overpropped vs underpropped - Page 2 - 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 Tree17Likes
Reply
 
LinkBack Thread Tools
  #11  
Old 12-15-2012
jrd22's Avatar
Courtney the Dancer
 
Join Date: Nov 2000
Location: San Juan Islands., WA, USA
Posts: 3,813
Thanks: 3
Thanked 14 Times in 14 Posts
Rep Power: 14
jrd22 will become famous soon enough
Re: overpropped vs underpropped

Lance- I'm not telling you what to do but using your analogy when you have your throttle wide open on the boat and you aren't able to achieve the manufacturers WOT rpm you ARE lugging the engine just like your Jeep when you have your foot in it and can't accelerate because of too much load. That extra load from too much prop/pitch is present throughout the rpm range but you can only tell when you try to reach WOT. Your engine is pulling it's guts out trying to get there but just can't (just like when you have to shift in your Jeep). By the way, your use of WOT in reference to your engine is incorrect. WOT is the manufacturers required maximum rpm under load, what you are referring to on your boat is the maximum rpm attainable with your over pitched prop at full throttle which is 600 rpm below WOT. Sorry, not meaning to go off on you, this is a very common misconception that results in various expensive engine problems. There would be absolutely no reason for the manufacturers to even bother to establish the WOT for their engines if it wasn't important.
__________________
John
SV Laurie Anne

1988 Brewer 40 Pilothouse

Reply With Quote Share with Facebook
  #12  
Old 12-15-2012
Senior Member
 
Join Date: Dec 2008
Posts: 646
Thanks: 0
Thanked 2 Times in 2 Posts
Rep Power: 6
lancelot9898 is on a distinguished road
Re: overpropped vs underpropped

Thanks John for the discussion. I understand that my WOT term is not the WOT that the mfg uses. It's just the max rpm that can be achieved at a given prop pitch.

Let me try to explain in a different way. As I go up the hill in my Jeep without down shifting I will slow down with the rpm going down to zero yet when I go into a headsea/headwind that is not the case for the boat. The boat will slow down yet the engine will maintain 2500rpm. Why is that? The Jeep has no slippage in that the tires maintain firm contact with the pavement while the prop on the boat is "slipping'. In other words the pitch of the prop is set for an optimal speed. I set mine for achiveing 6 knots at 2500 rpm and can approach hull speed at "wot" of 3000 under calm conditions. I just don't want to hear the engine screaming at 4000 rpm which is the max intermittent rating and is what I can achieve under no load. While a max prop allows for setting different pitches it does not allow a variable pitch while underway. I guess we will have to agree to disagree on this and note that I'm not advocationg anybody follow me on this.

This brings up another interesting point on engine operation. In my Perkins workshop manual, it mentions replacing a zinc in the heat exchanger. There is no zinc in that heat exchanger. I found a good perkins mechanic who agreed that there is indeed no zinc since the rubber end gaskets on the end of the exchanger isolated the core from the metal of the rest of the engine. My point is that even a workshop manual can be wrong...and in some cases even I.

Best Regards
Reply With Quote Share with Facebook
  #13  
Old 12-15-2012
Senior Member
 
Join Date: Feb 2010
Location: Narragansett Bay
Posts: 8,693
Thanks: 10
Thanked 113 Times in 107 Posts
Rep Power: 6
Minnewaska will become famous soon enough Minnewaska will become famous soon enough
Re: overpropped vs underpropped

Just to be certain, you must check WOT rpm under load. If you try to do so at the dock, in neutral, you will get a different result and its inapplicable.

When you are overpitched, you will get too low a result. I will echo that you confirm the accuracy of the tach, as they are indeed notoriously wrong. Lancelot, it could even be the case with yours.

Backpressure from overpitch is just clearly bad for a motor over the long term. Some engines will be more finicky than others, but getting way with it is like deciding to take up smoking because you know of one 90 year old that smoked their entire life.

Ironically, overpitching the prop helps maneuverability around the dock. That will probably convince some to take their chances. Being overpitched at cruising speed for a lengthy period of time, probably isn't that bad. But getting there is just like leaving a stop sign in 4th gear. Doing that thousands of times just ain't good.

Not sure why one would be fixated on cruising at a given rpm. It should be whatever the motor wants to be at proper settings. Don't like the noise? Well that's obvious, but there are no engine noises that I'm good with. Necessary evil.
__________________

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

Jeanneau 54DS

In the harsh marine environment, something is always in need of repair. Margaritas fix everything.
Reply With Quote Share with Facebook
  #14  
Old 12-15-2012
kellysails's Avatar
Puget Sound Pounder
 
Join Date: Nov 2008
Location: Bainbridge Island, Port Madison, WA
Posts: 593
Thanks: 16
Thanked 6 Times in 6 Posts
Rep Power: 6
kellysails is on a distinguished road
Re: overpropped vs underpropped

Regarding engine noise, ref to my last post of the change I made.

Quote:
Originally Posted by kellysails View Post
@20 degrees
WOT no load 3400rpm
WOT under load 3400rpm

@22 degrees
WOT no load 3400rpm
WOT under load 3250rpm
FWIW
My wife and I both noticed that after we made the change to the prop, the engine sounded slightly different at cruising rpm. It seemed a bit smoother, a bit lower sound tempo. Oddly we thought "it" sounded "happier", not so whiny
Reply With Quote Share with Facebook
  #15  
Old 12-15-2012
jrd22's Avatar
Courtney the Dancer
 
Join Date: Nov 2000
Location: San Juan Islands., WA, USA
Posts: 3,813
Thanks: 3
Thanked 14 Times in 14 Posts
Rep Power: 14
jrd22 will become famous soon enough
Re: overpropped vs underpropped

Kelly- what is your engine manufacturers spec for WOT. Full governed rpm is the rpm at full throttle in neutral, the governor controls the maximum rpm to prevent damage to the engine. If you were able to reach the max governed speed under load you were either way underpropped, or you have a lot of hp. Sounds like by increasing pitch you actually put the engine under load which would make a difference in sound.
__________________
John
SV Laurie Anne

1988 Brewer 40 Pilothouse

Reply With Quote Share with Facebook
  #16  
Old 12-15-2012
captbillc's Avatar
ancient mariner
 
Join Date: Jul 2008
Location: duluth ,minnesota
Posts: 421
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
captbillc is on a distinguished road
Re: overpropped vs underpropped

running with the greater pitch at cruising RPM gives better fuel economy & less engine wear. as long as the engine can keep the same RPM under heavier load conditions & without smoking it is not overloaded.
__________________
Liberals are people who care about others. Freethinkers are not constrained by the myths of religion.
Reply With Quote Share with Facebook
  #17  
Old 12-15-2012
Junior Member
 
Join Date: Dec 2012
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
srah1953 is on a distinguished road
Re: overpropped vs underpropped

I have a Yanmar 3JH5CE with SD50 saildrive. How do I know what RPM should be at WOT. The only figures I can find is that the manufacturer quotes a maximum output figure at the crankshaft which is 39hp @ 3,000 RPM. With the original fixed prop, I can get around 3,200 RPM. What does this mean in terms of being over- or under-propped?
Many thanks
Reply With Quote Share with Facebook
  #18  
Old 12-15-2012
Member
 
Join Date: Feb 2009
Location: Jupiter, FL
Posts: 56
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
amarinesurveyor is on a distinguished road
Re: overpropped vs underpropped

jrd22 has it right. I see that many still have the opinion that putting a bigger prop or a steeper pitch in your prop will reduce engine wear and save fuel, and they choose to disregard engine manufacturers and mechanics. Everyone thinks they know better. You all can do whatever you want with your boat, but I think you'll find that with the correct or slightly under-pitched prop you will get the least engine wear and the best fuel economy. Yes, with an over-pitched prop you actually burn more fuel and can't develop full horsepower.
The analogy I like to use is: having a boat with an over-pitched prop is kind of like riding you bicycle in high gear (you know how hard it is on your legs, well it's like that for the engine that is over-propped).
Reply With Quote Share with Facebook
  #19  
Old 12-15-2012
Senior Member
 
Join Date: Sep 2009
Location: Kingston Washington
Posts: 501
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 5
Waltthesalt is on a distinguished road
Re: overpropped vs underpropped

Difference between cars and marine diesels is that the diesel you control the governor and the governor controls the throttle, on a car you just control the throttle. So a marine diesel won't exceed the max governor setting. If you can't get up to RPM, also black smoke, too much pitch, if RPM gets up too quickly and hull speed is down too little pitch
__________________
Walt Elliott
Kingston WA
Puget Sound
Cal-29
Reply With Quote Share with Facebook
  #20  
Old 12-15-2012
kellysails's Avatar
Puget Sound Pounder
 
Join Date: Nov 2008
Location: Bainbridge Island, Port Madison, WA
Posts: 593
Thanks: 16
Thanked 6 Times in 6 Posts
Rep Power: 6
kellysails is on a distinguished road
Re: overpropped vs underpropped

FYI
Quote:
Originally Posted by jrd22 View Post
Kelly- what is your engine manufacturers spec for WOT. Full governed rpm is the rpm at full throttle in neutral, the governor controls the maximum rpm to prevent damage to the engine. If you were able to reach the max governed speed under load you were either way underpropped, or you have a lot of hp. Sounds like by increasing pitch you actually put the engine under load which would make a difference in sound.
Yanmar 4JH5E naturally aspirated (non-turbo). Yes, we are hitting the governed rpm rate (3400 rpm). Prior to making the prop pitch change we easily hit the governed rate under load. So our yard, who does all the warranty work for Beneteau, agreed that increasing the pitch would be a worth while activity. The two degree increase really helped my speed at cruising rpm. My Max Prop actually allows 1 degree pitch changes, the yard actually thinks it could go another 1 degree, but I am happy where we are. The engine sounds happy, so I am happy. So under load WOT is 3250 from a max no-load of 3400, sounds good to me.

FYI
4JH5E is 54 hp and we have an SD50 saildrive. Beneteau Oceanis 45. 23,500 lbs.

Last edited by kellysails; 12-15-2012 at 09:53 PM. Reason: added fyi
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



All times are GMT -4. The time now is 07:26 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.