RPM change when boat pitches

[Skipper]

This is odd. In neutral at idle, the motor RPM dips when the stern pitches down in swells. It's instantaneous, matching the pitch duration and decreasing further with depth.
For a long time I thought it just an audio perception or tone fluctuation from varying contact with water or something. But then recently when it was very apparent in deep troughs, I looked at the tach, and sure enough it was dipping 100 RPM or so.
Universal 2 cyl. Hurth 50. Maintained. Cutlass, stuffing box, coupler all recent. Anyway shouldn't matter in neutral. Smooth runner no issues. It has always done this since I've owned it.
I thought about fuel pressure from pitching tank but it has an electric feed pump to the injector pump. It goes Racor/pump/secondary filter/injector pump. Typical setup. Seems pressure would be constant.
Ideas?
I don't think it's a problem. Just curious.

 

[smokey73]

When the stern dips does the exhaust exit go below water? It might be increased back pressure. I know back pressure in a car exhaust will choke it down and I would assume the back pressure would reduce the rpm a little in a boat. 100 rpm is not that much of a drop.

 

[Skipper]

When the stern dips does the exhaust exit go below water? It might be increased back pressure. I know back pressure in a car exhaust will choke it down and I would assume the back pressure would reduce the rpm a little in a boat. 100 rpm is not that much of a drop.

I don't think so. But it's certainly a good thought. I'll eyeball it next time.
The RPM drop starts at the initiation of the downward pitch, long before the exhaust port would touch the water (seemingly). Then the RPM increases with the upward swing in harmony with the altitude. Almost as if there were a pitch level sensor managing the RPM.

 

[Gene S]

Well, the prop is in the stern. Maybe it is water pressure changes on it. Just guessing.

 

[Captn Scotty]

The pitching may be affecting the governor?

 

[capta]

Well, the prop is in the stern. Maybe it is water pressure changes on it. Just guessing.

He said it happens in neutral, not in gear, so I doubt the prop is part of the problem.

 

[JamesG161]

It might be increased back pressure.

That is my guess too.
The exhaust muffler water will shift aft too. This will give a larger back pressure on engine versus a calm sea state.
Jim...

 

[Stu Jackson]

and sure enough it was dipping 100 RPM or so.

My tach is marked at 500 rpm intervals. If yours is similar, say a standard 4 or 5000 rpm tach, how can you tell? Do you have a digital tach? If not, this appears to be a non-issue, and perhaps only one of perception.

He said it happens in neutral,

Not sure I understand being in large enough swells to even begin to notice this issue in neutral. I'd be moving to stay sane.

 

[capta]

Actually, as I sit here rereading this thread with the generator running behind the bulkhead forward of the chart table, I notice the sound of the generator change as the boat rocks in the gentle swell.
It sounds exactly as if the gene has lost a few RPMs and then recovered. This has always happened, it's nothing new and only this thread brought it into my conscious mind.
I know for a fact that the gene RPMs are NOT varying. Perhaps it is the Doppler effect?
Unless your engine begins 'hunting' (revving and slowing considerably at idle), which would indicate an introduction of air into the fuel system, I'd not be too concerned by this.

 

[kloudie1]

I am going to venture one here.. gyroscopic precession ..?? governor weights are like a gyro when engine is running.. tip the rotating shaft up/down and the weights want to move a little out of plane, which may be moving the fuel injector rack just a smidge..

 

[Skipper]

My tach is marked at 500 rpm intervals. If yours is similar, say a standard 4 or 5000 rpm tach, how can you tell? Do you have a digital tach? If not, this appears to be a non-issue, and perhaps only one of perception.

Not sure I understand being in large enough swells to even begin to notice this issue in neutral. I'd be moving to stay sane.

It's graduated in 200's (pic). Definitely moving during the incidents. Rock steady when water is flat. I think it's a non-issue too, but can't figure it out which befuddles me.
Yes I like to be moving but I see the fluctuation when stopped to bring sails down into the wind and swells after starting motor to transition to entering harbor.

 

[Skipper]

I didn't think about the governor. That's possible. Also water in the exhaust moving back and forth is interesting too; with the stern down, the motor is pushing water uphill towards the muffler which is forward of the motor in my setup. The hose makes a u-turn behind the motor/riser and runs under the motor towards the muffler. The exhaust then goes up and runs along the top of the engine bay going aft to the stern port.
Edit: added pic

 

[centerline]

It's graduated in 200's (pic). Definitely moving during the incidents. Rock steady when water is flat. I think it's a non-issue too, but can't figure it out which befuddles me.
Yes I like to be moving but I see the fluctuation when stopped to bring sails down into the wind and swells after starting motor to transition to entering harbor.

the needle on an electronic tach can move a bit with the sway of the boat, in the same way it can move a bit if you are holding it in your hand and rotate the housing, while its powered up or not...its an inductive instrument so the needle is not connected with a linkage like a mechanical tach would be.... and, unless one is using a digital tach, and the change is sustained, 100 rpm is hard to distinguish on a stationary engine in a shop, let alone one running on a rocking, pitching boat...

but its also true that a lower powered 2 cylinder engine will be more affected by any gyroscopic precession, as suggested by kloudie1.... as will any water surge within a waterlift muffler system while the boat pitches...
and when first reading your post, I was thinking along the same lines as capta, where the doppler effect gives the impression that the rpm is changing. and the sway of the boat is causing the needle to dip slightly.... it could be some combination of all the effects, and im sure that you could do some experiments and tests to find out the real reason, but I would also bet that in the end, it wont make any difference in the performance of the boat or how much enjoyment it can offer, than it would have if you never would have noticed the difference...

 

[SEMPERAVANTI41]

I think it is the governor. As I understand the exhaust system with a waterlift muffler, it is never a constant flow of water. The water builds up in the muffler/exhaust hose loop and then gets blown out. If the exhaust were the culprit of the rpm dipping, wouldn't that happen all the time?

 

[Ron20324]

Maybe a long shot, but maybe during the pitch cycle, the oil goes up the cylinder and bogs down the piston until the oil is all pushed out of the cylinder?

 

[Skipper]

I think it is the governor. As I understand the exhaust system with a waterlift muffler, it is never a constant flow of water. The water builds up in the muffler/exhaust hose loop and then gets blown out. If the exhaust were the culprit of the rpm dipping, wouldn't that happen all the time?

The more I think about it, the more I agree on this theory.
If you saw it, you'd see and hear that the RPM genuinely drops, and in precise unison with the pitch angle. Which wouldn't occur in this fashion based on the comparatively imprecise exhaust-water volume and movement.

 

[SEMPERAVANTI41]

Just another thought, perhaps the springs on the governor have weakened over the years?

 

[Skipper]

Just another thought, perhaps the springs on the governor have weakened over the years?

Could be. Although I think that would affect performance more than pitch sensitivity.

 

[centerline]

As I understand the exhaust system with a waterlift muffler, it is never a constant flow of water. The water builds up in the muffler/exhaust hose loop and then gets blown out. If the exhaust were the culprit of the rpm dipping, wouldn't that happen all the time?

there is a constant flow of water in to the system from the seawater pump as long as the engine is turning over, but the exhaust gas needs enough pressure to "lift" the water out. if there is not enough pressure to discharge it, the water can actually fill the system and flood the engine with water. (as when the engine wont start, but one keeps cranking on it trying to get it to run)... but even with a properly operating engine and waterlift muffler/exhaust system, there is ALWAYS standing water in the muffler. it never entirely discharged unless there is a serious problem....
when the engine is running, the water does build up a within the muffler, then when the bubble of exhaust gas builds up enough pressure to where it has to escape, it carries some of the water out with it, but there is always more water being held within the system than is being discharged with each exhaust bubble.... when the engine is running at speed, the exhaust gas bubbles become smaller and more numerous, but they still have to pass under the water that is being held within the muffler before escaping, which is what helps to deaden the sound..

on a small, naturally aspirated diesel engine, the exhaust gas back pressure should never exceed about 1 to 1.5 psi, and with this small amount, a surge of water, at 8lbs per gallon could hold back the exhaust gas long enough to cause a change in a low horsepower engine when running at a lower idle.... the surge is only momentary, so its not harmful to the engine, and if the engine was running at operating speed, the flow of exhaust gas would be so great that a properly sized system would not surge enough to have any effect whatsoever.... and a larger engine/more cylinders and much heavier moving parts, with a higher flow of exhaust gas may never have any noticable effects either...

constantly higher back pressures due to an exhaust system that is too small or blocked, can do permanent damage to an an engine if the engine is run at operating speed for long periods while trying to gain back the extra horsepower that is lost due to the small exhaust. but not if the exhaust is sized properly and the back pressure is only a quick surge pressure.

 

[alexco38]

Almost all inboard motor mounted at an angle down towards the stern. If you oil level is a little high there might not be any problem if the sea is flat. But when the boat pitches stern down, the oil level stays level thus rises against the aft cylinder. The oil foams and rpm drops.