Repowering a 54' Donzi Sportfish

Look at who did the test and if it was really knts. Often sales sponsored data is in mph (bigger numbers).

LOLOLOL, hull design and weight. Look up the last Speed Merchant that Tribute built. It was knocking on the 50 knots door step with 2-1000 hp C18's and was 62'.

There were a few other old SF that were very fast, the 57' Revenge's......the 58' Jim Smiths were deep into the 40 knot range with 8v92's and 700 HP.

No clue about how realistic the speed numbers are on that particular boat, but the 67 gph @ 1800 is nothing to write home about, even assuming that she's powered by 1150s.
According to Cat prop demand tables, your new puppies should burn 28.6 gph @ 1800, i.e. a whopping 10 gallons less than 67, overall.
Which btw suggests a not so remarkable hull efficiency - but it takes much more data to try to make a meaningful comparison, anyhow.

Ref. batteries, did your boat have dedicated starting batteries for each engine, maybe?
If so, I'd consider getting rid of one and use also that space for yet even more house capacity.
I know that separate starting batteries is a popular setup, but if there is one redundancy which imho is totally useless, it's this one.
Of course there must be a control allowing to connect the house bank in parallel in an emergency, but I consider that as a given.

Well, in this case forget what I said!
Out of curiosity, is that a standard Donzi setup?
I never came across a boat with 24V DC for engines start that wasn't 24V also for house services.
The only usual exception is the genset, with its own 12V battery.
But that's irrelevant, of course.

24Vdc gen-sets are still available. You can still upgrade your new ones to 24V easily.

We have 32, 24 & 12 on our boat. All working well but just to many freaking batteries.
In a couple of years we just may be all 24V. Not because of 32Vdc issues (none), just to many batteries. Then one DC to DC converter/charger and a 4D or G31, 12V battery somewhere for what lil 12Vdc junk that's left.
This could cut the battery count almost in half.
16 Batts on board now.

Interesting. Multi-voltage battery banks aren't actually that common, among EU builders.
Wherever 12V-only components are present in an all-24V boat (mostly for some instruments), they are typically handled through DC-DC converters.
Another trick sometimes used to facilitate the use of 12V bulbs in 24V boats (particularly before multi-voltage led bulbs were available, and most lights were halogen) is to wire them in pairs, each with a serial connection.

Anyway, nothing wrong with separate battery banks (and chargers, consequently) for 12 and 24 volts, of course.
And I can imagine that re-wiring a whole boat designed with that logic can be a big job, maybe not worth doing.
At the same time, it's an additional complication which is nice to get rid of, if feasible.
In this sense, I certainly understand why CRaplh is thinking to refit his boat for 24V-only.
I always thought that whoever invented the KISS principle must have been an experienced boater...!

It IS for a sportfish. Cruise RPM is going to be around 2050 and 33 knots at 86 GPH on that 57' Spencer (2.6 GPNM), 1800 is just efficient cruise. Just do the math, at 1800 rpms the 57' Spencer is burning 2.4 GPNM at 1800rpms/67 GPH with an old hull...…….

In comparison a new Hatteras 59' GT is doing 26-28 knots at 89 GPH at 1800 rpms 3.3 GPNM, 2100 32-34 knots at 126 GPH or 3.8 GPNM with the base C32/1600 HP cats (taken from Hatteras performance sheet). Most all of the custom SF boats are as fast or faster than the production boats, utilizing less HP and 2/3 the fuel burn per mile...….most do hold as much fuel, or if less, still have the same or better range due to the increased efficiency. The interiors are just as nice if not nicer than the production boats just not heavy. To put this into perspective the Hatteras 59' GT or 58' Viking burn as much fuel at their 35 knot cruise, as a early 2000's 75' Jim Smith SF with full tuna tower that I once worked on at it's 35 knot, 150 GPH cruise.

What I've seen from the production builders is they made great efforts to reduce weight in the hulls, and some in the FB area's, but then they lose all of that savings by still building a very heavy interior. For example the custom guys will take a 2" thick granite kitchen counter and cut it down to 3/8" thick (+/-) and adhere it to a lightweight coring material so you still have the 1 piece 2" bullnose yet it's 15% of the weight, whereas the production builder will take the 2" thick granite counter top and install it.

I did some math, actually. That's why I checked the Cat specs for the C-18, according to which two of them should burn 57 GPH at 1800 rpm, not 67.
Now, I'm well aware that prop demand curves are just estimated, and they are only valid when applied to displacement boats, if you are interested in the whole rpm range.
But in my experience, if we leave aside the transition speed when a planing boat is not yet over the hump, they are a decent reference also for planing hulls.
Which for 2300rpm max engines like these, normally means anywhere north of 15 /16 hundreds.
Anyway, no 57' hull could ever make 28kts without being steadily on the plane, so in the conditions that we are debating for that Spencer, a comparison with the engine manufacturer prop demand curves at 1800rpm does make some sense, for a rough estimation of her efficiency.

And I'm talking of efficiency in absolute terms, mind - not related to speed.
That's why I didn't considering GPNM numbers.
As an example, let's assume to have two different boats A and B, powered by the same engines, both achieving the same speed and burning the same amount of fuel at WOT (obviously, since in both cases the engines are running at max rpm and 100% load) . Let's say that they also make the same speed at 2000rpm.
At this point, you could think that A and B must have both the same fuel burn at 2000rpm, but evidence shows that this is rarely the case.
Depending on the hull shape, weight, and other factors, it could well be that the prop demand in boat A is higher than in boat B at that specific speed, and this translates in a higher engine load for the first, hence eventually a higher fuel burn.
Now, matching exactly the engine manufacturer's curves at any cruising speed can only happen by coincidence, of course.
But I am indeed aware of planing boats whose fuel burn at cruise speed matches nicely the manufacturer prop demand curves, as well as others where the fuel burn is significantly higher, or also lower.
And this can only be due to different overall efficiency, all other things being equal.

Anyway, if we want to bring back speed into the equation, in order to consider fuel efficiency vs. distance and range, we must also consider weight, otherwise 2.4 GPNM means nothing.
According to a review which I found online, the Spencer 57 weighs 47k lbs.
Wich is impressive I must say, because my boat, of similar size and with lighter engines, weighs 59k lbs, no less.
But in spite of that, at the same speed, I "only" burn 2.1 GPNM.
Bottom line, also in this respect I'm still not hugely impressed by those Spencer 57 numbers.
My experience with Sportfish boats is very limited though, so if you tell me that she's better than average in this specific category, I take your word for it.

I'm really looking fwd to hearing from Dockmaster about his own findings upon seatrial.

My fault. I found your temp liquor locker. Oh, Good stock..... Hic-up...