Repowering a 54' Donzi Sportfish

I agree that they look good, but based on them, at WOT that boat was running with a prop slip of 24.5%.
Which is way too high, to the point of making me wonder if there isn't something wrong somewhere...

That was also 1995, so propeller technology has come a long way.

Yeah, I saw that too, the magazine 54 Donzi test boat theoretical zero slip speed was all the way up to 44 knots if it in fact had 45" of pitch. Not sure how to explain it other than relying on someone else's data and data collection methods.

The Veem's with Interceptors are a different style and their effective pitch varies according to the color of interceptor strips - very hard to quantify but a safe bet is that we will see a gain in efficiency. At 40" of effective pitch, our favorite repower candidate is showing a theoretical zero slip speed of 42 knots, so 32 knots WOT is about 23% slip. Pretty conservative hedge IMO.

Absolutely.
As the old saying goes, there are only three ways to find the right prop: test, test, test.

Having said that, if I may throw in my 5c, beware of nominal pitch, because it doesn't mean a lot, per se.
I mean, as I pointed out, there's probably something wrong in the above table, but that's not necessarily the 45' pitch.
For instance, if the gearbox ratio was actually 2.25:1 instead of 2.0:1, that would have meant (all other things being equal) a prop slip down by about 10%, vs. the 24.5, which was just a sort of acid test showing that something wasn't quite right.
Actually, even if they didn't mention the gearbox model, back then MAN V12 engines were mostly mated with the BW165, which was only available with lower reductions, not higher.
So, it's very unlikely that the gearbox actually had a higher reduction - but you see what I mean, anyhow.

For a boat like yours, I think it's reasonable to aim at a prop slip somewhere between 10% and 15%, as a rule of thumb.
Less than 10% would be almost a miracle.
Somewhat more than 15% would still be acceptable, but once you reach 20% or more, you are into poor transmission efficiency territory.

Now, your ZF gearboxes have a 1.97:1 ratio, and you are going to spin props with a nominal 41' pitch.
So, let's see what's what, but with a small caveat first.
My own prop calculator gives slightly different results, when compared to the numbers in post #142 of PacBlue:
My theoretical zero slip speed for a 40" prop spinning at 1167.5 rpm (i.e. 2300 rpm engine with 1.97:1 reduction) is 38.5 kts, not 42.
Consequently, reaching 32kts at WOT would mean 16.7% slip, not 23%. A very acceptable number, anyhow.
Happy to give more details on my algorythm, if PB is interested.

Back to the point, let's consider the effect of a 41" nominal pitch.
Theoretical zero sleep speed becomes 39.4 kts, and if the boat could actually top "only" 32 kts at 2300 rpm, the prop slip would be almost 19%.
Still acceptable, but I would expect the combination of those props and that hull to do better than that.
At 33/34 kts, the prop speed would be 16% and 14% respectively.
And this is where my money would go, if I should place a bet.

I'll follow the actual results with interest anyway, that's for sure!

This dated article on HMY's website shows a 26/27 knot cruise and 30+ knot top......with 1050 mans. That would be my guess as to actual numbers with those engines and the size. Also probably what you will see with the CAT's, perhaps a tad more.

https://www.hmy.com/yachts-for-sale/donzi/54-convertible/

I used straight pitch instead of a measure pitch taken at the 0.7R as I don't have those details handy for Veem's, Frances Helices and others prop manufacturers. It results in higher slip numbers but it is more important to keep the same approach within the data set as it is just a comparison. The math is basic and goes like this: (Advertised Propeller Pitch x Shaft Rpm) / 1056 for mph

There are other comparisons/similitudes on speeds including weights, speed coefficients and others, and I suspect the weight numbers have some play in them as well, as the factory dry weight for the 54 Donzi is stated as only 51,000 lbs. and the weight at haul out for this boat is 76,000 lbs. wet with owners gear.

So it comes down to picking a common propeller slip method, looking at other similar sized and power boats as well as some basic experience factors. Regardless, I think the numbers are conservative and there is enough margin to have even better results, I tend to follow the camp of under promise and then over deliver, keeps everybody happy in the long term.

The calculator I just used. 1.97 gear ratio, 15% slip, 2350 rpms, 40" pitch= 33 knots. 20% slip= 31 knots WOT
2000 rpms (should be 80%) 28.7 knots with 15% slip, 27 knots with 20% slip

From my experience, under 15% slip is really un-realistic in a boat of this type and closer to 20% slip is more realistic I think.

A comment from the Michigan Wheel Site:

"Slip
Slip is the difference between actual and theoretical travel of the propeller blades through water. A properly matched propeller will actually move forward about 80 to 90 perfect of the theoretical pitch"

You have to be careful on what online marine propeller calculator you use. It can be tailored to certain styles of propellers and you need a clear definition of the inputs/outputs and what data set they use.

Here's a Viking 55 example that by my calculations works out to be in the 8% Slip Range:

Yup PB, I agree that consistency is more essential for comparisons than any theoretically better accuracy.
Couple that with what I replied to Dockmaster about test being the one and only safe way to find the best prop, and you will see why I have a funny feeling to have started a somewhat useless debate - for which I apologize with the OP.

But I'll try to better explain my train of thought anyhow, just for sake of clarity.

Firstly, I'm also taking the nominal pitch declared by the prop builder, because I have no way to check whether for instance a Veem 39.5" really is a 41" pitch prop or not - I'm just using the 41" mentioned by Dockmaster, as well as any other pitch declared by the boat tests.
I am actually aware of a few props whose actual pitch is consistently a bit longer/shorter than the nominal one, for reasons undisclosed by their builders, but they are neither Veem nor FH, and that would be further OT, so let's leave that aside.

Now, the basic math is obviously the same as yours, i.e. prop pitch x shaft rpm.
But it's your final coefficient for converting inches to nautical miles that I can't understand.
The complete algorythm I'm using (which btw I did myself with an Excel sheet after banging my head with some silly, and sometimes downright wrong, online calculators) is as follows:

[prop pitch x shaft rpm x 60 (=minutes in one hour)] / 72960 (=inches in one nautical miles).​

Applied to the Donzi 54 test, this turns out as:

[40 x 1,167.5 x 60] / 72960 = 38.4 kts of theoretical speed with zero slip.​

Whereas the shaft speed of 1,167.5 is calculated as 2300 engine rpm, divided by 1.97 ratio.
And 38.4 kts translates into a 13.4% slip, when compared to the actual measured speed of 33.3 kts.

As a side note, interestingly our numbers are getting closer when applied to the Viking test, where my algorythm above gives a prop slip of 8.2%.
And yet again, that makes me guess that there must be something wrong in those numbers.
Under 10% slip is surface props territory, totally unheard of for a sport fisherman, no matter how well designed.

Lastly, just for sake of another comparison, I can add the numbers of my own 56 footer.
She is a flybridge, non-sportfisherman boat, hence a bit lighter and less beamy (68,800 lbs / 16'4"), but not too different, overall.
Besides, back in her days, her hull was widely regarded as one of the most efficient, when compared to her typical competitors (Ferretti 57, Fairline Squadron 58, Princess 57).
Now, without bothering with all other details, she can achieve a 33kts max speed with 2x800 MAN, and a prop slip of 12%.
I'm not saying it's impossible to beat that, but to date, I've yet to find a boat with inline shafts whose performance at that weight/size/power is better.
In fact, unsurprisingly, she also performs much better than your 90 gph at 30 kts benchmark for fuel burn, since she's happy with 72 gph at that speed (and 83 at WOT).

That's another reason why I don't think it's too optimistic to expect Dockmaster boat, even if heavier and beamier, to achieve a similar max speed with no less than 700 more ponies in the e/r.

Nah, that would be way too boring - as opposed to your project, which is very interesting indeed.
Again apologies for the o/t, I just hoped it will be interesting to look at all these guesses in the light of your actual results, once available.
In the meantime, all the very best!

Very nice work, glad to see the project is moving forward at a rapid pace.

In regards to the mount and hanger brackets, preset the mount / retaining nut level to an equal distance on all four mounts for best performance (as you know). I like to have them right in the middle so you get maximum adjustability if needed.