Reusing BG 72C V Drives with 6BTAs

[yeungmicah]

G’Day from Sydney Australia
I’m new to the forum and a keen but armature, got to do it yourself, keen boat owner/mechanic ….. so go gentle and patient please 🙂

1989 42′ Mikleson Fly bridge cruiser, semi displacement, planes at 13knots and top speed 22 knots with the originally powered twin DD 8.2lt V8 220hp @3200rpm coupled to Borg Warner 72C V-Drives (don’t have the ratios as there is no labels).

While the boat is solid, the engines are shot and given they were never great engines I can’t replace with like for like.
So I went for the very very similar size Cummins 6BTA, because of 3 key factors on my repower.
1. Engine reliability
2. I got a great deal on fully rebuilt/condition (from the bottom end to top) pair up at a fraction of the cost of a Cummins ReCon
3. The Engine room space and fit. There is limited engine room hight and the 6BTA is 3.1″ longer and 1/4″ higher, plus, I have seen the same boat refitted with the 6BTAs

OK so my question
6BTAs (engine serial number 46410512) were originally specked at 330hp @2800RPM. The engine re builders have put in the 370 marine injectors and cylinders to spec to 370hp at 2800rpm. Note its got the P Pump.

I simply can not afford the ZF 280IV transmissions ($25K) and I know SBMAR do a conversion kit for the 6BTA to BG velvet drives.
However, I have been told conflicting information on if the BG72C will take the HP/Torque of the engines.#
SO please, what are my options
1. Will the BG72C handle the 6BTAs with the SBMAR conversion kits…. if Yes great problem solved! but…
2. if No, can the BG72C be modified?
3. If not, again, can I simply limit/dial the power back on the pump?

Thank you again.
Micah

Topic

[yeungmicah]

HI Tony
Thank you for the reply. Very useful.
I will go ahead and purchase the adapters under the operating guidance provided.
FYI yes the boat was propped to make the max RPMs.

Thank you
Micah

[Tony Athens]

If you use the same props that you had with the 8.2’s, then the STEADY STATE loading on the gears will not change.. However , using full throttle during the acceleration time could easily break the clutches loose as the 6BTA 370 has a substantial higher torque rise than the 8.2’s did in the 1600-2200 RPM range
(capable 682 at 1600 RPM & 732ft lbs at 2200 RPM) . Just keep that in mind when trying to get up on plane. Also, I am assuming your 8.2’s were propped to reach their rated RPM.

Tony

[yeungmicah]

Tony,
Can you let me know about point 2 and if I don’t change the props can I keep using the BW72c with the SB couplers?
Thanks

Tony Athens wrote:

#1—————- There is no such rating for a 6BTA 5.9…..It’s 355HP at a minimum rated RPM of 3000.. Propping should be for 355BHP at 3150 RPM if you expect good life.. The fuel spec is “build the fuel system to” is CPL2208 or CPL8457…………..

#2————Your BW V-drives will not even come close to holding up unless you prop to about 250 BHP at 3100 RPM.

#3————Were your 8.2’s RT & LH as to rotation?, or CCW (LH) for both.. The Cummins are CCW (LH) .. That’s another potential issue..

Tony

[yeungmicah]

Great. Thank you.
1. Thanks for the clarification
2. Can I clarify!!!! I’m using the same props so from reading the forums of engine life and props move boats (and this load the engine and in turn gearbox) engines just turn them. Does this mean that the engines will effectively be under proped and thus the gearboxes won’t be over stressed?
3. DD 8.2s are both CCW rotation as you look at the flywheel.

Thanks

[yeungmicah]

Great. Thank you.
1. Thanks for the clarification
2. Can I clarify!!!! I’m using the same props so from reading the forums of engine life and props move boats (and this load the engine and in turn gearbox) engines just turn them. Does this mean that the engines will effectively be under proped and thus the gearboxes won’t be over stressed?
3. DD 8.2s are both CCW rotation as you look at the flywheel.

Thanks

[Tony Athens]

#1—————- There is no such rating for a 6BTA 5.9…..It’s 355HP at a minimum rated RPM of 3000.. Propping should be for 355BHP at 3150 RPM if you expect good life.. The fuel spec is “build the fuel system to” is CPL2208 or CPL8457…………..

#2————Your BW V-drives will not even come close to holding up unless you prop to about 250 BHP at 3100 RPM.

#3————Were your 8.2’s RT & LH as to rotation?, or CCW (LH) for both.. The Cummins are CCW (LH) .. That’s another potential issue..

Tony

[Author]

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