Thumping noise when under power - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance
 Not a Member? 


Like Tree4Likes
Reply
 
LinkBack Thread Tools
  #1  
Old 05-28-2012
Senior Member
 
Join Date: Apr 2008
Posts: 454
Thanks: 12
Thanked 16 Times in 16 Posts
Rep Power: 7
Hudsonian is on a distinguished road
Thumping noise when under power

Last week I reinstalled my Yanmar 2GM and was quite careful with the realignment -certainly within .002" at the flanges. Now when powering forward I get a thumping sound at 2200 RPM or above; no sound when in reverse. When crawling around listening while underway, the noise seems to come from the hull. The cutlass bearing seemed fine when I put the boat in this spring. Where should I start?
Reply With Quote Share with Facebook
  #2  
Old 05-28-2012
Senior Member
 
Join Date: Jan 2011
Posts: 695
Thanks: 0
Thanked 18 Times in 18 Posts
Rep Power: 4
Frogwatch is on a distinguished road
Re: Thumping noise when under power

bad engine mount.
Reply With Quote Share with Facebook
  #3  
Old 05-29-2012
eherlihy's Avatar
Learning the HARD way...
 
Join Date: Jan 2007
Location: Boston Area
Posts: 3,531
Thanks: 113
Thanked 61 Times in 60 Posts
Rep Power: 8
eherlihy will become famous soon enough
Re: Thumping noise when under power

+1 on the bad engine mount.

possibly also check the prop-shaft, but the engine mount is where I would start.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

USCG Licensed OUPV Captain, ASA 101/103/104/105 Certified Instructor - Also certified in Recreational Marine Electrical Systems
Reply With Quote Share with Facebook
  #4  
Old 05-29-2012
Maine Sail's Avatar
Senior Member
 
Join Date: Jan 2003
Location: Maine Coast
Posts: 5,367
Thanks: 11
Thanked 122 Times in 90 Posts
Rep Power: 15
Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice
Re: Thumping noise when under power

Sounds like the shaft "whipping" and hitting the shaft log or worn mounts sagging moving to much under power and allowing the shaft to make contact with the shaft log.. Do you have a two blade prop, folding prop?? Before you started the alignment the shaft was dead center in the shaft log, correct..??
__________________
______
-Maine Sail / CS-36T


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.




© Images In Posts Property of Compass Marine Inc.


Reply With Quote Share with Facebook
  #5  
Old 05-29-2012
Senior Member
 
Join Date: Apr 2008
Posts: 454
Thanks: 12
Thanked 16 Times in 16 Posts
Rep Power: 7
Hudsonian is on a distinguished road
Re: Thumping noise when under power

Thank you all for your helpful responses. I have a two-bladed Martec prop, which is in good condition. I had the prop rebalanced as part of recent routine maintenance.

Since I pulled the engine in the water and have a face seal rather than a stuffing box, I thoroughly braced the shaft in position before pulling the engine so that the shaft would move forward. I didn't check to see that the shaft was dead center in the center of the log. I will do so when i check the mounts later today
Reply With Quote Share with Facebook
  #6  
Old 05-29-2012
svHyLyte's Avatar
Old as Dirt!
 
Join Date: Nov 2008
Location: Tampa Bay Area
Posts: 2,731
Thanks: 13
Thanked 105 Times in 100 Posts
Rep Power: 6
svHyLyte is on a distinguished road
Re: Thumping noise when under power

In addition to the foregoing suggestions, you might want to take a look at the shaft strut. We had a similar issue that proved to be the shaft strut moving up-and down as the prop rotated. It doesn't take much movement to generate quite a loud thump.

FWIW...
davidpm likes this.
__________________
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
Reply With Quote Share with Facebook
  #7  
Old 05-29-2012
DrB DrB is offline
Senior Member
 
Join Date: Mar 2007
Location: New England
Posts: 966
Thanks: 0
Thanked 7 Times in 7 Posts
Rep Power: 8
DrB will become famous soon enough
Before we fix the problem, more info

OP, you say that the "thumping" happens at 2200 RPM or above. Does it get worse as you increase the RPM, stay the same or diminish? Are you sure that the prop is clean? Is it new? If so, are you sure it is weighted and balanced right? If a new prop, correct pitch? Do you have zincs between the prop shaft log and the strut or after the strut between the propeller and the strut? Are they new or are they worn? Could they be loose? If you put the transmission into neutral and rev the engine, do you feel the vibration? If it was the motor mounts, I would think you would feel it somewhat.

While the shaft is turning, when looking at the turning in the stuffing box into the shaft log is it bouncing around a lot?

Answers to these will shed more light on the problem. I don't necessarily think it is a bad mount.

DrB
davidpm likes this.
__________________
If you liked my advice/response, give me some rep points. If you didn't......well what do you expect, it was free.
Reply With Quote Share with Facebook
  #8  
Old 05-29-2012
Senior Member
 
Join Date: Feb 2005
Posts: 1,077
Thanks: 1
Thanked 1 Time in 1 Post
Rep Power: 10
sawingknots is on a distinguished road
Re: Thumping noise when under power

it also could be a bent/warped shaft
Reply With Quote Share with Facebook
  #9  
Old 05-29-2012
Maine Sail's Avatar
Senior Member
 
Join Date: Jan 2003
Location: Maine Coast
Posts: 5,367
Thanks: 11
Thanked 122 Times in 90 Posts
Rep Power: 15
Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice
Re: Thumping noise when under power

Quote:
Originally Posted by Hudsonian View Post
Thank you all for your helpful responses. I have a two-bladed Martec prop, which is in good condition. I had the prop rebalanced as part of recent routine maintenance.

Since I pulled the engine in the water and have a face seal rather than a stuffing box, I thoroughly braced the shaft in position before pulling the engine so that the shaft would move forward. I didn't check to see that the shaft was dead center in the center of the log. I will do so when i check the mounts later today
These props are notorious for causing shaft whip. I have owned them and know it all too well. If you have a longish shaft between the cutlass bearing and the gear box output flange these two blade props can literally turn the shaft into a jump rope between the two bearings. If your shaft was not 100% centered in the shaft log this "whip" can cause the shaft to thump the shaft log at certain RPM.

Folding or many feathering props move the blade center considerably further aft of the cutlass bearing. This coupled with the fact that it is a two blade prop, non geared etc. can lead to vibration and shaft whip issues.


I am personally currently dealing with this on our own boat with a Flex-O-Fold two blade prop. See; FOLDING PROP REPORT The calculations put her on the borderline for whip, and I took the chance... Missed.... The shaft is just flat out too long for a folding two blade and it causes MASSIVE vibrations at certain RPM. This despite the shaft being true to .001" and the alignment also to .001" and the prop having been run through a Prop Scan tool to confirm balance.. The prop is coming off this week and the three blade will go back on..



Blades further aft cause more vibration:


Here's the off set on a fixed prop and an exaggerated illustration of what shaft whip looks like.


Here's a photo of a customers boat that was experiencing massive shaft whip thumping. We were able to get rid of the thumping and get everything to .001" including alignment and shaft tolerance.

Notice the position of the shaft passing through the shaft log.
SchockT likes this.
__________________
______
-Maine Sail / CS-36T


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.




© Images In Posts Property of Compass Marine Inc.



Last edited by Maine Sail; 05-29-2012 at 10:50 AM.
Reply With Quote Share with Facebook
  #10  
Old 06-01-2012
Senior Member
 
Join Date: Apr 2008
Posts: 454
Thanks: 12
Thanked 16 Times in 16 Posts
Rep Power: 7
Hudsonian is on a distinguished road
Re: Thumping noise when under power

Thank you all for your suggestions. After work yesterday my visual inspection the engine mounts and vigorously rocking the engine by hand found no problems with motor mounts. In hopes of determining whether the prop shaft is centered I disconnected the two flanges, pushed the prop shaft aft from the output flange on the transmission, and wiggled the prop shaft side to side and up and down. The face seal precluded a definitive determination of whether the prop shaft is on dead center of the stern tube but it looked pretty good. So I decided to look at it from the outside. After rebolting the flanges together, I motored in and put the boat in slings to inspect the prop, zincs, shaft and how centered the shaft is where it enters the stern tube. Based upon an eye ball inspection everything looked okay except the shaft was about 1/64" off center on the horizontal plane. Rotating the shaft by hand didn't reveal any bend in the shaft. I ran out of time so I put it back in the water and motored back to the mooring with the engine cover off so I could inspect the engine mounts under load. I ran it up to 2,600 RPM's and the mounts appeared to be fine and there was NO THUMP!
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Hooking 110 power cord to shore power? dvuyxx Electrical Systems 16 06-02-2012 09:35 AM
A thumping sound under foot LuckyMon Gear & Maintenance 8 07-27-2011 01:36 PM
Bumping/Thumping Noise-Advice Needed FishFinder Diesel 13 06-28-2011 03:03 PM
what was that noise? deniseO30 Gear & Maintenance 2 06-10-2009 10:30 AM
Head Sail Leach Cover Thumping jayseadee General Discussion (sailing related) 2 06-28-2008 08:12 AM


All times are GMT -4. The time now is 10:35 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.