Wikipedia provides insight on this question of bronze use:
http://en.wikipedia.org/wiki/CopperIn particular, copper is the major component of bronze. In 1999, copper hit a 60 year low price of $.60/lb. Which also means in 1939 the price of copper started a six decade rise in price. With the entry into the war in September 1939, the British began buying raw materials from the US as well as materiel; thus, starting the increase in copper prices.
In the Epilogue to Ian Dear's 4th edition (1999) "Enterprise to Endeavor--The J Class Yachts" he lists the demise of the American J Yachts: "Whirlwind (wood) and Enterprise were scrapped in 1935; Weetamoe in 1938, Vanitie and Resolute in 1939, Rainbow in 1940, and finally, Ranger and Yankee in 1941. "
Whirlwind never sailed after 1930, nor did Enterprise because of rule changes. Weetamoe and Yankee sailed competitively through at least 1937. Rainbow was the most competitive of the earlier J's with Ranger. Ranger was built of steel at Bath Iron Works.
Harold "Mike" Vanderbilt changed his focus from the J's to 12 meters and began campaigning "Vim" in 1939.
According to Herreshoff of Bristol, in 1937 the firm lost nearly $17,000. The 1938 Hurricane severely damaged the works, the Haffenreffer's repaired the damage and offered the business for $300,000. Unable to find a buyer, the firm continued to build boats mostly on order. The firm rebounded during the war building 100 military vessels from 1942 through 1944. Only 8 sailboats show up on the build record during the war.
With the owners scrapping the big boats because they no longer wished to pay storage fees for boats they would not use, one should assume there was plenty of bronze available for use, even as the price was starting to rise. Although Steve suggests the decision to use iron or steel was made in the 30's most likely for business reasons as was using vendors for casting.