Fuel usage

PhilipF. My boat is an NB820 from Norway which is a Nimbus lookalike and much the same weight and size. Ours also has a 160hp D3 but a duoprop outdrive rather than a shaft drive. At displacement type speeds in Poole harbour - say 8 knots- we burn a gallon and a half an hour which is extraordinarily reasonable? Last Summer across to France and the river Somme we cruised at 3100rpm for three and a bit hours, which is about 21knots, and burned 7gph. Up the Somme to Amiens we used about 1 gph at 5 to 6 knots. Down to the Wet Country to Dartmouth we usually average around 6 to 7 gph. cruising at 20 - 22 knots. If we go to 4000rpm WOT and 28 knots, we are up to 9gph which is still a lot less than most on this forum use, I suspect. Hope this is helpful.
Martin T.
 
Excellent. That sounds like good consumption versus speed.

As others have said, to compete with that fuel consumption, the Nimbus would have to keep to low speeds...

I might suspect though that the NB820's outdrive system will be more efficient than the Nimbus's skeg, shaft drive prop and large bladed rudder providing a lot of drag; and the Nimbus has a small stub keel forrard of the prop's skeg which will add more drag, meaning she won't often lift up onto the top of the water and plane properly with only 160 hp.

Probably the designer never intended her to plane, but wanted to take advantage of the charactersistcs of a semi-planing hull.

I guess the NB820 maybe a little lighter displacement too.
All the best.
 
The rated RPM for this engine is 4k, so if it will pull only 3.5K on the stops then with a clean bum I would begin to suspect over propping, but presumably it has the original prop from its build so I find that a bit difficult though not impossible to believe.

That said the power produced between 3.5 & 4K hardly changes at all and the torque really declines dramatically after 3k all things considered I would have thought one of these should be revving at 3.5k for cruise efficiency and 4K maxed out so it may after all be worth looking at the prop or having the motor checked for output.
 
Very helpful set of replies - thanks indeed to you all.

Accept and agree with what is being said. I think I was pushing the boat too fast - much more economical if I'd been going slower. At the speed I was running the engine, the log was reading 13kts.

I shall just have to plan on taking a little longer on my journeys. Ideally of course I should always try and make use of tides, but not always easy to do this, when one also has the weather to contend with.

Philip
 
you have had a lot of information here but I am not sure you have reached the right conclusion!

1. the larger the wake you are making the more energy is being used to do this (rather than increase your speed) thus between 9-12 knots for your hull are not efficient
2. cruising in normal conditions with the tabs fully down only tends to make your bank manager happy - there are reasons you might do so associated with improving an otherwise bad situation but they also tend to fall into the above speed range which is to be avoided unless travelling in a 12knots speed restircted area when the bar is closing /forums/images/graemlins/wink.gif
3. You don't mention what your engine and hull set up actuall do achieve at various settings - ie hull speeds for 2000, 2300, 2600, 3100, 3400, 3900 and WOT This would be usefull in bringing theory and practice together. For example I do 6kn @1300rpm 8 @ 1600 9@1900 15@2200............I don't cruise at anything between 1300 and 2000 except in extreme conditions and needs!
4. I would guess your optimum cruising speed would be based on 3700 rpm engine revs unless you want to cruise at 6 knots (in which case you have the wrong engine for that really.......)
5. I don't think you hull will have the extremes I have as illustrated above and 22 lph for 13 knots isn't really that bad for a boat your size - what were you expecting? Top performers of that size will use about 1.3l/pm at planning speeds
 
I suppose you could all do what I did when I brought my Ocean 30 single screw down from Great Yarmouth to Walton on Thames, fuel cost was around 30 gallons, speeds up to 45 knots!

Mind you, it was on a low-loader - does that count?
 
I seem to be getting on average around 15gals to the hour (which is about 15 miles) or about 5 gals a mile /forums/images/graemlins/shocked.gif this is based on consumption on the same trip several times last year - I dont have a fuel flow meter (yet) but simply took distance/time and fuel used at different speeds /forums/images/graemlins/frown.gif it would appear that at 22 - 27knts I use just under half a tank to do 42nm - thats 43gals (on refill) over 43nm - this took 3 hours at that speed (average) so 43gals in 3 hours is 14.33 gals / hr - on several trips it changed slightly so thats why the 15 gals an hour average - which is about what the literature says it should be so although quite hefty on fuel - it is correct and expected ... I did I have to say on one occasion when state was f0 and mirror like sea - got to 35knts and trimmed out beutifully she used just over 1/4 tank !!! but that does not count. /forums/images/graemlins/grin.gif
 
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I've got a Navman fuel computer, can't seem to get calibration right!

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Yep, join the bloody club! Mine works fine for either set of senders individually, but try to link the two and I have been getting nothing.

Except today, where the port engine seemed to be working, but the starboard one wasn't. Then the bloody unit keepd re-setting it self befor finally going blank.

Paul from Navman is going to come and have a look on Sunday - will post what he finds.
 
. Dont think it's just you. One minute boats are doing 10 miles an hour. Next there doing 13. Then there doing 15 miles on fifteen gallon, next it's five gallons to the mile. /forums/images/graemlins/confused.gif
 
I "know" that per the fgures my AD41 necks 45l/hr at full bore and that if I stick to 7knots or less this drops like a stone but I've no real mental picture on consumption at 1500 or less rpm apart from doing Teddington to Rochester on about 15 gallons. Being a 27 footer singls shaft not too dissimilar to the original poster's boat. I've no fuel flow meter so consumption estimates are based around the gauge needle, which is porbably no better than guessing about climate change:-)
 
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