However if you increase the weight of a boat w/o touching the throttles your speed will decrease and the prop/s will reduce rpm and fuel consumption will actually decrease. Just fewer piston strokes to inhale fuel.

Eric, you have not been reading my posts...

That is not a true statement. The governor will try to maintain set engine speed. It does that by adding fuel. So without touching "throttles"(they aren't throttles in the traditional sense), increasing load your speed might decrease but RPM will be the same and fuel flow will increase. They call it a governor on a diesel engine for a reason. It sets and maintains engine speed by metering fuel flow.

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Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B...Sold
Meridian 411...T-Cummins 450C

As to hull speed, I call it the magic speed of the vessel and each vessel, depending on the size, weight, LWL, and conditions at the time, all have this "Magic Speed' to where the vessel travels easily with the engines just loafing.. It's real typical to see 2-3 MPG of many slippery hulls in the 30-45Ft range travelling in the 7-10Kt. range. Plus, as a bonus, you can typically enjoy that wine a cheese along the journey..

Cheers,

Tony

Tony,
Welcome to TF.
Calling "hull speed" magic causes all kinds of problems for people trying to understand it. We would probably be better served if we were to ban the expression hull speed. Please don't use it again. Kidding of course but HULL SPEED IS NOT MAGIC.

Those Cat fuel curves have some strange bits. Consider the Max Power curves for the DM3308-00. Fuel consumption is higher at 1600 rpm and 202 hp than at 1800 rpm and 227 hp, and then higher still -- about twice as high! -- at 1400 rpm and 176 hp.

None of the other curves are like that. Is there something unique about the DM3308-00?

Something is wrong with those numbers. Never seen an engine where the gph went nuts like that at lower rpm.

Regarding governors: On my Cummins 450, I can be running at 1600 barely planed at about 14kts and about 6 or 7gph, about 5psi boost. Pull shift to N and engine just goes up to 1700 and sits there, boost zero. Probably 1gph burned just to keep it spinning and pumping all the juices.

Wow! With this much dissent you would think that I was a knuckle dragging mouth breathing sail boater...

So what I am gathering from the multitudes of posts are the following:

That since I extrapolated the fuel burn at 1100rpm, that the data is incorrect and the other RPMS must be as well.

That the CAT prepared data on the engines that are installed in my boat are not correct.

The travel logs that I keep showing distance traveled and fuel purchases are incorrect.

That I provide "fabricated number based on multiple choices."

Does this pretty much sum it up or did I miss something?

It's funny that I did not see one single post that would be remotely helpful in disproving that the CAT graphs were incorrect, or showing me where the math error was for the extrapolation from 1400rpm down to 1100rpm (remember, there is just the assumption that my math is wrong here as well as that I obtained this data from somebody else that did the work.)

We could debate this for days and the aviators among us could delve into all we know while quoting Aerodynamics for Naval Aviators and how it's not an apples to apples comparison to reference aviation and boating in the same sentence unless invoking the names of both Poseidon, Davey Jones, Chuck Yeager and the Wright brothers all at the same time or somebody, for the love of recreation could simply point the apparent "failed" father towards a document that would prove that the very basis of this argument, the CAT graphs, are incorrect.

The burden of proof rests on the prosecution. If the graphs don't fit, you must acquit. At this rate, I would have better luck pushing a wet rope uphill than to convince some of you otherwise.

What you are missing is that the engine at various rpms is responding to the boat's load curve, which varies with every boat.

The Cat curves are an approximation based on the exponential prop curve. An approximation at best. Sometimes a boat does follow along closely to the approximation, but that is not often.

So using a few data points your boat did come close to the Cat prop curve approximation. That's great, but if you had flow instrumentation and did an actual instrumented run and plotted the boat load curve, you would see how different it was from the theoretical prop curve.

Curves matching at a couple points does not mean much.

"So what I am gathering from the multitudes of posts are the following:
1.That since I extrapolated the fuel burn at 1100rpm, that the data is incorrect and the other RPMS must be as well."

The chart is a hypothetical representation of what your engine would burn if it had that exact loading curve. Did you believe that your boat was tested to make that chart or that your boat will behave exactly like any other boat with those engines? Do you believe that your fuel burn will remain the exact same per rpm if you add 4" of pitch to your props?

"2.That the CAT prepared data on the engines that are installed in my boat are not correct."
Same as above - the chart represents one possible loading of the engine. It is not representative of any particular boat it is a mathematical illustration.

"3.The travel logs that I keep showing distance traveled and fuel purchases are incorrect."
No - your travel logs are likely perfect.

"4 That I provide "fabricated number based on multiple choices.""
The laws of physics are pretty inflexible - it costs more hp to go faster, hp per unit of time costs more fuel, your mpg are way to flat to show the mounting resistance that boats hull has when approaching its calculated hull speed.

"At this rate, I would have better luck pushing a wet rope uphill than to convince some of you otherwise" You could make this same post over at boatdiesel - it will not go to well there I know for sure.

If you take your boat out at say 6 knots for say 100 miles and then return at say 10 knots your fuel use per mile will nearly quadruple. So there is a significance in knowing how this actually works.

If you start your fixed speed diesel genset and run with no load it will be using about 0.2 gph and when you switch on the AC units you can watch the governor add fuel as it goes up to about 0.6 gph for your AC to run. These engines are not fixed fuel per rpm so you cannot make that comparison.

Here is one boat test on a similar Sea Ray as yours - I am sure with a little time and searching you can get a few more as well.

Not trying to pile on here but I just took a quick look at your son's numbers and I think that you might have made the wrong conclusion regardless of using cats dyno numbers fuel consumption numbers.

His numbers clearly show you will get better range at lower speeds and no two speeds are equal.

As far as putting an extra fine point on fuel numbers in general I don't feel it's worth the effort and certainly not necessary. I've run the numbers repeatedly for our boat using the data output from our electronic engines and stored every second in a database. In addition we have used our calibrated day tank and a stopwatch to verify. Thirdly we always keep a ships log which records things like water temp barometric pressure wind speed and direction fuel use sog location course steered temperature data, which fuel tank is being utilized, er temp exhaust temps and about a dozen more items. With literally thousands of data points I feel really comfortable just using the averages. For example how much fuel did you buy last year how many hours did you run and how many miles did you travel. Simple is good. Now a more interesting question for me is how many gallons are really left in the bottom of your tanks when you think they are empty and do you really want to use it?

Vessel Model: 50` US Navy Utility trawler conversion

Join Date: Nov 2015

Posts: 1,065

Quote:

Originally Posted by sunchaser

Remembering this is a boy's school experiment --

Are there any TF ites brave enough to post how they would do this fuel burn experiment? Keeping in mind that an audience of perennial naysayers awaits, some of whom need help keeping track of holding tank levels.

I would borrow my buddy's whaler with a 250 hp outboard, hook up a towline to my boat with a load cell or scale to measure the tension on the towline, and tow my boat with the transmission in neutral at various speeds plotting speed vs force as measured by towline load.

Given that fuel burn is almost a linear relationship with thrust which equals towline load you could plot fuel burn against speed. No absolute numbers, but a curve that would be credible, I.e. increasing speed from x to Y consumes z℅ more fuel.

When we talk about cars, we always talk about MPG. That's a fuel efficiency measure saying how far the car can go on a gal or litre of fuel.

Any for some mysterious reason on boats we talk about GPH, and think it's a measure of efficiency. It's not. It tells you exactly nothing about fuel efficiency.

MPG, or NMPG is the only way to measure fuel efficiency, assuming that's what you are interested in. GPH is meaningless unless you divide it into your speed.

Any notion of an efficiency sweet spot on a boat is a complete myth. The efficiency sweet spot is tied up at the dock. As soon as the boat starts to move, the NMPG starts going down. And it keeps going down as you go faster. There is no high point in the NMPG curve. It's all down hill. I have never seen a trawler that doesn't behave this way. The slower you go, the better your NMPG numbers will be. The faster you go, the worse they will be. All you get to pick is the speed that you are willing to pay for.

That said, planing boats have some interesting properties. Once on plane, their NMPG numbers tend to flatten out. They are still down hill, but not by much as speed increases. My old Grand Banks was a planing hull, and between about 12 kts and 20 kts, there wasn't a huge reduction in NMPG, ranging from about 0.5 NMPG down to 0.4 NMPG over that speed range. The numbers that BandB posted show similar behavior. My rule of thumb was that if I was going to go 12 kts, I might as well go 20 kts because it wasn't going to burn much more fuel to make my trip.

If you look at outboard powered planing boats, there actually is an NMPG highpoint in the curve, and an efficiency sweet spot - at least in the planing speed range - but I think that's as much about the efficiency curves of gas vs diesel engines as anything else.

Twist,
For all practictical purposes you're right.
But I wonder .. Could a boat get better nmpg 2, 3 or 4 knots. No useful reason to know as there are very few of us happy at 6 knots much less three.

Any for some mysterious reason on boats we talk about GPH, and think it's a measure of efficiency. It's not. It tells you exactly nothing about fuel efficiency.

MPG, or NMPG is the only way to measure fuel efficiency, assuming that's what you are interested in. GPH is meaningless unless you divide it into your speed.

.

I found it odd too when I started coastal boating and I've never thought that way. Only two ways I think, nmpg and range, which is a result of nmpg.

I guess if you're just talking about spending time without real interest in destinations then gph makes sense. Also, if you are displacement and only travel in a very narrow speed range, it might. Still just doesn't tell me much that I feel the need to know or intend to use. I don't care that if I slow from 26 knots to 15 knots my gph will go from 79 gph to 33 gph. If I'm on a trip where I'm fuel challenged I do care that my nmp goes from 0.33 to 0.45 and that my range goes from 274 nm to 378 nm.

I thought at first the gph was perhaps a thing for slow boaters, trawlers and the type. However, I see captains of sportfishing boats using it. So, I don't know why.

Twist,
For all practictical purposes you're right.
But I wonder .. Could a boat get better nmpg 2, 3 or 4 knots. No useful reason to know as there are very few of us happy at 6 knots much less three.

One issue here is that the engine will burn a certain amount of fuel just to spin itself. And since most boats have way more engine than needed, you get down to super low loads and like half the fuel is burned just to spin engine, the other half to push boat. So nmpg might not be much better.

And yes, it would annoy both the engine and the crew.

Twist,
For all practictical purposes you're right.
But I wonder .. Could a boat get better nmpg 2, 3 or 4 knots. No useful reason to know as there are very few of us happy at 6 knots much less three.

Can't find many tests to indicate what might happen. Did find a Swift Trawler with a 370 hp Volvo.

Glad to pop in once in a while an try an offer a different way to look at things...

As to the term "magic", just a term that I use that describes how easily each particular hull can move forward under the vessel & weather conditions at that time . It can vary even with the same boat...Really, all that term means is just:

"Magic" for that boat based on the the particular hull and conditions at that time, the owners wallet as to MPG going from Point A to Point B, his need for getting someplace (time value), and of course the comfort of the journey itself, and what the passangers wants to enjoy along the way...........May or may not be "hull speed" based on an equasion, but certainly 7-10 kts is close enough for 90+% of boats talked about here fall into that range when being on "plane" is really not an option.........................

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