Many moons ago, Tad Roberts (naval architect who frequents this site) recommended Beta Marine if we ever wanted to repower our 30' Sundowner Tug.
The PO of our boat put in a 100hp Yanmar 4JH2-UTE and I believe it's too big. We usually run it at 2700rpm which is not much past half throttle, and the turbo doesn't really wind up till just below 1/2 throttle. This gives us 7 to eight knots depending on currents, etc, and uses about 1.3 gph. Adding throttle past that point does add speed but the arse end squats badly, the bow starts plowing a huge wake, and I imagine fuel consumption would increase quickly. It is maybe two/three inches from the forward bulkhead in the engine room.
If we ever repower it would probably be the Beta Marine naturally aspirated 75hp or the 85hp turbo...would have to research a bit.
I'm a letter carrier and can recognize the sweet, smooth running sound of a Kubota diesel engine in a backhoe doing work in someones yard from a street away