Larger Props For Efiency And Engine Life When Running Big Engines Slow

[chris]

Very interesting thread to me. I am currently trying to figure out what props to use on my 48MY. Most of my running is done at 10kts. I have to turn 1500rpm to acheive 10kts. I would like to acheive 10kts at 1000 rpm and have the eng. temps at 180. I am now running 26x21x4bl props with 1.5:1 Allisons. 6v92's rated at 425hp.
Chris

[Brian Degulis]

Re setting the governers prevents overloading because the governer hold the RPM below the point where overload can Happen. In my case overload starts a 1620 RPM so the governers are set at 1600 RPM so when everthing is clean the governer will dictate top RPM not the prop load.

When I spoke of running cold I should have been more specific. The water temp should always be within range no matter what speed your running. The problem with running very lightly loaded is the combustion temp is to low this happens even if the water temp is right on. The only way to increase the combustion temp is to increase load. To increase load for the same speed you must reduce RPM with larger props this will load the engine more at a lower RPM. Every engine has a rating for the HP that can be produced at any given RPM the closer you get to that rating the better.

To de rate I simply re set the governers for a max Rpm of 1600. The only benifit of going to smaller injectors would be a slightly better atomization but the gain would be very very small. In doing this I know that the posability exists that I may have to change it back to OEM when it comes time to sell right now it's just props and re setting the governers to get back to original.

[Brian Degulis]

[QUOTE=krush;79750]But does that prevent from creating an overload condition? You could be over fueling and overloading the engine at that "max rpm" if the prop is too big.


Now I understand your question 1600 RPM was used as the MAX RPM for the prop calculation. After I did the change I used exhaust temp to verify that the calculation was correct. The engines ran within parameters up to 1620RPM beyond that the EX temp goes up above the range as does turbo boost and air box pressure. After doing the check I set the RPM to 1600.

Brian

[akintosyali]

A motor sailer by Hatteras???

[SKYCHENEY]

Here's the motorsailer:

http://www.hatterasowners.com/Brochu...SYArchives.htm

[Jackman]

Doesn't overloading occur across the performance curve no matter what rpm you limit yourself to with a prop that is too big or pitched too high for factory spec WOT?

[REBrueckner]

"Doesn't overloading occur across the performance curve..."

Seems like Brian's test data reports a resounding: not all!!

In fact his test data shows most of our engines are UNDERLOADED at lower RPM as evidenced by lower exhaust temps and less than ideal efficiency, and are only close to proper loading (and exhaust temps) in the 1800 to 2000 RPM range. "Overloading" would occur in Brian's case over 1600 RPM where he began testing at higher than normal exhaust temperatures....hence his limiting RPM to 1600 RPM.

It appears that Brian would be unable to use them at higher RPM. Sounds good for extended hull speed cruising so far.

Had he attempted to use even larger props, then he likely would have over loaded his engines and this would have been reflected in higher than rated exhaust temperatures.

I got wrapped up in all that data a bit too much: All Brian actually did was to put on bigger props. Anybody can do that and run at lower RPM and be more efficient while losing top end speed. His insight was that after the change he tested for proper operating exhaust temps and mechanically limited his engines (to 1600 RPM) so that his engines can't be accidently overload.
Again I say: BRAVO!!!

[krush]

Now I understand your question 1600 RPM was used as the MAX RPM for the prop calculation. After I did the change I used exhaust temp to verify that the calculation was correct. The engines ran within parameters up to 1620RPM beyond that the EX temp goes up above the range as does turbo boost and air box pressure. After doing the check I set the RPM to 1600.

Brian

Yeah, I just wanted to verify you "shifted the curves" so that prop HP and engine HP meet at 1600rpm.

[MikeP]

One issue that troubles me a bit re the data - Conventional wisdom for long boat engine life is to deprop slightly, thereby INCREASING the eng RPM for the same power output and reducing the load on the engine. It has been posted here as well as on sites such as Boatdiesel and numerous other places.

The data presented and conventional wisdom seem to recommend the opposite for best boat engine life. They can't both be right.

Life is so confusing...

[Brian Degulis]

One issue that troubles me a bit re the data - Conventional wisdom for long boat engine life is to deprop slightly, thereby INCREASING the eng RPM for the same power output and reducing the load on the engine. It has been posted here as well as on sites such as Boatdiesel and numerous other places.

The data presented and conventional wisdom seem to recommend the opposite for best boat engine life. They can't both be right.

Life is so confusing...

They are both right but for diffrent purposes. You might consider reducing pitch if you run at or near WOT because in that range your are loading the engine at or near 100% of it's rated load.

In my case I have a boat with 1300 HP and at my normal operating speed it only requires 300 HP to push it. So I was operating an extremly lightly loaded engine. The larger props go a long way to load the engines better at a lower RPM while producing the same thrust that the smaller OEM props produced at a higher RPM. But I can no longer produce 1300 HP I'm down to about 900 at the new lower RPM.

The whole concept becomes very clear if you take a look at the power/ prop curves. Think of it as shifting gears in your car or truck you simply change the ratio of RPM and thrust produced at the prop. In a sense your changing the boats operating parameters to better suit the way you plan to use it.

Rob is exactly correct you could just go to bigger props and change nothing on the engines but then it would be possible to overload if you push the throttles up to much. It's basically just good engineering to prevent that from being possible.

Brian