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

[lionelz]

OK where do I start.

We see on this forum many conversations about engines, best speed, fuel economy over prop, under prop etc etc.

These often cause some debate as people have many different opinions. However when people do discuss these they often use technical language that I do not understand. I saw a Link the Portofino put up a few days ago of an article explaining much. However having read it I still did not fully understand it all as much of it was in technical term. I have no mechanical knowledge whatsoever and do not go anywhere near my engine in fear of breaking it.:nonchalance: Having said that I have a desire to have a basic understanding and to try and get the best out of my engine and the most out of my fuel.

The one think that I often see is Latestarter saying is Propellers move boats not engines, so what I am going to do is lay a out a few questions/observations/statements in some very layman terms to see if what little I do know is correct or not. Can people then please point me in the right direction and agree of disagree. I understand there are many variables but for the sake of ease where needed everything is the same conditions and environment.

1. The rotation of the propeller moves the boat. The engine just turns the propeller.

2. every boat and engine configuration will have its optimum set up that works together.

3. to turn the prob the engine needs to have RPM but most importantly it has to use it Horse Power to turn it.

4. The bigger the prop the more HP is needed to turn it. and there for the

5. a small prop requires less HP to turn it

6. By over/under propping a boat, it effects the amount of HP that is need to turn the Prop the amount of revs the engine can produce.

7. By over propping the boat may move faster but More HP will be needed and you may not get full revs and fuel burn will increase.

8. By under Propping, the boat may move slower, less HP is needed however you will increase revs but reduce the amount of HP needed fuel burn will decrease.

9. The more HP the engine is producing the more fuel it will burn.

10. The higher the RPM the more fuel is burnt. although, again the is offset by the amount of HP the engine is needing to produce.

11. the amount of revs is not fixed to speed in the water and conditions may vary and neither is the HP being used. i.e. 3000 revs may give a speed from 18-22 knots using 150/180 HP depending of tide/wind and other factors.

12. The more the boat weighs the more HP is needed to move it. so therefore the boat will burn more fuel.

13. what is Torque?? what does it do?

14. what is the effect of the same boat set up but with a bigger engine, does it just make it go faster? does it burn more fuel? if both boats were in the same conditions travelling at the same speed, would the fuel burn be the same?

15. Stress on the engine is deemed by how much HP (in relation to the HP of the engine)it is having to produce not necessarily by its RPM.

I have many more strange and little questions, too many to put into this one post. Again thanks for any guidance/advice/pointing in the right direction. I am just asking so that I can try and follow some of the more technical conversations

Lionel

 

[kashurst]

I think you mostly have it right,
torque is the rotational force created by the engine, you need it to turn the prop
power is the ability to continually apply that torque. (power = torque x speed)

if you can imagine a rubber band powered aeroplane, when the band is wound up you can feel the propeller wanting to turn against your finger - that's torque. When you let the propeller go, the energy stored in the rubber band is released and the prop will spin for a 5 - 10 seconds - that is power, A boat engine creates high levels of torque all across the rev range, as the revs increase more power is available to drive a load/turn wheels/spin a propeller. So the engine can spin the prop faster and faster as revs increase.

As LS says, props move boats, to make the boat move the prop must rotate, as the prop turns it creates a sucking and a pushing force to try and move forwards through the water. Water is viscous and boats are heavy so the prop will be hard to turn - so you need a big daft engine to do it, that can provide the big rotational force to start turning the prop and sufficient power to keep turning the prop all the time.

With a planing boat you can put more and more powerful engine(s) in and with the right gearbox and props it will go faster and faster.

with a displacement boat, that won't happen, once you hit maximum displacement speed the bow will rise up and up and up and up until the stern is down in the water and you will sink.

 

[BruceK]

Basically right

regarding 14

14. what is the effect of the same boat set up but with a bigger engine, does it just make it go faster? does it burn more fuel? if both boats were in the same conditions travelling at the same speed, would the fuel burn be the same?

To put it simply as possible...

Not necessarily, all depends on prop and bigger diesel engines tend to turn slower turning bigger props diameter and pitch requiring more torque and not necessarily all about hp.

And

Internal combustion engines run at roughly 40% efficiency so a bigger engine is liable to be less efficient than a smaller engine at medium loads unless the smaller engine is being pushed very hard in which case the bigger engine will often surprise you at being more fuel efficient.

 

[CharlieAlpha]

Lionel you might benefit and enjoy a 1 day marine engine course so you have a better idea many sea schools run them as do SeaStart who I think of as the AA for motorboat owners.

 

[lionelz]

Lionel you might benefit and enjoy a 1 day marine engine course so you have a better idea many sea schools run them as do SeaStart who I think of as the AA for motorboat owners.

Have done one, thanks, it was ok in the sense that I know what things look like and how to change the parts, the one I done in Gosport was very sailboat engine orientated,

 

[jfm]

Lionelz, you've touched on loads of engineering and physics there and a full reply would be long. Broadly and directionally you are along the right lines.
It's worth understanding the dimensions: torque is a rotational force; its dimension is newton metres. If I put a spanner on a seized nut and stand on the spanner, I'm applying a torque but I am doing no work and developing zero horsepower.

The twisted elastic band stores energy, measured in joules (fwiw). There is zero direct correlation or relationship between torque and energy; they are dimensionally different, like metres and kilos. The energy you have on your boat has nothing to do with the engine and everything to do with the amount of fuel you are carrying (the twisted band and the structure around it also produces a torque and a power output, so in that sense is analogous to an engine and a fuel tank, rather than just a fuel tank)

HP is the amount of work done/energy converted per time unit, per second. A 1HP engine will get my car up a hill but take 15 minutes, but a 600hp engine (aotbe) will do it in 1.5 seconds. There is therefore a direct relationship between HP and energy, in that HP is (dimensionally) energy per second. AOTBE the 600hp engine burns 600x the litres per hour of the 1HP engine of course - that is precisely what a big engine is meant to do - burn more fuel per second and output more kinetic energy per second energy

The "props move boats not engines" adage is a bit noddy and LS knows that and says it with a bit of tongue in cheek. It's an adage worth remembering because your boat wont work well unless it has the right props, but don't take it more seriously than that, otherwise you'll be saying the wheels move your car, which is equally true.

So a few comments on your list:
/3 - you need torque to turn a prop, not power. you only need power if you want to turn it fast. Remember, power is a "per second" value, whereas torque has no time in its dimensions so a mouse can turn a propeller if given a long enough lever and all day to do one revolution, but it takes big power to turn the thing at 1000 rpm or whatever
4+5, only true if you mean turn it at the same speed
7 No in practice, because the engine just wont make the extra power as the governor limits the fuel injected per firing stroke
8 Yes
9 Absolutely yes, if you add "per second" to the end of your sentence. Making more HP is all about burning more fuel per second
10 no not at all, unless you assume AOTBE ie the torque is roughly the same. A good example to think of is a standard genset that ALWAYs runs at 1500rpm. If the electrical load increases because you turn on the kettle, the rpm stays 1500 but the engine makes more HP because more fuel is injected per firing stroke to increase torque. Torque increases, rpm stays constant 1500, HP increases
11/ exactly correct (except tide has no impact). Factors that would affect speed and fuel burn at a fixed 3000rpm would include weight, hull dirt, trim, wind, wave resistance, etc
12/ Yes, if you assume a constant speed
13/ see above
14/ depends. You gotta define the question better. If you assume same speed and same engine friction and other friction/drag, then aotbe the fuel burn rate in the big and small engines will be identical. Alternatively, if you run both engines WOT then the big engine will make the boat go faster, burn more fuel per second, but identical fuel per mile apart from the fact water and air drag will increase the fuel per mile somewhat (but not a huge amount in the 20-40 knot speed range)#
15/ Broadly correct just for the power producing components, but this is a deep topic. Piston deceleration forces at the bottom of the stroke depend on RPM not fuel burn, for example, and many internal engine components do not see the power output at all as they are internal components (eg pumps, camshaft, valve gear) whose stress depends entirely on RPM and not on HP being developed. Big topic really.

Sorry for the level of detail above - I want sure what you were asking for or expecting

 

[lionelz]

JFM thank you very much and to everyone that has commented At least I am traveling in the right direction on understanding the very basics. Which is what I am after. That's actually been very useful to me

 

[BruceK]

I respectfully disagree with JFM's answer to 14 as being over simplified, not incorrect, just AOTBE is too big an ask. It's like quoting Newtons 1st Law of motion in this equation for the mass of boat but ignoring it for the inertial mass of the engine. But would concede that theoretically if both boats are equal then the amount of energy required to drive them would be the same hence the calorific value of fuel burn would be the same. However the inertial mass of the rotating parts of a bigger engines mandates significantly more energy spent and therefore more fuel inefficient. This of course is for the same revs and same prop on both. Nothing is ever that simple though. Smaller engines rev higher, have more gearbox reduction to develop similar HP and then drive smaller props, bigger engines rely more on torque to achieve the same values at lower revs with bigger props. So sizing here has an impact. For argument's sake, put a 1000cc 3 cyl engine in the average family sedan for eg a Mondeo, a 1600cc 4cyl and a 2600cc V6 engine. You'd expect average fuel stats to show the fuel efficiency mpg to be smallest through to largest displacement given my argument AOTBE but I think we'd all agree the 1600 wins followed by the 2200 and the 1000 is last because it is overworked. Only in theory would they all be identical.
Nor do bigger displacement engines make for a faster boat. Again within balance an engine capable of revving higher turning a smaller diameter but higher pitch prop will go faster unless you start playing with gearbox reductions again to convert torque to rpm. Nothing is ever simple

 

[jfm]

Yep, not disagreeing. I suppose I'd say you gotta answer the theoretical question on an AOTBE basis then adjust for reality, but if you start allowing for reality where do you stop? Especially if you're trying to explain the basic concepts. Tricky one - no worries!

All that said, one quick comment on your post: the high inertial mass of a massive engine doesn't burn much more fuel. the reciprocating parts cancel each other out (ie the deceleration of one piston accelerates the others) and the kinetic energy in a massive spinning crankshaft is a small quantity and is fixed, and comes back in the overrun. The extra internal load of a big engine is just lots of friction and add ons (bigger water pump, bigger gear box, bigger valve springs whose energy isn't generally recovered, bigger alternator), but it isn't mass

And I'm not sure the 1000cc Mondeo would be last - if you ran the cars on an identical run with the same starts/stops/hills etc I'd expect the 1000cc to use the least fuel - "over worked" is a bit of a bar room concept tbh

 

[Portofino]

Q14
14. what is the effect of the same boat set up but with a bigger engine, does it just make it go faster? does it burn more fuel? if both boats were in the same conditions travelling at the same speed, would the fuel burn be the same?

Depends on which engine fits the prop curve the best ie it's rated Hp is nearer the max and it bisects it ( prop curve ) at its rated rpm .
The one (within reason *) that fits the prop curve the best will likely have a lower or not far off GPH
The one that can not meet its optimal Hp likely to be the " small" is most likely overloaded or heading that way and most likely have a bigger GPH , to go the same speed .
It's not the available Hp -it more like how much actual HP is used to Turn the prop -to keep speed the same .

* comparing 7or 9 L 6 cylinder with a 12 or 13 L - 6cylinder --not with a 24L v12

 

[petem]

Good thread Lionel (and replies).

 

[Portofino]

Q15 "stress"
Petrol is volatile most oil isn,t .Compress air enough at nearly 2x as much as a petrol engine and it gets hot ,real hot -inject oil ( you choose -chip fat , veg oil , peanut -or diesel ) it ignites - wth greater heat energy than a petrol .
This excess heat s a mechanical killer .
Too much and it kills a diesel engine -key is controlling it ,keeping it within safe tolerances of the various metals .
If you overload a marine diesel engine trying to go too fast with a boat prop pitched on the edge ( too big for marketing reasons -to get a nice top speed when new and shiney hull , with no cruising stores and 1/4 tanks ) too heavey , too much fouling -Hull and prop -then the prop curve and optimum rpm are unable to meet -fuel /air ratios go to cock EGT rises -

which is real " stress". I think you mean The engine is unable to meet its rated rpm -prolonged it will shorten the engine life .So "stress" if that what you mean? -is related to rpm @ overload

In a road vehicle you can change gear down as you feel the engine labour .In a boat you cannot it's one gear -prop pitch -
Here's something I found from the tinternet below ---

We've already mentioned that excessive EGT can cause engine damage or turbocharger damage, but let's get more specific. Which parts will fail first is a matter of the design and materials used in the various parts of the turbo-diesel, but usually it starts with the turbocharger. Under sustained excessive EGT, the square corners at the outer ends of the vanes, where the material is thinnest on the turbine wheel, can become incandescent and then melt, resulting in a rounding off of the square corners. If you or your mechanic finds this indication before anything more serious happens, consider yourself very lucky, because shortly after the tips melt, the turbine wheel goes out of balance and wipes out the turbocharger bearings, which may or may not result in shaft failure and destruction of the turbine and compressor wheels. Excessive EGT can also erode or crack the turbine housing. In extreme cases, high EGT can drive the turbocharger into an overspeed condition that exceeds the designed operating speed due to the additional heat energy. When this happens, either the turbine wheel or the compressor wheel may burst. If the turbo doesn't go first, excessive EGT, if sustained, will damage the pistons. Such damage can include piston deformation, melting, burning, holes, cracking, etc. This damage is cumulative, so if you slightly burn a piston top, the engine may continue to run without problems, but the next time you run excessive EGT more damage may be done, and so on, until failure occurs. Piston failure can be catastrophic -- that means very expensive. At a minimum, an engine overhaul will be required, and that too is expensive. Excessive EGT can also cause exhaust manifold and cylinder head cracking. Exhaust valves can fail from high EGT as well. Among the first engine parts to suffer damage will be those made of aluminum since aluminum has a lower softening and melting temperature than steel or cast iron. Diesel pistons are aluminum, and a growing number of diesels also use aluminum cylinder heads.

So the big question is, what constitutes excessive EGT? If everything is working properly, 1250º to 1300º F. is a safe turbine inlet temperature, even for sustained running, mile after mile. Above 1300º F. things can start to get edgy. Remember, excessive EGT damage is cumulative. Over 1400º F., you're usually gambling against a stacked deck and it's only a matter of time until you lose. The higher the EGT, the shorter that time will be.

 

[[2068]]

So why are incredibly expensive marine diesels not fitted with EGT sensors / alarms?

 

[MapisM]

11. the amount of revs is not fixed to speed in the water and conditions may vary and neither is the HP being used. i.e. 3000 revs may give a speed from 18-22 knots using 150/180 HP depending of tide/wind and other factors.

11/ exactly correct (except tide has no impact). Factors that would affect speed and fuel burn at a fixed 3000rpm would include weight, hull dirt, trim, wind, wave resistance, etc

I think you are a bit at cross-purposes on that one.
In fact, on one hand I suspect that while Lionel wrote "speed in the water" (and jfm was surely thinking of speed though the water when he said that tide has no impact), he actually had in mind speed over ground.
Regardless, even assuming to focus on STW and not SOG, in principle it is NOT correct to say that at 3000 (or whatever) rpm the speed can vary depending on weight, wind, etc.
It's the power (hence the fuel burn) required to keep the props spinning at 3000 rpm that can and does change, but for any given hull/props/rpm, the STW remains constant.
Actually, there are factors which, on top of demanding more power for a given RPM, can also reduce the STW (typically, fouled propellers for instance), but that's because also the prop slip can be affected - which is a different matter altogether.

 

[Portofino]

So why are incredibly expensive marine diesels not fitted with EGT sensors / alarms?

They are perhaps read this

http://www.ybw.com/forums/showthrea...gine-revs-at-max-load-Governor-or-props/page6

And LS1 asked the Q re VP in here too
Suspect it's an " internal " issue regarding not wanting to bite the hand that feed you to manufactor not wanting to 8iss ff the client -boat builder, who,s trying to sell boats and nervous when Dave Marsh takes a new boat out for a mag test ?
BTW I was trying to be polite with "glass 1/2 full 1/2 empty "quip in my initial analysis ,of the data -the fuel data gives the game away
http://www.ybw.com/forums/showthread.php?457327-Volvo-D9-575-Full-revs-fuel-usuage-real-world

 

[Portofino]

I think you are a bit at cross-purposes on that one.

Regardless, even assuming to focus on STW and not SOG, in principle it is NOT correct to say that at 3000 (or whatever) rpm the speed can vary depending on weight, wind,

he,s right read this go to
"think and think again " the pic of the skier at the bottom - V important can lead to overloading -I suggested it to Bart - on this hard top thread -( all in all the mods over his owner ship not just rhe proposed HT ) bit nobodys listening -or maybe just waking up
http://www.sbmar.com/articles/understanding-low-power-troubleshooting/

 

[jfm]

I think you are a bit at cross-purposes on that one.
In fact, on one hand I suspect that while Lionel wrote "speed in the water" (and jfm was surely thinking of speed though the water when he said that tide has no impact), he actually had in mind speed over ground.
Regardless, even assuming to focus on STW and not SOG, in principle it is NOT correct to say that at 3000 (or whatever) rpm the speed can vary depending on weight, wind, etc.
It's the power (hence the fuel burn) required to keep the props spinning at 3000 rpm that can and does change, but for any given hull/props/rpm, the STW remains constant.

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.

 

[jfm]

he,s right read this go to
"think and think again " the pic of the skier at the bottom - V important can lead to overloading -I suggested it to Bart - on this hard top thread -( all in all the mods over his owner ship not just rhe proposed HT ) bit nobodys listening -or maybe just waking up
http://www.sbmar.com/articles/understanding-low-power-troubleshooting/

This is kinda weird Portofino. You say I'm wrong, then post a link to an article that actually agrees with me and disagrees with the MapisM words that you quoted. No worries

 

[BartW]

........-I suggested it to Bart - on this hard top thread -( all in all the mods over his owner ship not just rhe proposed HT ) bit nobodys listening -or maybe just waking up

Don't worry PF, I take alle comments here seriously !
but don't alway's react as I have nothing knowledge-able to add,
But I have to admid beeing from abroad, that I don't alway's understand your english wording, you must have a very strange local dialect

as regards to the engine load in my old tub,
I have a gut feeling that the big upgrades in my boat during the years have not changed much on the engine load,
at least not in the same amount as say a completely full tank, versus empty tank.
the indicators I have; performance / smoke / fuel burn / cooling water temp, ... have'nt change noticeable
and these are clearly noticeable with a full tank !

now after learning about engine load and EGT in the thread you are referring to,
after adding the bladder tanks (2000l extra fuel) I'm a bit worried,
so plan is to add EGT gauges (next winter)
also just because of freekyness
for the time being I will use her very gentle, that was the plan anyway after the 2 x engine rebuild,
and for sure when 7500l of fuel onboard !

 

[Portofino]

This is kinda weird Portofino. You say I'm wrong, then post a link to an article that actually agrees with me and disagrees with the MapisM words that you quoted. No worries

no "he " is you -excuse my carp English

weight kills speed - if overloaded the rpm stays the same -boat slows -but EGT -load and GPH shoot up .
those that have the sensors to record this see this - can see this, and make throttle adjustments or should .

Its the prop slipping more -that one can not tell -there is no prop slip gauge -so poor diesel engine gets thrashed -unknowingly to the operator .
In a vehicle you can change gear when you sense the engine labouring .
this partially explains why a road diesel might make 1M km,s and a the same block in a boat dies @ 700 -1400 hrs . EGT too high for too long .

Another boaty bit is the SWAC sea water air cooler - (road vehicles do not have one ) if this starts to clog - (both sides - scale in the seawater side or/and oil grime in the air side) -then the charge air temp risers and the air going in is less dense -so power is less and the ideal match of HP on the prop curve is lost + tendancy to open throttles more = more unseen overloading = shorter life span