Most economical cruise revs 5.0 GXI/DP

[ricky_s]

I've googled this one to death but can't find the answer.
What is the most economical (ish) revs for a Volvo Penta 5 GXI/DP 270hp 2006 engine?

In the harbour it will go on for ages at bugger all revs/4.5 knots when I am just pottering round.

When at sea it planes at just after 3000 revs which gives around 17 knots with a clean bottom.

Would I be better going 25 knots with just under 4000 revs or as slow as I can plane?

 

[oGaryo]

I've googled this one to death but can't find the answer.
What is the most economical (ish) revs for a Volvo Penta 5 GXI/DP 270hp 2006 engine?

In the harbour it will go on for ages at bugger all revs/4.5 knots when I am just pottering round.

When at sea it planes at just after 3000 revs which gives around 17 knots with a clean bottom.

Would I be better going 25 knots with just under 4000 revs or as slow as I can plane?

I don't have the same setup (boat, engine, drive & prop all different) so not sure how helpful this will be but the chap who had the boat prior to myself fitted a fuel flow meter and discovered the best fuel consumption was at 24knts with Merc 5.0Lmpi & Alpha 1 drive fitted.. prior to that he was convinced that 21knts was the most economical

 

[omega2]

The general rule of thumb is If you can pull max revs as per book then cruise at 200 rpm below that.

 

[AndieMac]

The general rule of thumb is If you can pull max revs as per book then cruise at 200 rpm below that.

Ricky's motor is a 5 litre MPI petrol V8, I think you may be referring to a diesel.

I would have thought revs close to 3500 ish.

 

[Solitaire]

I've googled this one to death but can't find the answer.
What is the most economical (ish) revs for a Volvo Penta 5 GXI/DP 270hp 2006 engine?

/QUOTE]
Before we had the web we used to work it out for ourselves! Go out on your boat and see what your own set up does. Take a note of what fuel you put in and how much fuel you use based on your usegage. Set up an Excel spread sheet with a formula for both hours run and NM travelled - then you will know what your boat does.

I did this years ago and decided that it was really a waste of time. Go boating, enjoy it and if you can't afford it....................................... sell it!

 

[MapisM]

What is the most economical (ish) revs for a Volvo Penta 5 GXI/DP 270hp 2006 engine?

I'm afraid that the answer to your question is it depends. From the hull, that is.
I've seen tests where the same engine/outdrive installed on different boats gave the best MPG at VERY different RPMs/speeds.
Otoh, you should be able to check that yourself on your boat, in a much simpler way than Solitaire suggests, because the GXi ECU sends the instantaneous consumption to the multifunction dial that you surely have on your dashboard.
I'm not sure if it can also be directly interfaced with the GPS to show the MPG in real time. Maybe not, in which case you should note the GPH at any given RPM, together with the speed in KTS at the same RPMs, which you can see on your GPS.
From there, just divide the KTS by the GPH a any given RPM.
Job done, you've got a list with the MPG at each RPM.
The RPM corresponding to the higher of these numbers is the answer to your question for YOUR boat.
Mind, the results can also change depending on the boat load, so you might wish to make such test with light and heavy load, for better accuracy.
Also, you should make these test with no current/wind.

As an alternative to the above, just follow the last recommendation from Solitaire, which is very sensible indeed!