I was curious about how many hammers we would lose by switching builds, so I did some testing. All test were done with SIP & switching to a Work Boat with knowledge of fishing. I worked Corn until size 2, then worked Corn & FPH while completing 1st WB, then netted a northern Clam and worked the Clam Boat & FPH.
With both a Warrior (initial build time 22 turns) and a Worker (initial build time 23 turns) I started getting warnings about losing a hammer in 5 turns on the initial build at turn 16, but in each case I was able to complete the 1st WB and resume building the Warrior or Worker without losing any hammers. Warrior completed on T25, Worker completed on T32.
With a Barracks as the first build (initial build time 75 turns), I completed the 1st WB and never got a waring about losing hammers. So I started a 2nd WB, and completed it without getting a warning. I started a 3rd WB, and on turn 44 I got a warning saying I would lose 1 hammer on the Barracks in 19 turns. On turn 46, the 3rd WB was completed, and the Capitol grew to Size 4, but there were still 17 or 18 turns remaining before I would lose a hammer on the barracks build. So I started a Settler at Size 4, working 3 Clam Boats and the FPH (except for the first turn, where I worked 2 Clam Boats, unimproved Corn & FPH). On T56, I whipped the Settler for 2 pop w/ 42
overflow, and started completing the Barracks. City #2 founded on T59 and Barracks completed on T61 without losing any hammers.
Someone may want to try and verify these results, but it appears we can manage to not lose any hammers on our initial builds. With Warrior or Worker, it just requires completing the initial build after the 1st WB is complete. Also haven't looked at if its worth letting those 10
evaporate in order to get Work Boats out faster, but apparently if we go with a Barracks as the first build, we don't have to worry about that.
Haven't tried this with a Settler as the initial build.