Overdue question on fuel efficiency

[vodzurk]

Hi everyone,

I've a small 21-foot planing mobo, and have long since forgotten to confirm what I believe to be true... but can I just run it by a few people?...

Is it true that in this type of boat, maximum fuel efficiency is at the slowest speed possible?

I'm just wondering in the event that fuel starts to run low or weather means I have to drop off the plane.

I know I could test this... but that'd take days, and I prefer being on the plane.

I'd expect the efficiency to look something like this...



... where you can see the sudden increase in efficiency (I call it "going into warp") when fully on the plane... but is that efficiency still less than plodding along at 4kts?

Obviously I'm excluding any current or wind effects.

Sorry to ask such a dumb question... just it seems something i should know whilst planning my excursions .

 

[jrudge]

Yes.

As long as you are at or below the hull speed you litres per nautical mile will be very low. I would imagine your consumption would be similar for any speed below hull speed.

Once on the plane it depends. Many boats have a pretty flat curve so that litres per nautical mile is very similar regardless of speed. My s65s most efficient planning speed was 29knotts but the only way to find that out will be with fuel flow meters.

 

[jwilson]

On a four-stroke engine low speed, minimum wash is probably most fuel efficient. May not hold true with two-strokes. Best combination of speed and efficiency is enough power to fully plane, then ease off till just still cleanly planing. A lot of planing hulls drag transom badly at low speed, so possibly not hugely better at 4 knots than 20.

 

[Hot Property]

What boat/engine do you have?

I did some sea trials a couple of weeks ago on Southampton water when it was flat calm - tried 3 different props to see which is best. 13", 15" Enertia and Revolution 4 - all Mercury, on 200 HP L6 Verado, 23ft Sportscruiser around 2.2 tons all up.

Whist I wasn't interested in low speed consumption I agree that anything below your hull speed (I estimate yours at 5 knots - square root of 19ft hull length x 1.34 = 5.84 mph = 5.07 knots) will be very efficient.

I haven't got round to plotting the fuel consumption VS speed yet but for my boat it seems that 3.6 statute miles per gallon is about the best I can achieve with the 15" Enertia.

Propping is a black art and my garage has a few props in it. (Hope my wife doesn't sell these for what I said I paid for them!!!).

In the end you need a fuel flow meter which has been calibrated - My mercury Smartcraft gauge reads around 10% higher than actual when I brim to brim the tanks with a known quantity of fuel.

 

[wellf100]

Power boat and Rib magazine has lots of reviews of similar sized outboard powered boat with fuel consumption figures. It's true that it's most efficient at slow speeds of course however i am often surprised at how little difference there is compared to the most efficient planing speed. Especially when compared to diesel powered boats. From what i can see you have to be virtually at tick over to get much, if any, benefit.

see this recent article:
https://www.powerboatandrib.com/201...v-mercury-200hp-verado-fuel-consumption-test/

on this rib with the latest twin 225hp v6 mercury's fuel consumption is 3.6mpg at 600rpm (3.2 knots!) and 3.4mpg at 28 knots!

where as with older 200hp supercharged verados it's 5mpg at 650 rpm (3.6 knots) and 3.3mpg at 32 knots. Presumably the better nmpg at tickover is due to it being a v4 I think instead of the new motors being a v6.

So there's lots of variables, but the bottom line is to get any significant benefit by going slow in a petrol outboard boat you have to be going very very slowly!!

 

[vodzurk]

Interesting, thank you everybody.

So I guess, if desperately low on fuel...

- If there's the slightest tide I'm going against, either consider anchoring, or stay on plane, but only 1kt over.
- If there's the slightest tide I'm going with, run at 5kts.

What boat/engine do you have?

It's a Sealine 195 - ie 19.5ft hull I believe. Engine is 150hp Volvo Penta AQ151 on a leg.

 

[jrudge]

Just bear in mind the only only time this should be a consideration is when you are close to the range of the boat and weather etc has been against you.

 

[QBhoy]

Oh I love this kind of thing. Spent years being so geeky about efficiency and prop selections etc. Love it.

Similar to another reply, I have the smartcraft software which indicates exactly what my engine is using at any given and exact rpm and gps speed. It’s very interesting.
Generally you’d be safe to say that on most planing boats similar to yours...the most efficient running is either just above tick over at around 800-1000 rpm or just above minimum planing speed. The truth can vary slightly and there are exceptions to the rule. If I can bore you with some details...haha

My boat is 18ft planing performance hull with a 300hp V8 Mercruiser MPI. She is ridiculously efficient at certain stages in the Rev range.

Her most efficient condition in a lab would be at tick over rpm, but in the real world, at such low speeds, you are subject to influence from tides and wind etc. So most efficient I’ve found is at around 1000 rpm where in most cases she will potter at around 7 mph from memory.

Her second most efficient running is at around 2600 rpm @upwards of 30mph and will use early 20’s litres per hour, from memory....but the exception is that it isn’t always comfortable to run at over 30mph in a short sharp chop and the boat is easily led sideways in a thwart wind.

The one I do know exactly is below. This is my usually favoured running

At 2200 rpm she will be just above planing speed @24/25mph gps and using around 18 litres per hour at that. Incredible economy from such an engine.

Propellers have a huge deal to do with it too. Generally the less blades you have the more efficient, as you will know. I’ve experimented with so many different props and pitches on this boat and have settled for being pitched more towards top end pace and low rpm cruising speeds. Not so great for accelerating but I don’t dare open her up from a standstill anyway. She only has the wee alpha drive.
On the other end of the rpm range...she will happily empty the tank in just over an hour at full throttle.
At 5000 rpm she will usually see @65-67 mph and have a thirst for about 80 litres per hour !!

I’ve bored you all enough !

 

[QBhoy]

What boat/engine do you have?

I did some sea trials a couple of weeks ago on Southampton water when it was flat calm - tried 3 different props to see which is best. 13", 15" Enertia and Revolution 4 - all Mercury, on 200 HP L6 Verado, 23ft Sportscruiser around 2.2 tons all up.

Whist I wasn't interested in low speed consumption I agree that anything below your hull speed (I estimate yours at 5 knots - square root of 19ft hull length x 1.34 = 5.84 mph = 5.07 knots) will be very efficient.

I haven't got round to plotting the fuel consumption VS speed yet but for my boat it seems that 3.6 statute miles per gallon is about the best I can achieve with the 15" Enertia.

Propping is a black art and my garage has a few props in it. (Hope my wife doesn't sell these for what I said I paid for them!!!).

In the end you need a fuel flow meter which has been calibrated - My mercury Smartcraft gauge reads around 10% higher than actual when I brim to brim the tanks with a known quantity of fuel.

View attachment 76483View attachment 76484View attachment 76485

Hot property.

How did you get on with the props ? How did you find the enertia on a cruiser ? Don’t think I’ve heard much about them on a cruiser.
It’s an incredible thing on my boat certainly. Across most of the spectrum it out performs anything else I’ve tried. Really lends itself to a performance hull as long as you have the power to turn it and let it do its thing with the lift it can generate.

 

[Hot Property]

Hi Qbouy,

The Enertia seems good, it grips just as well as the rev. 4

Maybe I'm expecting too much from a 200 hp outboard on a 2.2 ton boat. Acceleration is stunning due to the supercharger so no complaints there!

The hull of the Saver 690 isn't like a Fletcher's by any means, however a cruiser is a compromise at the best of times.

I'm pulling the boat out for a scrub and antifoul soon and will repeat the tests to see if any difference.

I still can't obtain WOT though and it's starting to bug me!!!

This is the boat with a very typical Italian angle to it!!

https://m.youtube.com/watch?v=YvPANfLcuGQ

 

[gsturgeon]

We have a fuel computer and it shows that you need to avoid the window between the hull speed and planing

 

[Hot Property]

We have a fuel computer and it shows that you need to avoid the window between the hull speed and planing

Yep, it's either 5 knots or 15+ for us...

 

[vodzurk]

Just bear in mind the only only time this should be a consideration is when you are close to the range of the boat and weather etc has been against you.

That's absolutely why I'm checking .

We're hoping to do a week-long excursion again this summer (Portishead to Tenby - previous trips' vids in signature link). Each leg is ~65-85 miles (long leg being Swansea-Tenby-Swansea). We've found that we use max 2/3rd of a tank per leg last year on the same run. Conditions were good last time... so would just like to have the correct course of action in my head if weather turns against us (I'd always err on not making the trip if it looks anything less than good).

So with that in mind, it's good to know that if waves pick up such that we can't stay on plane, drop to ~6kts, and we can keep at least the same (if not better) efficiency .