rustybarge
Well-Known Member
Just guessing you do a bit of pootling, fishing, trolling, picnicking, barbecuing...........
The costs of pulling the throttles back
- Thread starter dylanwinter
- Start date
Just guessing you do a bit of pootling, fishing, trolling, picnicking, barbecuing...........
Firefly625
Well-Known Member
Just guessing you do a bit of pootling, fishing, trolling, picnicking, barbecuing...........
No, not a lot. Swmbo just wants to get where we are going, so it's 20 minutes pootling up Hamble river then throttle up to cruising speed until we get to some Solent destination. No trolling, fishing allowed...
BartW
Well-Known Member
I know this is not relevant to your boat but I did this for my existing boat about 3 yrs ago using the lph data from the Cat engine instrumentation and noting the GPS speed. I chose a flat calm day with the boat having about 50% fuel/water and blackwater tank loads and 3 people on board to represent an average load. The hull was clean and at every speed, I optimised the boat speed using the trim tabs. The nmpg v speed curve looks like this
3 things surprised me. First, at D speeds, the fuel consumption climbs significantly from minimum in gear speed of about 7.7kts to a fast D speed at about 10kts at which the fuel consumption is more than doubled. Second, at least on my boat, there doesn't seem to be an inefficient hump speed at which going faster uses less fuel. Third, there is little difference in fuel consumption between 15kts and 25kts and between 20 and 25kts, the fuel consumption in nmpg terms is constant. I suppose this makes sense in that a planing boat design like mine must be optimised to cruise in the 15-25kt range.
FWIW, I then used these figures to construct a range v speed curve for my boat. The figures given for range assume zero safety margin
Just a bit of a caveat here. I don't know how accurate the lph data from the engine instrumentation is, especially at v low speeds. However, based on actual fuel consumption over the 4 seasons I've owned the boat, it does consume fuel at an average rate of around 0.65 - 0.7nmpg so the engine instrumentation data is there or thereabouts
thanks for posting Mike, interesting,
thats the kind of graph I wanted to make,
(although I would use liter per Nautical mile, but all the same principle)
about your observations,
1) indeed the increase in consumption from lo to hi displacement speed is remarkeable !
2) yes I experienced that for a bigger boat there is not something as going over the humb,
could do the test on my Karnic, but she has so much headroom on power that she never feels to struggle going over the humb, so I guess a similar curve.
I think that with a petrol engine, the curve would look different, you would need more torque, more RPM to go over the humb, and then ones on the plane, you can bring down RPM, ... and consumption (?)
3) yes that was also my experience with the Karnic, there is a planing speed range where the consumption / speed does not change,
higher planing speed, is boat more out of water, is less water moved, is less drag, ....
I just found a "power curve" of my MAN engines,
the propellor power curve indicate:
a little less than 150l/engine @ 2000RPM
about 25l/engine @ 1000RPM
this is exactly confirming our experience,
so I can construct the approximate consumption graph, using this prop power curve, by taking note of RPM / speed figure
D
Deleted User YDKXO
Guest
Be careful with prop curves because in theory they are only valid for a specific boat/engine/prop combination. For example, a prop curve for a D boat having the same MAN engines as Blue Angel would be very different to a prop curve for a SD or P boat with the same engines. In the case of Caterpillar, they do publish prop curves but they make it plain that the curves are for D boats. My last 4 boats have had Cat engines and in every case, the prop curve overstated the fuel consumption at a particular rpm compared to what I thought the boat was consuming. Before relying on the MAN curve, you need to know for which type of boat it has been prepared
jfm
Well-Known Member
Deleted User, while I haven't written down the data I think your graphs are pretty accurate as regards their shape. From watching my litres/per mile readout for many hours cumulatively I am sure that:
1. D burns less fuel per mile than P
2. There is a big range of litres per mile figures within the D spectrum. Faster D (but still less than hull speed) burns much more fuel per mile than slow D and range at 5 knots can easily be 2.5x range at 10 knots or whatever
3. Once you are planing the curve is nearly flat. You get similar mpg whether planing slow or fast
all of which is exactly what your graphs say
1. D burns less fuel per mile than P
2. There is a big range of litres per mile figures within the D spectrum. Faster D (but still less than hull speed) burns much more fuel per mile than slow D and range at 5 knots can easily be 2.5x range at 10 knots or whatever
3. Once you are planing the curve is nearly flat. You get similar mpg whether planing slow or fast
all of which is exactly what your graphs say
MapisM
Well-Known Member
100% correct, in principle.
We already debated the matter of prop demand curves in this old thread (just in case B would be interested in more details):
http://www.ybw.com/forums/showthread.php?178775
Otoh, the bigger/heavier the boat (and BA is neither small nor light), the less important this distinction becomes, so I would think that whatever prop demand curves MAN published shouldn't be much far from the truth.
B, maybe you could post the sheet you found? Or just check whether it shows the "exponent", as the correction factor used to calculate the prop demand curve is usually called. Quite likely 2.5, 2.7 or 3.0.
Besides, true as it is that Cat normally assume 3.0 (which relates to D boats) in their prop demand curves, there are around also Cat spec sheets where 2.5 is used instead.
Frinstance, this is a C32 1900hp sheet whose prop demand curves are calculated with a 2.5 exponent:
http://www.pon-cat.com/PageFiles/19...25) SPEC SHEET (2.5 PROP DEMAND EXPONENT).pdf
Now, if you compare these numbers with those calculated with the 3.0 exponent on the same engine, you will see that the differences are (obviously) relevant only at very low RPMs.
Bottom line, taking also into account that for BA it would probably be more appropriate an exponent nearer to 3.0 than to 2.5, I'm not surprised to hear that B experience matches nicely the MAN prop demand curves which he found.
As an aside, I'm more surprised to hear that on your boats the Cat curves overestimated the actual fuel burn.
In fact, for P hulls the 3.0 factor is surely too optimistic, and regardless of whether for your boats the ideal factor would have been 2.5, 2.7 or whatever, any lower factor actually means a higher, not a lower load (hence fuel burn), at any given RPM other than the max.
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D
Deleted User YDKXO
Guest
You are quite right, Mapism. Another senior moment. It was the max power curve which overestimated my fuel burn and the 3.0 prop curve which underestimated it so my actual fuel burn was falling between these 2 curves and yes a factor around 2.5 sounds about right
MapisM
Well-Known Member
LOL, no worries, and welcome to the club!
