G'day Chop
The 6BTA's are a great engine, and it is what I will repower my Berty with when the time comes.
The big advantage is the nice flat torque curve of the Cummins. It makes more torque at 1600rpm than your current 70 series Volvo makes at peak torque.
At your cruise revs, assuming you can use the existing gear boxes (I know MG506 is ok at 320hp, the 370 Cummins might be touch and go), you will make over 100ft/lbs more torque with the Cummins and significantly more HP. You will be able to swing a lot more pitch, as long as your existing shaft size can handle it.
You are currently using about 500hp to do 15kts, that really won't change a lot, but instead of using 500 of 540 available HP and running at around 93% load, you can run 500 of 740 available HP, at a load factor of 68% for the same performance. A much better scenario for long engine life.
You will probably do even better, as the Cummins will be a bit lighter.
Fuel burn will be directly related to the HP extracted, so if you stick to 15kts, you can expect pretty much the same burn you have now.
The problem with the burns at certain RPM's on the charts they publish, is it depends on the load they use in the "propeller curve". If you use the 19L/100hp you won't be far off.
What type of hull do you have? What speed is the hull really happy at?
I know my Bertram hates dragging its bum at the 18kts I get around at, once I gets over 21kts it really comes alive. The wake flattens out, the ride is miles better and it is generally just a nicer boat to be in. My problem is I don't have the HP to cruise there.
Basically, if you have the Cummins there and the extra coin to do the repower (may need a change in gears, shafts and definitely props), I would do it for sure.
The difference in spare parts price alone would have me do it haha Volvo make nice engines, but they are rapists when it comes to buying spares.
Decisions decisions
Cheers,
Myles