Is this preventive maintanence

[Toronto]

All this talk of too much power from the motors has me worried.
I have a 91 Hatteras with 8/92. Low hours, very strong, but 720HP a side.
Should I consider getting these reduced to 650-675 and is that just injestors. I have no idea what I would be in to to reduce the power.
I enjoy 8-9 knots, once in a while, for cleaning purposes I run it for 30 minutes at 1800-1900 for a clean out.
Total summer hours are about 65-100.
Lake Ontario fresh water only.
Any thoughts are welcome.
Must say I really enjoy my visits to this site, even if it does create a worry I never knew I had.
Thanks, Toronto

[Sonny]

If you used all 720hp on a regular basis, then you'd probably be looking at maybe a 2000hr life span. As mentioned in other threads, it's heat that will kill your engine, so if you spend most of your time running off-boost, then you aren't making a lot of heat and should theoretically get a lot higher TBO (time between overhaul) than if you mostly ran on-boost. If you don't have boost guages then it'll take a little research to find out at what rpm you start making significant boost.

[Genesis]

The EASIEST way to reduce the output of the engines is to remove pitch from the props.

This will reduce the maximum demand from the engines yet does not require changing the injectors. The governor will prevent overspeed.

If the injectors are in good condition there is no particular reason to change them - just reduce the prop pitch if you wish to reduce output.

[saltshaker]

Sounds like you are running them easy and may not need to do anything. 720HP sounds like Covingtons. If so there were a few significant upgrades for those. I believe one of them was to a larger heat exchanger for the 6V92's and I think the 8V92's. Do you know which, if any, were made?

As Karl said, decreasing pitch will decrease engine load. You may want to do this for slow cruising. Karl could explain in more detail, but you want to keep the RPM's in operating range for the turbos and to keep engine temps from falling too low

[krush]

As Karl said, decreasing pitch will decrease engine load. You may want to do this for slow cruising. Karl could explain in more detail, but you want to keep the RPM's in operating range for the turbos and to keep engine temps from falling too low

To do what you just mentioned would require increasing pitch. Brian made a thread about this a year or two ago. He increased pitch and to ensure he didn't overload the engines he reduced the max RPM on the governors.

[MikeP]

Just don't run the engines at WOT and they won't put out 720HP.

You can do all that repitching but why? I would never complain about having 720HP available if I wanted it and I PERSONALLY would not reduce the boat's capability. It might come in handy sometime to be able to go 18+ knots when you have cleverly made the boat where it can only go 10...

I think it's a poor decision to take a boat designed for one use and try to make it into something it is not. These are NOT great displacement boats, they are OK displacement boats compared to other planing boats but they were designed to plane. I wouldn't take away that capability.

[Brian Degulis]

The EASIEST way to reduce the output of the engines is to remove pitch from the props.

This will reduce the maximum demand from the engines yet does not require changing the injectors. The governor will prevent overspeed.

If the injectors are in good condition there is no particular reason to change them - just reduce the prop pitch if you wish to reduce output.

Do the exact opposite if you want to run slow with less worries of cold incomplete combustion. Decreasing pitch will make your situation much much worse. If you increase pitch remember you will no longer be able to run up to rated RPM without over loading but you will improve displacement speed cruising substantially

Brian

[REBrueckner]

As noted, no worry mate!!! You only develop high horsepower at higher RPM....

Peel back a turbo blanket on two turbos and check your exhaust temp....check temperature at cruise with an infrred thermometer...acording to posts here you'll get a close reading...over about 500 degrees is desireable according the Brian's prior post....
eight knots may be a bit slow..but when you speed up if smoke clears in a minute or two likely your are burning fuel and not washiong cylinders...

At 100 hours annually you'll die before those engines!!!! waaaaay before....

[GJH]

See? Wasn't that simple? Nothing like consensus from all the experts to ease one's troubled mind!

[Genesis]

Just don't run the engines at WOT and they won't put out 720HP.

You can do all that repitching but why? I would never complain about having 720HP available if I wanted it and I PERSONALLY would not reduce the boat's capability. It might come in handy sometime to be able to go 18+ knots when you have cleverly made the boat where it can only go 10...

I think it's a poor decision to take a boat designed for one use and try to make it into something it is not. These are NOT great displacement boats, they are OK displacement boats compared to other planing boats but they were designed to plane. I wouldn't take away that capability.

There's a difference between de-tuning to improve life and de-tuning to try to turn a planing boat into a displacement hull. Not the same thing.

If you have an engine that can produce 700HP @ 2300 RPM it might only be able to produce 500HP @ 2000 RPM. "Just throttle back" doesn't solve this, in that the engine may only be making 500HP but at the lower RPM it is also at a higher percentage of its maximum output for that RPM.

All things being equal if I want 500HP out of that motor and it can safely operate at 2300 RPM on a continual basis I prefer to get it at 2300 RPM, not 2000.

If you look at the EGTs and internal pressures under both scenarios (500HP at either 2000 or 2300) you'll find that the engine is showing less sign of stress at 2300 than it is at 2000, all other things being equal (again).

Of course if 2300 RPM is not really where the engine can be safely run over the long term then "all other things AREN'T equal!"

As a consequence if the "maximum" speed available with 720s is 21kts, cruising at 19, but you're happy with a maximum performance level of 19kts, cruising at 17, down-pitching to drop the boat's maximum speed to 19kts likely detunes the engines by 100-150hp. That will have a very significant impact on operating life assuming you intend to run ON PLANE, and making those 17kts at 2000 RPM instead of 1700-1800 will likely result in better engine life. I'm willing to bet that if you instrument those two boats you would find that at the same speed the engine operating at 2000 RPM has lower EGTs and is overall "happier" in terms of engine stress .vs. the one operating at 1700, even though both are burning the same amount of fuel and the boat is travelling at the same speed.

The other thing to be aware of is that mechanical Detroits cannot adjust their injection timing for RPM. That is, the fuel takes a finite amount of time to ignite once it is injected; that ignition delay is in fact significant and is part of the computation when it comes to injector height (timing) on these motors. Unlike an electronic engine that can dynamically adjust for BOTH load and RPM, a mechanical engine's fuel injection timing is load-sensitive only. That is, the mechanism inside the injector that controls start and end of injection points is incapable of sensing RPM - only the position of the rack, or fuel delivery required. As such except at the precise combination of both output and RPM that the designers set the timing up for you are either injecting early or late compared to the optimum point, and both hurt efficiency and increase stress. This is one of the reasons, by the way, that overloading these engines is so damaging; you wind up injecting EARLY which causes extremely high combustion chamber pressures.

You can take this too far, as Brian is implying; de-pitching to run an engine at DISPLACEMENT speeds is likely the wrong move, but unfortunately due to how prop demand curves work pitching UP probably doesn't get you where you need to go either, unless you're going to run "partly in the hole." The right solution for long-term TRUE displacement operation is to get a boat and engine combination matched for that or refit to have some sort of auxiliary power plant that is used ALONE for displacement operation - where THAT engine can run in its power band and keep internal temperatures in their ideal range.

The "10 kt" thing with these boats, however, is not really operating as a displacement vessel. Displacement speed is well below that and many of these boats (mine included) were not far off dead idle at their maximum true displacement speeds.

I maintain that if you have a boat with "hopped up" engines and are happy with a couple of knots slower performance the best way to accomplish that and improve engine service life is to downpitch the wheels by an inch or two.