Running warm or not?

[post42]

I have 735 h.p. 892's . When cruising at 1800 -1900 rpm's my starboard temp gauge reads up to 245 degrees. The gauge in the engine room, which I believe is mechanical, reads 185. The port reads around 185 on both gauges( bridge and engine room). Correct me if I'm wrong but there is no way the gauge reading 245 could be correct. Wouldn't I blow the engine or do serious harm? It runs fine and keeps good oil pressure. What do you guys think? Is it usually a gauge or a sending unit? What should I change? Thanks for any advise you can give.
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[SKYCHENEY]

That reading is obvisouly wrong. Swap the gauges at the helm first or use some jumper wires. This will tell you if it is the gauge or the sender that's acting up.

 

[MikeP]

Yes, it's definitely the gauge! The mechanical eng room gauges are the only ones to pay any attention to.

Does it start at a low reading and steadily climb as the engine warms up? If so, kinda sounds like the wrong sender for the gauge; has it always done this or did it just start acting this way?

 

[Genesis]

The gauge is obviously hosed.

However, 185F is the TOP of the safe range. I would investigate that; ideally you want to run in the 175-180F range at cruise.

 

[post42]

Thanks for all the advise. Karl I'll look into that. I just bought all new power cool so I'm going to flush and fill with new. I'll keep you posted.
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[Genesis]

THe problem is almost certainly on the raw water side. See my "on engine cleaning" procedure posted here in the tips section if you want an easier alternative to pulling the H/E cores. It may or may not do the trick, but it sure is easier than ripping the system apart.

 

[post42]

Karl I was wrong. Ran the boat again today and the mechanical gauges in the engine room were reading between170 and 175. Actually closer to 170. That actuallyseems cold to me.
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[Boss Lady]

Karl I was wrong. Ran the boat again today and the mechanical gauges in the engine room were reading between170 and 175. Actually closer to 170. That actuallyseems cold to me.

Count your blessings, 185 is the do not exceed temperature. What rpm do you cruise at?

 

[post42]

I cruise between 1800 and 1900. I'm reluctant to go under 1800 because I don't want to lug them. Or at least I've read I should not lug them.
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[krush]

I cruise between 1800 and 1900. I'm reluctant to go under 1800 because I don't want to lug them. Or at least I've read I should not lug them.

This is not the first time I've heard this. People don't want to lug the engines, but my engineering backround disagrees slightly. Doesn't a prop just absorb the power u put into it? I've heard of overloading occurring on large diesel engines in ferries because the captain likes to "slam" the throttles, but is running at 1/2 throttle really "lugging"? If the prop acts like a pump, it absorbs whatever power is transfered to it.

I don't know, I could be completely wrong, but I think if you measured the toque in the shaft and the load on the bearings, it wouldn't be any more at lower RPM than higher.

http://www.irrigationtutorials.com/pump.htm

 

[Genesis]

170-175 is great. Don't bitch - you're doing good.

As for lugging, the only thing to be aware of is that you don't want to run where there is no boost on a turbo engine but you're plowing. If the boat's out of the hole and on plane, its fine. Ditto if its settled (but that's slow!)

There is a "no-go-zone" for long term running which is where the boat is pushing a huge wave, stuck in the hole, and the engines are working their butts off but there's no boost. If you have Pyrometers you'll see sky-high EGTs - that's what you need to be worried about.

As the boost comes up the EGTs come down dramatically, then start rising again.

Prop demand curves are typically logrythmic, not linear. Its not a truck.

 

[krush]

Prop demand curves are typically logrythmic, not linear. Its not a truck.

Yeah, I know they aren't linear. That's why the lower the RPM, the less fear of "lugging". I think you meant exponential instead of logarithmic though.

At a certain RPM the prop is always going to absorb the exact same amount of HP, correct?



Edit: found this: M= motor hp output P = Prop power absorbed

article: http://www.ossapowerlite.com/tech_library/fuel_efficiency/fuel_efficiency.htm

 

[Genesis]

Well, not exactly.

A prop that is in a boat tied to the dock will absorb more power than a prop in a boat that is moving with the input RPM being equal.

This is why having a dirty bottom can cost you RPM (and not just speed)

 

[krush]

That's getting into much more complex analysis which is probably way beyond the scope of this forum.

When tied to a dock, the prop is not moving "forward" in the water, so the kinetic energy associated with the moving water (when the boat is moving) is not there. You're comparing static thrust vs dynamic thrust.

If the RPM is equal, how is it absorbing more power? The motor is spinning at the same RPM and thus is putting out the same amount of HP regardless of the speed of the vessel. Wouldn't the RPM be different in each case?

 

[Genesis]

No.

Diesels have governors on them; you set an RPM and the fuel is modulated to deliver that RPM. If you load the engine more, the RPM will remain constant but fuel delivered will increase.

 

[Jackman]

I've always been told that the prop is the diesels giovernor.

 

[Avenger]

I've always been told that the prop is the diesels giovernor.

Only to the extent that the engine will not deliver any more HP than the prop is absorbing. If the prop loads the engine past its capability the engine governor becomes the limit.

 

[Genesis]

Scrod's got it kinda backwards but Jackman, nope.

You can go up to a Detroit, out of gear, and ram the throttle wide open. It will go to the high RPM limit ("high idle") and no more.

Do that on a gas engine and it will disassemble itself unless it has a rev limiter in the electronics somewhere.

When you set a throttle position on a diesel with a variable speed governor (as is found on a boat) you are selecting an RPM, not a power level. The governor varies the fuel flow to achieve the RPM you selected. If the RPM selected is not achieveable because thee engine cannot produce that much power then obviously the RPM will not be achieved, but otherwise, it will be and fuel will be modulated as required to maintain that RPM level.

This same sort of governor arrangement is used with diesel generators, and is typically set for 1800 RPM. The governor holds the engine at 1800 RPM irrespective of load (within the engine's power output capability, of course.)

This is very different than what happens with a gasoline engine where you modulate airflow with the throttle and the computer (or carbuerator) meters fuel to meet the airflow setting you have selected.

If you take a gas engine at part throttle and take the engine out of gear, it will race (and may explode due to overrev.) If you take a diesel at part throttle out of gear the RPM will not appreciably spike.

This sort of governor is not used on diesels in road applications because it would make driveability very difficult; in road applications on a diesel you have what is called a "limiting speed" governor which operates far more like a gas engine throttle in practice (you are selecting a power level rather than an RPM) but with a cap on the RPM the governor will permit to be achieved.

 

[Avenger]

Okay, fine. I should have written a page or two to explain it.

All I meant was that at a given RPM setting, prop load is the governing factor of engine output until load exceeds engine capability. Then the engine governor controls output. That's why I said "only to the extent." It's not backwards, it's just semantics.

Picky, picky, picky.

 

[krush]

No.

Diesels have governors on them; you set an RPM and the fuel is modulated to deliver that RPM. If you load the engine more, the RPM will remain constant but fuel delivered will increase.

I know how a governor works...i've had a few apart before. You are tossing another variable into the equation that doesn't need to be there. Unless you have gone and put a window in the governor, you can't tell how much fuel it is requesting to match the RPM.

I made the comment with the assumption that HP is held constant.

I honestly don't know what we are discussing anymore! However, I will stand by my first statement that HP absorption in a prop is primarily a function of RPM...and you can not directly compare static to dynamic conditions because they have different factors involved.

And many gasoline engines have governors, they are called lawnmowers

This new designed jet drive has a variable sized intake that takes advantage of the kinetic energy in the flow of water that I was speaking of: http://www.marinejettech.com/