Fuel Efficency

promocop

promocop

New Member
Oct 7, 2007
425
MDR
Boat Info
2005 390 SunDancer
Engines
8.1 Merc's
Does anyone have fuel burn/efficency charts for a 2005 390 SunDancer? My friend says that at higher RPM's that my boat my actually be MORE EFFICIENT then at lower RPM's and that slowing down TOO much to conserve fuel is actually wasting more fuel? True? Thanks.
 
It depends on if you are looking at fuel used from point A to point B.....in which case there is a sweet spot that will get you from A to B most economically (there is a time component involved). However the lower the rpm almost always equals a lower fuel burn rate.
 
ALWAYS? My buddy swears that the 'sweet spot' on the dancer is around 3300 RPM's and that much lower then that equals lower RPM's but MUCH efficency. In other words slower to get from A -B but buring more fuel to get there!
 
Lower RPM = a lower burn rate, but not necessarily higher efficiency.

Efficiency isn't linear. In boats like ours, the 2500 RPM range is brutal from an efficiency standpoint. Even 3000 RPM is LESS efficient than the mid-3000's.

By the time you get to 4000, efficiency has already begun a downward curve.

The cruising sweetspot is somewhere near 3500.
 
He has 8.1's. I agree. That is a big boat for the gas engines, so you'll want to keep them running pretty fast to keep that boat well planed out and efficient. It should pretty easy to determine. Just try it at a variety of RPM's. Take a calculator.

At 3000 RPM, divide your speed by your total fuel flow. For relative purposes, Smartcraft should work fine. Miles per hour divided by gallons per hour equals miles per gallon. Bump it up to 3250, trim it up, and do it again. Try 3500, 3600, 3700, 3800, 3900 and 4000 RPM. I bet your sweet spot is around 3800.

If this seems obvious, sorry.
 
There are usually two efficient spots for a boat. The first is the speed at which the boat does not exceed its hull speed and is operating like a displacement hull. Most of us don't like that speed because it is too slow, the longer the boat the higher the that speed is. The second spot is when the boat is up on plane. The speeds in between those two are the least efficient in terms of fuel consumption. Inbetween those speeds the boat is pushing a big wall of water and producing a very large wake. So rather than riding on top of the water, plane speed, the hull is angled upward and pushing the hull against the water. I can't tell you where those speeds are for your boat but you can figure them out.
 
If you want your tank of gas to give you lots of time on the water = run low rpms and go slow. However it takes you longer to get from A to B so you may ultimately burn more fuel. You must think, do I want more time or more distance from a given amount of fuel?
 
I almost ran out of fuel on labor day weekend. I slowed down to conserve fuel but at what cost. Someone must have a fuel burn chart
 
I learned the hard way - believing that a slower planing speed is the most efficient.

I had to burn a lot of gas to figure it out.

I think of a lot of boaters out there claim they can "feel" the sweetspot. I doubt it. I think what they're feeling is simply what they're accustomed to - regardless of the data.
 
This chart is for the 380DA.

Sea condition, tide, amount of water and gas and how much stuff my wife loads up the boat with, all affect my ultimate gas usage.

3500 rpm's does seems to be the best overall.

RPM MPH Knots GPH MPG
3000 19.1 16.6 26 0.73
3500 26.6 23.1 33.2 0.80
4000 31.4 27.3 47.1 0.67

Your boat is little bigger, and heavier but the engines are the same.



RPM
MPH
Knots
Total GPH
MPG
NMPG
Range
NM Range
 
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We had this discussion here a few years ago. Slowing down in a planning hull does not necessarily save fuel... in fact, you need to understand where the "sweet spot" is on fuel consumption vs. speed to figure out the best efficiency for a planning hull. I think the thing that brought this up a few years ago was someone saying that they could tell people were conscious about fuel costs because the were backing down the throttles... that may in fact increase fuel needed to get from point A to point B.

Think of water skiing (if anyone has done that). When the ski boat is going slower, it requires more work to hang on to the rope and up on the skies than if you were going faster... a water ski is a planning hull... However, there is a point where the less drag on the hull does not compensate for the decrease in engine efficiency... that's the "sweet spot"

However, Jim told me not to respond to this post.
 
Mr Salt said:
There are usually two efficient spots for a boat. The first is the speed at which the boat does not exceed its hull speed and is operating like a displacement hull. Most of us don't like that speed because it is too slow, the longer the boat the higher the that speed is. The second spot is when the boat is up on plane. The speeds in between those two are the least efficient in terms of fuel consumption. Inbetween those speeds the boat is pushing a big wall of water and producing a very large wake. So rather than riding on top of the water, plane speed, the hull is angled upward and pushing the hull against the water. I can't tell you where those speeds are for your boat but you can figure them out.
Click to expand...

+1 Mr Salt.

My 340 (8.1s) is a lot smaller than your great diesel boat, but at 3350 rpm I use 30 gph for 26.5 statute miles per hour (.88 mpg). In a "displacement" or "hull" cruise at 1700 rpm, I use 9 gph for 11 statute mph (1.2 mpg).

I recall the hull speed formula for a displacement hull (think sailboat, cruise ship, etc) is:

Hull speed = Square root of waterline length multiplied by 1.34.

This gives you the theoretical hull speed in knots in a displacement cruise.
 
Four Suns said:
Only if the oil has been changed per the manufacturers recommendations.
Click to expand...

+1! perfect!

It may have to do with the square root of the manufacturer's recommendations times the quantity of oil added times the filter capacity (only if you don't change the filter when you change the oil!).

Am I on the way to my first "infraction"?
 
OK... Here's the discussion from some time ago:

http://clubsearay.com/forum/showthread.php?t=12607&highlight=yachting

The guy from Yachting magazine had his head up his ass...

Sorta like that guy that worked for Boeing or something and talked about wave drag on boats and how the stern would rise out of the water or some crap like that... he had a 240 DA if I recall... lived out west... wait a minute...
 
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