juma
New Member
- Sep 14, 2009
- 58
- Boat Info
- 31 sundancer 2000
- Engines
- mercruiser mpi 5.7 v-drives,westerbeke 4.5
I'll like to know what's the best cruising RPM setting for my 2000, 310 da ,mercruisers EFI 5.7 300hp with V-Drives.
best cruising RPM's
- Thread starter juma
- Start date
Theresamarie11
New Member
Some people say 3800, but I'd say it depends on the conditions and where you get best economy.
We can get almost 2.8 mpg at around 3200 RPM on 1-1.5 ft seas but if it's really nice and calm we'll go faster because we still get around 2.5mpg almost to WOT and it's very comfortable. For us, the conditions of the water will dictate where you end up running. A fuel flow meter is really essential to determine where your boats best economy, for the conditions, will be had.
Terri
We can get almost 2.8 mpg at around 3200 RPM on 1-1.5 ft seas but if it's really nice and calm we'll go faster because we still get around 2.5mpg almost to WOT and it's very comfortable. For us, the conditions of the water will dictate where you end up running. A fuel flow meter is really essential to determine where your boats best economy, for the conditions, will be had.
Terri
Sundancer
Well-Known Member
- Jul 20, 2005
- 2,909
- Boat Info
- 34 year old CLASSIC 300 DA, towed almost anywhere behind the Duramax Dually Crew Cab.
- Engines
- 16 cyl, 700 cu. in./Alpha I's
Generally it's slowest speed you can remain on a "crisp" plane. Typically around 3,300 rpms plus or minus depending on load, props and weather. Crisp, meaning not mushing through the water and tabs adjusted for a level ride but not fully deployed (unless this is required for some reason on your boat?). I usually set the rpms and then monkey with the trim on the Alpha's and trim tabs noting the speed changes on the GPS. If I've gained too many rpm's, then I'll back them down a bit and reset everything. Getting on plane with the engines tucked under and the tabs fully deployed I'll adjust the rpm's to around 3,200. From there I can gain two miles an hour(maybe more!) without touching the throttles and just trimming while watching the gps.
boatrboy
Well-Known Member
I am not trying to be adversarial but I find that hard to believe (2.8 mpg). I have a 1999 270 with the same power as you. I have a fuel computer connected to my gps and have checked it the old fashion way on a longer trip. I cruise at around 1.6-1.8 mpg at 27 mph. The only place I have ever seen your numbers in the harbor. Are you sure its not a typo?
Mark
Theresamarie11
New Member
Mark, you're welcome to be adversarial (I'm an engineering manager...I'm used to it). I'm going by the Standard Horizon fuel flow gauge and how close my fill ups are to the gauge usage (so far they're within the limits of error of the gauge - 5%). But I have to admit, I've not done a truely scientific evaluation.
I don't live on the Ocean, but on Lake Champlain, and others I've corresponded with seem to be in the same ball park. I've not corresponded with anyone who gets mileage as low as yours. For me, 2.5 is typical (calm or 1'/seas) where 2.8 is with the current (typ S>N on our lake). I can get down to the low 2's if I'm trimmed badly or weather is bad, but it's far from normal. 1.X is nothing I've seen yet, even at WOT.
Maybe my gauge is under-reporting, but then I'd expect my fill ups to be inconsistent with my usage numbers and so far they don't seem to be.
Or maybe you're 'harbor' weather is what I see as typical on Lake Champlain. The lake is only 10mi wide at it's widest point so we don't get big waves or big rollers (fetch is not that long).
Also, I never cruise that slow (27mph) unless the weather is expecially bad.. When you're at 27mph, what is your RPM? What is your prop pitch? Do you have a bravo3?
My prop pitch is 24" with a bravo 3. Boat is trailered and very clean. My bottom has no paint but does have a very smooth epoxy coat.. it's like glass.
We have a few more weekends before winter, and I'll try to do a little more scientific study (the limitation being how consistently I fill my tank up each time). I'll report back.
boatrboy
Well-Known Member
Funny - I am an engineering manager too
Anyway - my prop is 22p w/a 2:0 ratio drive b3. I think my cruise rpm is about 3500 at the 27 mph. Now I do have bottom paint but I was just very surprised to hear your numbers. IIRC others here with the same boat has similar numbers to me. Besides, engineers never believe what they hear........... at first 
magster65
New Member
3500 rpm (or very close to it)... which is maximum torque / volumetric efficiency.
Theresamarie11
New Member
Funny - I am an engineering manager too
Cool... what field? My field has nothing to do with boats (Semiconductor Development) but nevertheless, as you know, when you have a mind for engineering you can problem solve and design solutions across across many disciplines with a bit of practical knowledge to get you going. BTW.. I agree with you on never believing what you hear... I question everything too. :grin:
To compare on RPMs, At 3100/3200 RPM I'm running between 29.X-31.X MPH. And it all depends on the water.
After reading this I did did a couple of web searches and found some folks who were getting around 2.0/2.1 mpg (both single 454 and twin 4.3's...so almost the same mileage).
To me this isn't much of an mystery (at first blush) given your boat is bigger (wider/longer) and quite a bit heavier than the 1' would indicate and has things like bottom paint, which I understand, can effect speed and efficiency. It's about 1300 pounds heavier dry. Which would indicate it probably has bigger tanks as well and all the things that goes with more area (more stuff added on to that dry weight).
Anyways, I'll do more semi-scientific tests over the next couple of weeks to see if I have an issue with my numbers.
