Speed wheel and compression advice

[Proper Job]

It's all part of the public service

[flatplane8]

I think it was a reasonable system in its day for engines that ran at fixed speed, less so where variable speed/load is required. To be 'correct' one could be up and down to the engine room constantly depending on what speeds were currently required. We generally cruise at 750rpm-ish so the valves should be screwed in. But coming up to a lock 1000rpm may be needed in reverse, so in theory the valves should then be wound out to low.

 

I guess the pragmatic way is to set the valves for the expected majority of the load and go from there.

[by'eck]

I think it was a reasonable system in its day for engines that ran at fixed speed, less so where variable speed/load is required. To be 'correct' one could be up and down to the engine room constantly depending on what speeds were currently required. We generally cruise at 750rpm-ish so the valves should be screwed in. But coming up to a lock 1000rpm may be needed in reverse, so in theory the valves should then be wound out to low.

 

I guess the pragmatic way is to set the valves for the expected majority of the load and go from there.

 

That's what I do. Worst case scenario is a lock intensive part of the Thames whilst travelling upstream on low comp. Lots of upturned noses from the big stinkpot owners as you pull out of a lock in a haze of smoke.

[Proper Job]

 

That's what I do. Worst case scenario is a lock intensive part of the Thames whilst travelling upstream on low comp. Lots of upturned noses from the big stinkpot owners as you pull out of a lock in a haze of smoke.

 

Sounds familiar

[FadeToScarlet]

I think it was a reasonable system in its day for engines that ran at fixed speed, less so where variable speed/load is required. To be 'correct' one could be up and down to the engine room constantly depending on what speeds were currently required. We generally cruise at 750rpm-ish so the valves should be screwed in. But coming up to a lock 1000rpm may be needed in reverse, so in theory the valves should then be wound out to low.

I guess the pragmatic way is to set the valves for the expected majority of the load and go from there.

Compromise with one pot on high, and one on low?

[by'eck]

Compromise with one pot on high, and one on low?

 

But imagine the quandary with a JP3

[George94]

 

But imagine the quandary with a JP3

 

When feeling lazy I will just set the starting cylinder to high compression and leave the other two on low. After a minute or so, that cylinder is set to low compression too. My engine is hand-start.

 

Works a treat.

 

On the tidal Thames I always use low compression because I know that at some time I will need full power.

[MtB]

 

But imagine the quandary with a JP3

 

1½ pots on high, 1½ pots on low. Easy. Where's the quandry?

 

MtB

[frangar]

Just in case anyone is tempted to think that the posts about running one in high and one in low are not in jest....please don't try this at home!....the unbalance in the forces won't be very good for the well being of your engine!

 

Cheers

 

Gareth

[Proper Job]

The unbalanced force at the helm is the worst thing for my engine

[frangar]

The unbalanced force at the helm is the worst thing for my engine

Very good!

 

Cheers

 

Gareth

[Proper Job]

What the manual states about the High/Low compression valves:

 

Compression ratio Change-over valve (JPM only)

This is a device to give a high compression for starting, and a lower compression for normal running

The valve screwed “IN” gives high compression and “OUT” gives low compression.

Do not run under heavy load with high compression.

For long runs at half load or less, use high compression.

Always see that the Valve is either Fully in or Fully out.

Keep the screw thread clean and bright

 

For continued slow running over long periods at speeds below 850 r.p.m. the Change-over Valves should be screwed in. (JPM only)

 

 

I can't see how a JP3 will ever operate in excess of half load and above 850 rpm on canals

Even a JP2 might struggle be be at half load, > 850 rpm for the majority of canal work.

[Timleech]

I can't see how a JP3 will ever operate in excess of half load and above 850 rpm on canals

Even a JP2 might struggle be be at half load, > 850 rpm for the majority of canal work.

 

 

If it's a propulsion engine driving only a propellor and nothing else, with full load at 1200 rpm, then 850 represents more like one third of full load. If full load is at 1000 rpm, it's nearer two two-thirds (0.6x).

 

Tim

[Proper Job]

Engine load is not necessarily relative to engine speed.

 

A JP2 throwing a decent sized blade may be fairly close on load/speed aspects, but I doubt many JP3s have a sufficiently large enough blade to fully load the engine

 

edit: obviously we're talking NB's here

Edited April 24, 2014 by Proper Job

[Timleech]

Engine load is not necessarily relative to engine speed.

With a propellor, it is precisely relative to engine speed, following approximately a cube law curve

 

A JP2 throwing a decent sized blade may be fairly close on load/speed aspects, but I doubt many JP3s have a sufficiently large enough blade to fully load the engine

 

edit: obviously we're talking NB's here

Agreed

[frangar]

Don't also forget that given the trend for larger alternators they also put a fair size load on the engine especially if geared correctly whatever the engine speed. Mines only 70A at 12v but it still makes the engine work harder at full load.

 

I think the moral is use the changeover valves regularly as you never know when you might need them.

 

Cheers

 

Gareth

[flatplane8]

I agree the valves should be used to keep them free, however I think I'll stick to leaving them in high compression as we don't exceed 900rpm unless on the tidal Thames. The exception being bursts to stop at a lock, but they are not for long.

 

Are you guys with JP2s running at over 900 rpm for most of the time?

[Proper Job]

I agree the valves should be used to keep them free, however I think I'll stick to leaving them in high compression as we don't exceed 900rpm unless on the tidal Thames. The exception being bursts to stop at a lock, but they are not for long.

 

Are you guys with JP2s running at over 900 rpm for most of the time?

 

Of all people, I'd have thought that you could run your JP3 in low compression for most of the time. Misterton gives it some work to do.

 

You're running in the upper rev range and the engine should be well loaded

 

In deep water (usually the Thames/Cherwell), I run Ocelots JP3 at 700-800 rpm to give it a bit of a work out. This is usually in low compression.

[flatplane8]

I think (and I'm aware that my knowledge is Internet based) that the load on the engine is a function of the prop dimensions, which produces thrust. The vessel dimensions such as length, beam draft and weight will determine what that thrust will do in terms of speed.

 

I know our engine smokes quite a bit on low, so I'll experiment with high comp at 900rpm and below. I've also yet to run it in anger with a closed cooling system, so it's possible higher operating temperatures will enable low comp at sub-900 rpm speeds.

[George94]

Just in case anyone is tempted to think that the posts about running one in high and one in low are not in jest....please don't try this at home!....the unbalance in the forces won't be very good for the well being of your engine!

 

Cheers

 

Gareth

 

That's fine, I agree, but I don't believe it will do any damage to start the engine in the way I described. We are talking about low revs in neutral for a very short time.

 

The changeover valves are a great invention, and I am surprised more makers didn't use them. Perhaps there was a patent.

[Proper Job]

Kelvin, of course, did the opposite and reduced the compression so that the engine could be started on petrol.

 

It was then raised to allow it to run on diesel

 

[George94]

I think (and I'm aware that my knowledge is Internet based) that the load on the engine is a function of the prop dimensions, which produces thrust. The vessel dimensions such as length, beam draft and weight will determine what that thrust will do in terms of speed.

 

I know our engine smokes quite a bit on low, so I'll experiment with high comp at 900rpm and below. I've also yet to run it in anger with a closed cooling system, so it's possible higher operating temperatures will enable low comp at sub-900 rpm speeds.

 

My reading of the manual suggests that you should be in low compression if you are running at 900. That is close to full revs, and constitutes a high load, unless you are very under-propped.

 

If your boat is anything like mine, you will lose precious little speed going at half those revs, and you will save a lot of fuel. Of course, if going against the current you may have no choice, but you should then (I think) be in low compression.

 

My rule of thumb is to use high comp on canals (no current, and speed restrictions) and low on rivers (the opposite).

[Proper Job]

My rule of thumb is to use high comp on canals (no current, and speed restrictions) and low on rivers (the opposite).

 

I'll second that

[MtB]

Kelvin, of course, did the opposite and reduced the compression so that the engine could be started on petrol.

 

It was then raised to allow it to run on diesel

 

 

Not always!

 

There are diesel start variants of their K and J series engines that come without the carbytooter, magneto, spark plug and petrol combustion chamber.

 

Far less bling on them consequently, and I can't remember how they start straight to diesel.

 

MtB

[BEngo]

 

 

Far less bling on them consequently, and I can't remember how they start straight to diesel.

 

MtB

Generally not very well unless in really good order. Otherwise it's the same as any diesel- Decompressor on, wind it up, drop the decompresssor, keep winding till it goes or until the winder is kn@ckered.. With a starter motor decompressor is optional.

 

 

N