Repropping to lower top speed??

First question, do you have a way to read engine loads?
Taking pitch out will reduce the loading of the engine thru the whole power band. Like a lower gear on your multi speed pedal bike. It is easier, but less efficient.

The E's pretty much pull the injectors back when not required so I do not see really any gains in unloading the engines more than your doing by going slow.

If anything, maybe, maybe,, maybe a micro more pitch.
Do you have an old spare prop set to experiment this idea with?

Running 12-14 kts is just about the worst possible speed as you are pushing a big wake and actually heavily loading the engines. At 14 kts I bet you re burning about 50gph.

either run at 9.5 to 10kts on 18 GPH or just get on plane and run 19kts. At that speed you should be burning about 60/65gph

the above suggestions are based on running a Johnson 70 skylounge with 1400hp 3412Es a few years back . I used to run the boat between miami and Nantucket fall and spring running 10kts 90% of the time and with 15’ on plane at the end of the day.

it ain’t broken, don’t break it trying to fix

70'-ish overall, ~65' or so at the waterline? I'd say you'd be best off no faster than 10.8 kts... and any higher than that (until you're on plane) you're just pushing water and wasting fuel. Even better, 8.8-9.5-ish kts.

Just yesterday our diesel tech was telling me our engines should be at 92% load at our rated WOT of 2300 RPM. (Which means actual WOT ought to be approx 2350-ish.) And also that being overloaded at any RPM is overloaded at all RPM. (Presumably it follows that being underloaded at any RPM would mean being underloaded at all RPM.)

Your engines will be different, but your manuals and your techs can advise on appropriate prop tuning.

And then once you've got those tuned, if they're not already, run slow or comfortably on plane.

-Chris

On the pins (WOT) on both engines, 200GPH? You may not be loaded correctly.
Brother Pascal has a great point, Tap on a real Cat shop. (Not Pantropic).

For what it's worth, I have fairly new CAT engines. They are not 3412E's but I believe similar principles apply. I too find myself running at slower speeds more often. It is more relaxing for me and, of course, uses much less fuel. I asked CAT Engineering directly if running at slow speeds would cause any harm to my engines. They said absolutely not. Just run them up once or twice a day for a short period to clear the turbos and get the stack temp up and you will be just fine.

As Pascal said - "it ain’t broken, don’t break it trying to fix"

I put 4000 hours on the 3412Es of that 70 Johnson I mentioned in my earlier post. When it sold, they had over 5000 hours and oil samples were perfect. Most of these hours, probably 80% were at 10/10,5 kts about 1100 rpm. As long as the engine is running at operating temp (180) it will be fine with a short speed run every few hours.

i know many old fashioned mechanics love to say that diesels need to be run hard all the the time but they also love being called to fix them…

i much prefer buying a boat that has been run gently than one where most of the hours where logged at 75% load. There is a reason why most manufacturers incl Cat specify hours or gallons used for maintenance intervals, whichever comes first

Yeah that 200gph number was off… I don’t think the 2000hp MTU drink that much on the 120 I run these days.

reducing pitch will increase RPM at hull speed but I don’t think it is going to increase load.

If you never run the motors up because they burn too much fuel you're going to be paying a hell of a lot more for major overhauls. Those motors were designed to be run at 80% load. You can run them slow speeds (1000 rpms and under) for around 6 hours at a time, but you need to run them at cruise for 30 minutes to spool up the turbo's and clear out the carbon...........if you can't afford the fuel, don't buy a yacht. Fuel is going to be one of your lesser expenses.

Speaking of A 70’ Sky-Lounge with 2x 1,350 hp:

I operated one in Fort Lauderdale some years ago and got exactly the same 29 knots at full power, same as the OP. (Johnson 70, the owner wanted to go full blast while cruising)
Optional 1,350 hp MAN engines, up from the stock MTU 1,150 hp.
At idle with both in gear it was too much for the No Wake Zones between Bahia Mar and the Cut, we had to go in and out of gear on one all the time, not sure what the speed was, 7 -8 knot or above with both in forward.
Otherwise easy to maneuver: Bow and Stern thruster, Sleipner 25 hp, since then changed name to Side-Power.

The OP is talking about a Neptunus 70 not a Johnson although performance should be similar.

the J70 I ran had the optional 1400 hp 3412Es. Real good performance. Funny you should mention the stern thruster.. on that one Johnson didn’t open a limber hole from the compartment in which the ST was installed to the main bilge. Over the years that filled with water and drowned the thruster. Never bothered fishing it as that would have meant removing all the steering gear and its massive SS shelf. Really no need for a ST anyway.

You are NOT going to get 10-11 knots more efficient by running the engines to a higher RPM. You will burn more fuel. It takes X amount of fuel to push the boat at a certain speed. If you are this concerned about fuel burn and cost, you should go buy an RV. Do you know how much it costs to re-prop a boat that size?????? Then you'd never be able to sell it. You need to run the motors at cruise for 30 minutes every 6 hours at a minimum.

In all fairness when most of us read this

“and I normally run slower than that running 12 - 14 kts or so” or 200gph WOT on 3412Es

it does raise questions about the person’s experience.

i really don’t see what you re going to gain by replacing props and raising RPM at hull speed. You re not going to save any fuel, if anything the friction losses from the higher RPM is going to increase fuel burn.

All of these things you say, make no sense from anyone experienced. Even the title "repropping to lower top speed". 12-14 knots is the most INEFFICIENT speed you can run as your boat is completely plowing...... you either run at cruise (or near it on plane), or 10 knots and under (hull speed) while keeping the motors at 1000 rpms or less, then running 80% for 30 minutes every 6 hours to clean the turbo's and cylinder walls of all of the carbon. It's most efficient while keeping your motors LONGEVITY and not crudding them up full of carbon and destroying the engines so they need rebuilds. They are designed to run at 80% load all of the time, not 1000 rpms, NOT 1400 RPMS, and no idle. You have engine displays, it's simple MATH to figure out how many GPH you are at each RPM/speed. 1400 rpms is A LOT more fuel per mile than 1000 rpms Probably 45 GPH at 14 knots versus 18 gph at 1000 rpms 10 knots.

Because you bought a yacht designed to be run at 20+ knots with engines that are designed to be run at 80% load all day everyday. Not idle speed, not 1000 rpms, not 1400 rpms, not 10 knots ALL of the time. So you come on here asking about spending $30k to change props to run at 1400 rpms for the sole reason of saving fuel and ONLY doing 10-11 knots. You basically bought the totally wrong boat for what you intend to do because you refuse to run at cruise and burn a little fuel for 30 minutes every 6 hours, or run the engines as they were designed. Yet your asking everyone for advice, but arguing with what people advise you.

For a boat like yours, hull speed is about 10.5 to 10.9 kts. One single knot over that is going to cause your fuel burn to jump 60-70%. 2 knots and you ll be burning over twice the fuel. At 13/14 kts you will be burning about the same that you would if you were fully on plane at 17kts.

your Cat displays show fuel burn… plot it every 100 rpm and note the GPS speed in a neutral body of water (no current and minimal wind)

12-14 kts on a boat that size is just about the worst possible speed.

I used to run a 70’ skylounge with the same engines (almost… they were 1400hp not 1350 but that doesn’t make any difference). I ran that boat up down the east coast from Miami to Nantucket back and forth for 5 years. I did all the calculations regarding fuel and running 10.5 kts for most of the day with a high speed run at 20kts at the end of the day was the most efficient.

for instance let’s look at a 200 nm trip to the Exumas from miami:

- 10.5 kts all the way would be 19 hours or 380 USCG

- 20kts would be 10 hours or 800USG

- 14kts > 14 1/2 hrs > 725 US

- 10.5 kts for 15hrs / 20kts for 2 1/4 hrs> 300usg/180usg or 480 total

- 10.5kts for 10 hours / 20kts for 4 3/4 hours > 200usg / 370usg or 570 usg total

as you can see running at 12/14kts is the absolute worst combination of fuel, time and load on the engines.

Not really intending to flog a dead horse, but...

If I've done the math right, using Pascal's fuel/time figures, it looks to me like his:
10.5 kts = .525 NMPG
14.5 kts = .280 NMPG
20 kts = .25 NMPG

If yours is similar, then 14 kts would be significantly less fuel-efficient than 10.5 kts, and less fuel/time-efficient than 20 kts.

If I fiddled with the numbers properly... Haven't had enough coffee yet today...

-Chris