From sterndrives to jets to surface drives

Two years ago I converted my Draco 2500 Twincab from twin Mercruiser 3.0 Ltr LX's to a pair of Berkeley Jets, powered by the same engines. A major operation, which involved adding a box shaped extension to the transom to obtain the necessary 1ft extra lenght in the engine bay. The results were very disappointing for several reasons. Despite rudders under the nozzles, the steering response at slow speed is very poor (sluggish) and the efficiency of the jets is about half that of sterndrives. In the Adriatic sea, where my boat resides, oysters and other marine growth have taken posession of the impellers and the steering nozzles, making things even worse. And last but not least, the alloys used for the various parts seem to contain so much zinc that the whole jet body is rapidly dissolving.

Since large holes had to be cut in the transom and bottom to accomodate the jets, returning to stern drives is no option. Besides, they also cannot cope with warm seawater and need frequent maintenance.

This fall, I plan to remove the Berkeleys and the engines, mold half circle tunnels in the bottom and transom and install prop shafts. It will be some sort of surface drive at speed, fully submerged when going slow. There will be 2 rudders to each side of the prop, similar to the Q-SPD drive has, but somewhat smaller and made from grp.

To start with, I purchased a pair of VW 1,9 TDI engines and Technomarine gearboxes plus various marinizing parts and just started work on making bell housings because in their cars VW used a one-piece gearbox/bell housing that also held the starter motor. Engines and gearboxes will be close mounted on silent blocs and there will be a double universal joint to the prop shafts.

To minimize drag, I intend to use a stainless steel tube of sufficient strenght to house the prop shaft without the need for a strut. The free length will be abt. 2 ft. Because I never had much faith in water as a lubricant, I want to use ball bearings at the outside, protected from the water by 2 SKF neoprene molded simmer rings, one facing outwards, the other inwards and use angled ball bearings at the transom side to take up the thrust, sealed with another simmer ring facing inwards. The tube itself will be kept oil filled from a small tank just above the water level. Since all outboard and sterndrive makers use such seals, I do not see why a prop shaft should be in a water filled tube with only a stuffing box at the inner end.

Any comments or suggestions are of course more than welcome.