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kelandmads

bmc 1.8 running revs

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Hi new to this forum and hope I can get some help.I have a 70ft narrowboat with a BMC 1.8 I need to run it at around 3000rpm to get any real momentum does this seem high.

Cheers

Kel

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Ridiculously high!

Are you sure whatever you are using to measure RPM is reading correctly ? My 1800 I doubt will rev to that when it is in neutral!

 

What RPM does your rev-counter show when idling (should be 650 to 700).

 

Unless it is very deep draughted or poorly designed, the fact that the boat is a full 70 feet will not mean it needs hugely more power than in a somewhat shorter boat. Our 50 foot leisure boat with a BMC 1800 reaches good cruising speeds by no more than 1500 RPM, and on most canals if pushed to anywhere near 2000 RPM would be flying with a heavy breaking wash.

 

If it is genuinely 3000 RPM, then something is I'm sure wrong, like a prop that isn't matched to the gearbox reduction, or the box itself is slipping, (as possible examples).

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Hi new to this forum and hope I can get some help.I have a 70ft narrowboat with a BMC 1.8 I need to run it at around 3000rpm to get any real momentum does this seem high.

Cheers

Kel

Something's wrong

 

Either your revcounter is completely b*ggered

Or it's not been fitted correctly

Or you're seeking innappropriate 'momentum.

 

 

How many waterskiers are you towing?

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Could be something simple like a badly fouled propeller, large build up of debris in front of the rudder etc. I would also check that your not pushing along something substantial under the bow below the waterline. Can you see lots of prop wash behind the stern? Is it less than normal?

 

After checking the above I would check the drive train, prop. shaft coupling, gearbox clutches slipping (check oil level), it's also possible but unlikely that the prop. key has sheared and is slipping on the shaft taper.

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Hi, we run a 1.8bmc in our 40ft NB and have just fitted an axiom prop to suit and normal cruising revs are 1800 to 2200, this doesn't seem to be overly working the motor and gives us good fuel economy, previously the boat was over propped and would cruise at 1200 revs but be traveling too fast at tickover.

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Some who has fitted a Axiom to do slower


Hi new to this forum and hope I can get some help.I have a 70ft narrowboat with a BMC 1.8 I need to run it at around 3000rpm to get any real momentum does this seem high.
Cheers
Kel

I am surprised that a BMC diesel will do 3000 rpm regardless of what its trying to drive, is that flat out of do you still have a few revs in hand? I suspect your rev counter

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It will rev to 3500 quite easily but makes far too much speed and wash, tickover is 800 revs on the rev counter, it seems to read ok ?.

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Hi, At 3000rpm your engine would sound like it is trying to self destruct, on idle in neutral, what does your rpm counter read?(should be around 500rpm). I would guess full throttle in neutral would not exceed 2600 to 2800 rpm if rev counter is correct. At idle in gear 2mph should be achieved,if not a fouled, slipping propshaft or slipping gearbox would be where to look next. If you can access the prop and observe the speed of it from idle to full revs it may give a clue to what is happening. You do not state if you have just purchased the boat or if the problem has developed,this info would help.

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Our 62ft heavily built boat reaches a comfortable cruising speed at 13 to 1400 revs on the canal, lees with the stream on the Soar and few more revs against it. The only time I've had to go above 2000rpm was when reversing out of a berth into the flood stream at Sileby. So 3000rpm is far too high!!

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Could be something simple like a badly fouled propeller, large build up of debris in front of the rudder etc.

Without a badly fouled prop many BMC 1800s would struggle to get close to 3000 RPM in gear.

 

With a badly fouled prop - not a chance!

It will rev to 3500 quite easily but makes far too much speed and wash, tickover is 800 revs on the rev counter, it seems to read ok ?.

A BMC's tick-over shouldn't actually be as high as 800 RPM normally - as I said the specified range in the operators manual is 650 to 700 RPM.

 

So if idele is actually cirrectly set, you tachometer could be reading 23% to high. If that were the case, when reading 3,000 RPM, the real figure might be more like 2,440. Still high but not as outrageous as 3,000!

on idle in neutral, what does your rpm counter read?(should be around 500rpm).

No it shouldn't !

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hi

thanks for all the replys.i have ditched the butties and the waterskiers.

the speed I am trying to go is to keep up with people walking.

the engine ticks over at 1200 rpm(tried reducing it but the engine shakes)

gearbox is a prm 160 oil is a little high if anything.

i have checked prop for damage / debri etc and all is clear(approx. 20" prop)

I can see the propshaft turning but do not know how fast/slow its going.

there are still some revs left in hand.

I have had the boat a while but not gone anywhere until recently and it does seem to be getting worse.

cheers again

kel

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Almost definitely the rev. counter.

 

It appears to be reading double what it should.

 

Now in the dark recesses of my (car) mind I have a thought that some rev counters can be set for two,four, six or eight cylinders

 

May or may not be relevant.

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Almost definitely the rev. counter.

 

It appears to be reading double what it should.

 

Now in the dark recesses of my (car) mind I have a thought that some rev counters can be set for two,four, six or eight cylinders

 

May or may not be relevant.

 

Most modern rev counters are actually driven from the "W" connection on the alternator, and are adjusted according to number of poles on the alternator, and the pulley ratio between crankshaft pulley and alternator pulley.

 

Ours has an (about) 6 position switch to set a range based on those parameters, but then has a further fine tuning of a potentiometer to get the calibration completely correct.

 

If the rev-counter says the engine is doing 1,200 RPM at tick-over, but reducing that any further results in lumpiness, then I agree, on the face of it, it could be reading roughly double the correct number.

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Is a rev counter really needed on a BMC 1.5 or 1.8 diesel engine in a boat ? I've managed without one on my boat for 26 years although I'm from an era that associated rev counters with go faster stripes , leather rimmed steering wheels , seat covers with padded sides etc.

I don't believe the vans that these engines were fitted to had rev counters so what advantage is there with fitting one in a boat . Seems to me to be another thing to go wrong as appears to be the case here . Water temperature guage , oil pressure guage and ammeter I wouldn't be without but thats enough guages for me to look at .

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I did say it was in the dark recesses of my mind.

 

Not a good place to go.sad.png

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I don't believe the vans that these engines were fitted to had rev counters so what advantage is there with fitting one in a boat .

The most useful thing about mine is actually the hour counter part, which is the main reason I fitted one.

 

OK I know you can buy hour counters on their own, but the incremental cost of having one in a tachometer was not a lot higher.

 

Well it was a good theory, but already for the brand I went for, (Faria),the hour counter has jumped forward without explanation on two different examples. I have now had two replacements, so am on my third, but it is well out of warranty now, so if it happens again I would say I have to live with it. (The tacho continues to work faultlessly).

 

A tacho can be useful if you think "am I thrashing this more than usual", because sometimes when you actually look down, the speed you are actually running the engine at may be significantly different from the one you think you are running it at.

 

But not essential certainly, and I don't have one on our older boat, (but have recently fitted a mechanical hour counter).

 

An hour counter allows you to easily know when the next engine service is due, without having to actually record how long you cruise each day, (or run the engine for for any other purpose).

  • Greenie 1

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Whilst not arguing with the general nature of the replies, I do think the revving capability of a (good) 1.8 is being maligned. Calcutt's online manual quotes maximum governed revs are 4900.

Arthur

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I tend to agree with that, out of

gear a quick blip of the throttle shoots up to 4000 revs with ease, I worked on these engines in cars and vans for my sins and they were no donkeys then !!!!!!

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Agreed here too. My 1.8 will easily rev past 3500 in gear and there's been times I've needed to push past 3000rpm to punch tides / currents. I just have to watch the engine temperature creeping up though.

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my bmc 1.5 ticks over too fast and I have to go in and out of neutral when passing moored lines of boats.

how and what can I adjust to slow the motor down?

roddy

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6 minutes ago, roddy brooks said:

my bmc 1.5 ticks over too fast and I have to go in and out of neutral when passing moored lines of boats.

how and what can I adjust to slow the motor down?

roddy

 

Follow the cable that comes from the speed control lever (the other cable goes to the gearbox)

 

This cable will link to a lever on the engine. First thing is see if you can find the stop for the lever - does the lever go back against the stop at idle? If not, you need to adjust the cable

 

If it does, it's the stop that needs adjusting

 

Don't be surprised if the engine starts performing like a bag of spanners - these engines are not great at low idle speeds

 

Richard

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