One for tech guys, and help needed by me Latestarter/VP/Portofino etc

[jfm]

no "he " is you -excuse my carp English

OK! That rather changes things 180 degrees. I need a translator!

weight kills speed - if overloaded the rpm stays the same -boat slows -but EGT -load and GPH shoot up .

I agree that, to a degree, but it wasn't what we were discussing and isn't what O/P asked. RPM only stays the same if the max fuel delivery rate set in the ECU or by the fuel system is sufficient to maintain that RPM; in other words, GPH cannot just keep going up - it hits a ceiling. When it hits that ceiling any more load on the boat reduces RPM (and speed, and GPH, and EGT) until a new equilibrium is found. The ceiling isn't theoretical or distant: most boats are set up on sea trials to be just nudging the ceiling at WOT, and a bit under it at cruise speed

 

[Portofino]

OK! That rather changes things 180 degrees. I need a translator!

I agree that, to a degree, but it wasn't what we were discussing and isn't what O/P asked. RPM only stays the same if the max fuel delivery rate set in the ECU or by the fuel system is sufficient to maintain that RPM; in other words, GPH cannot just keep going up - it hits a ceiling. When it hits that ceiling any more load on the boat reduces RPM (and speed, and GPH, and EGT) until a new equilibrium is found. The ceiling isn't theoretical or distant: most boats are set up on sea trials to be just nudging the ceiling at WOT, and a bit under it at cruise speed

Agree but an overloaded boat never reaches WOT -suffers in silence ----erh until BANG
that's the the point of all this stuff - I get the impression folks just don,t know

 

[jfm]

It reaches WOT, or more precisely (because there is no throttle on a diesel engine and we only use "WOT" as shorthand) it reaches 100% rated fuel delivery per injection event, but it never hits rated RPM. It is actually debatable whether this is truly "overloaded": the engine is developing less BHP. But that is semantics I suppose.

 

[Portofino]

It reaches WOT, or more precisely (because there is no throttle on a diesel engine and we only use "WOT" as shorthand) it reaches 100% rated fuel delivery per injection event, but it never hits rated RPM. It is actually debatable whether this is truly "overloaded": the engine is developing less BHP. But that is semantics I suppose.

I think it is

 

[lionelz]

All ok re STW/SOG - simple mix up. Forums!
You are not correct as regards the bit highlighted above. Sure, as load increases, the fuel burn will increase because the governor puts in more fuel to maintain the 3000rpm, but speed will also fall. It must, for the following reason. The force required to push the boat at X knots increase if you add weight or headwind to the boat. If the engine maintains precisely 3000rpm, where does the extra force to push the boat come from? It comes from increasing the AOA of the prop blades to increase their lift, and with a fixed pitch prop that means a higher prop slip %, which means the boat speed falls as a new equilibrium is found between the increased drag of the boat and the force of the propellor.

I think that's what I was trying to say. It was also the fact that if you had a heavier loaded boat than normal to get to 3000RPM you would burn more fuel and especially if you wanted to get to your normal speed,

 

[jfm]

I think it is

But what do you really mean by overloaded, in a proper engineering analysis sense?

My engine comes in 1925hp and 1000hp versions, and a few in between. Lets say the 1925 is installed in a boat fully to spec, and reaches rated RPM at 2300rpm at WOT. That's ok in your book

Now let's imagine another boat with the 1000hp version and it is over propped so at WOT it only makes 1900rpm not the rated 2200rpm. In your book it is "overloaded", right? But is the overloaded 1000hp engine going to smash up in fewer WOT hours than the correctly-loaded 1925hp? So what exactly, in an engineering sense, is "overloaded"? How much of it is bar room talk and how much is real, at least in the sense of the engine smashing up or failing?

 

[Portofino]

But what do you really mean by overloaded, in a proper engineering analysis sense?

My engine comes in 1925hp and 1000hp versions, and a few in between. Lets say the 1925 is installed in a boat fully to spec, and reaches rated RPM at 2300rpm at WOT. That's ok in your book

Now let's imagine another boat with the 1000hp version and it is over propped so at WOT it only makes 1900rpm not the rated 2200rpm. In your book it is "overloaded", right? But is the overloaded 1000hp engine going to smash up in fewer WOT hours than the correctly-loaded 1925hp? So what exactly, in an engineering sense, is "overloaded"? How much of it is bar room talk and how much is real, at least in the sense of the engine smashing up or failing?

Overloaded for me is when the EGT is excessive so,s to cumulatively or permanently shorten the life span of the diesel engine-,that would not otherwise happen if the rpm was reduced so to bring the EGT inline with the manufacturer s recommendations

Heat kills diesels in boats -

 

[lionelz]

Sorry for ignorance what does EGT stand for

 

[BartW]

Sorry for ignorance what does EGT stand for

Exhaust Gas Temperature

 

[MapisM]

The force required to push the boat at X knots increase if you add weight or headwind to the boat. If the engine maintains precisely 3000rpm, where does the extra force to push the boat come from?

I'm not sure to follow your reasoning, J.
Let's use the simpler way to take prop slip out of the equation - don't you agree that a car can keep the same speed when going uphill at any given rpm, just by using more energy (fuel) to produce more power AND torque?
Up to a limit of course, but that's already agreed, as I understand.

That said, yes, of course my statement that you quoted in bold is not 100% correct as such.
I'm well aware that on boats (as opposed to cars!) the prop slip is a factor, but as I said, that's a different matter vs. OP point #11.
In fact, there's a reason why I mentioned fouled props as an example of a relevant change in prop slip: any other factor which one way or another increase drag (load, dirty bottom, wind, whatever), hence the force required to move the boat, are MAINLY reflected in a different prop demand curve rather than in a different prop slip, in principle.

Mind, I agree that the prop slip can be affected, hence reducing speed, but that's marginal (if not meaningless) in any proper setup, where you can't realistically hit the point where the slip changes significantly - which means that the props aren't doing their job properly anymore.
For instance, by sticking your Sq78 props on a 300T tug, they would probably still be able to move the thing, up to a point/speed (veeery slowly, and obviously with a higher prop slip compared to a Sq78 anyway), but after that point, no matter how much power could be applied to them, they would have no other choice than cavitate, hence skyrocketing the slip practically to 100%, and not moving the vessel anymore.
But when a hull/engines/props setup is used within its envelope, so to speak, the prop slip is pretty constant.

The proof of the pudding is in the slip measurement - in this sense I disagree wholeheartedly with Portofino when he says about slip that "one can not tell -there is no prop slip gauge", because the prop slip is extremely easy to measure even without any dedicated instrument once you know the rpm, gear ratio, prop pitch and speed. Btw, indeed there are also instruments that calculate and show the prop slip in real time - I can think of one made by Livorsi, for instance.
In fact, in several occasion while playing with my old lake toy, I did make such calculation.
Interestingly, her hull was always mirror like (never been antifouled, always stored when not used), so perfect for comparisons, but her top speed was seriously affected by load:
When lightly loaded (two people, little fuel) she could hit 71mph @ 5100rpm, with a 11.8% slip.
With full tank and 6 people onboard, the best she could achieve was 66mph @ 4700rpm, but the prop slip remained EXACTLY the same. Well, I never bothered checking the second decimal tbh, but you see what I mean.
Otoh, I cruised at 3000rpm most of the time - and that always translated in 42mph or so, regardless of the boat load. In other words, again, the prop slip was unaffected (though fuel burn was, of course!).

Last but not least, re. the webpage linked by Portofino, I gave up reading after I reached this para:
This water skier would be an example of a force EXTERNAL to the engine. Cut the rope and the RPM stays the same but the vessel picks up 2+ Knots
If you CAN’T buy into this, then my answer is also very simple. YOU ARE IN DENIAL, and lack the most basic understanding of how a vessel’s performance interacts with the engine’s capabilities
Now, I might lack the most basic understanding, but one thing is for sure: the writer never helmed a skiboat to pull a skier through a slalom course, and has zero clue of what actually happens, period.
Besides, saying that by not pulling the skier anymore the vessel picks up 2 knots at constant rpm, he's implying that the prop slip is reduced by more than 50% the moment the rope is cut, which is beyond a joke.
And I just made this calculation assuming the 36mph tournament speed, while most skiers are typically pulled at lower speed, in which case the 2 knots increase would imply an even higher prop slip reduction, go figure... :ambivalence:

 

[MapisM]

as regards to the engine load in my old tub

Oi B, only a few of us are allowed to call their boats "old tubs".
BA is firmly in the gin palace league - and a very elegant one at that!

 

[superheat6k]

A brilliant thread and responses, but the OP's request for things being kept simple was lost after the first few threads, then too much emphasis on abbreviated and initialled terms. EGT principles will fly over most people's heads, and is primarily why manufacturer's ignore it.

To explain to a non-techy technical things sometimes requires similes that are technically (at a higher level) incorrect, but are useful to allow a more basic understanding of a principle. So we end up on a discussions of the semantics of the finer details of how engines / props / hulls / loading / sea conditions / wind conditions that flies right over the OP's he'd (and many others that might be interested).

So the purpose of my tenpeneth here is to request to remember the OP's original request and therefore try to condition your response in a way he can relate to the information and principles being described (which are I would add all good so far).

I'm no expert on Volvo Penta or Cummins, Yanmar, etc. But I do have a reasonable understanding of just how they work and where to look to find a problem on any of them and then deal with it. Great that there are experts on here for the knowledge and experience of specific engines that we can all tap into. Lets try to help those who do not get engines, as once they are out there sooner or later they will need to know.

 

[jfm]

I'm not sure to follow your reasoning, J.
Let's use the simpler way to take prop slip out of the equation - don't you agree that a car can keep the same speed when going uphill at any given rpm, just by using more energy (fuel) to produce more power AND torque?
Up to a limit of course, but that's already agreed, as I understand.

That said, yes, of course my statement that you quoted in bold is not 100% correct as such.
I'm well aware that on boats (as opposed to cars!) the prop slip is a factor, but as I said, that's a different matter vs. OP point #11.
In fact, there's a reason why I mentioned fouled props as an example of a relevant change in prop slip: any other factor which one way or another increase drag (load, dirty bottom, wind, whatever), hence the force required to move the boat, are MAINLY reflected in a different prop demand curve rather than in a different prop slip, in principle.

Mind, I agree that the prop slip can be affected, hence reducing speed, but that's marginal (if not meaningless) in any proper setup, where you can't realistically hit the point where the slip changes significantly - which means that the props aren't doing their job properly anymore.
For instance, by sticking your Sq78 props on a 300T tug, they would probably still be able to move the thing, up to a point/speed (veeery slowly, and obviously with a higher prop slip compared to a Sq78 anyway), but after that point, no matter how much power could be applied to them, they would have no other choice than cavitate, hence skyrocketing the slip practically to 100%, and not moving the vessel anymore.
But when a hull/engines/props setup is used within its envelope, so to speak, the prop slip is pretty constant.

The proof of the pudding is in the slip measurement - in this sense I disagree wholeheartedly with Portofino when he says about slip that "one can not tell -there is no prop slip gauge", because the prop slip is extremely easy to measure even without any dedicated instrument once you know the rpm, gear ratio, prop pitch and speed. Btw, indeed there are also instruments that calculate and show the prop slip in real time - I can think of one made by Livorsi, for instance.
In fact, in several occasion while playing with my old lake toy, I did make such calculation.
Interestingly, her hull was always mirror like (never been antifouled, always stored when not used), so perfect for comparisons, but her top speed was seriously affected by load:
When lightly loaded (two people, little fuel) she could hit 71mph @ 5100rpm, with a 11.8% slip.
With full tank and 6 people onboard, the best she could achieve was 66mph @ 4700rpm, but the prop slip remained EXACTLY the same. Well, I never bothered checking the second decimal tbh, but you see what I mean.
Otoh, I cruised at 3000rpm most of the time - and that always translated in 42mph or so, regardless of the boat load. In other words, again, the prop slip was unaffected (though fuel burn was, of course!).

Last but not least, re. the webpage linked by Portofino, I gave up reading after I reached this para:
This water skier would be an example of a force EXTERNAL to the engine. Cut the rope and the RPM stays the same but the vessel picks up 2+ Knots
If you CAN’T buy into this, then my answer is also very simple. YOU ARE IN DENIAL, and lack the most basic understanding of how a vessel’s performance interacts with the engine’s capabilities
Now, I might lack the most basic understanding, but one thing is for sure: the writer never helmed a skiboat to pull a skier through a slalom course, and has zero clue of what actually happens, period.
Besides, saying that by not pulling the skier anymore the vessel picks up 2 knots at constant rpm, he's implying that the prop slip is reduced by more than 50% the moment the rope is cut, which is beyond a joke.
And I just made this calculation assuming the 36mph tournament speed, while most skiers are typically pulled at lower speed, in which case the 2 knots increase would imply an even higher prop slip reduction, go figure... :ambivalence:

MapisM, I sort of agree plenty of that but on the core point I'm disagreeing. The boat is planing with 3000rpm, and you then add more weight (or headwind, or whatever). You said the governor will inject more fuel to maintain the 3000rpm and that is 100% correct. You also said STW will not change, and I'm afraid that is never correct. STW will fall because prop slip increases - the amount is pretty small of course. A propellor produces a force to push the boat, and if RPM stays constant then the only factor affecting that force is AOA, aka lift, aka prop slip. If you increase AOA and prop slip, at fixed RPM, you get more force. A heavier boat undoubtedly needs more force to push it than the same boat light, so AOA must increase as you add load, and for that to occur then STW absolutely must fall if RPM stays the same. It's a small amount in our day to day lives, I admit, but it definitely happens. In summary, STW absolutely does not stay constant, and that's the only thing I'm disagreeing with you on.

The car analogy doesn't work because there is no pumping of fluids by Newtonian action on a vane. A constant RPM impeller(or even centrifugal) pump is a better analogy: if you increase the head or the back pressure that it is pumping against, the flow rate will fall, ie the speed of the fluid will fall, increasing AoA of the vanes/blades, which is perfectly analogous to a reduction in STW on a boat

 

[Portofino]

Sorry for ignorance what does EGT stand for

Arh -Exhaust Gas Temperature .

Rises because as the load 1* exceeds 100 % or on my book ( see T47 thread attached above ) exceeds 90 % for me a no go area -buts that's personal -more fuel is pumped in , engine cannot burn it properly to increase rpm/rev up , like a petrol engine does - it's over fuelled and gets hotter-starts to damage things .-exhaust gas temp increases -see my post somewhere above showing it at various Rpm,s /boat speeds .
A marine diesel is unable to rev up due to excess boat wieght , drag ,fouling , prop pitch etc , it spins sure , but the engine kinda stops reaching full revs ,known a s WOT wide open throttle ,
Idea is to try and match rpm to whare the Hp 2 * bisects the prop demand curve -boils down to prop pitch all other things being equal ?
1* load see JFM,s description
2* that's not necessary max Hp -more the Hp you want to take out of what's available .-depends what's on the table -which engine

For Superheat6 -good post BTW # 32 thank you

Basically what this thread is about ( now a conglomeration of a few -same topic ? ) from my view is highlighting to others like omegalite, that if Fairline for example- claim top speed guess 32-33 knots - in a T 47 -then knock my estimate. Off - 8 knots - ( no science ) then in my view its best to cruise 8 knots less in this case a T47. 24-25 knots which is where he was -
VP do not fit EGT or load guages , they could .

Or for those fortunate to that have EGT and load. Guages -MAN as an example -look at them and throttle accordingly .

Given a choise -appreciate that does not happen in boat purchase -go for the biggest engines
And/or try not to overload the diesel engines .

In a vehicle say a cement mixer with a volvo D12 has 340 Hp . It shifts 30 tons of conreate every day for 1million Km,s with just normal consumables oil filters ' tyres ,clutch etc .
When going up hill and down dale ,the driver can sence when it's labouring so changes down .
Just before he changes down his foot stays in the same place as it slows for hill the same amount of fuel goes in , but the engine in the same gear .momentarily say 10 seconds the fuel/air ration goes to cock too much fuel -the temp in the exhaust gas sky rockets for 10 seconds -untill the down change .Once in the right lower gear as the hill s climbed the fuel air ratio recovers .
What's all the fuss with fuel/air ratio ? I here you ask ?
Basic s
Take a bowl of a volatile substance -petrol and any oil say chip fat , coco nut , diesel .
Line then up on the kichen table all 4 bowls ,Take a match to them ,only the petrol goes up -boom .
The oils do not ignite .
So how do you get the oil to ignite ?
Heat it ?
How ?
ans - Compress air - we talk about diesel and petrol engines as they are the same ,in the sense one costs less at the pumps , we kinda nowadays morph a BMW 520d with a 520i in a road vehicle .
Mentally we are thinking cars - what else have got as a yard stick ? -not much .
We ( forum members then take up boating )
With the grater MPG from diesel etched on our brains from our road experiance -it's normal .
What we fail to realise or most boaters do - is that to get that "oil " to ignite requires the air to be compresses almost 2x as much as a petrol . Compress air and it gets hot ,I mean real hot .Petrol goes up in the bowl on the kitchen table remember with out any help of compressing the air .
This extra heat in a diesel needs to be carefully "managed " as said get it wrong ar/ fuel ratio and in a diesel and you start to see a even bigger rise in the Exhaust Gas Temp (EGT ) -leading. Physical damage -basically components melting .
How,s this happen in a boat not a cement mixer ?
Gears -you can change down in the vehicle To keep the same spped up a hill as opposed to just slowing den in the wrong higher gear - and in a boat the prop slips more , just turns but eventfully finds a max rpm LESS than when it was new . Extra drag , fouling, wieght etc prevents , it reaching full WOT as when new - But and it's it's the clincher -the boat engine fuel pump does not know this ( nor the operator without EGT / load. Instruments ) -so the EGT risers -remember the air is hot allready to get the oil to burn in the 1st place .
In the boat -think of golf -do 18 holes erh pick ONE club for the hole round. .
In a boat there is no caddy ,jumping in and changing the prop .its one prop fits all conditions .
What conditions ,I am just off for trip ? To Yarmouth ,what's changed ?
Wieght , drag , hull fouling , prop fouling ,etc .
So what ?
Thing is the " caddy " has given you one club - one Prop pitch -in a competive market to sell the boat the " caddy"Is concoius of stats and in a one upmanshippy way wants to beat if possible the nearest deemed rival at the next boat show .
So the pressure is on for speed . Too much pitch or on the edge .
That's why some do not publish EGT, and are vague about " load "
New boat given to MBY mag for test , everybody waits with bated beath for the result !
Boat " Performs " -sales rocket .
1-2y later maybe 2nd owner -heavier , caked in poorly applied antifoul , mid season , fouled up - owner try's to reach the speed sales man and mag have lead him to believe .Erh -nope
Opens throttles to make it go faster -but inlike a vehicle he can not tell if it's labouring ( talking planning boat here ) Does Hp and prop demand curve meet now same as was when new -maybe ?
Because of the excess fuel and cocked up ratio EGT risers to culmuative damage levels .
At 3 old sells boat now with 400 hrs , at 7 y old now 700 hrs another owner , 900 hrs. Chances his arm at those now old mag reports - Several owners later @2000 hrs BANG .
All because it's got no gearbox or caddy -just one prop chosen to sell it , that the owners thought its ok to give it "tight wire " all day long -cos brochure said so.

Mean while mr Cement Mixer 9 y layer plods on with approaching 1million km -up hill and down dale
Dead reliable diesel engines

 

[lionelz]

Portofino Thanks that's a great and simple explanation. I now have a much better idea on what is meant by that. I very rarely take my boat to WOT which happens to be 3300 RPM if I do its does 27/28 knts. I normally travel and try to keep the RPM at 3000 which means it cruises at 18/22 knots. It was my engineer that said do this for two reasons Fuel economy and it's was not good to travel at WOT very often. You have just explained why. I know that it is not always necessary to know why but it's nice to have a basic understanding. Mine is a 240 Yanmar pushing a 3.5 ton boat I understood that it was working much harder than if it was trying to drive a car. It had never even occurred to me the affect of just having one gear! (It's a shame there was not a way they could put gears in a boat that could be changed in the same manner as a car).

Prop slippage has been mentioned a lot, are we talking about the cavitation caused by its movement and what it does to the water? Simple way to describe for me where it moves the water it creates a vacuum and the faster it rotates it's a bigger vacuum or is it something different altogether?

Lionel

 

[Beyondhelp]

Arh -Exhaust Gas Temperature .

Rises because as the load 1* exceeds 100 % or on my book ( see T47 thread attached above ) exceeds 90 % for me a no go area -buts that's personal -more fuel is pumped in , engine cannot burn it properly to increase rpm/rev up , like a petrol engine does - it's over fuelled and gets hotter-starts to damage things .-exhaust gas temp increases -see my post somewhere above showing it at various Rpm,s /boat speeds .

Sorry this is not right, very quickly as its late, and I need sleep.

Petrol engiens have max egt when under moderate cruise loads whilst still maintaining around 14-14.5:1 AFR (air fuel ratio by weight).

Unless designed as a 'lean burn engine' when under WOT loads fuelling will be somewhere around 12:1-13:1 and EGT will be lower.

An engine (any engine) does not know the load placed upon it. There are just three conditions.
Engine power > Load, RPM rises.
Engine power < Load, RPM falls.
Engine power = Load, RPM stable.

'Overloading' by over propping/wrong gear on a (petrol) engine will mean that its power curve will be incorrectly matched to the application (obviously) and full power output will not be reached. However, an engine with a correctly programmed engine management system will take this without issue. EGT's will not rise to damaging levels, even on a petrol turbo, unless the engine ran lean (they almost always run a little rich, especially carb engines).

The only potential problem that may arise is if due to so called 'overloading' lets say there is not enough air flow through a rad, and the engine is flat out at half rated rpm pulling say a heavy load up a hill, that on a weak system the thermal cooling capacity of the cooling system may be exceeded. But this is a cooling issue, not an engine overloaded one.

As for engine loading/wear due to 'overload', RPM is a much bigger player than 'overloading'. As example, Turbo charge a 2.0 car engine which for example would make 150 HP without a turbo, I can make one of those reliably make 250HP with a turbo, keeping the same power band, where as trying to do the same with NA tuning would mean revving the engine to 8000+ rpm where loads on everything are so much more.

Diesels are different, their EGT goes up as more fuel is applied (up to a point) and yes - enough to cause damage, but any moden electronically controlled/common rail engine should again be programmed so that under all conditions there are none that put the engine into a situation where thermal capacity of any of the parts is exceeded.

Just before he changes down his foot stays in the same place as it slows for hill the same amount of fuel goes in , but the engine in the same gear .momentarily say 10 seconds the fuel/air ration goes to cock too much fuel -the temp in the exhaust gas sky rockets for 10 seconds -untill the down change .Once in the right lower gear as the hill s climbed the fuel air ratio recovers .

Simply not true in an engine with electronic fuel control.

FWIW people say stressed smaller engines produce less MPG then larger ones not stressed. Under the same conditions this would not be the case. As above most (petrol) engines run richer as the load gets higher, for both power and egt reasons, but match the AFR of two engines get a 4.0 and a 2.0 at WOT producing 150hp, and the 2.0 would be by far more efficient.

Another thing I noticed in this thread above, is that someone mentioned engines typically are 40% efficient. Definitely not the case for consumer available engines. I'd be surprised if any of these diesel ones exceeded 25% and the petrol 20%

 

[jfm]

Prop slippage has been mentioned a lot, are we talking about the cavitation caused by its movement and what it does to the water? Simple way to describe for me where it moves the water it creates a vacuum and the faster it rotates it's a bigger vacuum or is it something different altogether?

Lionel

No, prop slippage isn't anything bad like cavitation etc. It is an intrinsic feature of all props; it is impossible to run a boat without what we call prop slippage. It means just the angle of attack of the blades. Think of it as a waterski or an aeroplane wing: you have to have it at a slight angle to the direction of travel as you push it through the fluid, in order to generate lift. An aeroplane with simple flat wings made of plywood will not fly if the winds are not tilted, relative to the plane's direction of travel. You need that tilt in order to get high pressure air under the wing. The similar tilt in a propeller blade is what creates the "lift" and it is manifested in the fact that the prop travels less than its pitch, per revolution. For convenience we call that slippage and measure it as a percentage, but don't be fooled by the term "slippage" - it is not a bad thing and it is meant to happen

In general props have nothing to do with vacuums. The movement of the blades with an AoA (angle of attack) creates high pressure water on one side of the blade and low pressure water on the other. This is what moves the boat.

It's best to think of high pressures rather than vacuums when you're dealing with fluids. The propeller doesn't work by having the low pressure water suck it; it works because the high pressure water pushes it. You can't pull things with string that is made of water but you can push things with water. (Tensile strength of water is ~ zero)

 

[Beyondhelp]

Portofino Thanks that's a great and simple explanation. I now have a much better idea on what is meant by that. I very rarely take my boat to WOT which happens to be 3300 RPM if I do its does 27/28 knts. I normally travel and try to keep the RPM at 3000 which means it cruises at 18/22 knots. It was my engineer that said do this for two reasons Fuel economy and it's was not good to travel at WOT very often. You have just explained why. I know that it is not always necessary to know why but it's nice to have a basic understanding. Mine is a 240 Yanmar pushing a 3.5 ton boat I understood that it was working much harder than if it was trying to drive a car. It had never even occurred to me the affect of just having one gear! (It's a shame there was not a way they could put gears in a boat that could be changed in the same manner as a car).

Prop slippage has been mentioned a lot, are we talking about the cavitation caused by its movement and what it does to the water? Simple way to describe for me where it moves the water it creates a vacuum and the faster it rotates it's a bigger vacuum or is it something different altogether?

Lionel

An engine is most efficient (generally) at peak torque output. This is simply as this is the point in which the most amount of air is passing through the engine and thus is available to burn with fuel on a per cycle basis. However due to the way in which a boat reacts with the water, its unlikely that maximum hull efficiency speed will match with the engines peak torque output.

Prop slip if its not obvious, is simply the difference between the theoretical amount of travel per prop rotation vs actual travel. At slow speeds water acts fluid and is moves around the blades, much energy is simply wasted and ultimately dissipated as heat due to friction in the water. As speed increases the viscous effect of the water starts to act and it begins to act as a solid around the prop that slices through it pushing the boat forward. Typically slippage % are somewhere around 10%. Knowing the prop pitch, engine RPM and gearbox ratios, its possible to calculate prop slip.

 

[jfm]

Diesels are different, their EGT goes up as more fuel is applied (up to a point) and yes - enough to cause damage, but any moden electronically controlled/common rail engine should again be programmed so that under all conditions there are none that put the engine into a situation where thermal capacity of any of the parts is exceeded.

Yup that is critical - your post was mostly petrol but diesels are different from petrol. The diesel EGT rise occurs because at WOT (ie maximum fuel injection per cycle) in an "overloaded condition" the rpm is too low, which makes the turbo slow, which reduces the charge air pressure, which then means the large fuel injection isn't burnt during the firing stroke due to lack of air (=black smoke) and there is after burn of the fuel/air mix in the exhaust stack which increases EGT. I don't think we are disagreeing though

FWIW people say stressed smaller engines produce less MPG then larger ones not stressed. Under the same conditions this would not be the case. As above most (petrol) engines run richer as the load gets higher, for both power and egt reasons, but match the AFR of two engines get a 4.0 and a 2.0 at WOT producing 150hp, and the 2.0 would be by far more efficient.

Alleluia! As mentioned above in the context of 1000cc, 1600cc and 2300cc Mondeos :encouragement:

 

[Beyondhelp]

Complete agreement, except to say that the diesel engine would if controlled electronically reduce fuel and thus prevent an over fuelling condition from happening in the first place indeed most mechanical pumps with boost fuel enrichment should also be capable of doing so.