Overpropped; What to do? - Page 3 - 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 > General Interest > General Discussion (sailing related)
 Not a Member? 


Like Tree7Likes
Reply
 
LinkBack Thread Tools
  #21  
Old 04-16-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,544
Thanks: 81
Thanked 53 Times in 45 Posts
Rep Power: 7
MedSailor is on a distinguished road
Re: Overpropped; What to do?

I bought a boat with an undersized engine and an over-propped prop. To complicate matters further, I have an engine that is governed for 4000rpm but only rated for continuous usage at 3000rpm under load. I've given this question a lot of though and asked a lot of experts. It's amazing to me the number of conflicting opinions on the matter!

From what I've distilled, here are my salient take away points:

With a clean bottom and a warmed up engine, with good fresh fuel, and an accurate tachometer, you should be able to reach your max rated rpm with WOT (wide open throttle or full throttle).

For me, the debate was 3K or 4K. I've settled on 3k being the only number that made sense. So, for me, at full throttle after a haulout, I should be able to reach 3,000rpm. Check and make sure your Yanmar is rated to 3,600 rpm continuous working rpm. It might just be governed at 3,600 and able to run for "intermittent usage" at that rpm. If so, that's not your target.

Now, if you're ABLE to reach max-rated RPM you can congratulate yourself because you're not driving up hills in 5th gear and ruining your engine. What about "cruising RPM"? Most of us want to go hull speed. Ideally, with the prop you have now pitched perfectly, the RPM that pushes you at hull speed will coincide with max efficiency of your fuel/torque/HP curves. But does it?

In order to hit the above RPM perfectly, it requires the correct SIZED engine. If it's too big, your cruising RPM will be low on the efficiency curves. If this is the case, do not despair, as you can push the boat faster than hull speed and you have the option to (inefficiently) turn extra diesel into a little extra speed when you need it.

Of course, with an oversized engine comes a price. It will be running too cool most of the time, causing carbon to build up in places you don't want it. The easy solution to this problem is to run the engine for 4 min or so at 80-100%max rpm at the end of each time you use it. That will keep the carbon from building up.

Now if you're like me, and you have the correct pitch figured out to keep your engine healthy, but at that pitch, your boat goes less than hull speed at cruising RPM, you have the choices of: 1. Accept that you will motor slower. 2. Run at higher than recommended rpm possibly wearing out the engine sooner. 3. Over-prop the engine slightly, and risk wearing it out further.

So, check your Yanmar manual closely. What's your MAX CONTINUOUS RATED RPM? That's the one you want to be able to hit at WOT. 3,600 seems a little fast for continuous usage to me, but then again Yanmar has been doing a lot of things in the last 20 years that I don't really approve of...

MedSailor
__________________

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


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by MedSailor; 04-16-2013 at 11:35 PM.
Reply With Quote Share with Facebook
  #22  
Old 04-17-2013
TQA's Avatar
TQA TQA is offline
Bombay Explorer 44
 
Join Date: Apr 2009
Posts: 2,563
Thanks: 0
Thanked 43 Times in 39 Posts
Rep Power: 6
TQA is on a distinguished road
Re: Overpropped; What to do?

Quote:
Originally Posted by sony2000 View Post
Internal combustion engines are the most overall efficient at 75% of maximum RPM. The math is easy, 2700 rpm. It's a rule.
Err not so. An IC engine is at it's most efficient when it is run at it's maximun torque RPM.

We are talking fuel efficiency here. If you have some other definition of 'efficient' then what you say may be true.

So if you want your boat engine to be operating at it's maximun efficiency you need to 'over prop' it.

Just like driving down a flat road in your car. You want to be in 'overdrive' or a high gear.
lancelot9898 likes this.
Reply With Quote Share with Facebook
  #23  
Old 04-17-2013
Senior Member
 
Join Date: Jan 2013
Location: Montreal, QC, Canada
Posts: 255
Thanks: 0
Thanked 12 Times in 11 Posts
Rep Power: 2
sony2000 is on a distinguished road
Re: Overpropped; What to do?

In quoting me you dropped the word "overall". This term is to encompass efficiency related to
1. maximum range on available fuel.
2. a level of RPM that causes above normal wear on the engine.
3. time to destination.
4. maximum torque.
5. hull speed.
An over propped engine ends up operating at a higher manifold pressure, resulting in a more frequent, destruction of head gaskets.
Reply With Quote Share with Facebook
  #24  
Old 04-17-2013
casey1999's Avatar
Senior Member
 
Join Date: Oct 2010
Location: HI
Posts: 2,845
Thanks: 4
Thanked 17 Times in 17 Posts
Rep Power: 4
casey1999 is on a distinguished road
Re: Overpropped; What to do?

How does a prop shop re-pitch a prop?
Reply With Quote Share with Facebook
  #25  
Old 04-17-2013
TQA's Avatar
TQA TQA is offline
Bombay Explorer 44
 
Join Date: Apr 2009
Posts: 2,563
Thanks: 0
Thanked 43 Times in 39 Posts
Rep Power: 6
TQA is on a distinguished road
Re: Overpropped; What to do?

Quote:
Originally Posted by sony2000 View Post
In quoting me you dropped the word "overall". This term is to encompass efficiency related to
1. maximum range on available fuel.
2. a level of RPM that causes above normal wear on the engine.
3. time to destination.
4. maximum torque.
5. hull speed.
An over propped engine ends up operating at a higher manifold pressure, resulting in a more frequent, destruction of head gaskets.
I am confused here. You talk about manifold pressure. Now this is relevent to a superchrged or turbocharged engine but NOT a normally aspirated diesel as there is no form of restriction to the inlet manifold opening; it runs wide open all the time.

Also I do not see how time to destination figures into this. I wonder if you are using data from an aircraft fitted with supercharged petrol engine fitted with a device to control mixture strength and boost pressure plus a variable pitch prop.

Finally in my experience almost all cylinder head gasket failures can be traced to engine overheating from coolant failure, some are corrosion related.
Reply With Quote Share with Facebook
  #26  
Old 04-17-2013
OPossumTX's Avatar
Senior Member
 
Join Date: Jul 2011
Location: Alvin, Texas
Posts: 155
Thanks: 20
Thanked 4 Times in 4 Posts
Rep Power: 4
OPossumTX is on a distinguished road
Re: Overpropped; What to do?

Most "prop shops" have a "bargain board" of props they have taken in trade or salvaged from damaged props left by customers. Often you can get very good deals swapping for something on the "board". It is also a great place to pickup a spare if you are inclined to keep such unnecessary clutter about.
Reply With Quote Share with Facebook
  #27  
Old 04-17-2013
Senior Member
 
Join Date: Jan 2013
Location: Montreal, QC, Canada
Posts: 255
Thanks: 0
Thanked 12 Times in 11 Posts
Rep Power: 2
sony2000 is on a distinguished road
Re: Overpropped; What to do?

Lets say an overpropped engine lowers it's WOT rpm, but brings its operating RPM much closer to this lower WOTrpm, the result is a higher fuel consumption and greater wear per hour.
Time to destination, usually suffers if you are trying for the greatest MPG.
Detonation blows head gaskets.
I was listing a number of reasons to counter the statement that " to get maximum efficiency we should be overpropped". There are many maximums and some are opposites.
Reply With Quote Share with Facebook
  #28  
Old 04-18-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,544
Thanks: 81
Thanked 53 Times in 45 Posts
Rep Power: 7
MedSailor is on a distinguished road
Re: Overpropped; What to do?

Quote:
Originally Posted by casey1999 View Post
How does a prop shop re-pitch a prop?
In the old days it was with a hammer and a jig. Now it involves something similar but I believe there is more jig and hydraulics and less hammering.

Obviously from my response, I've never worked at a prop shop, but one thing that might be relevant information is that for a well made prop in good condition one can usually have 3 inches of pitch added or subtracted. Trying to add or subtract more will depend on a lot of factors and may or may not be able to be done.

I had my 12" prop changed to a 9" and now it's in the shop again to be changed to a 10". (turns out the perfect sweet spot is probably 9.6" but I want 10").

If you want to change your 22" pitch prop to a 9" you'll be out of luck.


MedSailor
__________________

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


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #29  
Old 04-18-2013
Senior Member
 
Join Date: Jan 2013
Location: Montreal, QC, Canada
Posts: 255
Thanks: 0
Thanked 12 Times in 11 Posts
Rep Power: 2
sony2000 is on a distinguished road
Re: Overpropped; What to do?

casey1999,
to repitch a prop the blades should be twisted when heated up. More of an angle or less of an angle. Then compare it to a jig of angles for that particular prop.
Suppose you have a 12X9 prop. The 12" diameter is measureable by you. The 9" means that in a tub of Jello, one rotation of the prop will advance it 9" in the Jello. Lessen the angle of the prop blade, and it only will go 7" for example.
The prop pitch can also be changed by cutting of equal amount of the blade, but that is a road far less travelled.
Reply With Quote Share with Facebook
  #30  
Old 04-18-2013
TQA's Avatar
TQA TQA is offline
Bombay Explorer 44
 
Join Date: Apr 2009
Posts: 2,563
Thanks: 0
Thanked 43 Times in 39 Posts
Rep Power: 6
TQA is on a distinguished road
Re: Overpropped; What to do?

Quote:
Originally Posted by sony2000 View Post
Lets say an overpropped engine lowers it's WOT rpm, but brings its operating RPM much closer to this lower WOTrpm, the result is a higher fuel consumption and greater wear per hour.
Time to destination, usually suffers if you are trying for the greatest MPG.
Detonation blows head gaskets.
I was listing a number of reasons to counter the statement that " to get maximum efficiency we should be overpropped". There are many maximums and some are opposites.
Sony2000 please be clear that I am not having a go at you personally but you are making statements here which are not correct.

All the following relates to a typical sailboat auxiliary normally aspirated diesel engine.

First of all you talk about WOT this usually means a wide open throttle. I do not like this used in reference to a diesel engine. Typically a throttle is a butterfly valve in the inlet manifold which coontrols the air flow into the engine. Diesel engines do not have them. The inlet manifold is not restricted in any way.

The engine speed lever connects to the governor fitted to the fuel injection pump. The governor controls the amount of fuel pumped into each cylinder. If the engine rpm differs from the one set by the engine speed lever then the governor either reduces the fuel supply or increases it to attempt to match the RPM.

Lets say you have a prop which allows an engine to achieve its maximum governed RPM . This will be beyond the point at which the engine generates maximum power.

With 'more' prop you might have a situation where at ' full fuel' the engine would attain the rpm at which it develops maximum power. This would be desirable if you were power boat racing.

With even 'more' prop you get to a situation where at ' full fuel' the engine would just attain the rpm at which it develops maximum torque. This is the rpm at which the engine is most fuel efficient. The engine wear rate will be less too. These are basic engineering facts based both on simple physics as well testing in real life situations.

In practice on a sailboat you would not select a prop which held the engine down to the rpm matching peak torque in flat water with no headwind. Normally it will be somewhere between peak power and peak torque.

Detonation. Basically this is not something that effects diesel engines. See

A supercharged petrol engine running lean with high manifold pressure and high cylinder head temp will be close to detonation.
Quote:
The prop pitch can also be changed by cutting of equal amount of the blade, but that is a road far less travelled
.

No the prop pitch can not be changed by trimming the blade. If you reduce the diameter the pitch remains the same. If you make the blades narrower then by removing some of the leading and trailing edges, the pitch remains the same. If you were to make the blade narrower by trimming only the trailing edge you would get a very tiny change in pitch say 1%.

Oh yes I better enter a couple of qualifications on the above just in case we have any serious diesel heads around.

The governor type I refer to is the constant speed type found on almost all modern engines. If you have an old Gardiner you have something different.

If you have a pneumatic governor then you do have a throttle butterfly in the inlet manifold. I think the last one I saw outside the lab. was on a Merc 200D from the late 60s. They are an invention of the devil as most failures lead to an engine overspeed [ runaway ] which is pretty alarming. I used to create one in the lab. Students would run for cover. Anybody out there got one?

Gahh I used to teach and research in this stuff and spent hours running diesels on dynos. Retired now. No more trying to fill empty heads.
MedSailor likes this.

Last edited by TQA; 04-18-2013 at 01:06 PM.
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
overpropped vs underpropped groundhog Gear & Maintenance 45 12-19-2012 10:20 PM


All times are GMT -4. The time now is 05:23 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.