Best fuel burn speed on outboard cruiser (25 foot)

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LadyJ

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Hoping to find some mathematician boat owners who have outboard fuel burn experience. I've a 175hp single on a 25foot Merry Fisher and planing a longer trip around the inner Hebrides so fuel efficiency is a real issue. I'm used to cruising at 25 knots in the knowledge the fuel berth is always close enough but won't have that benefit. I burn 9 gallons an hour at 4900 revs but if I throttle back to 4200 and just stay on the plane my speed drops to c. 18knots but burn to just under 6 gallons. What I am unsure of is whether it's better doing displacement speed of 6/7 knots in the areas I have more distance and lack of fuel or is it better to get on the plane? Tank is 60 Gallon (280 litres). Any fuel advice for Islay, Gigha and Jura welcome as well as I can cycle/walk with Gerry Cans if a petrol station is on the islands but finding that out on the web is not as easy as I'd hoped!
 
My two most efficient speeds are six knots and around twenty six knots. At six knots I'm doing six/seven litres per hour. At twenty six knots I'm doing about fifty three litres per hour. I find that I need to properly get on the plain and wait for the bow to drop, I then trim up the engine for better speed. Obviously conditions dictate when I can do this.
 
In practise, you get the best mpg just above idle and at the lowest speed where you keep her on a stable plane.

In other words, always keep the weight as low as possible, go idle or fire her up to get on plane then reduce to the point where she maintains a stable plane (commonly slightly faster than you think).
Any behaviour with bow high or dragging a wash means spending valuable fuel at that doesn't move you forward.
 
Whitelighter said:
Boat will be much more efficient at true displacement speed.

True displacement speed for your boat is likely to be quite low though as it's based on waterline length - I'd guess at around 5knts
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Thanks for help, indeed at 5knots the rule burn halts from 7knots.
 
Spi D said:
In practise, you get the best mpg just above idle and at the lowest speed where you keep her on a stable plane.

In other words, always keep the weight as low as possible, go idle or fire her up to get on plane then reduce to the point where she maintains a stable plane (commonly slightly faster than you think).
Any behaviour with bow high or dragging a wash means spending valuable fuel at that doesn't move you forward.
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Appreciate the comment and am trying to establish the 'just on the plane' speed. I find at 4200 revs she's a bit bow high, whilst 4500 brings it down it's by no means level and indeed doesn't get level until 5200 and nearly 30 knots. Surely the 'just on the plane' speed will be nearer the 4200 thank the 5200
 
wipe_out said:
Wouldn't the litres/gallons per mile be the more relevant number here to work out which speed will give the desired range?
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Agree, now trying to establish most economic plane and displacement speed then will work it out from there.
 
Getting the bow down helps plane at modest speed. Trim the engine in or redistribute load to help and experiment to find the best compromise

If aiming for highest speed you need to allow the bow to lift out of the water to minimize resistance.
 
Spi D said:
Getting the bow down helps plane at modest speed. Trim the engine in or redistribute load to help and experiment to find the best compromise

If aiming for highest speed you need to allow the bow to lift out of the water to minimize resistance.
Click to expand...

And this also applies for economy - if you look at the MBY figures in a lot of their tests, best economy is 25-30kts rather than just over hump speed.
 
LJ

We worked out our fuel consumption figures for our Shetland F4 (1090 kg) with a Mariner Optimax 75 (170 kg). You can see the graph on our blog below at Equipment | Main Engine, about halfway down. Figures were the average of runs in opposite directions to take account of any tidal flow.

As others have said, the best fuel efficiency was at low engine speeds with the boat going at displacement speed or less. Worst efficiency was just before the boat started to plane, at about 3000 RPM in our case. When the boat was planing, fuel efficiency improved again, but did not reach that obtained at low speeds.

Your figures with a bigger boat and engine will obviously be different from ours, but I would think the shape of the graph would be the same.

To answer your second question, petrol is available at Port Askaig (Islay) and Craighouse (Jura). Not sure about Gigha. We carry 80 litres, and in all of our trips in the Inner and Outer Hebrides, have never had trouble getting petrol, or running out. Stick to displacement speeds and go with the tidal currents rather than against them and you will go for miles. We were getting 5 NM/litre at one stage coming down the Sound of Luing with the engine just ticking over in a 7 knot current!
 
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