1991 Hatteras 50 C

I am not familiar with that age 50' SF Hatteras. I always seemed to run 52' etc. I would guess cruise speed would be around 23 knots with those engines. Fuel burn will be 90 GPH at cruise. It should be a good sea boat. I've run newer 50' Hatteras SF from the 2001-2004 era and they were a pretty good sea boat, but I don't think it's the same hull as 1991.

Here's a great link for Hatteras brochures and magazine articles:

https://www.hatterasowners.com/Brochures/50C3/50C3Archives.htm

Here is a snapshot of your model boat test:

Keep in mind that the boat test was without a tower or gear, so probably a bit optimistic on speed and fuel burn,

Detroit Diesel’s love a cruise at 1850 to 1950 rpm, any more your are increasing maintenance costs by shortening time between overhauls.

It's all about load and varying load with them. Not necessarily RPM. I've heard varying opinions from many well knowledgable sources. Vary load factors will kill a Detroit a lot harder than running them a little hard. It really depends on what kind of boat they're installed in. For example if they're on a sportfish and running a set of DDEC's at 80% load gives you a constant load factor. It's better to run them there than 100 rpm's less, where the engine rpm's are climbing and lowering and load is going up 4-5% going up a wave and dropping 4-5% coming down the wave along with varied SOG. This is what really kills a Detroit. Running a 92 series JT slower at say 1800 rpms doesn't necessarily do them any favors either as the giant injectors JT put in them don't meter the fuel as well as at say 1950. But really it's about going by EGT's on older motors and load on DDEC engines and making sure they're propped to do 50 rpms over WOT.

The longest running set I've been around in a TI/TA configuration was a 1987 63' Ocean with 12v92 ta's…...motors went over 6300 hours on original engines towing a 35' Marlago behind it at 1950 rpms everywhere, overpropped and all.

J, you are all over the place.

These engines are not DDEC’s, he won’t be able to measure load. They are not 92’s either so no need to introduce that topic here as well.

If he has pyrometers, he can monitor exhaust gas temperatures.

But keep it simple, have it propped right to 2350 rpm,s, cruise it at 1850 - 1950 rpm, and resist the temptation to dial up anything in the 2000+ rpm range and you will keep those 71’s happy.

1850 - 1950 rpm is the sweetspot

It all depends on the bottom and how much weight you add. I would expect given the age and everything 23-24 knots at cruise as the boat did 24 knots at 1800 new. Generally 5-6000 hours (or more) are pretty normal out of a set of 12v71 TA's in a SF if they've been taken care of. If it has 15 coats of bottom paint on the bottom that will definitely slow it down 1-2 knots. I had the bottom soda blasted back to gel coat on a 7 year old express MY and with a barrier coating and 2 coats of new bottom paint, the boat picked up 2 knots.

Fill the boat up with fuel and water, empty the waste tank. Have your bottom cleaned and then check these two things: After a good warm up at the dock, check the maximum engine rpm while you are in neutral. This is the no load rpm and confirms that you have a good fuel rack position and good throttle linkage allowing a full thottle position. This no load rpm in neutral should be in the neighborhood of 2500 rpm.

After doing this at the dock, take the boat out for a sea trial. Record what the maximum rpm is under load, you want to achieve something over 2300 rpm, preferably 2350 rpm to ensure you are propped correctly.

I would then follow this schedule:
1750 rpm for my light cruise, 1850 rpm for my normal cruise and 1950 rpm for my max cruise. If the engines are putting out their rated power you should be covering 21 to 25 knots in that range.

In the states, we would typically have a Marine Surveyor for the boat and equipment and a separate engine survey done by a reputable Detroit Diesel servicing center. That means 2 different survey reports.

The following is the Factory Performance Curves for a new engine:



You take Fuel from the Propeller Curve to get the approximate gallons per hour consumption:

1750 rpm > 22 gph each, 44 gph total
1850 rpm > 26 gph each, 52 gph total
1950 rpm > 30 gph, each, 60 gph total

Older engines may not achieve these factory values exactly.

Sorry I'm late...
I have a 1994 50c which is the same layout but I don't know about hull modifications.
I have 12v 183 MTUs with approx 1000hp.
At 1950rpm with a medium/light load, I see 28-30kts
She's a bucking bronco in a head sea and in calm water when I run her up faster, she tends to do a slow chine walk until I get it perfectly balanced with the tabs. maybe it's a little too fast for the hull? In the head seas, I quickly learned to slow down to what I had been doing in my previous boat, a Bertram 50 that cruised at 23-24kts and then it's very comfortable.

We LOVE this boat. Fast, economical as 50' 30kt boats go and handles wonderfully in most sea conditions (i just slow down a little in a big head sea. Very little bow steer in following seas and she eats up quartering seas.

Everything is for sale at the right price if youre looking over here

Thank you, too! Great DD info to know.