Wide Open Throttle

[psneeld]

Block checked....but the least trusted check.
Head totally redone.
Manifold pressure checked and all new gaskets.
Hang around big boys with lots or top pro experience and great advice.
So......

 

[rslifkin]

That definitely sounds like something weird going on. Personally, I'd keep running it and just keep my eyes out for a reasonably priced engine for a re-power (either another of the same or something else suitable).

 

[Art]

Brand new Perkins in 1965 - Constantly overheated at above a slow cruise. Used it for a while that way. Dad ended up having BIG fight with Perkins HQ. Finally they sent two tech specialists. Ended up there was hairline crack in the head. New head - problem solved.

 

[psneeld]

That definitely sounds like something weird going on. Personally, I'd keep running it and just keep my eyes out for a reasonably priced engine for a re-power (either another of the same or something else suitable).

Just to keep all the advice I have received in perspective to apply to corrective actions...


What are your engine maintenance/operational qualifications?

 

[rslifkin]

Just to keep all the advice I have received in perspective to apply to corrective actions...


What are your engine maintenance/operational qualifications?

Probably a lot less than some on here, but basically working on boats, cars, etc. since I was old enough to help with anything. And plenty of stunts like pulling the heads off an engine for work before a weekend event, getting it all back together, giving it a short test run the night before the event and then putting a couple hundred very hard miles on it the next day (didn't break anything).

 

[Nomad Willy]

On a displacement hull, wouldn't you potentially hit a point where another bit of RPM and power comes in the form of the boat pushing a bit more water, maybe going 0.1 kt faster (potentially within measuring error) and experiencing more prop slip?

Yes but it’s still a curve and more power will result in more speed (even it just a bit) but adding one hp over and over will eventually render the boat uncontrollable.
There was a chap on the Willard owners group that had an 80hp 4cyl Cummins engine. Seriously overpowered and it was capable 8 knots. So for double the power you get 25% speed gain. Hull speed is 7 knots and I cruise at 6 knots. I remember this guys boat in the accompanying picture .... was a mountain of water 8’ or so aft of the stern.

Kinda like whan I hit a bull humpback. You stop promptly but there’s no broken windows. So the chat about a FD hull running into a brick wall isn’t nonsense (close) but there’s a grey area just above hull speed.

 

[psneeld]

Probably a lot less than some on here, but basically working on boats, cars, etc. since I was old enough to help with anything. And plenty of stunts like pulling the heads off an engine for work before a weekend event, getting it all back together, giving it a short test run the night before the event and then putting a couple hundred very hard miles on it the next day (didn't break anything).

Thank you for being honest... some on here have bragged about their experience but when challenged vanish into thin air.

 

[markpierce]

On a displacement hull, wouldn't you potentially hit a point where another bit of RPM and power comes in the form of the boat pushing a bit more water, maybe going 0.1 kt faster (potentially within measuring error) and experiencing more prop slip?

Yes, the last 200 RPM increases speed one-tenth of a knot on my boat but at the expense of going from 72 to 100% load, or a 60% increase in fuel consumption. Normally cruise at 43% load, one knot below hull speed.

My boat has a weight of 14 tons.

 

[SteveK]

Thank you for being honest... some on here have bragged about their experience but when challenged vanish into thin air.

BUT have they stayed at a Holiday Inn Express?

 

[Delfini]

Here’s how it was explained to me

You can rule out the timid folks who refuse to verify that the engine is able to make its full potential. You certainly should be able to run an engine (that is at operating temperature (180- 185 degrees)) up to the load-governed 2500 RPM. The hull design dictates a normal "range" of RPM that you will operate the boat; however, that does not mean that you should not be able to run at higher RPM to beat weather, or make a bridge on occasion.

Consider the testing more like a "health check" to verify that the following is occurring:

A) The vessel is propped correctly
B) The temperature does not vary by more than five degrees either direction
C) The oil pressure does not drop more than five pounds
D) Water flow increases with the RPM increase

The results of this periodic testing does more than "blow out carbon" as it accomplishes the following:

A) Verifies that the engine is properly loaded at all RPM
B) Verifies that there is not a restriction in the fresh water or sea water systems that will eventually result in an overheat
C) Verifies that you will not have excessive bearing, bushing, and ring wear due to a lack of oil pressure
D) Ensures that your exhaust hose will not delaminate with higher exhaust gas temperature.

The keys to a successful load govern RPM test are as follows:

1 - You use a hand held (or verified) tachometer. Do not depend on the electronic gauges in the panel
2 - Be certain that the engine is at operating temperature
3 - Be available to bring the RPM back down if the engine temperature climbs, pressure drops, or sound manifests itself.
4- Consider do this when you are not too far from your home port, just in case something is not right.
5 - You document the readings for your log book to compare for future testing.

The most important consideration regarding the engine carbon build up involves running below 1500 RPM for long periods and/or running with a propeller that is too large for the application. These two factors can easily cause an excessive buildup of carbon in the exhaust system that can be helped by periodic increases in RPM until the problem is resolved by changing operating habits and adjusting (or replacing) the propeller.

 

[Nomad Willy]

Delfini,
You take your propeller loading very seriously.
I do too but I consider my instrument tach to be goodenough. Could be my bad.
And after all these years maybe I wouldn’t wanna know if the tach was wrong. I liked the way it felt best when I was 100rpm underloaded. Putting that prop back on soon.
My coolant temp has always indicated about 205 and the engine’s worked perfectly for many years.

 

[Delfini]

Wot

Hey Big W.
I,m new, never seen my prop. I know its there and the rig runs sweet but this WOT thing has a few views and made me wonder so I ask a guy and he emailed me. Made sense, So I posted it.
Being new and hearing guys never ever and others promoting hammer down
Made me question the feeling I got when we cranked it up.

Not sure about temp. 190 - 205 sounds like a good cup a joe.

 

[BruceK]

As Art says, there are reasons peculiar to boats you might use WOT, eg heat exchanger function and prop correctness testing. But running a 40 yo not rebuilt engine at WOT would bother me. There are parts that are fine at normal or even higher operation rpm, say 2000rpm on a Lehman, which might be stressed at WOT. It`s not meant to be destruction testing.

 

[Art]

In my 400 hp, 500 foot pound, 430 cid big block, 1967 Buick Wildcat... everytime I floor it on the hwy and she comes out of passing gear [2nd gear] at 75 mph - she hits WOT for a couple seconds!

Then... when I once buried her needle [on a wide open straight road] at 125 mph [figure I likely went about 10 mph over that #] she was screaming at or near WOT for a few seconds.

Of course; that was only 10,000 miles past the time I'd had a renowned race car mechanic rebuild my "Cat's" domed head engine. Just had to see what she'd really do!

All those times put together of a freshly built high performance engine reaching WOT don't amount to more than a minute or two at that top end rpm.

For my boats I hit WOT very seldom and only if needed for one reason or another. I'd say that I could probably count on my fingers the times of running a boat WOT in 50 years. Although, that's not including o/b's. Those little tykes don't mind WOT nearly as much as inboard engines. Even with those o/b's, although WOT is more often utilized... I still do non't maintain that rpm for but a minute!

Bottom line is that most engines are not designed to run at WOT for periods of time.

 

[psneeld]

This passed on by an expert engine guy who see our sized normal propusion engines used in genset applications all the time....


"Every day I see the same engines that power most of the boats discussed on that group operate at 1500 or 1800 rpm constantly at loads varying from 50 or 60 horsepower up to several hundred for thousands of hours. They don't suffer from excessive "carbon buildup" and they don't change rpm between zero and full load. If that same engine is used for propulsion and is propped to absorb whatever power it is capable of developing at a given rpm as indicated by the manufacturer's power curve then it will be operating happily within its limits.


If the boat performance doesn't meet desires when operating the engine within its limits then either change the engine or the gearbox/prop combination or get a different boat."

 

[rslifkin]

I'll agree that going WOT to blow out carbon is unnecessary. Running under any reasonable load should take care of that. And it won't be an issue in the first place (for most engines) unless you spend a lot of time basically at idle.

On the car side of this discussion, it looks like there's some confusion on what WOT is. It's full throttle. As in, pedal all the way down or throttle lever all the way forward. In a boat, WOT will very quickly get you to max RPM. But that doesn't mean max RPM is the only way to be at WOT. In a car, for example, you can be at WOT well below max RPM.

 

[Art]

I'll agree that going WOT to blow out carbon is unnecessary. Running under any reasonable load should take care of that. And it won't be an issue in the first place (for most engines) unless you spend a lot of time basically at idle.

On the car side of this discussion, it looks like there's some confusion on what WOT is. It's full throttle. As in, pedal all the way down or throttle lever all the way forward. In a boat, WOT will very quickly get you to max RPM. But that doesn't mean max RPM is the only way to be at WOT. In a car, for example, you can be at WOT well below max RPM.

Sustained WOT test to me means reaching top speed at max rpm available with throttle at full extent of travel. Although it could be said that WOT [i.e. throttle at full extent of travel] is actually in place while acceleration occurs... WOT tests on any engine are usually not performed until full available rpm and speed has been reached.

At least that's how I see the picture!

 

[BDofMSP]

Relationship to Rating Definition?

This seems like a good place to get a related question answered. Cummins defines their "Intermittent Duty" rating as:

Intermittent Duty (INT)
This power rating is intended for intermittent use in variable load applications with a load factor of 20-40 percent. Full power is limited to two out of every eight hours of operation. Reduced power operations must be at or below 80 percent load. Also, reduced power operation must be at or below cruise speed (rpm). Cruise speed (rpm) is dependent on the engine rated speed (rpm)​

High Output (which most recreational engines are rated) is limited to one out of every 8 hours at full power.

What is "Full Power" in this context, and how does it relate to WOT? If they are the same, it seems like worrying about 10 minutes here or there seems silly.

Thanks,
BD

 

[Xsbank]

The manufacturers designate a wot setting at some level where they can rate the engine’s power (120 hp max, for example) where the engine can run without exceeding any of the design parameters and cause damage. Its carefully set so the engine will live its design life with minimal or no damage. My engine is rated “continuous” and it can run at 2100 rpm for 24 hours a day. Its noisy though and burns a lot of fuel. The high-output versions of my engine can run at 2500 rpm for one hour out of 24.

Wot can reveal fuelling, cooling and drive train faults and is therefore very useful for determining the health of your propulsion system. Why be afraid of it?

 

[BDofMSP]

As I thought.

So the Cummins 6BTA 315 is Intermittent rated at 2800 RPM, which implies that it can be run at 2800 RPM two out of every eight hours without damage or concern (unless some other problem exists obviously). The rest need to be run "at or below cruise speed (rpm). Cruise speed (rpm) is dependent on the engine rated speed (rpm)".

What would "cruise" rpm be in this context?

Thanks
BD

 

[rslifkin]

Cummins rating definitions can be found here: https://www.sbmar.com/technical-information/cummins-diesel-ratings-definitions/

So for your engine rated at 2800, cruise RPM would be 2600 or less according to Cummins.

 

[BDofMSP]

Perfect, thank you.

So besides massive fuel bills, are you giving up engine life if you run regularly around 2600? I feel like the answer should be no, based on the rating, but that contradicts a lot of what I read here. Like if you knew that the PO ran the boat like that, you should run fast and far.

Just trying to understand what is real vs lore here.

Thanks,
BD

 

[rslifkin]

Excluding very low loads where there may be under-loading issues on some engines, yes, running it harder will give it a shorter life. However, up to a certain level of output, it'll be able to provide that and still live a long, happy life.

 

[Nomad Willy]

Perfect, thank you.

So besides massive fuel bills, are you giving up engine life if you run regularly around 2600? I feel like the answer should be no, based on the rating, but that contradicts a lot of what I read here. Like if you knew that the PO ran the boat like that, you should run fast and far.

Just trying to understand what is real vs lore here.

Thanks,
BD

No.
Long engine life has to do w a lot of variables. But rpm run has little to do w it and usually running a higher rpm is almost always accompanied by lower loads and almost always lower tempetures as well.

The wear on the cylinder walls and pistons is IMO a direct result of sideways forces transferred from combustion forces w the offset crankpin. So a little less pitch, or lower gear ratio resulting in more rpm will result in extended piston, piston ring and cylinder life.

Many disagree w me on this. I remember Marin Faure who said engine life is a result of how many times the piston rises and falls. Simplistic idea I think. Wear needs to be related to work done that is directly related to piston forces.

So I think engine load is the dominant controlling factor regarding engine wear. Not rpm.