Looking at 56 Hat's 8V92's Problem??

[Brian Degulis]

No you read it correctly by going to a larger prop you will not be overloaded thru the entire RPM range. If you take a look at an engine/propeller power curve you will see that the engines power curve is pretty linear. In other words if you have an engine that is capable of producing 200 HP @ 2000 RPM then it's capable of producing about 100 HP @ 1000 RPM. If you look at the propeller curve you will see that's it's not linear if you have a propeller that requires 200 HP to drive at 2000 RPM at 1000 RPM it requires around 50 HP. So in this example when turing the prop at 1000 RPM the engine is only loaded to 50% of it's rated load for that RPM.

Most boats are propped to allow the engine to develop full rated power but at anything less than full rated power the engine is under loaded the lower the RPM the more under loaded it is. Being under loaded is not a realy a bad thing but it's not the best and most efiecient thing either.

What the original poster is considering is how to get the best efiecency at hull speed. By increasing the prop diameter or pitch he will get more thrust at a lower RPM. The engine will be closer to rated load at a lower RPM and he will get the benifit of a larger slower turning prop. These things combined will equal better fuel milage and longer engine life when running at hull speed.

Butt there is no free lunch by doing this he will no longer be able to run at the RPM he could with the original props. He will change the propeller power consumption curve it will now intersect with the engine power curve at a lower RPM and this will be his new and lower WOT. How much lower depends on how much larger the new props are. The governer should be adjusted to this new RPM to prevent the posability of overloading.

So basically you have a boat with engines that are way to big for running at hull speed. Changing to smaller engines isn't practical so look at this as customizing an engine propeller combination that will make running at hull speed more practical and efeicent. But you will lose the ability to use all the power the engine is capable of producing. Usually when this is done it's set up so the engine is now fully loaded at 1800 RPM (the work boat rating) instead of 2300 RPM the pleasure craft rating.

I should qualify my statments I have done this with DD Naturals and I know it works. With TIs I believe you would also have to factor in the turbo boost to insure that at hull speed your not at that point where your loaded up but turning to slow for suficient boost.

Brian

[Genesis]

This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.

The best bet with turbos if you want to "de-rate" the engines is to DOWNPITCH the wheels. This will reduce the demanded output and thus the boat's speed, moving the RPM for a given speed higher. It will not help fuel burn materially however.

If you intend to operate at displacement speeds this is a waste of time and money. It really only applies as a "good idea" if you intend to operate at planing speeds but wish to "de-rate" the engines - it is the cheapest and easiest way to do so, and involves only changing the wheels.

[Danali Star]

[QUOTE=Genesis;68637]This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

Genesis,

I plan to run pretty much at displacement speeds most of the time. If I am running at 1100 to 1200 rpms is this not considered lugging the engine down? I understood your advice was to run at these lower rpms and leave the engines alone. Is harm done by not allowing the turbos to "spool up" while underway?

Also, in your last paragraph you mentioned something being a waste of time. I believe you were referring to making changes thru the props but wanted to be sure.

Dan

[JLR]

I have the 56 MY with the 892's. Bought the boat with 1000 hours and rebuilt both engines when they hit 2,000 hours. Contrary to the wise advice on this forum, I run mine at cruise speed, (200 off the pins) all the time and think the boat and the motors are great. I burn a total of 47 gallons per hour at cruise and generally run about 16 kts. That type of cruising is what suits me. As far as the boat is concerned, the more people you put on it, the bigger she gets. It is a perfect boat from our perspective. Hope this helps.

[Brian Degulis]

[quote=Genesis;68637]This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

That's not correct in a stock boat they spool up @ 1600-1800 RPM because that's where the engine gets closer to rated load. Turbo's react to the heat that's created by the load not just RPM. If you increase prop size you lower the RPM at which the engine is coming near rated load that creates heat and causes the turbo to provide boost at a lower RPM.

You need only to look at the engines power curve and you will see that @ 1200 RPM you are using a lot less power than the engine is capable of safely producing (without lugging).

Brian

[Genesis]

[QUOTE=Danali Star;68643]

This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

Genesis,

I plan to run pretty much at displacement speeds most of the time. If I am running at 1100 to 1200 rpms is this not considered lugging the engine down? I understood your advice was to run at these lower rpms and leave the engines alone. Is harm done by not allowing the turbos to "spool up" while underway?

Also, in your last paragraph you mentioned something being a waste of time. I believe you were referring to making changes thru the props but wanted to be sure.

Dan

No, running there is fine. But I would not change the props with your intended operational regime. The gains from doing so in terms of fuel consumption will be modest and you will destroy the ability to run the engines up if it ever becomes necessary (e.g. to get away from a storm)

My point on pitching DOWN (depitching the wheels) is that if (1) you intend to run on plane and (2) are unhappy with your engines being too high-strung the cheapest way to address this is to depitch the props. This will unload them and effectively "turn them down" in maximum output without changing anything else (e.g. injectors, etc)

[ADaily3224]

Removing Pitch:

It's easier on the engines, if you drive your standard shift car up a hill you'll need to down shift to allow the engine to run at a higher RPM but with less load.

If you remove pitch from a propeller it's like shifting down a gear. You won't save fuel but your engine life will be extended.

Your boat is going up hill all the time, the more weight and growth on the bottom determines how steep the hill is. At low speed it has little consequence, it's the opposite at high speed.

Tony D

[Trojan]

Turbos don't operate on heat expansion. They operate on exhaust gas pressure and volume... That is why you have such a difference in exhaust size, input verses output. You can't drive up a turbo at low RPMs (spool up). But you can lug a diesel at the low end and build up enough heat to cook everything and never spool up the turbo. Lugging at low RPMs is bad for any diesel. Unless you run your boat by thermal couples. Loading your engine by over propping at low RPMs is asking for trouble. As Genesis states the fuel use will not get better but most likely get worse. Because of the linearity that is the reason they tune at WOT. This prevents over loading through the entire HP range. HP does not move the boat Torque does, but to maintain the torque you must have the HP. Look at the torque range and then try to balance the prop to the torque/HP. curve. Then you might save some fuel at a lower range RPM.
BILL

[doc g]

I would take the 1271tis over the 892tis any day . The 61 is a great boat and with a liveaboard I would not want to be with out the cockpit. We spend more time in the cockpit than almost anywhere else on the boat. The access to the water ,line handling,fishability,diveabilty etc are things that make the CMY's so nice. I personally will never own another motoryacht without a cockpit . Besides what's difference in cost of a rebuild 8 vs. 12 cylinder. The 12s will last longer before rebuild anyway and most that are on the market have been rebuilt at least once by now. Prices are about equal 56 vs 61 also. Of course I'm biased and all this is just IMHO!!!..........................Pat

[krush]

Turbos don't operate on heat expansion. They operate on exhaust gas pressure and volume... That is why you have such a difference in exhaust size, input verses output. You can't drive up a turbo at low RPMs (spool up). But you can lug a diesel at the low end and build up enough heat to cook everything and never spool up the turbo.

Your statement is almost 100% incorrect. Turbos operate off excess heat energy in the exhaust gas stream.

The turbine allows the exhaust to expand and extracts work from heat. The enthalpy (simply put, heat energy) change is do the the change in temperature and expansion of the gasses against the turbine (modeled as isentropic expansion usually). Mass flow is constant....velocity has almost no effect on the whole thing.

A turbo can spool at low rpm.