Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 21-08-2012, 20:52   #1
Registered User

Join Date: Dec 2008
Location: New Orleans, LA
Boat: 1993 Lagoon 37 TPI
Posts: 124
Propulsion Puzzle

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.)
__________________

__________________
Mike Sibley is offline   Reply With Quote
Old 21-08-2012, 22:10   #2
Registered User
 
Reefmagnet's Avatar

Join Date: May 2008
Location: puɐןsuǝǝnb 'ʎɐʞɔɐɯ
Boat: Nantucket Island 33
Posts: 2,737
Re: Propulsion Puzzle

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
__________________

__________________
Reefmagnet is offline   Reply With Quote
Old 21-08-2012, 22:15   #3
Registered User
 
Nemo55's Avatar

Join Date: Feb 2012
Location: Punta Banda, Ensenada. and Canada
Boat: 28Ft Piver Encore, Tri-Maran, Anchored in San Diego.
Posts: 728
Send a message via Skype™ to Nemo55
Re: Propulsion Puzzle

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.
__________________
"The Truth Shall set you Free....But First it will Piss You Off"

http://nemo1955.blogspot.ca/
Nemo55 is offline   Reply With Quote
Old 21-08-2012, 23:05   #4
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,152
Re: Propulsion Puzzle

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.
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 22-08-2012, 00:03   #5
Registered User

Join Date: Nov 2011
Location: Fethiye Turkey
Boat: Lagoon 440
Posts: 3,164
Re: Propulsion Puzzle

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
__________________
"Political correctness is a creeping sickness that knows no boundaries"
Lagoon4us is offline   Reply With Quote
Old 22-08-2012, 00:09   #6
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Re: Propulsion Puzzle

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...
__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Old 22-08-2012, 03:15   #7
Registered User

Join Date: Nov 2011
Location: Fethiye Turkey
Boat: Lagoon 440
Posts: 3,164
Re: Propulsion Puzzle

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...
__________________
"Political correctness is a creeping sickness that knows no boundaries"
Lagoon4us is offline   Reply With Quote
Old 22-08-2012, 03:22   #8
Registered User
 
Nemo55's Avatar

Join Date: Feb 2012
Location: Punta Banda, Ensenada. and Canada
Boat: 28Ft Piver Encore, Tri-Maran, Anchored in San Diego.
Posts: 728
Send a message via Skype™ to Nemo55
Re: Propulsion Puzzle

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.
__________________
"The Truth Shall set you Free....But First it will Piss You Off"

http://nemo1955.blogspot.ca/
Nemo55 is offline   Reply With Quote
Old 22-08-2012, 03:35   #9
Registered User

Join Date: Nov 2011
Location: Fethiye Turkey
Boat: Lagoon 440
Posts: 3,164
Re: Propulsion Puzzle

Quote:
Originally Posted by Nemo55 View Post
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.....
__________________
"Political correctness is a creeping sickness that knows no boundaries"
Lagoon4us is offline   Reply With Quote
Old 22-08-2012, 04:25   #10
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Propulsion Puzzle

Quote:
Originally Posted by Lagoon4us View Post
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.
__________________
Andrew Troup is offline   Reply With Quote
Old 22-08-2012, 07:24   #11
Senior Cruiser
 
colemj's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,049
Images: 12
Re: Propulsion Puzzle

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
__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 22-08-2012, 08:16   #12
Marine Service Provider

Join Date: Aug 2008
Location: North Carolina
Posts: 252
Re: Propulsion Puzzle

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.
__________________
NCboatrx is offline   Reply With Quote
Old 22-08-2012, 10:22   #13
Registered User

Join Date: Nov 2011
Location: Fethiye Turkey
Boat: Lagoon 440
Posts: 3,164
Re: Propulsion Puzzle

Quote:
Originally Posted by colemj View Post
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 :-)
__________________
"Political correctness is a creeping sickness that knows no boundaries"
Lagoon4us is offline   Reply With Quote
Old 23-08-2012, 21:02   #14
Registered User

Join Date: Dec 2008
Location: New Orleans, LA
Boat: 1993 Lagoon 37 TPI
Posts: 124
Re: Propulsion Puzzle

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.
__________________

__________________
Mike Sibley is offline   Reply With Quote
Reply

Tags
propulsion

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 16:55.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.