Optimum speed for best fuel consumption?

[DPH]

Cheers for that.

Would you run for a long time at 3500 rpm though? I feel 3200 is a sensible limit for long cruises, but I've got no idea if I'm correct in that assumption.

<hr width=100% size=1>

 

[oldgit]

Surely any boat is most economical at the designed displacement hull speed.
Can get 1.5 gph at creepy crawly speed but gets much worse going any faster.

<hr width=100% size=1>If it aint broke fix it till it is.

 

[Deleted User YDKXO]

Absolutely right, tcm. Max hull speed in displacement mode for the P360 is going to be about 7.8 knots (1.34 x sq.rt LWL with LWL assumed to be 34') so most efficient speed is going to be about 4 - 6 knots. I think 10knots for the Fairline 50 is a bit higher than the most efficient speed 'coz max hull displacement speed is about 9 knots assuming 45' LWL
The other most efficient speed for a planing hull but not nearly as economical as displacement speed is just beyond the hump speed which is probably about 12 -14 knots for the P360 so the most efficient planing speed is going to be about 15 - 17knots

<hr width=100% size=1>

 

[jfm]

Re: Volvo digi display, fred drift

Thanks observer. Dont go to the trouble of digging out the manuals, we are stitched up with EVC so the software is probly a bit different from EDC. I will case up the boat this weekend to decide if these things will fit, I hope they wont becos I just discovered they are £2500 for the pair, and I just blew my boat pocket money on a new 10 inch Raymarine display unit this afternoon

<hr width=100% size=1>

 

[jfm]

Why low speed = more mpg

I agree tcm's low speed analysis, but I dont think it has anything to do with displacement design speed.

If (and it's a big if, which I'll return to) a propeller has the same slip ratio in all steady state circumstances, then the best mpg is in gear at idle speed. This must be so, becos any given say 100 mile trip is going to take a fixed number of propellor revolutions whether at 4 knots or 40 knots if the prop is a fixed pitch and always slips say 20%. That in turn means a fixed number of engine revolutions for the 100 mile trip, whether 4 or 40 knots. So, the best mpg is given if the engine sucks in the least amount of fuel per revolution. Self evidently that occurs at idle throttle setting, as the whole purpose of the throttle control is to squirt in more fuel per revolution.

Now this theory is 100% accurate for a car (ie, max mpg is ALWAYS idle speed in top gear) but it can fail for a boat if the hull/prop dynamics result in more prop slip at low speed than high. I dunno if that occurs, but I doubt it.

So, max rpm is at idle speed in gear. (Pretty boring way to sail though)

<hr width=100% size=1>

 

[hlb]

Hence my 18 knot figure. 13/14 would be ideal, but it's forever going to be sinking down again. Then strugeling to plane on the same throttle settings. So a bit faster.

<hr width=100% size=1> Get Yer Flags ear</font color=red>
Haydn

 

[Deleted User YDKXO]

Re: Why low speed = more mpg

Yes, I agree with you, the best mpg is going to be at in gear idle speed which on something like the P360 is going to be about 5 knots
All I was saying with regard to max displacement speed was that every boat has a max displacement speed commonly calculated as 1.34 x sq rt of waterline length which in the case of the P360 would be about 7.8knots and that fuel consumption climbs considerably as max displacement speed is approached. So it would be a proportionately much less efficient to run the P360 at, say, 7 knots, rather than at 5 knots even though the boat is in displacement mode in both cases
Obviously, planing and semi-planing boats have the power to create hydrodynamic lift and therefore lift themselves out of displacement mode into planing mode but at the penalty of a huge increase in fuel consumption

<hr width=100% size=1>

 

[ianainge]

Re: Why low speed = more mpg

Sorry Mike and Matt the figures say different, TCM is correct about the 50 Ft fairline, as per my original post trawler yachts and 50ft planing boats appear to have the best consumption at about 10 knots, whereas 35-42ft have the best consumption at around 2000- 2300 rpm about 22-24 knots. I have just received the figures for a Princess 38 and fairline 40 with same engines as mine, figures from boat test in MBY, least efficient figures are for 7-10 knots and 29-33knots. best are around 21- 23 knots. MBY figures not mine.

<hr width=100% size=1>

 

[Deleted User YDKXO]

Re: Why low speed = more mpg

Ian, whats the basis for your statement that 50' planing boats achieve their best consumption around 10knots and 35-42' boats around 22-24knots? I think thats a very sweeping statement
I looked at the old MBY test reports for the Princess 38 (MBY july 1999) and the Fairline Phantom 40 (MBY July 2003) -because I'm an anorak and I've kept back issues! The mpg figures in the P38 report only go down to 16.2knots/1900rpm and in the Phantom 40 report down to 12.9knots/1800rpm so where did you get the figures from for the lower speeds? And then the Phantom 40 report does not support your statement that the best consumption for this size of boat is 2000-2300rpm/22-24knots as the best consumption of 1.43mpg is at 16.6knots
Your quoted least efficient figures of 7-10knots and 29-33knots basically support what I and others have been saying. 7-10knots is pushing the max hull displacement speed and 29-33knots is flat out both of which will be inefficient
I'm still with jfm on this one. The absolute best fuel consumption for any boat will be the minimum in gear idle speed which for most boats is around 4-6knots

<hr width=100% size=1>

 

[adarcy]

Re: Nearly right

I regret to say it's all a bit more complicated than that.

There are 2 scenarios to best MPG ie needing to get somewhere in a non-raggie mode = high displacement speed or semi-displacement or planing alternatively the absolute max MPG and range regardless of speed ie to make it home or cross the pond.

Scenario 2 - displacement speed has higher MPG than the other types and in essence the slower the better. For the moment disregarding tides + wind, true economy depends on minimising drag as any excess of power over drag will accelerate until they balance out at higher speed.

Following jfm's argument I regret I disagree, it doesn't really matter whether the propellor slips or not (they all do they're not geared in the water) nor how much. What matters is how much thrust is applied to the boat by accelerating a column of water backwards (actually causing suction in front of the blades but never mind it's easier to think of), how much fuel it takes to give this thrust and the drag holding the boat back determines the speed and thus the distance for the fuel used.

As the hydrodynamic drag goes up ? by the cube of the speed, the slower the better. As the speed increases so will the drag more than proportionally even before one considers entering semi-displacement or planing mode, so idle is the most economical.

Regarding ianainge's comment, I believe that it is not easy to measure fuel consumtion accurately on large diesels at idle as so much of the fuel is being recirculated back into the tanks rather than consumed. How the boat builders or mags do it, I don't know but any engine manufacturers fuel consumption graphs and propellor curve law graphs show idle to be better than revs for 10 knots.

Of course, powerful engines may mean idle speed is 6+ knots on fast boats so trying to go slower than in-gear speeds by trolling valves or shifting in and out is comparatively inefficient.

Scenario 1 - Although the MPG falls dramatically whilst running in semi-displacement mode or just climbing onto the plane, it then falls dramatically as the boat settles - usually 2-3 knots above true planing speed. hlb is quite right saying 17-18 kt if you plane at 14-15 so as to avoid getting knocked off the plane would give the best "real life" consumption. This is because the whole idea of a planing hull is that it takes more power (eg higher drag) and converts it into lift as it goes through the planing process but, from then on the drag and, crucially the increase in drag with increasing speed, is very much smaller than with other hull forms.

So, if you want to get somewhere in a reasonable time, the best MPG from any speed above when the trim starts rising on a planing hull, say 7-10 knot according to waterline length (vide Deleted User), is 2-3 knots after it has "settled" on the plane. Occasionally there are "unusual" hulls ( see the recent Riva in MBY) and racing hulls which have different characteristics.

So, we are all agreed then? Well actually NO.

All the boats that I think we are talking about (eg P360s) have twins and it would be more economical to run on one engine at or, if necessary, just above idle.
Consider a boat with one or both engines at idle in neutral, zero miles I agree but one case consuming twice as much as the other. The fuel is being spent overcoming internal engine, pump and accessories (alternators etc) drag. When the engine(s) are put into gear their fuel consumption will go up slightly as the govener/electronics maintain the same revs with the gearbox and prop drag. Now say two engines each giving X shp drive the boat at 4-6 knots, one at the same X shp may drive it at 3-4 kt for nearly enough half the fuel consumption. Even if one opened the throttle enough to drive it at the 4-6 kt one engine producing 2X shp will consume less than two producing X shp at idle as there is only one set of internal engine losses to consider.. There should be the same underwater drag only minimally increased by having to vector the rudders to maintain a straight course.

The 3-4kt one engine or, less good, 4-6kt twin engine will still require our intrepid ianainge to read/use the tides or tack across them and he will, no doubt, have rigged his aft canopy and bimini to catch the wind so that he may have a safe homecoming on minimal fuel.

Well at least he didn't ask about oars or the relative merits of a 2 or a 2.2 hp outboard on a 36footer!

<hr width=100% size=1>

 

[Deleted User YDKXO]

P360

Ian, there's a test of the Princess 360 in the Sept 1993 issue of MBY although not with the 63P engines. PM me if you want a copy

<hr width=100% size=1>

 

[jfm]

Who is nearly right?

Adarcy, yuo misunderstand what I said about propellor slip. I'm afraid you are not allowed to disagree with me . What I said in the constant prop slip hypothesis was entirely hypothetical and was self evidently, axiomatically, true. So sorry, but you cannot disagree I didn't say prop slip is constant on a real boat.

A couple of other things
1. yes most diesels recirculate the fuel back to tank but it's dead easy to measure fuel burn because seperate sensors measure fuel flow and return and subtract to get the net burn. This is standard gear on many modern diesels (all EDC, EVC volvos, frexample)
2. I think you have suck and pressure the wrong way - fluids only ever push, they dont pull. Try making a tow rope out of water, it wont work. Try putting water into a hydraulic cylinder and it will work. This doesn't really matter though.
3. you say mpg=max when speed=min but your reasoning contains only half the relevant factors (though, coincidentally you still get the right answer). The half of the argument you do use is that drag rises by perhaps cube of speed, so the slower the boat the less drag to overcome by burning fuel. That's 100% true. However there's another factor which is that a big diesel is thermally less efficient at low rpm than high, so by travelling at low speed you inflict the extra fuel cost of lower thermal efficiency. Now, as it happens the drag curve is steeper than the thermal efficiency curve so by going slowly you take 3 steps forward (less drag) and one step back (poor efficency), and net net you still have more mpg at slow speed as you said (only not entirely for the reason you gave)

<hr width=100% size=1>

 

[[2068]]

Re: Nearly right

#####
Following jfm's argument I regret I disagree, it doesn't really matter whether the propellor slips or not (they all do they're not geared in the water) nor how much. What matters is how much thrust is applied to the boat by accelerating a column of water backwards (actually causing suction in front of the blades but never mind it's easier to think of), how much fuel it takes to give this thrust and the drag holding the boat back determines the speed and thus the distance for the fuel used.
#####

I might be being thick here, but isn't "how much fuel it takes to give this thrust" the key statement here ? You say that slip doesn't matter, but at low rpms there is going to be huge amounts of slip which is churning water outside the "tunnel", rather like driving with a slipping clutch. Trying to follow where the energy is going, the slipping clutch heats up, and a slipping prop churns and heats the water. The fact that slipping is lowering the thrust (and hence load) on the engine isn't the point, the wasted energy is.

I suspect that props being more efficient at higher revs, vs. increasing drag at higher speeds is what gives some boats an almost flat consumption curve at planing speeds.

dv.

<hr width=100% size=1>

 

[ianainge]

Re: Why low speed = more mpg

Mike

Wasnt meant to be a sweeping statement i looked back through the cpoies of MBY mags and found that out of the 50 footers with planing hulls that there were tests on, the best consumption was at low speeds sub 10 knots like semi -displacments whereas the smaller planing hulls achieved a better consumption at higher speeds, there are obviously going to be execptions to the rule, Im no expert on any of this just reported my findings publish by MBY to try and get the answer to my original question about my boat , As for the test figures for the Phantom and fairline They were faxed to me by a fellow forumite not sure of there origin (assumed MBY)I will confirm where they came from and if required send them on to you.

<hr width=100% size=1>

 

[mjf]

Re: Why low speed = more mpg

Ian, They are P38 MBY July 1999, Fairline Phantom 38 (not 40 but with 40 loa!) again MBY undated.


Can I ask something? Looking at the thread - could this be why some delv. skippers only run long distances on one engine - changing the motive power every 12 or 24 hrs?. If so, then for me the 1/2hr run from Portsmouth up to port solent could save some fuel by running on one engine.
You actually still get good SOG if you carry the tide and I was surprised how good the steering was too running just one unit.

I hasten to add I did this once to balance the engine hours after a small service issue with one that caused the inbalance of running time. But.....

<hr width=100% size=1>

 

[tcm]

portsmouth

The signage says that you have to go through porchester lake "as quickly as possible". Mind you, the speed limit is 10 knots, so i spose you have to go through at 10knots.

Also, [pedantry] note that according to the almanacs the speed limit in portsmouth harbour is speed through water, not speed over ground [/pedantry]





<hr width=100% size=1>

 

[adarcy]

Re: Cringe quake

but fools rush in...... jfm

I am most pleased you are still convinced by your hypothesis...........<BG>

1. Yes I agree (I was trying to be polite to ianainge/MBY tests or his memory of them)

2. <<fluids only ever push, they dont pull>>
another interesting hypothesis - I assume we agree for shorthand that the fluid's innate self doesn't do it but the force exerted by pressure is what we are talking about.
<<(actually causing suction in front of the blades but never mind it's easier to think of)>>
Fluids being (almost) incompressible transmit pressure eg your hydraulic cylinder but they sure suck eg osmotic pressure across a semi-permeable membrane or sucking fluid up into a syringe
Axshully I was again trying to limit my ramblings by using shorthand but the front face of props do pit because of microcavitation where the pressure is reduced (?partial pressure of vapourisation - I forget) just as I believe it is now widely accepted (the last refuge of a scoundrel) that aeroplane propellors work by reduced pressure in front of the blade rather than compressing the air behind the blade. Honestly though I did say <<never mind>>

3. You couldn't be more correct, accurate and succinct
In my example of X and 2X shp I was only trying to get people to focus on "losing" the inherent energy waste of having a second engine running but you are spot on (IMnotsoHO) that the one running engine will have a better specific fuel consumption when putting out 2X than X shp.

Friends I hope or do you want another round?

<hr width=100% size=1>

 

[terryw]

Max revs for KAMD300 is 3950 RPM, but I think it is approx 100 RPM less for the KAD300. Cruise should be at about 80% of max.

<hr width=100% size=1>

 

[adarcy]

Re: Fuel to thrust

Thank you Dave

the fuel "cost" of getting a certain amount of thrust which will equal the drag (at a steady state speed) is what matters.

To take your other point <<props being more efficient at higher revs>>
obviously high-speed props, I honestly don't know.
In an Einstein action/reaction sense, it shouldn't make any difference if you consider your effective thrust as shooting a column of water backwards ie as if your were stationary or the prop metaphorically winds the boat forward as a screw thread on an incompressible fluid it is still the effective thrust we are interested in.
Intuitively (so it's probably wrong) a steep pitched prop will work more efficiently when it's going forwards through the water but logically (and peace to jfm for switching to columns of water) if your prop jets water astern at a certain speed Y knts then if the prop is moving forwards at n knots does your column not get "pushed behind" the boat at (Y-n) knots? Maybe I have just thought of the "proof" that it is suction that is doing the work (ducks)

I think we need a fluid dynamicist but AFAIK props are still a black art.

<hr width=100% size=1>

 

[Observer]

KAD300 max engine speed is 3800rpm but I disagree with 80% of max engine speed as a 'fixed' cruise speed. That would be 3040 rpm on 3800 rpm max which is way too low. The plate which comes with the engines recommends 200rpm below obtained top speed which would be 3600 rpm. I'd set that as max. cruising rpm with a range down to about 3000 rpm.

<hr width=100% size=1><A target="_blank" HREF=http://aflcharters.co.uk>Dream Dancer</A>