BMC / Thornycroft 108, Working rpm's

[wrisberg]

Just getting more and more hours on my boat, with the new engine. The old BMC 1800 had its operating rpm at, 2900rpm. With a maximum of 3200.

 

My newly installed / rebuild engine, seems to be waaaay from that. The marinisation is done propper, on this one, so thats okay.

 

I have been cruising for a couple of weeks, adding a little more throttle on each trip. Now I have found my "sweetspot" I Guess.

 

At 1800rpm, it sounds nice, and sails nice, around 7 knots. The maximum at WOT is 2300rpm - I would have thought more, but I have changed the propeller also. in Neutral it easily revs to 3500.

 

Does this sound okay to you guys?

 

Any of you have had this engine and remember you cruising rpm's ?

 

 

Kind regards

 

Jonas

[Flyboy]

It sounds like you have changed to a larger/higher pitched prop. What size prop. did you have before & what is it now?

 

Is your tachometer calibrated correctly?

Edited April 25, 2016 by Flyboy

[Richard T]

On our narrowboat we cruise at between 1500 and 1800 rpm going up to 2100 against the flow on rivers for short periods.

[Tony Dunkley]

Just getting more and more hours on my boat, with the new engine. The old BMC 1800 had its operating rpm at, 2900rpm. With a maximum of 3200.

 

My newly installed / rebuild engine, seems to be waaaay from that. The marinisation is done propper, on this one, so thats okay.

 

I have been cruising for a couple of weeks, adding a little more throttle on each trip. Now I have found my "sweetspot" I Guess.

 

At 1800rpm, it sounds nice, and sails nice, around 7 knots. The maximum at WOT is 2300rpm - I would have thought more, but I have changed the propeller also. in Neutral it easily revs to 3500.

 

Does this sound okay to you guys?

 

Any of you have had this engine and remember you cruising rpm's ?

 

 

Kind regards

 

Jonas

 

Off load maximum rpm is adjusted and fixed by the settings made to the hydraulic governor built into the DPA type injection pump used on these engines.

 

If an engine in good condition, with a pump correctly calibrated to give full rated power output, can't achieve within 200 - 300 rpm of governed off-load rpm when under full load, then it is being overloaded.

 

In the case of a boat engine the overload can only arise from two possible sources; either excessive frictional resistance to rotation/power absorption within the gearboxes and sterngear bearings due to a mechanical defect or misalignment, or an oversized propeller, ie. excessive diameter and/or pitch for the horsepower and torque available at the gearbox output coupling.

 

Rpm figures, maximum,cruising or whatever else you want to call them, for similar engines in other vessels have no influence or relevance to the way your boat and engine performs.

Edited April 26, 2016 by Tony Dunkley

[wrisberg]

On our narrowboat we cruise at between 1500 and 1800 rpm going up to 2100 against the flow on rivers for short periods.

 

Sounds like I am in the right range then.

 

 

 

#2 Flyboy

Tachometer is calibrated.

Changed to a 14x16, from a 10x16 propeller

[Flyboy]

 

Sounds like I am in the right range then.

 

 

 

#2 Flyboy

Tachometer is calibrated.

Changed to a 14x16, from a 10x16 propeller

It looks to me like you may have overdone it with the larger prop. I guess you are using a 1:1 ratio gearbox.

[wrisberg]

It looks to me like you may have overdone it with the larger prop. I guess you are using a 1:1 ratio gearbox.

 

 

It looks to me like you may have overdone it with the larger prop. I guess you are using a 1:1 ratio gearbox.

 

It's a 2.51-1 ratio.

 

1800 should be the range, right ?

[Tony Brooks]

Whatever you call "the range" is specific to you and your boat as Tony D pointed out.

 

You seem to have lost maximum in gear revs because the prop is a bit over pitched. However (from memory on other threads) you seem to be able to run at pretty much your hull design speed. You also seem happy with the cruising revs.

 

To my mind the boat is not so over propped that you are Ilkley to run into charging or overheating problems so be happy with it.

[wrisberg]

 

Whatever you call "the range" is specific to you and your boat as Tony D pointed out.

 

You seem to have lost maximum in gear revs because the prop is a bit over pitched. However (from memory on other threads) you seem to be able to run at pretty much your hull design speed. You also seem happy with the cruising revs.

 

To my mind the boat is not so over propped that you are Ilkley to run into charging or overheating problems so be happy with it.

 

 

I was meaning, " the range " within the engine are designed to work - between 1800-2200, with 2200 as maximum.

 

Would it make more sense, to prop down a bit and reach like 2800, with 2500 is the operating rpm ' s ?

[Tony Brooks]

 

I was meaning, " the range " within the engine are designed to work - between 1800-2200, with 2200 as maximum.

 

Would it make more sense, to prop down a bit and reach like 2800, with 2500 is the operating rpm ' s ?

 

I seem to recall that the manual for the 1.5 had the maximum revs as 4400 so 2200 seems a bit low to me as the maximum revs. However as you will almost certainly be speed limited by wave making and you seem to be pretty much there without making black smoke I do not see any problem. If you can go slow enough for your needs on idle, can reach the hull design speed and the engine sounds OK it would seem to be fine.

 

That is not say the going for a smaller prop and thus higher revs for a given speed would not do wonders for battery charging.

 

Think about that engine in a van. It would be fairly low geared so drivers would thrash it along main roads revving far faster then 2200 or 2800. The engines never were designed for marine use, they just proved to rather good at it and also robust. I think you are worrying over nothing.

Edited April 29, 2016 by Tony Brooks

[wrisberg]

Thanks for your reply's

 

I have no problem charging, not at all.

 

I'll stick to the current configuration.

 

 

Only problem, that isn't really a problem is the temperature rises when idling, not overheating, but rising a lot.

 

 

thanks for this great forum!

 

Cheers from denmark

 

Jonas

[Tony Brooks]

Thanks for your reply's

 

I have no problem charging, not at all.

 

I'll stick to the current configuration.

 

 

Only problem, that isn't really a problem is the temperature rises when idling, not overheating, but rising a lot.

 

 

thanks for this great forum!

 

Cheers from denmark

 

Jonas

 

I assume that this is a wet exhaust direct raw water cooled or heat exchanger engine. If so that points towards low raw water flow that is turn suggests air leaks on the inlet run to the raw water pump, a worn, faulty or undersized raw water pump, or just possible a blockage downstream of the raw water pump. I assume you descaled the exhaust mixing elbow when you overhauled the engine and also cleaned and "rodded through" the heat exchanger and core.

 

I am more inclined towards air leaks or pump problems.

[wrisberg]

New raw water pump, Jbsco, New impeller, PLENTY of flow when idling. Its a solid string of water at 1200rpm, from the exhaust.

 

Everything cleaned when refitting.

 

brand new mixing elbow

 

Air leaks or pump . . . yeah, it could be. - but it's unlikely i think

The Engine starts right away, and doesn't give either white or black smoke, when cold, or when full loaded.

No smoke, at all. It settles on the rpms instantly when taking the gas of. No leaks or anything other. It runs as smootht as I could imagine a engine from that time to run.

If it was to lean, it would heat up when running with higher rpm's, right ?

 

Maybe I'll just have to live with it. It doesnt overheat, it settles at like 90-95.

 

It does push a little coolant out when idling for a long time though, I have a hose from the cap to a container. Then I pour it back, every once in a while . . . . its like 0.05 - 0.1 liters. a little cupfull. If i doesnt pour it back, the running temperature is a bit higher, not 80, but 85 degrees. >then I can tell if its time to pour some back on. It only uses / pushes this out If I have been in idle for a long period . . . fishing or something. So I am not that concerned over it. A car has a expansiontank for this, this engine don't, so it must be normal i guess ?

 

 

Jonas

Edited May 13, 2016 by wrisberg

[Tony Brooks]

Without knowing which Jabsco pump your engine is fitted with one can't be sure about the pump's bearings and sealing arrangements - if any.

 

If the seal behind the impeller/wear plate is worn they can suck air down the shaft. If its a plain bearing pump and its worn it can suck air down the pump. If the end cap, wear plate or pump body that is behind the impeller is worn the pumping volume will be reduced. This usually shows up at low revs.

 

If yours is a standard heat exchanger engine then it does have an expansion tank, just like old cars. You should not fill the fresh water side to more than 25mm below the filler neck. That gap is for expansion and on a heat exchanger engine it shoudl be enough but there is nothing to stop you fitting an expansion tank.

 

If its direct raw water cooled it dos not need an expansion tank.

 

If you get a pressure cap with a rubber seal right under the cap part and make sure the filler neck is in good condition then if the little overflow hose runs right to the bottom of your container when the engine cools it should suck the coolant back into the manifold.