FREE SAILING HYDROFOIL MODEL "SØFOIL 4" BY SØREN STRUNTZE
The photo shows the hydrofoil model named "Søfoil 4" designed and built by Søren Struntze of Copenhagen, Denmark.
As can be seen from the photo, the model runs foilborne via a single two blade propeller driven by a Monoperm electric motor via a thin inclined drive shaft with energy provided by a single large NiCd battery.
The model is free sailing in that it is not equipped with a radio control unit. The rudder located at the centre of the model aft is positioned at a slight angle such that when it is released it will run in circles until its battery runs low. Typically, the model is used on a small lake where it is released from a boat such that it can run without hitting any obstructions. A pressure switch has been incorporated at the transom such that when the model becomes hullborne after battery power runs low, the power is automatically cut and the model can be retrieved easily to the boat from which it has been launched.
The hull of the model is 420 mm long with a breadth of about 120 mm. The model has a mass of about 400 grams when complete with battery. Both the bow and stern foils are of a surface piercing design though the stern foil virtually runs in fully submerged condition with only the pair of outboard support struts connecting it to the hull penetrating the water surface. The model runs very smoothly and quietly and can handle waves well relative to its small size. As can be seen in the photo, the model cuts through the water with a minimum of spray from its foils, struts or propeller shaft. The foils are finished in high gloss paint to minimise their drag. With a set of three quickly exchangeable battery packs, the model is able to sail about two miles in total.
The "Sø" in Søfoil means lake in Danish, the "4" indicates this is the last of a series of four prototypes. The model was built in 1967 and Søren Struntze took the photo in about 1970.
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