Most Economical Speed

[Bigplumbs]

I know this is a very wide question but all you chaps with more experience than me can you give me an idea of the best speed to get the most distance from a tank of fuel. I don't need actual knots I mean more throttle position. ie

Get on plane them back off to just below displacement speed or
Slow Displacement
Or 3/4 throttle.
etc

Just some sort if idea would be nice.

This is not about saving money it is about haw far I might get on my 105 ltr tank (19 footer with Mercruiser 3 lty Alfa 1 gen 2 leg)

Regards

Dennis

 

[rbcoomer]

It will be a little different being a 4-stroke, but I've found all mine (2-stroke outboards) have been best at low planing speeds (17-20kts). The worst is likely to be just above displacement and before she planes as you'll be pushing a bow wave that she'll be trying to climb. The big difference with the 4-stroke is the low-speed displacement stuff. All the two-strokes I've had drink heavily at that speed, but the 4 stroke should be better - provided you stay below hull speed! If a limit on a river is 6 or 8 knots, many will try and do that, but if hull speed is less (at 19', yours will be around 5.88kts) you'll be wasting fuel - do 5kts if you can't plane!

 

[superheat6k]

Hull speed = ~ 1.34 x Sq Root LWL

LWL is the waterline length so on a 19' LOA boat around 15', so around 5-6 knots.

Fuel consumption per mile will be lowest below HS. As soon as you start to exceed HS the boat initially has to lift up over its own bow wave, using a lot more power, as the hull starts to plane its consumption will then reduce a bit. Once on the plane the power required will broadly increase with the square of the speed, with the consumption broadly following this curve.

If you want to work out your endurance / range deduct 1/3 of the tank quantity. The last 10% is virtually unusable anyway and differing sea conditions can adversely affect consumption even below HS.

if you keep a log of your trips out, time / distance, and refuels you can quite quickly establish approximate consumption for your boat at differing speeds.

 

[dpb]

This will give you some idea, you might find a better match if you look further.

http://www.boattest.com/oem/32/boats/162/0/on/640x360

 

[wipe_out]

My guess is probably at or just over idle so 800-1000 rpm if you want a rev position for lowest fuel consumption..

 

[mlines]

Rpm	 Mph	Miles / L
Idle  	0	0.0
600  	3.6	1.1
750	4.5	1.1
1000	5.7	1.0
1250	6.8	0.9
1500	7.8	0.7
1750	8.4	0.6
2000	10.1	0.5
2250	13.5	0.6
2500	18.8	0.8
2750	22.9	0.9
3000	26.6	0.9
3250	30.1	0.9
3500	33.4	0.9
3750	37.0	0.9

5.7Litre 350 MAG

 

[Deleted User YDKXO]

Most economical speed is generally minimum in gear speed ie idle speed in gear and if you've got 2 engines thats with one engine only. Its simple; the faster you go the more it costs you