Repowering a 54' Donzi Sportfish

At 76,000 lbs. you may be pushing my numbers a bit, so I would expect a drop of up to 1 knot on WOT / Cruise.

For an additional reference, a 58 Donzi ran 37 knots WOT / 30 knots cruise when repowered with MTU 12V2000's rated at 1480hp each.

https://www.**************/boats/1999/donzi-58-repowered-2007-3616704/

The older gen 55 Hatt has a beam of 17’-6” beam with a 4’-10” draft including a speed sucking keel.

The 54 Donzi has a beam of 17’-4” with a draft of 4’-4”.

The below the waterline cross sections of the Donzi bottom is a better shape for speed and efficiency, with the Hatt having more wetted surface.

The reference 55 Hatt’s were built from 1980 to 1989 while the 54 Donzi’s were available from 1989 to 1997 and represented a next generation design for that class.
Draft can be quite a few things, being the lowest measured point below the waterline, and can be the bottom of rudders or props or keel in some cases. Fair body draft is the depth of the centerline of the hull without a keel and would be pretty close for these two boats. Structurally, the Donzi’s have proven there ability to take on seas at higher speeds with better more fuel efficient designed bottom.

The Hatt’s of that era were just not built for speed with hollow aft hull bottom (concave) sections and the penalty of the keel. They were overweight for their waterline and preferred to bust through seas at the expense of ride comfort/speed. The next gen 55 Hatt came out in 1999 and was a different animal with high hp 3412 Cats and a much higher cruise and top speed, but they have always been known as thirsty designs, even the newer models.

One benchmark to look at is how much fuel does it take to go 30 knots. Over 90 gph is a thirsty less efficient design, under 90 gph is a much better design and usually has the higher speed potential. The Hatt’s are usually north of 90 gph, some drastically so.

?????? This measure REALLY depends on length. What length are you referring to? 55'? Because the longer the boat, no matter how efficient, if it's a normal SF it's going to consume more fuel to do 30 knots.

The biggest issue I see with the modern Hatteras SF is the size of their tunnels due to the reduction and props (diameter) they choose to use. I believe they lose a lot of stern lift and some speed because of it. Then again same sized Vikings aren't really any different and they have a different bottom design. I believe it's mostly because the production boats are so heavy compared to the customs.

Length doesn’t matter. Stuffing high horsepower with the resulting high fuel consumption into any design does not necessarily make it an efficient design. 30 knots/90 gph = o.333 nmpg, that’s the magic number. There’s some bandwidth, +/- 10 gph, so the gradient is 80 - 100 gph.

Consuming 120 gph at 30 knots puts you at 0.25 nmpg and so on, that’s a fuel sucking hog IMO. That’s only 250 nm range on every 1000 gallons, can’t see the efficiency in that.

Some SF’s are below 0.20 nmpg and that just plain sucks, there is a better way.

You'd be hard pressed to do 30 knots in a 90' at 90 gph. Length and associated drag and weight when going larger takes more HP. Yeah, most of the custom SF are very fuel efficient compared to the production SF. Jim Smith's Marlena is very fuel efficient for it's size...….as well as most Jim Smiths, Tribute built Dick Moroso's last Speed Merchant that was very impressive in terms of speed and fuel burn...….Bayliss', etc. etc.

Not too concerned about 90 footers.

I found a 1995 Boating test on a Donzi 54 and the numbers look pretty good:



They even hit my Benchmark criteria for 30 knots at 90 gph or less!

I think you are in good shape, the Veem propellers are a not so secret weapon in getting to your speed targets. Makes me want to stick with my initial numbers.

Call bob roscioli and ask him, but I bet the boat had a hardtop and outriggers. If you look at the photos in the article, that is how the boat wouldve been when they did their test. Inside of the boat would’ve been empty of any additional gear (tackle, galley stuff), but every speed test I’ve done for any manufacturer is without trim tabs, so usually another knot or two above test data IRL guess that accounts for the additional owner weight. My guess is unless you have a tower AND a lot of gear you may be slightly underpropped, and your speed numbers on the light side of actual. Will be very interesting to see.