Considering Repowering Again

I agree with all of this. But my question is why even bother repowering in the first place if you're happy with how it performs now and everything is running well. If something grenades, then do the repower, is what I would do. I love C18s and feel they're are great choice. Yes, you'll gain probably 3 knots and lose some fuel burn at cruise (85 GPH versus ?), but on the troll and 1000 rpms and under the Detroits are a lot more efficient than the C18's.....so depending on your usage you might not see any fuel gains overall.

OP stated hard to find parts DD and old DD techs.
I luv new fancy things but also in the school of never kicking a sleeping dawg.
In Alaska it may be if it's not broke, don't fix it.
But, it's still his check book.

It's been 55 years sense I was in Alaska (Army baby). If you can not find a DDC tech, Tap on us.

Good post and understood.
Slap Dab....
You have a dream and plan; Lets get this in motion.
How can I help from North / east Florida?

That being said, then go for it. I'd go with Onan's over NL's. They just seem to have better computer monitoring and computer system on them. Nothing wrong with the NL's either. I'd prefer NL's over Onan's on boats that run lots of hours on them per year. But Onan's for your application.

KISS
Northern Lights..

I'd rather have one that can run the entire boat and a smaller one. If you sell the boat with 2-9kw's it would be very undesirable to anyone in a Southern location. Here we'd want 2-20's. But 1 20kw and a 9kw would be ok for buyers, but not the preference.

NL's aren't KISS anymore, they're computer controlled and everything just like anything else to meet EPA mandates.

Uh? Installing a new 20 and 12 in a Hatteras now.
There is a fancy & expensive power control option.
NL is still simplicity at it's finest.

That's great. The new ones I've seen all had an electronic display and computer control.

You need that fancy junk for the inverter to auto start the gen-set.
Nice LCD panel local and remote like the Oh-Nos.
The NL gen-set does not need this gizmo to run.
Still two lil relays in the connect box that we've seen for over 20 years now.

That's what I'd recommend, even if just for resale. A 9kw and a 20 kw.

The 1.757 gear is better as noted but puts you in a slight dilemma as the safety factor for the AQ 22, 2.5” shaft falls below the industry standard of 5.0 for your application, as it is now 4.0.
Not much you can do about it other than accept it as is or upsize the shafts which is another expense.
You will also be getting a different prop with the different gear ratio, but have awhile to go for that decision.

The short answer is you will have to wait and see and it depends on the condition and material of your existing propeller. When it gets hauled let us know the model make a size with a picture, and what your prop shop says.
One other thing to keep tabs on - try and mark your waterline with tape as soon as possible. Then when you relaunch, check where the tape sits on your new waterline, to get an idea if you are lighter or heavier and by how much. Tape it at the transom corners, maybe at the aft salon bulkhead area and the bow, just a simple eyeball test to see how she refloats.

All
Please note Dockmaster has opened a new thread for the install of the new Cat systems and gen-sets.

Follow the install at;
https://www.yachtforums.com/threads/repowering-a-54-donzi-sportfish.31923/

Since we all enjoy spending other peoples money, May I make some suggestions while your on the hill;

Replace strut thru bolts. Inspect every bolt and hole for signs of issues.
If your shafts are in good shape and in good alignment, just replace a few bolts at a time.

Have the shafts well inspected.

If your getting new shaft couplers to mate with the new clutches, pull the shafts, have them well inspected and properly fit and true the new couplers.

New shaft log bellows/hoses. Service/update log glands.

Inspect rudder post and logs. Service/update log glands.

All hard to reach thru hull hardware inspected.

Clean, clean, clean, clean and paint the bilges.

Consider low bilge water type pumps in the best locations with the large de-watering pumps.
Including sumps for condensate accumulation. Keep the pretty bilges dry.

You are already making comments on a new R/O system.
Lots of comments on YF about control panels and cost of service on some models.
KISS here also for the long run.

KuChing, KuChing $$