That lower unit puts the age at late 47 to 1949. It is the unit used by the 50 horse storm boat motor and uses a .988" prop shaft. In 1950 they changed to a slimmer unit that is faster on a light boat. The early Speeditwins used an Elto pattern that was very fast. They had some problems with the water intake being covered with weeds. They modified the case making the water intake wider and adding a pipe tapped hole at the top with a plug screwed in. The idea was you could take out the plug and screw in a length of pipe (with the end cut at an angle) so you could pile up six inches of weeds and still pump water. These pre-war cases used a .788" prop shaft. The Storm Boat Motor lower Unit had wings on the gearcase to protect the prop when running up on beaches. This added drag made the ’46 and 47 Speeditwins the slowest ever built. Knowledgable owners took a hacksaw to these protrusions.
When the 5,000 left over lower units from the last Storm Boat Motor contract (for the invasion of Japan in 1946) were used up, the later castings were wingless. Clyde Wiseman (Wysco Piston Co.) made a copy of the Elto pattern gearcase that was used in APBA "C" Service racing. He had a "Heavy duty" model with a heavy duty pinion bearing and .988" prop shaft for the BigFour. Dad’s modified Big Four on a Lyman lapstrake boat clocked 54 MPH through a measured mile using that Wiseman gearcase. One of Dad’s modified gear cases was modified By the factory (They took the sharp point off the nose & shortened the case above the exhaust outlet) and put on the 1950 Speeditwins and Speedifours. Several of our AOMCI members Big Fours are using those 1950 lower Units. The last models of these cases had a water scoop behind the prop. The Elto pattern is still the fastest and the one to race. . . 😎
