Reply
 
Thread Tools Display Modes
 
Old 03-17-2016, 11:10 AM   #41
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,174
Quote:
Originally Posted by Baker View Post
Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat. David is speaking theoretically.

I have a question......why in the heck are Floscans so damn inaccurate when it measures the fuel being burned as well as the fuel being returned???
Because of subtraction.

Say supply is 11gph and instrument is +/- 2%. that means measured flow is between 10.78 and 11.22.

Say return flow is 10gph and instrument is +/- 2%. that means measured flow is between 9.8 and 10.2.

Let's say the instrument error is on the high side of the supply instrument and the low side of the return instrument.

Supply is reading 11.22. Ret reading 9.8. Calc'd flow is 1.42gph. Real flow is 1.0gph.

So two instruments with 2% error each can result in a total error of 42%.

Part of the reason I'm not a fan of flowscans, et al. You can tweak them for one spot, but how good are they at other spots???
__________________
Advertisement

Ski in NC is offline   Reply With Quote
Old 03-17-2016, 11:30 AM   #42
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 9,916
Quote:
Originally Posted by Baker View Post

I have a question......why in the heck are Floscans so damn inaccurate when it measures the fuel being burned as well as the fuel being returned???
They're not when properly maintained and interpreted correctly.
__________________

BandB is online now   Reply With Quote
Old 03-17-2016, 11:32 AM   #43
Senior Member
 
AKDoug's Avatar
 
City: Kenai, Alaska
Country: USA
Vessel Name: Melanie Rose
Vessel Model: 1999 Willard PH
Join Date: Feb 2013
Posts: 471
Floscans have an adjustable set of switches on the back of the unit for fine tuning that are intimidating enough that I think most people just accept whatever inaccuracy the unit has when it comes pre-set. I had two different units in the past, both over read my fuel consumption, and I never made any effort to play with the adjustment switches. My over read was in the range of 5-7% over 92 gallons.

They were both on a planing hull that went through the tank of fuel in less than 10 hours, so there seemed to be little point in getting closer to perfect. That was my experience with Floscan anyway, and having to adjust two of them for engine and return line seems even more complex...
AKDoug is offline   Reply With Quote
Old 03-17-2016, 11:37 AM   #44
Guru
 
djmarchand's Avatar
 
City: East Greenwich Yacht Club, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,325
As Ski noted above, Flowscans are notoriously inaccurate at low loads.


The Flowscan installation instructions tell how to calibrate them, but I don't know how you measure real world fuel burn to have a basis for calibration unless you have a small, calibrated day tank to measure fuel over a short term. And as Ski notes you can only calibrate at one point and I suspect that the Flowscan meter isn't totally linear.


I did see an interesting posting on another thread about an incredibly accurate flow meter- 0.25% (of full scale presumably). It was based on a positive displacement meter, unlike Flowscan which uses an electronic sensor. Each individual meter is factory calibrated to achieve that level of accuracy.


Using Ski's example above and assuming an absolute accuracy of 0.25% of 50 gph full scale flow which is .125 gph, then the worst that it could read would be 0.125 high on the supply and .125 low on the return or an error of 0.25 gph. That is still a significant error while measuring flow rates in the 1-2 gph range, but not bad at all at higher loads.


David
djmarchand is offline   Reply With Quote
Old 03-17-2016, 11:57 AM   #45
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 12,668
Baker wrote;
"Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat."

Good input Baker,
Many SD hulls are hard to tell from planing hulls and almost as many are hard to tell from FD so there is a wide range of SD hulls having very significantly different performance re FD, SD and planing.

I think though if you jumped from one knot below HS on a GB 32 (SD) and noted the fuel burn increase (%) and did the same on my Willard the Willard (FD) would suffer the greatest increase. Or is that not so?
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 03-17-2016, 11:58 AM   #46
Veteran Member
 
Paul Swanson's Avatar
 
City: Ventura CA
Country: USA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 74
[I][I]Paul Swanson wrote;
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."

It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.

-----------------------------------------------------------------------------------
We are getting pretty technical here but I think a turbocharger produces more power per displacement (volumetric efficiency) for the reason you stated but the thermodynamic efficiency is about the same, except for the compression ratio thing. Turbocharging does raise the effective compression ratio since there is already an over pressure in the cylinder before the normal compression from the piston occurs. That is why turbocharged engines generally start with a lower compression ratio than normally aspirated engines. It avoids too much pressure. It is also why it is ill advised to simply bolt a turbocharger onto a NA engine.
Paul Swanson is offline   Reply With Quote
Old 03-29-2016, 01:26 AM   #47
Newbie
 
City: Crystal River
Country: United States
Vessel Name: Ailyn
Vessel Model: Mainship 36DC
Join Date: Mar 2016
Posts: 4
The turbo isn't getting enough exhaust flow at low rpm to help with the power. That is why many new engines have two turbochargers, one with a wheel for the low rpm range and a second designed for the higher rpm's. Under 1500 rpm, most older turbo engines get little help. A boost gage will give you a better idea about what a turbocharger is doing.
CmdrDick is offline   Reply With Quote
Old 03-29-2016, 01:35 AM   #48
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,309
Quote:
Originally Posted by CmdrDick View Post
The turbo isn't getting enough exhaust flow at low rpm to help with the power. That is why many new engines have two turbochargers, one with a wheel for the low rpm range and a second designed for the higher rpm's. Under 1500 rpm, most older turbo engines get little help. A boost gage will give you a better idea about what a turbocharger is doing.
It is called a "sequential turbo" and is not generally found in the boats we are talking about here. It is fairly common in cars. We operate our cars in a wide range of RPMs and power bands. So to avoid turbo lag and then a rush of power, they put the turbos in sequence...to give a more linear throttle response. Boats we generally set the power and are done with it. I guess all I am saying is sequential turbos don't really help in our application. You will see them in very high powered marine diesels....the "MTU range". I am not that familiar with the newest marine diesels so maybe they have small marine diesels with sequential turbos.....
__________________
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B
Baker is offline   Reply With Quote
Old 03-29-2016, 06:02 AM   #49
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 15,251
"but I don't know how you measure real world fuel burn to have a basis for calibration unless you have a small, calibrated day tank to measure fuel over a short term"

Many cruisers have a "normal" cruise RPM range (ours is 1000 to 1500, but most time its 1200).

A tiny tank is not required , just fill the tank and when its refilled adjust the fuek burn reading to the actual gallons on the tank fill.

Nothing to it and the set becomes more accurate after a few fills.

With a tiny 200G fuel cap , after the loop we could estimate the next fill with in a gallon or two.

The biggest inaccuracy is probably from NOT filling the tank to full to the top because of the sheen police should a tea spoon be vented.
FF is offline   Reply With Quote
Old 03-29-2016, 08:58 AM   #50
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 12,668
FF,
I'm suprised at you.
That would represent all engine running and there's a lot of running that is not at cruise speed. And like you many don't cruise at the same rpm much of the time.
Makes it look like your boat's really efficient though .. and many like that. That's why you get so many unbelieveably low fuel burn numbers .. among others.

Actually the small tank would deliver if done right. Small enough and big enough (one or two gallons?) and plumbed to be switchable .. in the system .. out. Just a lot of trouble to rig up. Way more accurate than observing the #of gallons put in the fuel tank and the reading of the hour meter.

By the way it would be nice to know who you were quoting in your opening comment above. I'd like to see who said that and the other relavant info related.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 03-29-2016, 10:30 AM   #51
Wannabe
 
Britannia's Avatar
 
City: SF Bay Area
Country: USA
Vessel Name: Stillwater
Vessel Model: Kadey-Krogen 54
Join Date: Dec 2014
Posts: 695
For me the only real interest in getting an accurate fuel burn rate is for ocean crossing. That's when it will really matter how accurately I can measure fuel consumption.

With 1200 gal tanks I can't realistically check at fill up - plus I'm not looking for an average - I want to know what the burn rate is at various RPMs in different sea states.

What I plan to do is install a small day tank or even temporarily plumb in a small gravity feed plastic tank. Then use that to measure the accuracy of my Flo Scan (or Maretron equivalent if I buy one). Once I have confidence in the electronic measurement system I can use that to monitor fuel consumption in various ocean conditions so I can get the best possible idea of what my real world consumption will be.

Richard
Britannia is offline   Reply With Quote
Old 03-29-2016, 10:48 AM   #52
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,174
One technique is to have a day tank that is kept solid with fuel, with engine both drawing from and returning to that tank. Then put the flowmeter in the makeup line to that tank. Then only one instrument required and accuracy should be quite good.
Ski in NC is offline   Reply With Quote
Old 03-30-2016, 05:06 AM   #53
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 15,251
"For me the only real interest in getting an accurate fuel burn rate is for ocean crossing."

Was on Block Island one holiday weekend when a charter fish boat went for fuel.

He knew very well how much fuel the boat would take , the dock pump read far higher .

After a short discussion with the dockmastrer it was decided there would be no cost for the fillup.

All knowledge is power.
FF is offline   Reply With Quote
Old 03-30-2016, 06:44 AM   #54
Guru
 
Dougcole's Avatar
 
City: Carrabelle, FL
Country: USA
Vessel Name: Morgan
Vessel Model: '05 Mainship 40T
Join Date: Jan 2008
Posts: 680
[QUOTE=manyboats;428383]FF,
I'm suprised at you.
That would represent all engine running and there's a lot of running that is not at cruise speed. And like you many don't cruise at the same rpm much of the time.
Makes it look like your boat's really efficient though .. and many like that. That's why you get so many unbelieveably low fuel burn numbers .. among others.

Actually the small tank would deliver if done right. Small enough and big enough (one or two gallons?) and plumbed to be switchable .. in the system .. out. Just a lot of trouble to rig up. Way more accurate than observing the #of gallons put in the fuel tank and the reading of the hour meter.

But isn't knowing your average rate of fuel burn over a long period at many different rpm, speeds, conditions, of the greatest value? That's how boats are run pretty much all of the time. For me there is very little value in knowing how much I'm burning in flat water at 1873 rpm as opposed to how much I burned per hour at normal speeds and conditions over the course of 300 or more gallons.
Dougcole is offline   Reply With Quote
Old 03-30-2016, 07:36 AM   #55
Guru
 
dhays's Avatar
 
City: Gig Harbor
Country: United States
Vessel Name: Kinship
Vessel Model: North Pacific 43
Join Date: May 2015
Posts: 3,358
I see two reasons for wanting to know fuel burn at various RPMs. One is simply being able to calculate how much fuel is left. On my boat, I burn less than 1 g/h and have a 40 gal tank. So I try to make sure I fill up as I approach 40 hours on the clock. I normally only need to fill up twice a year. (400 hours on the engine in the 5+ years I have owned her). The fuel gauge on the boat is laughably inaccurate and their is no sight tube. I was really sweating it this winter when I realized that I had run 38 hours since the last fill up and was 1 1/2 hours away from fuel. If I had either an accurate gauge or site tube, I really wouldn't care about fuel burn rates in this regard since I am never going to have to calculate max range.

Second reason is to figure out the most efficient speed to run at. At what point does the fuel burn vs boat speed really diverge from linear? I like to know that because I am inherently cheap. For this, I don't need accurate absolute numbers only consistent relative numbers.

So unless you are making long passages and need to know your range, I guess I don't see the point of knowing exactly what the fuel burn is.
__________________
Regards,

Dave
SPOT page
dhays is online now   Reply With Quote
Old 03-30-2016, 07:46 AM   #56
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 15,251
On a normal displacement hull the SQ RT of the underway LWL times .9 to 1.15 on a light really sweet hull is your LRC fuel burn.

Convert K to MPH if you normally use lubber miles to navigate , the norm inshore.

Very light boats do better bit an extra 2240 lbs (a ton) only requires 2-3 hp more.

At 16 hp/gallon , 2 hp is pretty cheap.
FF is offline   Reply With Quote
Old 03-30-2016, 07:49 AM   #57
Guru
 
City: Venice Louisiana
Country: United States
Join Date: Dec 2013
Posts: 1,097
For ocean crossing, knowing your immediate fuel burn is most important. Limited fuel capacity makes it important. Average GPH is not going to let you know that you need to slow down 2 knots in current conditions to make it across the pond with a decent reserve. My trawler has about 1000 gallons capacity now, and I plan to add another 500. Still, thats only 1500 gallons, Without accurate fuel management and knowing the burn rate "now" a 3000 mile crossing would be risky. I like the day tank arrangement for simplicity and accuracy. I have two 25 gallon tanks that gravity feed to the mains. They are fed from the belly tanks (125 gallons each) by gear rotor pumps. The belly tanks are fed by the bulk tanks. The day tanks have adjustable float switches that turn on and off the transfer pumps at preset levels. 2 gallons is the norm. A pump counter keeps track of the cycles. Check the time and reset the counter. In a few hours you have a very accurate GPH estimate.
kulas44 is offline   Reply With Quote
Old 03-30-2016, 09:09 AM   #58
Wannabe
 
Britannia's Avatar
 
City: SF Bay Area
Country: USA
Vessel Name: Stillwater
Vessel Model: Kadey-Krogen 54
Join Date: Dec 2014
Posts: 695
Quote:
Originally Posted by FF View Post
"For me the only real interest in getting an accurate fuel burn rate is for ocean crossing."

Was on Block Island one holiday weekend when a charter fish boat went for fuel.

He knew very well how much fuel the boat would take , the dock pump read far higher .

After a short discussion with the dockmastrer it was decided there would be no cost for the fillup.

All knowledge is power.
I didn't say I wasn't interested in how much fuel I have in my tanks.
Britannia is offline   Reply With Quote
Old 03-30-2016, 09:18 AM   #59
Wannabe
 
Britannia's Avatar
 
City: SF Bay Area
Country: USA
Vessel Name: Stillwater
Vessel Model: Kadey-Krogen 54
Join Date: Dec 2014
Posts: 695
Quote:
Originally Posted by kulas44 View Post
For ocean crossing, knowing your immediate fuel burn is most important. Limited fuel capacity makes it important. Average GPH is not going to let you know that you need to slow down 2 knots in current conditions to make it across the pond with a decent reserve. ...
My point exactly. Ocean currents and sea state my mean a considerable reduction in RPM in order to make the crossing with available fuel & reserve. Knowing the burn rate accurately becomes important if the passage is near the limit of your range.

Richard
Britannia is offline   Reply With Quote
Old 03-30-2016, 09:58 AM   #60
Guru
 
City: Seaford Va on Poquoson River, VA
Country: United States
Vessel Name: Old Glory
Vessel Model: 1970 Egg Harbor 37 extended salon model
Join Date: Jan 2016
Posts: 996
Quote:
Originally Posted by FF View Post
"For me the only real interest in getting an accurate fuel burn rate is for ocean crossing."

Was on Block Island one holiday weekend when a charter fish boat went for fuel.

He knew very well how much fuel the boat would take , the dock pump read far higher .

After a short discussion with the dockmastrer it was decided there would be no cost for the fillup.

All knowledge is power.
So the fuel dock was cheating stealing people's money everyone who filled up? Free fuel for the charter boat the price to keep it a secret?
__________________

sdowney717 is offline   Reply With Quote
Reply

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 01:30 AM.


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