Reply
 
Thread Tools Display Modes
 
Old 11-06-2013, 06:35 PM   #41
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
Huh? Lots o f interesting theories. After today traveling 93 miles and it appears by the gauge that I used 10 gallons of fuel, I am beckoning to wonder if rudder deflection, course correction and the freewheeling prop may have a negative effect. There were light winds and a following current. I know gauges can be off, I will let you know after I too that tank off.
__________________
Advertisement

Miss Donna is offline   Reply With Quote
Old 11-06-2013, 07:09 PM   #42
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Tim and Donna,

Tim,
The way it relates is that two engines will always burn more than one if they are running at the same rpm. And conversely one engine will always burn about half as much as two. Further one engine produces only half the thrust so the boat goes slower. And if it goes slower, it burns less fuel. And the slower it goes the more miles per gallon you get. It's all quite logical and you don't need the graph to know that.

But the graph gives you an accurate way to know how much slower the boat will go, how much the resistance is reduced and how much less fuel will be burned.

Donna,
Your latest readings give a nmpg of 93/10 = 9.3 mpg, (8.1 nmpg); fantastic! I think you must have your numbers scrambled.

Pete37
__________________

Pete37 is offline   Reply With Quote
Old 11-06-2013, 07:32 PM   #43
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
Pete, I hat to disagree with you however I believe it is .93 mpg . :-))
Miss Donna is offline   Reply With Quote
Old 11-06-2013, 07:42 PM   #44
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Tim,

Yes, the affects of propeller drag when freewheeling and rudder resistance are unknowns which must be evaluated before we can get an accurate answer. We think their affects are small but that must be verified by testing which is what we are doing.

However, we are getting some reports of dramatic fuel savings in other posts.

Pete37
Pete37 is offline   Reply With Quote
Old 11-06-2013, 07:54 PM   #45
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Donna,

Please check your numbers again.

Pete37
Pete37 is offline   Reply With Quote
Old 11-06-2013, 09:10 PM   #46
Guru
 
timjet's Avatar
 
Join Date: Apr 2009
Posts: 1,901
Pete, in my case my speed drops from 7.4 kts to 6 kts when shutting an engine down. That's an incredible gain in fuel mileage.

I can't fully explain why this is so. Your explanation that hull drag is reduced when speed is reduced from 7.4kts to 6 kts would offer some explanation, but it doesn't stand to reason that this reduced hull drag coupled with a greater than 50% loss of thrust would account for only a loss of 1.4 kts.

As you know you loose more than 50% thrust when shutting down an engine due to the thrust necessary to overcome the displaced rudder and windmilling propeller.

I'm guessing here but as a percentage, I think boats powered to plane may get better gas mileage single engine at displacement speeds than boats powered to operate at or below displacement speeds when operated single engine.
IOW, a trawler is already at or close to max efficiency at displacement speed so shutting an engine down doesn't help much. But a planning hull boat is not at max efficiency at displacement speeds so shutting one down gives a big improvement in gas mileage.

Perhaps some explanation may be that the bigger engines needed to power planning hulls are using as a percentage more fuel to overcome engine parasitic drag at low speeds than smaller engines. When an engine is shut down on a planning hull boat the fuel used to overcome this parasitic drag is eliminated.

As you can probably tell, I'm guessing.
__________________
Tim
Tampa Bay
Carver 355 ACMY Twin Cummins Diesels
timjet is offline   Reply With Quote
Old 11-06-2013, 11:39 PM   #47
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Tim,
You don’t have any specs listed on your boat so I had to look her up on Yachtworld. She’s a rare bird; only one listed. She’s 35.5’ long and assuming a normal hull form her waterline length (L) would be about 92% of that or 32.7’. That would make her speed-length ratio (SLR) at 7.4 knots 7.4/sqrt(32.7) =7.4/5.7=1.30. At 6 knots it would be 6/5.7 = 1.05. I’m going to assume that she’s a light trawler and therefore use the del=150 curve.
Run a line vertically up from SLR=1.05 to where it intersects the del=1.5 curve. You’ll find that the specific resistance is about 16 lbs/ton. Run a second line up from SLR=1.3 to where it intersects the del=150 curve and you will find the specific resistance is about 57 lbs/ton. So by shutting down one engine the specific resistance drops from 57 lbs/ton to 16 lbs/ton. You have cut your resistance to only 16/57 = 28% of the two engine resistance. Your change in speed should be (1.3-1.05) x 5.7 = 1.43 knots. That’s pretty close to the 1.4 knots you estimated.
The reason the curve is so steep between SLR=1.2 and SLR=1.6 is because this is where your boat is building up its wave train. Wave making resistance is the largest source of resistance.
You don’t give any hull weight or current twin engine fuel consumption at 7.4 knots so I can’t estimate the fuel burn rate but the single engine fuel burn should only be about of the two engine rate.
Everything sounds wonderful except that you are operating your engines at idle speeds and I doubt the engines will get up to their normal operating temperatures. Do that regularly and you’re going to damage your engines. The exhaust system parts will be the first to go. Look for transom soot. Later, if you don’t get your engines up to proper operating temperature you will have piston, cylinder and block damage. Check your temp gauges and make sure you are getting up to around 180F. Your engine manual will give you the exact temps the manufacturer wants you to maintain.
Keep in mind that the graph is most accurate for trawler speeds up to SLR=1.6 ( 9 knots for your boat). At planing speeds (SLR>1.6) the curve of specific resistance is nearly flat so there would be little difference in fuel economy between single and twin engine performance (provided you remain above SLR=1.6 for both modes of operation).
Parasitic drag is the drag caused by dragging the propellers, shafts, struts and rudders through the water. Parasitic drag is usually small potatoes compared to wave making and surface friction drag. But for high speed planing hulls it can become a significant factor.
Pete37
Pete37 is offline   Reply With Quote
Old 11-07-2013, 06:04 AM   #48
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
Good morning Pete and all, Pete is right again, don't I wish I was getting over 8miles per gallon. I used 100 gallons Pete.
You are never just doing a constant on this river. There are barges to pass, locks and currents. A couple times I will fire up the other engine to pass a barge maybe even bringing them up to 1800rpm for 10 minutes or so.
Miss Donna is offline   Reply With Quote
Old 11-07-2013, 11:51 AM   #49
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
Trouble on the run.

My port engine is 20 degrees hotter on the inboard manifold than the outboard side. Anybody?
Miss Donna is offline   Reply With Quote
Old 11-07-2013, 08:31 PM   #50
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Miss Donna,

Look for a faulty thermostat, blocked water intake, obstructions in the cooling system, broken hoses or blocked exhaust system. Your engines (like mine) are over 20 years old. With so many possibilities it isn't possible to be sure what the problem is. At 20 degrees it doesn't sound too bad. Monitor it closely and have a mechanic look at it as soon as possible. Shut it down if necessary.

Pete37
Pete37 is offline   Reply With Quote
Old 11-08-2013, 05:17 AM   #51
Guru
 
timjet's Avatar
 
Join Date: Apr 2009
Posts: 1,901
Pete, thanks for the reply. I want to download and print the graph when I have some time to look at it carefully.

I really am amazed that decreasing speed by just 1.4 kts decreases drag that much.

Miss Donna, Pete is quite right, look at the raw water system carefully. Start at the strainer and work your way back. If that checks out then start checking the fresh water (coolant) system starting at the thermostat. Good luck.

Likely if it was a sudden increase it's probably on the raw water side.
__________________
Tim
Tampa Bay
Carver 355 ACMY Twin Cummins Diesels
timjet is offline   Reply With Quote
Old 11-08-2013, 07:33 AM   #52
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
I may have coughed up a hair ball. It is now only 10 degrees cooler from one side of the port engine to the other. Inboard side manifold is around 197. While outboard is 187. Where exactly should I be shooting for temps and what is normal? Please! Anyone.
Miss Donna is offline   Reply With Quote
Old 11-08-2013, 09:22 AM   #53
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Donna,

There are two thermostats available for 6V92s; a 170F and a 180F thermostat. Most Connies seem to have the 170F thermostat which should keep the engines somewhere between 170F and 187F. But that is the water temp. With an IR pyrometer you are reading the surface temp of the manifold. That should be 5 to 10 degrees hotter but I've never seen an official number on manifold surface temps.

I don't think 6V92s develop hairballs. You're thinking of cats. But they do develop rusty manifolds; particularly after 27 years. A chunk of rust may have broken off and temporarily blocked the cooling system. I doubt your problem is on the intake side because that would affect both sides of the engine equally. Another possibility would be that one your risers has plugged up. But there's not much point in continuing the guessing game; keep watching the engine and shut it down if the temps get out of hand. I also doubt it has anything due to the single engine operation. More likely it's just because the engines are getting old. If you can hang on until the end of the trip it would probably be wiser to wait until you reach your destination to address this problem. How far do have left to go?

Pete37
Pete37 is offline   Reply With Quote
Old 11-08-2013, 09:28 AM   #54
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Donna,

In my last post I said "hotter" when I meant "cooler". I went back to the post and tried to correct it but apparently this forum has no way to correct a post once it is submitted.

Pete37
Pete37 is offline   Reply With Quote
Old 11-08-2013, 09:39 AM   #55
Veteran Member
 
Pete37's Avatar
 
City: Grasonville, MD
Country: USA
Join Date: Oct 2013
Posts: 52
Hi Tim,

The quickest way to download the graph is to install a copy of "Printscreen" on your computer. I think it's still free. From "Printscreen" you can get a paper copy or a file which you can manipulate with your photo editor.

Pete37
Pete37 is offline   Reply With Quote
Old 11-23-2013, 09:00 AM   #56
Veteran Member
 
Francois's Avatar
 
City: San Antonio, Boat is located in Corpus Christi
Country: USA
Vessel Name: Bluebonnet
Vessel Model: Chris Craft Constellation 500
Join Date: Nov 2013
Posts: 76
Hi Chris Craft forum members !
Since I am now a proud owner of a 1985 Connie 500, I plan to participate in this tread.
Pete, I followed the other Boat US forum for the Connie and was never able to post for technical reason (...). Now I'm happy to be able to participate.

the boat I bought from a older couple in Beaumont Texas was (and is still) named Bluebonnet. Not sure if you have the history, but you can add me to your list of CC500 owners.

Happy to be here !
Bye
Francois is offline   Reply With Quote
Old 11-23-2013, 02:10 PM   #57
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
I have been at Dog River Marina for over 2 weeks now. First I replaced the thermostat and hoses on the inboard side of port engine. Next we had the boat hauled to do the bottom. One larger area of delamination about 2x3'. And half a dozen blisters. I buffed and waxed while the yard did the bottom. Looking good, new zincs and prop speed, took strainers off and revealed those.
So get relaunched and ready to go the next morning, batteries and charger shot.
I had been preparing to replace batteries and also cut out plywood floor. This was the nasty part. 28 years of oil, fuel and grime. Reprinted the bilge bilge and battery boxes today. I will use 4-2"x2" SS angle to hold boxes. Going with a Mastervolt 40 amp charger.
Hopefully we will get traveling again next week, weather permitting.
Miss Donna is offline   Reply With Quote
Old 11-24-2013, 11:09 AM   #58
Veteran Member
 
Francois's Avatar
 
City: San Antonio, Boat is located in Corpus Christi
Country: USA
Vessel Name: Bluebonnet
Vessel Model: Chris Craft Constellation 500
Join Date: Nov 2013
Posts: 76
DIY swimming platform on Connie 500

Here's the pics of a swimming platform I built for my connie 500. It is 13' long by 3' wide. Munch more secure for my wife to help me dock the boat and my kids to walk on.
Attached Thumbnails
Click image for larger version

Name:	IMAG1041.jpg
Views:	130
Size:	64.8 KB
ID:	24985   Click image for larger version

Name:	IMAG1038.jpg
Views:	120
Size:	55.0 KB
ID:	24986  
Francois is offline   Reply With Quote
Old 11-25-2013, 08:15 PM   #59
Veteran Member
 
Francois's Avatar
 
City: San Antonio, Boat is located in Corpus Christi
Country: USA
Vessel Name: Bluebonnet
Vessel Model: Chris Craft Constellation 500
Join Date: Nov 2013
Posts: 76
I'm thinking to setup a Christmas tree in front of my boat for the holiday.
I'll upload pictures after this weekend cruise to Port Aransas Texas.
Francois is offline   Reply With Quote
Old 11-26-2013, 09:34 AM   #60
Veteran Member
 
City: ChicagoIL home, orange Beach, AL now
Country: USA
Vessel Name: Miss Donna
Vessel Model: Chris craft constellation
Join Date: Nov 2013
Posts: 52
Does not seem to be a way to start a new tread.

What size packing do I need for my Connie with a 2 1/2" shaft? I tied 3/8" and am no t having good results.
__________________

Miss Donna is offline   Reply With Quote
Reply

Tags
connies

Thread Tools
Display Modes

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





All times are GMT -5. The time now is 12:28 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012