Bertram 42 Detroit 6v-92TA Repower with Yanmar or Cummins

You can do a basic mock-up on your garage floor with either 1/4" plywood or doorskins, or if you are able to, on a CAD program.

Get the overall height dimensions of the 6V-92TA from a drawing, cut or draw the length/height on a large sheet, basically the profile of a 'box" that represents the length of the engine from the front end to the Rear Flywheel Flange (that eventually connects to the gear box) and the overall height. Then draw the centerline of the crankshaft, so that you understand the distances below crank centerline (usually to the bottom of the oil pan) and the maximum height of the engine. Get the dimensions for the Allison gearbox and do the same from the engine flywheel where it connects to the output flange of the gearbox. Note the "drop" from engine crankshaft centerline to the Allison Gearbox output centerline. Now do the same in a different color for your Cummins engine and Twin Disc or ZF Gear. You can use the 6V-92TA Crank Centerline and the rear face of the block at the flywheel as a reference point to "pin" the Cummins outline and new gearbox outline. You will then begin to notice any differences once you put the new gear on this "layout'. Note the difference of each gearbox output flange and this will begin to tell you how much, if any, of a down angle gearbox you will need.

The next step is to accurately transfer the engine/gearbox hanger brackets, realizing that they may be different from the inboard/outboard locations. You can put them on either side of your plywood. Include the bottom foot of each resilient mount and maybe even a really good approximation of the string tops, and the available height of the engine room.

If someone where to do this in CAD (a pretty simple exercise) you would just pick a reference point like the Gearbox Output Flange Centerline, much up the old and new to get the same idea. Maybe your engine or gear supplier can do this for you, or the shipyard that will do the retrofit?

PacBlue gives a very good description of how to model engines for layout/fit checking. In the custom world I always have the hangers fabricated to fit the boat vs. trying to adapt "stock" mounts which usually are made to fit flat and parallel engine stringers. Also the most common critical measurement is usually the bottom of the transmission base pan. Most builders try and get the engines as low as possible and that point is often the tightest. I just installed a set of MGX-5065A's and will warn you there are bosses on the gear base pan that are NOT on the CAD drawings for the gear. There are also gear cooler connections not represented on the drawing that could cause you headaches, try and lay eyes on a set before you go too far if possible!

Been bitten by that "CAD" bug more than once, and on those 550's to! Here's a set of Cummins/5065A's going in, notice the big offset on the mount heights to accommodate different height stringers and the offset of the gear housing. It is NOT symmetrical to the crank centerline!!!

And remember to include any engine oil pan and gear oil change fittings / valves that will tie into an oil change system and make your life easier.

I would be curious as to the price difference between the two Cummins engines, what is the percent difference? I agree that there are significant characteristics/differences in a 2600 rpm 8.3 liter diesel vs. a 3000/3300 rpm 6.7 liter engine.

My first inclination on pleasurecraft diesel engine selection (of equal or near equal hp) is to give priority to the following: (others may have their own personal rank)

1. Displacement - 8.3 is greater than 6.7 (obvious)
2. RPM ranked from first choice to last as 2100/2300/2600/2800/3000/3300; 2600 (8.3) preferred over 3000/3300 (6.7)
3. Fuel Consumption - 14.5gph (6.7-419bhp) vs. 16.9gph (6.7-473bhp) vs. 17.4gph (8.3-493bhp); all within a reasonable bandwidth, advantage to 6.7
4. Dry Weight - 1398 lbs. (6.7) vs. 1975 lbs. (8.3); advantage 6.7
5. Time Between Overhaul - 8.3??? vs. 6,7??? Would be good to know before engine purchase.
6. Technology advantages - Electronic Controls, Displays, available Information; Same?

Price is the 'floater'. It may mean everything or nothing at all , given one's personal budget. A smaller engine may have smaller raw water /fuel/exhaust fittings/connections and save some additional installation money, but you have to recognize what you may be sacrificing - the ability to carry extra load given changing sea conditions and usually engine life, especially if you plan on hanging onto it for a long period.

Another curious item to consider - the 8.3 is available in a Commercially rated version at 493bhp/2600 rpm. Is the Commercial Rating price point better than the Recreational Rating price point in your area?

Here is more food for thought, taken from Cummins Marine Performance Curves available from the internet and normalized by % Throttle to account for the differences in rpm. Some of the data may be preliminary and should be considered representative. I would focus mostly in the 60% to 85% Throttle range, as below 50-55% Throttle it is basically a "wash".
Torque is in ft - lbs.


Brake Horsepower vs. % Throttle:



And finally, Total Fuel Consumption in gallons per hour (2 x engines) vs. % Throttle, taken off of the Cummins propeller Curves:

This appears to be data from a factory Bertram 42 Sea Trial with Cummins 903VTA Engines (450bhp @ 2600 rpms). I would assume this is a light boat that was especially targeted to get 30 mph at WOT:

With the 6V-92TA data, rated 475bhp at 2300rpm, on the same graphs to give you some perspective. Note - this does not shed any light onto the weight savings gained with either of the repower engine/gear combinations:


Interesting in the 60% - 80% region:



Notice the estimated Fuel Burn is not too different for the QSC8.3 or QSB6.7 - 473bhp, but does not take into account weight savings:

Yes, but I would be more inclined to go with S60! Or better yet, how about a 10' cockpit extension with a set of IPS units under deck and a fantastic workshop where the Engine Room used to be??

I believe the Cummins QSC-600 is still in their current sales guide at the 3000rpm configuration. Maybe the best high price solution if cost is not an issue but speed is desired. A QSC-500 is a solid choice, but the QSB-480 is intriguing because of the weight savings. The Dry Weight of the DD 6V-92TA is 2,795 lbs. and the Dry Weight of a Allison M20 Gear is about 350 lbs. for a total Dry Weight of 3,145 lbs. per engine.
The QSB -480 Dry Weight is 1,395 lbs. while the QSC-500 Dry Weight is 1,975 lbs. without their respective gears.