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

Since we are talking lottery dream, no jackshafts, put the engines/pods under the deck of your fantasy cockpit extension and the engine room becomes a generator room with workshop, a mechanical guys dream. Add some fuel tank age. Lift up watertight cockpit hatch for stand-up access to engines. I would really like to see this done on a 58 Bert or Hatt MY one day.

Well understood, every decision has trade-offs and budgets to realize. You will gain an amazing amount of space in the engine room and the new QSBs will be much easier to service and access. The weight loss will be a great benefit, but you will have to manage the trim of the vessel. You may have to move batteries aft as well as the generator if space allows. You want the boat to trim about 2 - 3 inches down in the stern (or up in the bow, depending on how you are looking at it) with fuel fuel/water, no waste and all equipment/gear onboard. You can manage your trim underway with trim tabs, that is what they are designed for. By the way, I would recommend the dual ram types if they are not already on your boat. Ultimately, all this is necessary as you will eventually trim down by the bow as your burn the fuel in your aft fuel tank and you can back off on your trim tabs.

I can not stress how much better off you will be to get the boat to float right away, you do not want a deep-vee planning hull to trim bow down in the condition I mentioned above.

I have had success with either one of them, but mostly Bennett. I would not recommend the Vertical Interceptor types at the moment because of all the holes you drill. I like the advanced controls that are available now as well, have not had issues with hydraulic tabs.

You should definitely have a Heavy Duty Dual Actuator set-up and it should be at least 12" - 15" long when measure aft from the transom. For a 42' boat, a 24" to 30" wide (measured athwart ship) is usually sufficient but get it sized for your application. It is nice to have a safety lanyard in place if you are an aggressive captain backing down on sportfish, this will prevent the plates from being completely ripped off in extreme cases. When installed, make sure the aft edge of the Trim Tab is at least 1/4" above (higher) the transom when measure with a straight edge, this will give you a nice range of trim and helps if you hit that target of 2" Stern Down Static Trim when measured across a straight waterline at the dock.

What kind of cruise / top speed are you looking for?

Good luck with your purchase / installation. Assuming the gear ratio is 2.55? Sometimes you have to deal will practical availability in gear ratios so I would just go with it and pay a bit more attention to the final propeller selection.

At 3300/2.55 = 1294 shaft rpms, you will be running higher tip speeds. This can be handled by the right propeller selection, factoring, Blade Area Ratios, Number of blades, etc. Just have your propeller supplier run the numbers for a cavitation check prior to specifying the propeller brand and style. It will also be important to keep the Propeller Tip Clearance from the Bottom of the Hull in the region of 10 - 15% of the Propeller Diameter. What propeller are you running now - make/model/diameter/pitch?

Down Angle / vertical offset is a function of the engine installation angle and your stringer system.

Using the existing shaftline set-up, you have to mentally picture that you are going to match the Propeller Shaft Coupling Face with the Gearbox Output Flange Face. In doing so, you have limitations on the installed engine angle as well as clearance from the top of the engine and the bottom of the gearbox / engine oil pan. You need to consult the Cummins dealer for their Engine Angle limitations before you can accomplish this. You also need to be aware of how your engine mount brackets and resilient engine mounts fit into the overall system and current stringers. You may have to add metal platforms/towers to the existing stringers or cut them down and build up the appropriate supporting structure for your selected engine/gearbox hanger locations. I personally could not imagine going forward without a basic physical or CAD mock-up to ensure a practical fit.

The other item to be aware of is the exhaust outlet. You want to have enough space available to have a nice Exhaust Outlet Elbow that has the appropriate drop angle to tie into your existing outboard exhaust run.

Your Bertram modified - vee probably has a shaft angle of 10 - 15 degrees. So you will need a down angle gearbox to get the engine to sit closer to engine spec. The down angles are typically around 7 degrees, so you would be sitting around 3 degrees to a max of 8 degrees (assumption only) and you would need to check this to see what the engine spec is for at rest angles.

So you will need a gear with a down angle regardless if it has a vertical offset or not. A Vertical Offset may be necessary to gain clearance for the bottom of the gearbox to the hull and you will still need some access to the gearbox oil drain nut. Would need pictures to get a better idea.

Any updates or pictures from the OP?

My 42ft flush deck Bertram cruiser has 500 hours with Yanmar 6LYA 490 hp engines, these were fitted by the P O.

Installation is perfect, zero oil drips....bilge clean as a whistle.

Boat runs like a train

I wouldn't hesitate to go with the Yanmars.

Bahma

I guess this is probably too late to be useful... but FWIW I'd guess that 42 Bertram weighs in the neighborhood of 10K lbs more than our 28K-lbs 42, not counting engine/gear weights. ??

We have twin 6CTA8.3s, 450-hp version, and I don't think I'd want any less horsepower. Add 10K lbs... even more so.

-Chris

Nice! Good luck with the work!

-Chris

AQ 17 is generally used for applications like workboats or commercial craft that are used on a daily basis. They are susceptible to Crevice Corrosion when left unused for large periods of time, like you would expect for a recreational craft. I have never used AQ 17 for a yacht application because of this.

A 2" diameter AQ 17 or AQ 22 shaft falls below the recommended safety factor of 5.o ( calculated 4.3 @ 70,000 psi Torsional Strength). The calculations for a 5.0 Safety Factor show you would need a minimum 2.100" diameter shaft which pushes you to a 2-1/4" AQ 22 High Strength, because standard AQ 22 is only 63,000 psi Torsional Strength at sizes over 2" diameter while AQ 22 HS is 70,000 psi. I would have to recommend a 2-1/4" Diameter AQ 22 HS shaft if this was my installation.