D6-330 consumption in a 28 ft boat

[Macce]

I have a new to me Nord Star 28 /D6-330DPH, which I bought last August. I had about 1,5 moths time to use it before winterizing it. It is year model 2010.

All good, but maths as comes to consumption, do not sum up. With half tank and some gear on, it tops 35,5 - 36 kn, which is as it should. At 2500 rpm.the speed is around 22,5 kn. Test reviews tell lowest planin speed consumption being in 1.3 - 1,4 l/nm range. Now, my measured average consumption was almost 1,8 l/nm, that even involves parts driving in displacement speeds. I was avoiding hanging in semi-planing speeds, mostly planing in 2500 -2700 rpm range, and when slow, always in the range of 1000-1300 rpm. With 1200 rpm or so, it should take no more than one litre /nm.

With my previous boats I was easily able to meet the reported test consumption rates. This time rpms match for both cruising and top speed, but the consumption is selveral 10% higher than tested by the magazines. Interestingly, I found two other owners reporting online exactly the same cruising fuel flow figures, 1,8 l/nm

Anyone here with the same engine and similar boat/speed range, what figures do you get? Or, can the (playsibly) elevated fluel consumption be a symptom of something odd hapening in the engine? Acceleration is good, no smoke, no vibrations. My previous boat, with D4 260 took around 1,1 l/nm average, similar top speed of 35 - 36 kn.

 

[mcanderson]

Here you go - https://www.coastalmarineengine.com...r5W4rsk3SsTPRjZR3vrTV2nv4wMnZV4MRFM50FQQ02Qqr

I run twin D6-330s in my Bavaria 37HT and get pretty close to these.

 

[Macce]

Thanks Macanderson.

So it should be 27 ish liters per hour at 2500 rpm according to this. If 2500 rpm gives 22+ knots, it means sub 1,3 liters per nm. E.g. way lower compared to what I have. But that is "at calculated propeller load exp 2.5" whatever it means. What if the load differs?

 

[mcanderson]

There are so many variables that you won’t be able to get an accurate answer. You could pay for either an update to the software managing your engine so it displays the fuel flow, or fit the VP Easy Connect Volvo Penta Easy Connect app | Volvo Penta UK & Ireland or some other aftermarket engine monitoring system.

 

[Macce]

Yeah, I guess I need to do that to diminish my guesswork. First of all I need to cruise a bit more to get more miles and burn more fuel and look how the consumption looks like.

 

[NBs]

I'm thinking that your boat has more stuff that weighs vs a new test boat. Also, the underwater surface might be old antifoulig and rougher or slightly dirty vs new smooth antifoulig. From this difference in consumption and fuel measurement tolerance may vary, it is not an absolute truth.

 

[Farmer Piles]

Also watch your displacement speed if you want economy. your max displacement hull speed is probably only around 5kts. Anything more will guzzle fuel.

 

[sap_2000]

All good, but maths as comes to consumption, do not sum up. With half tank and some gear on, it tops 35,5 - 36 kn, which is as it should. At 2500 rpm.the speed is around 22,5 kn. Test reviews tell lowest planin speed consumption being in 1.3 - 1,4 l/nm range. Now, my measured average consumption was almost 1,8 l/nm, that even involves parts driving in displacement speeds. I was avoiding hanging in semi-planing speeds, mostly planing in 2500 -2700 rpm range, and when slow, always in the range of 1000-1300 rpm. With 1200 rpm or so, it should take no more than one litre /nm.

As you say, something doesn't sum up here.
I do quite a bit of delivery jobs for Sargo and used to do a bit for Nord Star also. I would say its impossible to get an average of 1,8 l/nm on plane or with mixed plane and slow displacement.
The Sargo 28 with D6 which weighs about 5 tons typically averages around 1,4 l/nm on a 3-400 nm delivery run with 99% of it is at 22-28 knots.
Combined planning and slow displacement will reduce the average. From memory the NS 28 had similar consumption figures.

I would get the fuel data option from VP. Its accurate to within 2%, usually less then 1% compared to doing the maths.
Is it possible that you didn't get your tank full at one fill up and that made the calculations wrong?

I can not see how its possible to get an average of 1,8 l/nm unless the bottom and drive is heavily fouled and/ or constantly doing 12-16 knots i heavy seas.

 

[Macce]

I can not see how its possible to get an average of 1,8 l/nm unless the bottom and drive is heavily fouled and/ or constantly doing 12-16 knots i heavy seaseas

Thank for your insights based on hands-on experience.

Yes, and if case of heavy fouling, the max rpm/ max speed wouln't have been Ok as they were.

As said, I need to re-measure and/or get the flow meter activated.