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The B29 came originally powered with an Atomic 4 gas engine. This engine was not geared down via a transmission and the prop turned at the speed of the engine—meaning since the prop was turning at relatively high speed it could be small and the aperture for the prop could be made small as well—in the case of the B29 the aperture is limited to a 12-inch diameter prop.. For direct geared gas engines that may be fine, yet a modern 2 or 3 cylinder Yanmar wants to throw a 16 inch prop at least. I have found a very happy medium between output power of a small modern diesel and the limitations of a 29. I use a three blade 12 inch by 12 inch pitch prop. The engine I repowered with is a 3GM30F with a 2.64:1 transmission. The engine makes a full 3200 rpm with no strain and provides hull speed to the boat at about 2100 rpm.
Now, the transmission is a Hurth, very common as is the gear ratio. However, if the gear ratio is different on your boat, you will need to adjust the pitch of the prop accordingly.