RPM's And Fuel Used Per Mile

[BandB]

Fwiw just about all of the data you see manufacturers throw around in advertising their boats ranges use data from the engine manufacturers. The boat test of the sea ray cited in post 69 does so as well. The data is somewhat useful but not as useful as a true propeller power curve which can better quantify the numerous real world variables all boats deal with while underway.

In my experience, very little of the data is based on what engine manufacturers provided. The post in 69 is based on a real test of the boat. The test information I cited earlier was all from actual tests. All the information I share in this regard is based on actual real world data. Yes, there are parameters. The tests indicate those such as "Acme 4 blade 23x26.5p, 3 persons, 30% fuel, 50% water, 50 lbs of gear, 78 degrees, 67% humidity, wind 15-20 mph, seas choppy."

Many of the boat and engine combinations have been around for a long time. Boats are regularly tested by boat manufacturers and by third parties. Now, for the information builders use in advertising, they do tend toward being conservative.

 

[FOG]

Good information. I now know how to achieve what I wanted.

Thanks to all for your feedback.

 

[Martin J]

Well jst did a quick check of miles covered and what I've ran through the floscans, and my 120hp 30 year old lehmans are doing .801gall per mile (both). In a previous post I mentioned some problems with high engine room temps and efficiency. We installed high cpacity exhaust fans changing the e.r. air every min and blowers pushing outside cooler air directly at the engine intakes. Ball aprk we are running at 20% less fuel at the same RPM and 3/4 knot faster. We've dropped the E.R temp by a good 25 degrees F. All the rest of the Engine room componenets, batteries, a/c pumps, water pumps are performong better. When I started this project to increase my efficency I found very little info generally. However I did find some helpful sites. Which showed some possible gains, wereas the engines temps have remained stabil the ability of the engines to breathe cooler denser air and get more bang for your buck has been very positive.

Previously when I got the boat E.R Temps could hit 130F now we cruise at 1800 (7.3kts) with E.R. temp around 103F..

 

[smitty477]

Hello Martin J


"Ball aprk we are running at 20% less fuel at the same RPM and 3/4 knot faster."


If you are running 3/4 knot faster at the same rpm it has nothing to do with engine room temps.
Taking that 3/4 knot increase at the same rpm observation further you will see less fuel burned per distance if you have increased speed at the same engine rpm.

 

[Nomad Willy]

Runnin on one engine IMO presents different conditions. Overloading is easy to do. I found that if I throttled up to the point where rpms increased and speed did not as much then backed off a bit the engine seemed not to rise in temp or sound loaded for what that's worth. Furthermore I backed off the point where steering became difficult. On my44' MY that was about 6 kts.

Bayview,
Sounds like running both engines would be better.

 

[cafesport]

I agree with smitty, if the boat is going faster it's because it's likely lighter, more slippery or the props are cleaner. Have a look at the article Tony cited in this thread called propellers move boats then take a look at this PDF from David Gerr about power curves: http://www.gerrmarine.com/Articles/EnginePowerCurves.pdf


I'm curious as to what temps your getting once everything gets heat soaked, say running 50 or so hours continuous. It's one thing to bring down the charge air temps. Cooling fuel and big chunks of metal are more problematic. Our er seems to run the same temps in Maine as the Bahamas once everything has been running for a few days. It's great for drying laundry underway.

Bayview if you are propped right there is plenty of reserve power available to run on just one engine. See the Gerr PDF in the link above. You can run it like you would run a controllable pitch prop and just watch the exhaust gas temperature to stay away from the edge of the curve.

 

[markpierce]

I posted that once and got a bunch of crap about it. One thing for certain though, in a displacement or semi displacement boat, your fuel economy goes down very quickly above hull speed.

My fuel consumption more than doubles going from one knot below hull speed compared to hull speed. The boat can't exceed hull speed. So, normal speed is hull speed minus one.

 

[Tony Athens]

Sounds to me like you found your "Magic Speed"..


Tony

 

[Martin J]

Hi Smitty
During my research I came across a engine research facilty, very comprehensive, thats were I came across the most easily understood research info spreasheets etc. You could plug in air temp, speed, altitude, air pressure a huge load of varibles. from this I was able to increase my HP at a given RPM to obtain better combustion , by the engines ingesting colder and denser air. Previously when in the open sea I would notice that the tachs would dip slightly as seas passed underneath the boat etc. from this we were aware we were not getting constant BHP (it was dipping and rising as the props came under load) due mostly by imgesting hot air at 130F. The calcs proved that the BHP would increase to near original Lehman test figures if we could get the temps down to under 100F. Previously the engines were ingesting hot thin air and making up for it by increasing the fuel and we were using higher RPM to make the speed. Basically running very inefficiently. The engine room was 15ft 6in x 9ft 6inx 6ft 4" not including Bilge areas The previous owner had just one extractor fan a small PAR which extracted som 350 cu ft of the total 950cuft every min. This coupled with the natural aspiration of the engines would not control the heat to a reasonable level. Currently I have some 1000cuft extraction and some 700cuft intakes aimed at the engine intakes, plus the vented exterior engine room deck door. Plus the engines themselves ingesting cooler air. This has provided better combustion and more BHP for any given RPM, with leaner fuel consumption etc. Constantly smmother RPM giving better speeds in a seaway etc.
If you look on line there is a very comprehensive manual by caterpiller concerning the need for cooler efficient engine rooms vs performance.
Some of this may sound wrong however in practice, the boats performance is up!
Unfortunately I cannot give you the links (they are in my desktop), but will add them to this forum when we return from cruising in January.
I am sure when I publish the links, many other members will be reviewing there engine room temps. Part of our log routine is to record E.R. Temps

Safe sailing!!

 

[Martin J]

Smitty

Heres the link to the Cat manual http://s7d2.scene7.com/is/content/Caterpillar/CM20160713-53120-44971

And a good article BySteve D'Antonio Venting the Engineroom - Professional BoatBuilder Magazine

 

[North Baltic sea]

I found a very accurate fuel consumption Nordig 37-39 Tug boat test made. I was surprised how low consuptions you can go, if necessary, NT 7 nautical miles 1 gal of about 6 knots. I like Boat test site because they have such a specialist factual.


link which you interested NT precise consumption figures.


Kippis!


Playing - Nordic Tugs Nordic Tug 39 (2014-) - | Nordic Tugs | Powered By BoatTest.com | 283

 

[smitty477]

"Hi Smitty
During my research I came across a engine research facilty, very comprehensive, thats were I came across the most easily understood research info spreasheets etc. You could plug in air temp, speed, altitude, air pressure a huge load of varibles. from this I was able to increase my HP at a given RPM to obtain better combustion , by the engines ingesting colder and denser air. Previously when in the open sea I would notice that the tachs would dip slightly as seas passed underneath the boat etc. from this we were aware we were not getting constant BHP (it was dipping and rising as the props came under load) due mostly by imgesting hot air at 130F. The calcs proved that the BHP would increase to near original Lehman test figures if we could get the temps down to under 100F. Previously the engines were ingesting hot thin air and making up for it by increasing the fuel and we were using higher RPM to make the speed"



Hello Martin J,


It is possible that you can make more hp at a particular rpm by adding colder intake air. It is possible that you are making better mpg by any number of methods.
But you cannot make better speed at a set rpm unless you are making changes to a propeller, transmission or the boats hull and/or running gear is cleaner.

You posted this relationship about rpm and speed again here....
"we were using higher RPM to make the speed"
RPM and HP produced in a diesel are not related closely at all. RPM and fuel use is mostly dependent upon the load on the engine and not the rpm.

 

[smitty477]

"I found a very accurate fuel consumption Nordig 37-39 Tug boat test made. I was surprised how low consuptions you can go, if necessary, NT 7 nautical miles 1 gal of about 6 knots. I like Boat test site because they have such a specialist factual."

I do not know anything about that particular model or the resultant numbers of fuel use vs speeds. But the relative % changes between 9 knots and lower speeds down to 4 knots is pretty consistent with just about any diesel boat I have seen. In other words it is typical that very large increases in distances covered per gallon occur with fairly small decreased in speed when you are under 9-10 knots. Often you can make very large changes in fuel use by going just a wee bot slower when at these speeds. It often makes boat comparisons hard as the data is not typically available to compare these details.

 

[Tony Athens]

Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


Tony

 

[sunchaser]

Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


Tony

Absolutely. In big head seas my smooth water average speed of say 7.9 knots at 1700 RPM will drop below 6.5 knots at same RPM. Climbing a large ocean swell shows the same, going uphill as they say. Then throw in 25 knots of breeze on the nose

 

[BandB]

Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


Tony

They all state clearly what the conditions are and you are correct they aren't done in nasty conditions. Now, the average person here doesn't boat in nasty conditions.

 

[sunchaser]

Now, the average person here doesn't boat in nasty conditions.

Correct, but ----. One person's average may well be another's extreme.

 

[BandB]

Correct, but ----. One person's average may well be another's extreme.

Very true. And Tony's right about the dangers of just taking numbers and not looking at situations. We have usage numbers over the life of our boats at various speeds. But none of those necessarily reflect the conditions we will face tomorrow.

I ran across someone in a panic because his fuel consumption was so much worse than he'd seen on performance charts. He'd just come south, the entire trip in the Gulf Stream, he'd had a strong wind against him the entire way, and he's been in 6-8' seas the entire trip. I realized how much trouble he was in though when I found out he didn't know the Gulf Stream flowed South to North. He had no idea the current against him.

 

[North Baltic sea]

I have only one long journey, this nordic tug. time to her home in the northern German into last fall. about 600 nautical miles and an average speed of 15 knots and I can say that the consumption of the same, as the test boat, but in 2600 The motor spins a little smaller than the test. wave was all the way from 5-10 feet swollen. upcoming boating season when the sea ice cover has melted to give me the whole truth. It's great that Tony seaboard commented here in the forum, I just got a new raw water pump him.