Fergus
New member
So should you do this for an hour every so often to clean out the pipes?
Engine at full revs?
- Thread starter Fergus
- Start date
pvb
Well-known member
I always tend to motor at high-ish revs. Doesn't seem to do any harm, and may indeed do lots of good.
Fergus
New member
I don't get much extra speed above 2000 rpm hence the question
vyv_cox
Well-known member
What Yanmar actually say in their workshop manual is: "If possible, periodically operate the engine at near maximum revs while underway. This will generate higher exhaust temperatures which will help cleaning out hard carbon deposits, maintaining engine performance and prolonging the life of the engine." I do this every two hours or so but rarely for longer than about ten minutes.
The 'Italian tune-up' is to run the boat pretty much flat out for half an hour. It will smoke heavily at first but gradually run cleaner. Many boatyards will carry this out for non-specific problems like 'it doesn't seem to be running as well as it did' and charge £100 for doing it.
The 'Italian tune-up' is to run the boat pretty much flat out for half an hour. It will smoke heavily at first but gradually run cleaner. Many boatyards will carry this out for non-specific problems like 'it doesn't seem to be running as well as it did' and charge £100 for doing it.
Tranona
Well-known member
You don't say what your engine is, but most modern engines max at 3200-3600, or even higher for some automotive derived engines such as the Perkins/Volvo MD22.
So, if you are reaching max speed at 2000 either your rev counter is way out or you are seriously overpropped (or possibly way overpowered).
If the engine is correctly sized to the boat then the optimum prop will achieve hull speed at close to maximum rated revs. You will then get a comfortable cruise speed at revs that equate to roughly 70% max power.
Fergus
New member
I re-engined (now vp d2-40) and have the same speed at same revs - get 7kn at 2k revs with which I am happy! Maybe overpropped but hey!
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Tranona
Well-known member
But hey! You should not be happy with that performance. You are probably building up big problems in the future by running the engine at such low revs and not loading it as it was designed. At those revs you are only drawing just over 25 hp but overloading the engine. You should be able to achieve at least 3000 out of the 3200 and a speed on a 41 footer of closer to 8 knots.
So, suggest you do the sums, or get somebody else to do them to fit the correct size prop. You may find feeding your boat's data into the Propcalc programme on www.castlemarine.co.uk a useful exercise to get a ball park size recommendation and then compare it with what you have. At a guess you probably need to come down an inch in diameter or 2 inches in pitch to get the revs up to where they should be.
These engines are designed to run at 70-75% power continuously and you do not seem to be doing that even as a maximum.
Lakesailor
New member
Engines have a sweet spot and most people will find and use it if the speed is what they are happy with.
However it is true that a bit of "de-coking" may be useful to clean out some deposits building up.
However do it sensibly.
Get the engine up to temperature and then have a high rev (not maximum) run, under load, for 5 or 10 minutes. Like Tranona says, 75% revs should be your target (2400rpm). Half an hour at maximum revs with no load is not right.
Whilst the accepted wisdom is that diesels are built to run at near maximum revs for long periods, the bedding-in period sets the running attributes of an engine. Higher than normal revs will induce some sideways con-rod float on the big end bearings, which if bedded down at lower revs may not suit the bearing surfaces.
Because of the higher compression loads in a diesel the big end bearings are very highly stressed.
In other words, it won't do a lot of harm, but may lead to shorter than possible engine life. Why do anything to shorten the engine's useful life?
However it is true that a bit of "de-coking" may be useful to clean out some deposits building up.
However do it sensibly.
Get the engine up to temperature and then have a high rev (not maximum) run, under load, for 5 or 10 minutes. Like Tranona says, 75% revs should be your target (2400rpm). Half an hour at maximum revs with no load is not right.
Whilst the accepted wisdom is that diesels are built to run at near maximum revs for long periods, the bedding-in period sets the running attributes of an engine. Higher than normal revs will induce some sideways con-rod float on the big end bearings, which if bedded down at lower revs may not suit the bearing surfaces.
Because of the higher compression loads in a diesel the big end bearings are very highly stressed.
In other words, it won't do a lot of harm, but may lead to shorter than possible engine life. Why do anything to shorten the engine's useful life?
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Fergus
New member
The engine runs sweetly -quiet and no smoke at 2k - uses 1.5l per hour - I'm not going to re-prop on that basis! It's a small 2 blade folding prop
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alahol2
Well-known member
I tend to run the engine up to full revs for 5 mins every time I leave the mooring (having allowed it to come up to temperature). This serves several purposes...
1) I hope that travelling at hull speed will wash off some of the silt that will have settled on the bottom of the boat.
2) The speed we get up to gives me a good idea of the state of the antifouling/fouling.
3) I hope it will 'clear the pipes' on the engine.
4) Any problem with the engine is likely to show up more readily.
5) I get to the harbour entrance a little bit quicker.
1) I hope that travelling at hull speed will wash off some of the silt that will have settled on the bottom of the boat.
2) The speed we get up to gives me a good idea of the state of the antifouling/fouling.
3) I hope it will 'clear the pipes' on the engine.
4) Any problem with the engine is likely to show up more readily.
5) I get to the harbour entrance a little bit quicker.
Icarus39
New member
Hi Fergus, I too have a new D2-40 on a 39ft boat - I run at around 2200RPM and no black smoke and engine runs sweet as a nut. From time to time I will run for a while at 2400-2500 but not for a long period. I also have a 2 blade prop but see a little more fuel consumption to you. I make about 6.5kts at this RPM - ideal for me.
Once I tried running at 2500RPM for a few hours - I spent the next hour alongside cleaning soot off the transom ... says it all. 2200 perfect.
For me, 2200RPM is 68% of max revs based on 3200 RPM max - fine
I work on commercial ships, the one I am on now runs diesels at 74% load with no problems day in day out ...
Tranona
Well-known member
You can't get away from the fact that if you cannot reach near full revs, you are overpropped and could do long term damage to your engine. If I had just spent several thousand pounds on a new engine I would want to use it as it was designed. A 2 bladed prop can be just as "big" as a 3 blade. Its "size" is determined by the diameter and pitch.
Tranona
Well-known member
Sounds like you are also overpropped. even running at full revs you should not get black smoke or soot on the transom. Getting it at 2500 means you are overfuelling the engine as the prop is putting too much load on it.
Icarus39
New member
I'm not so worried, I have the correct prop fitted by VP as was the engine. I can run right up to just under max revs which would not be the case if I was over-propped. I'm just a bit anal with my nice shiny boat ... soot to me is something your average person wouldn't even notice
I work in the marine industry, have done all my life and am particularly involved with engines under high load towing, all is ok for me
Tranona
Well-known member
I'm not so worried, I have the correct prop fitted by VP as was the engine. I can run right up to just under max revs which would not be the case if I was over-propped. I'm just a bit anal with my nice shiny boat ... soot to me is something your average person wouldn't even notice
I work in the marine industry, have done all my life and am particularly involved with engines under high load towing, all is ok for me
Sorry I was reading you could only get 2500. Sounds like it needs a good hard run at higher revs. 2200 is a good cruise speed for that engine. OK if you spend lots of hours, but running slow for just short periods like many people do can lead to coked up exhausts. The "Italian tune up" often referred to here really does work. I get no soot out of my 2030 at any revs despite it having done over 3500 hours, mostly cruising for several hours at a time at 2400 with the odd blast up to maximum.
Quandary
Well-known member
Our engine is a D1-30 with Volvo three blade folding prop. supplied as a package by Volvo to the builders of the boat. The engine is a marinised Japanese Perkins mainly used for static generators etc. I run it at just below 2000 revs. or just over if against wind or waves. It sounds sweet and very quiet, gets 6.2 to 6.5 kts. through the water and burns around 2 litres of fuel per hour. It likes going at that speed, it tells me so. Every 10 hours or so I push it up to about 2800 for 10 or 15 minutes. Occasionally, punching a strong tide I have run it at sustained 3000 revs. when it can achieve close to 8 kts. through smooth water, which seems to be almost above the theoretical hull speed, (On a good broad reach in flat water we can just manage the same) but it is noisy, the steps vibrate and it gets very warm and it then burns 7 litres per. hour.
It would take an an awful lot to convince me that it is overpropped or that running at around 2k. revs was not a good idea, though Volvo do advise running at 3000- 3200.
If the engine wears out a year or two earlier, the £5000+ worth of fuel I will have saved should go a long way toward the cost of a rebore and a set of bearings.
It would take an an awful lot to convince me that it is overpropped or that running at around 2k. revs was not a good idea, though Volvo do advise running at 3000- 3200.
If the engine wears out a year or two earlier, the £5000+ worth of fuel I will have saved should go a long way toward the cost of a rebore and a set of bearings.
Tranona
Well-known member
That sounds just right - as most Volvo saildrive boat combinations are. Your 2200 revs is producing around 20hp which is consistent with the 70% power rule of thumb. Your fuel consumption confirms this, and your prop also seems efficient based on the speed you are getting. You can reach full revs and just a bit over hull speed, so again just right. It means you can cruise comfortably and still have another 10 hp to deal with adverse conditions.
This is not the same as being unable to exceed 2500 revs at WOT as suggested in the example that started this off. The overpropping is disguised by the ability to cruise at low revs - which is no problem as the boat only needs less than 30 out of its 40hp to do that. However, the final 8hp or so can never be used because the prop is holding it back. If you do try to use it, all you get is overfuelling and black smoke. At the other end of the scale, you usually get far too much thrust at low revs making low speed work tricky. Overpropping generally makes little difference to fuel consumption as this is a direct function of power used.
When we replaced the saildrive in my boat the new one has the lower reduction designed for the engine you have which maxs at 3200 compared with my 3600. So I was on my own for props as Volvo don't have a recommendation for that combination. At the same time I was doing some testing of a number of props for a manufacturer. Mostly their recommendations were far too coarse pitch. In most cases I could get my 5.5 knots cruising at around 2000+/- (compared with my normal 2400), but as soon as WOT was tried they got nowhere near 3000, nor the 7.3 knots I normally get - just a load of black smoke and coarse running. On one run my tame engine man who spent a lifetime working for marine engine manufacturers advised we stopped because he was fearful of the damage comtinuous running like that would do to my engine.
The prop we ended up with is a 2 blade folder which gives virtually identical performance to the 2 blade fixed, except it peaks at 3400 instead of 3600. blades go in 1inch increments so the next size down would be too fine, so that is what we stuck with.
EuanMcKenzie
Member
not convinced with the theory of knackering an engine by not running it hard
Mine is 39 years old and is still fine
It is a Volvo MD21A and over sized - 50HP@3000 rpm for
It does 6 knots at circa 1800 RPM
It is sweet - It will die of rust and failure of the ancillaries long before it wears out. I do give it a short blast occasionally to blacken my transom but otherwise don't believe in the long term damage to your engine theory.
Have anyone actually worn out the rotating parts on their engine without doing long term cruising?
Mine is 39 years old and is still fine
It is a Volvo MD21A and over sized - 50HP@3000 rpm for
It does 6 knots at circa 1800 RPM
It is sweet - It will die of rust and failure of the ancillaries long before it wears out. I do give it a short blast occasionally to blacken my transom but otherwise don't believe in the long term damage to your engine theory.
Have anyone actually worn out the rotating parts on their engine without doing long term cruising?
pvb
Well-known member
I don't believe that a 28hp engine can possibly use 7 litres an hour.
Tranona
Well-known member
Think maybe you are misunderstanding the issue. Not suggesting that engines are run at or near max revs continuously. The rule of thumb of 70% max hp is a good one. The dangers are running with light loads at low revs for long periods leading to potential bore glazing and exhaust coking, and being unable to run at max revs because of overpropping (or overpowering).
In the example quoted, if you can only ever draw 30 or so hp because the prop is too big - why have a 40hp engine? As the other examples such as Quandry and Icarus confirm, correctly propped boats can achieve close to full revs if required and cruise comfortably at revs that normally equate closely with 70% max power, and also close to maximum torque and lowest specific fuel consumption.
Just look at the power curves of the engines if you want convincing.
