and also a higher final speed. The optimum tuning of the propulsion system to the ship determines whether the theoretical hull speed of the ship can be fully realized under engine power. The theoretical hull speed is calculated from the length of the waterline using the formula Vr=√(LWL/0.64). For the Bavaria370 Lagoon, a waterline of 9m results in a value of 3.75 m/sec. This corresponds to 7.29kn. With the existing engine and fixed pitch propeller, a speed of 7.2kn was achieved, roughly the hull speed. The three-blade folding propeller even reached a top of 7.8 kn, an improvement of 8% after all. The speed was measured with the ship's own GPS and care was taken to ensure that a stable condition had been established and wind influence on the measurement was avoided.

Improvement
So the propeller works with a much better efficiency, the engine power will be converted into thrust and propulsion faster: So now we achieve the same speed at a much lower RPM. In addition to the fuel savings, which allow a greater range, we notice a clearly noticeable reduction in noise levels and vibrations. The acceleration figures prove a significantly improved response; when reversing, the time to accelerate to three knots is halved! The turning circles in reverse remained more or less the same, and it is noticeable that the turning circle over port is only half as large as over starboard. This offers a clear strategy for mooring over stern (Roman Catholic) with mooring lines, as is common in the Mediterranean.