Propulsion Puzzle

[Mike Sibley]

OK, here’s the puzzle. Consider a catamaran with two engines – a 27HP engine on the port side and an 18HP engine on the starboard side. (Don’t ask why there are two different sizes.) The transmission gear ratio is the same for both engines and the diameter and pitch of the propellers are the same. When the port engine alone is used, the boat achieves 4.6 kt at 2000 RPM. When the starboard engine alone is used, the boat achieves 3.1 kt at 2000 RPM. What gives? Shouldn’t both engines provide the same thrust at 2000 RPM regardless of the difference in HP?

I am no mechanic, but the only thing I can figure is that the clutch must be slipping in the starboard engine. (I assume marine diesel engines have a clutch.) Is there anything else that could explain this? (The starboard engine runs smoothly and doesn't make any unusual noise.)

[Reefmagnet]

If the tacho's are accurate it could be the sum of vectors perhaps. Assuming both propellers rotate the same direction, the rudder angle needed to keep the boat going straight would vary in both situations as one direction / side negates the turning force whilst the other will add to it. The more angle, the more "braking" effect.

Other things could be related to how efficient the props themselves are pushing water. Structures near the props could be affecting water flow or, if a prop is damaged, it could also be more cavitation occurring on the "slow" side.

..just my guesses

[Nemo55]

Sounds like Reefmagnet has got it in one,,think of the Yaw characteristics of a multi engine Aircraft,,,and you will have the same tendency's to Yaw one way or another depending on the Rotation of the prop,,The torque backlash..Rudder compensation etc.

[David M]

Do both props rotate in the same direction or are they counter-rotating? If counter-rotating are they outboard or inboard turning?

You are correct, identical propellers turning at the same RPM will produce the same amount of thrust at the same hull speed.

[Lagoon4us]

Kinda siding with the YAW theory but just for giggles-
I am going to move to another anchorage in a couple of hours i'll run either engine at 2000 rpm and see if there is any speed difference, i'll use GPS SOG, are you using GPS?
Just thinking where your paddle wheel is if you are only using a log.... I'll let you know results...
Cheers Frank

[Ex-Calif]

Interesting to see Frank's results.

I can buy the adverse yaw if both props rotate the same direction but I am skeptical the props are the same with such a big split in hp ratings on the engines.

The backstory on the engines may be in play here...

[Lagoon4us]

Did a run at 2000 rpm both engines oils hot = 7.4 knots.

Port engine at 2000 rpm = 6 knots.

Stb engine at 2000 rpm = 6 knots.

Maybe tacho error, log error or props may differ? If same gearbox with same shaft rpm i would think you should get equal results...

Lagoon 440 Volvo 55hp legs..

Cheers we are anchored now need to get a cool Karlavoiche...

[Nemo55]

Gotta side with David M ,,if both port and starboard shafts are achieving the same RPM with the Same type and Pitch of prop,,then its a no brainer,,of course you are going to have even thrust.
Even with different size engines,,as long as you can maintain Matched RPM's
Prop walk or Gyro torque is another matter.

[Lagoon4us]

Originally Posted by Nemo55

Gotta side with David M ,,if both port and starboard shafts are achieving the same RPM with the Same type and Pitch of prop,,then its a no brainer,,of course you are going to have even thrust.
Even with different size engines,,as long as you can maintain Matched RPM's
Prop walk or Gyro torque is another matter.

That is why i just did a run to be totally sure, i tend to believe there is a false or misleading reading in speed or rpm.....

[Andrew Troup]

Originally Posted by Lagoon4us

That is why i just did a run to be totally sure, i tend to believe there is a false or misleading reading in speed or rpm.....

In addition to what's been suggested, I'd check the ACTUAL pitch (and diameter) of the props, as opposed to what's marked (props get repitched routinely... and even re-radiused at the tips)

and make sure neither prop is damaged or has more fouling than the other.

[colemj]

Put a strobe on the engines and get actual rpms. Small differences in alternator pulley ratios, or even different alternators, will cause large rpm changes. The 18hp engine was standard on that boat, and I suspect the PO changed one with a 27hp engine.

Mark

[NCboatrx]

Andrew Troup has it correct. The props must be different.

If they were the same and correct for the larger engine, the smaller engine would be labouring and having a hard time getting its speed up.

If they were the same and correct for the smaller engine, the bigger engine would be reving up to easily and not moving the boat well.

[Lagoon4us]

Originally Posted by colemj

Put a strobe on the engines and get actual rpms. Small differences in alternator pulley ratios, or even different alternators, will cause large rpm changes. The 18hp engine was standard on that boat, and I suspect the PO changed one with a 27hp engine.

Mark

Exactly :-)

[Mike Sibley]

I have ordered a laser photo tachometer and will measure RPM on the propeller shafts. This should allow me to eliminate some of the possible causes and find the answer to the puzzle. Thanks for all the suggestions.