Impact of tide on RPM

[rotrax]

Kicks in? Can the governor increase the fuel when its already at max throttle as per OP? If not when turning into the wind the boat speed will drop and the revs will slow due to the extra resistance and inability to deliver more fuel to keep the revs up?

The govenor's only purpose is to limit RPM. To stop engine damage by overspeeding.

It depends what system is in use, but the Govenor, mechanical within or external to the injector pump, or computerised within the ECB, supplies the amount of fuel required to achieve this.

When the Govenor 'kicks in' it will normally reduce fuelling, and so limit RPM.

Marine vessels dont go down hill in the way cars do very often........................ And gearboxes are invariably - but not always - simple ahead/astern jobs.

Tide, sea and wind can make significant difference to engine power required to maintain speed.

But we all know this. It is where the OP started...........................

 

[Adios]

Attempt at a drawing
Engine power curve and a few propeller power curve, both vs rpm.
A propeller matched to absorb all the available engine power at full rpm would have a curve like #1a: they cross at MAX rpm and MAX power.
If outside resistance increases (waves, etc) the curve will tilt to #1b. The engine cannot keep constant Max RPM as it would require more power than it is capable of providing, so rpm gets lower until available engine power is equal to power absorbed by the propeller: point 1B, at lower rpm.
If the engine remains fixed at MAX rpm, it means we are moving along MAX rpm vertical line. That can be given by a propeller curve #2a, which tilts to #2b with an increase in outside resistance. Curve #2a is for a (seriously) undrpropelled boat, imagine your boat with a tiny propeller: you can bring it to max rpm without problems (of course speed will be very low), if external resistance is applied then the propeller will absorb more power which the engine will be able to give while remaining always at max rpm.

View attachment 111484

Got it So if the OP was overproped he might be at the maximum RPM already, somewhere between 1a and 1b say, and when the forces on the boat changed into or away from the wind and waves he'd feel a difference either an easing of an over burdened engine and revs increasing or a slight reduction in the revs and an engine straining if turning into more resistance on the boat.

A boat with the right prop would be able to maintain the revs despite some variation of power, via the governor.

 

[Daydream believer]

On the contrary, we have three plausible mechanisms proposed.

The most plausible being :-

1) The Op is talking b..x
2) The engine is knackered
3) Lots of bees about

Personally i would go for No 2 but 3 does have a sting in the tail
Really had the forum buzzing with excitement.

 

[sfellows]

Indeed you did. My apologies for not spotting it sooner!

There is no need to apologise - I just think it's really funny how this has taken off when (IMO) it's really simple (or should be)! Perhaps though, it's just that I am simple!

 

[AngusMcDoon]

I'm surprised nobody has mentioned Coriolis yet, it must have some effect, which depends on the heading. upon the rotating machinery.

What about Doppler effect? Maybe the OP wasn't on the boat at the time the measurement was made, but on a rock observing the boat as it motored past and judging engine speed by listening to the sound it made. As the boat motored past with the tide the tone of the engine would sound lower as it receded than when it turned round and started motoring back against the tide towards the lonesome listener standing on his rock.

Of course, as the boat is in the tide's frame of reference and the observer is in the rock's frame of reference, which are moving relative to each other, there will be some relativistic time and length dilation going on as well. So many possibilities!

 

[ip485]

2) The engine is knackered

Bl**dy cheek.

Runs like a dream as long as I put a litre of oil into it every hour.

 

[rotrax]

The Bollinders semi diesel I once made a piston for - I cast it in a large paint tin using alloy from Hillman Minx Clutch Bell Housings - had a 'Hit and Miss' govenor system.

When the engine speed reached that set by the external govenor, the sort with rotating balls and a scissor action, it just missed injecting any fuel.

Which gave it a very unusual exhaust note.

Bonk, bonk, bonk, bonk, ---------------------bonk bonk--------bonk----------------bonk bonk bonk---------------bonk--------

After starting up, requiring an integral blowlamp to heat a platinum tube, it often blew smoke rings at tickover. The external flywheel was about five feet in diameter.

It was fitted in a 1922 Big Woolwhich canal motor boat.

 

[dunedin]

Have we forgotten to take account of the rate of spin of the earth - suitably adjusted correctly for latitude, of course?

 

[Gary Fox]

Have we forgotten to take account of the rate of spin of the earth - suitably adjusted correctly for latitude, of course?

I did mention Coriolis but I can't do the maths

 

[JumbleDuck]

The Bollinders semi diesel I once made a piston for - I cast it in a large paint tin using alloy from Hillman Minx Clutch Bell Housings - had a 'Hit and Miss' govenor system.

When the engine speed reached that set by the external govenor, the sort with rotating balls and a scissor action, it just missed injecting any fuel.

Sounds like a "hit and miss" system. On petrol engines they sometimes worked by holding the exhaust valve open when the speed was high enough and sometimes by shorting the spark. As you describe, they sound very odd at tick-over, but much more normal when loaded.

 

[anoccasionalyachtsman]

Something for @JumbleDuck to think about.

@ip485 mentions that he sees this in shallower water, so it got me thinking about how the gradient of velocity wrt. depth could be affecting things - this being more marked in shallower water.

@lustyd's thought on waves could be explained by the fact that forming a bow (and stern) wave depends on getting a cylinder of water rolling, and that will be easier if the surface is sliding with the boat, and vice versa. But I think that will just have an effect on the speed of the hull and has no direct effect on the propeller and I can't see why rpm would be so affected.

Further thought took me to the inflow to the propeller 'disc'. Normally the disc sees slower flow nearer the hull, but in an adverse current I wondered if the faster flow near the surface would compensate for the effect of skin friction, and that just by chance the inflow to the disc became more evenly distributed, hence loading the prop more effectively and so through to the engine? (Have we had an answer on how governors work yet?).

 

[rotrax]

IIRC from my Tech College days the govenor in a diesel inline injection pump is set to limit RPM by closing fuelling down just enough to achieve that aim. They have an external adjustment and require carefull setting on an expensive test rig.

What is interesting - to me anyway - is that a big inline pump on a test rig sounds just like a diesel engine running.

The govenor shoves the rack closed a bit. As the engine takes full air, the amount of fuel controls its RPM. Similar in a rotary pump, but would have a different mechanism as they have a cam, not a rack.

Some diesel engines for road vehicles had a butterfly in the intake manifold for supplying a vacuum for a brake servo.

Not terribly effective from direct experience.

 

[ip485]

Something for @JumbleDuck to think about.

@ip485 mentions that he sees this in shallower water, so it got me thinking about how the gradient of velocity wrt. depth could be affecting things - this being more marked in shallower water.

@lustyd's thought on waves could be explained by the fact that forming a bow (and stern) wave depends on getting a cylinder of water rolling, and that will be easier if the surface is sliding with the boat, and vice versa. But I think that will just have an effect on the speed of the hull and has no direct effect on the propeller and I can't see why rpm would be so affected.

Further thought took me to the inflow to the propeller 'disc'. Normally the disc sees slower flow nearer the hull, but in an adverse current I wondered if the faster flow near the surface would compensate for the effect of skin friction, and that just by chance the inflow to the disc became more evenly distributed, hence loading the prop more effectively and so through to the engine? (Have we had an answer on how governors work yet?).

Very interesting.

The prop is set well down and of course very near the stern (not a sail drive).

 

[davidej]

In theory I suppose the bow wave could be different if the water is already moving with you. How small is the drop?

No, the bow wave would be identical up-tide or down-tide, all other things (such as wind) being equal.

I cant believe I am reading this.

 

[Beneteau381]

The train analogy is good. Same as if you put the boat in a huge tank which was drifting with the tide and the boat was motoring in still water inside the tank. Speed (and therefore revs) would be the same. TIDE HAS NO EFFECT. Other factors, not tide related such as swell will have an effect. End of discussiony!

Sorry, waterflowover the prop does have an effect

 

[Beneteau381]

I would also be interested in how governors work in the real marine world.

I understand pretty well how a governor works on an aircraft propellor, and I suspect these are much more sophisticated.

My engine is turbo charged, and it is also quite noticeable as the turbo charger kicks in. I wonder if this is also a factor at the top end, as presumably the turbo almost certainly does not perform with linear progress.

most ofthe governors we come up against in our small auxiliary engines are centrifugal. Very simple

 

[Gary Fox]

I cant believe I am reading this.

Please explain how we can both be wrong!

 

[Gary Fox]

Sorry, waterflowover the prop does have an effect

Yes, except that the tide doesn't cause waterflow over the prop if you are under way.

 

[ip485]

So the boat is making exactly 5 knots at 3,000 rpm which is its maximium speed and maxium rpm into a tide of exactly 5 knots and the speed over the ground is exactly zero.

The boat turns around , the same parameters and the boat is moving over the ground at 10 knots.

There is no wind.

Are we all convinced taking into account all the other factors, the RPM will remain at 3,000 in both circumstances?

 

[Gary Fox]

So the boat is making exactly 5 knots at 3,000 rpm which is its maximium speed and maxium rpm into a tide of exactly 5 knots and the speed over the ground is exactly zero.

The boat turns around , the same parameters and the boat is moving over the ground at 10 knots.

There is no wind.

Are we all convinced taking into account all the other factors, the RPM will remain at 3,000 in both circumstances?

If you ignore the 10 knot relative headwind when she is going down-tide, then the RPM will stay the same in both directions.