Faster cruising speeds can save fuel

[DAKA]

We all know cruising at 6 knots is economical but boring.

We also know that hump speed is expensive.

I have argued a few times that 'just on the plane' is not the next best economic speed.

I think I now have the reason for this along with a theory that explains why flow meters don't help either.

Works with legs but I show shaft drive for ease

6 knots, boat level, props point almost level and propulsion is almost all to the rear of the boat.

Just on the plane , we have all seen boats at this awkward angle, look at the props , a lot of forward thrust is actually heading down to the seabed and not pushing forward at all.
This is supposed to be 18 knots, on the plane but not level yet, ( I don't have a photo of my boat at 18 knots but hope you get the idea)

A few extra revs and the boat is now level again and most of the thrust is back wards again, not much wasted propulsion heading to the bottom of the sea.

So why doesn't the Flow meter linked to speed help ?

The props draw their water from the bow of the boat and pull water past the log so the log has no idea that the boat is not actually pushing through the water as fast as the water passes the hull at (not prop slip, just the water needed to push the stream back wards, as witnessed in a narrow channel.



This explains why boats that are underpowered use more fuel than boats with larger engines.

Most economic cruising speed is 'Level' planning speed.

I know they are not the best of photos, and I know the angles could be wrong but I hope I have shown enough for you to understand the theory.

I have seen a lot of large Flybridge boats and Nimbus owners also this year at slow planning speeds that look at a terrible angle, they are not saving fuel and if they have Volvos I hate to think what state the tenders are getting in from soot.

Go faster and get there before you run out of fuel /forums/images/graemlins/cool.gif

 

[TigaWave]

we started measuring above 26knots, but got better mpg at 39 knots than at 26? but only by 0.4.
the graphs here http://www.h4marine.com/Data%20Charts-64.pdf

 

[[2068]]

There are several factors, and "thrust lost due to props pointing in the wrong direction" is definitely part of it, but also because the hull is producing huge amounts of drag with the stern dug in. The size of the resulting wake is an indication as to where the energy is going!

Also, prop slip decreases as speeds increase (there isn't much prop slip at 100kts).

It's still the case that the optimum speed is going to be different for each hull/boat combination.

dv.

 

[Chris_d]

I suppose it depends on your definition of "just on the plane", for me this is when the boat has leveled out and you can ease back on the throttles, so the lowest "level" planing speed is the most economical. However you may as well go faster if your engines have enough margin, as at the slowest speed in poor conditions your too easily knocked back off the plane, and speeding up again is not very econonmical.

 

[LittleShip]

We all know cruising at 6 knots is economical but boring.

Oh No It's Not! /forums/images/graemlins/tongue.gif

Tom

 

[DavidJ]

[ QUOTE ]

there isn't much prop slip at 100kts.


[/ QUOTE ]
hmmm! I'd like to see your evidence of this!
I believe that wind resistance is proportunal to the square of the speed so at 100kts this would be considerable. However I do agree that hull to sea resistance could decrease with speed because of a "skimming" effect, but I feel that the curve for this would plateau out fairly early on (say 30-40 knots at a guess).

 

[oceanfroggie]

Good post Daka

 

[DAKA]

[ QUOTE ]
We all know cruising at 6 knots is economical but boring.

Oh No It's Not! /forums/images/graemlins/tongue.gif

Tom

[/ QUOTE ]

Oh yes it is ......... /forums/images/graemlins/grin.gif


sorry Tom, just as I thought everyone was in agreement with me for a change , I could have perhaps worded the post a little more carefully.

I am a bit blinkered into thinking all forum members have Princess 360s (or similar Fairline equivalents).

I shall try harder to be more considerate to others with different boats /forums/images/graemlins/smile.gif

 

[oceanfroggie]

It depends. Some times cruising at 7-8kt displacement speeds is pure magic. Stick on the AP and chill out for hours watching amazing scenery go by (ie more time on the water enjoying it for half the price which is quarter of the price per hours enjoyment). Other times cruising at 15-22kt is great. Depends on the reason for the trip (ie A2B or the filling in the sandwich itself)

 

[[2068]]

Well, ok, maybe 100kts is not a good example, but it is something that is more of a factor at lower rpms, then flattens off to some constant(ish) value at higher rpms.

Example graphs HERE.

dv.

 

[Nick_H]

Can you clarify exactly what your theory is? It seems to be that a boat will be more efficient at 17 knots than it will at 16, if its not quite on the plane at 16. Probably correct, but hardly a revelation.

If you think that your average shaft driven planing boat will be more economical at 25 knots than at 17 (assuming again that 17 is fully on the plane), then I think you're just plain wrong in the large majority of cases.

A quick review of some old boating mags will give you mpg figures at different speeds for lots of boats, and i'm sure they don't rely on the boats (notoriously inaccurate) speed log to calculate the figures.

 

[[2068]]

No idea about heavier shaftdrive fly's, but some sportscruisers with sterndrives have a pretty much flat mpg figure once planing properly, and the optimum point can be higher up the speed range than you might expect.

But, not flat out. E.g. use the top 500rpm's on a KAD32 and you can see the gauge move.

dv.

 

[longjohnsilver]

[ QUOTE ]
We all know cruising at 6 knots is economical but boring.

Oh No It's Not! /forums/images/graemlins/tongue.gif


I'd second that, if I reach 8+kts I feel I'm flying!! Actually once went over 10kts with a bit of tidal assistance and engines opened right up. Positively supersonic /forums/images/graemlins/grin.gif
Tom

[/ QUOTE ]

 

[LittleShip]

I shall try harder to be more considerate to others with different boats

If you insult Little Ship when she is around she is likely to lean against you in the dock. /forums/images/graemlins/ooo.gif I'm with Ocean on this one I just love being on the water! the longer the better in most cases.

Maybe when I'm older and need to take medication I'll go rushing to port!

On a more serious note I asked a friend of mine to explain why it takes so much energy to keep a boat on top of the water and "she" started to explain the dynamics of it all.......... think thats what got me interested in slow boats, far easier to understand.?

Tom

 

[DAKA]

[ QUOTE ]
Can you clarify exactly what your theory is?

If you think that your average shaft driven planing boat will be more economical at 25 knots than at 17

[/ QUOTE ]

Yes , that is exactly what I believe is the case and what I know to be the case with my last three boats.

Assumes that the boat has adequate power to sustain 25 knots.
Most will have to make do with 22 knts

At 17 knots-22 knots boats are on the plane but props still pushing a lot of propulsion to the bottom of the sea.

 

[volvopaul]

Cant really agree with all of that , have you ever read fuel figures for an engine from say 2400rpm to 2800 rpm the increase is nearly double on some engines, ive had volvos meter in many times on sea trial, to be honest the last rpm of the range are the most frighteneing of all, mid rpm after the boats planing is the most sensible speed say on a tamd41 series from 3000 to 3500 anymore after that then its a massive increase against speed achieved.

 

[gjgm]

These are Windy/Volvo figures. Seems to suggest as you might think. The faster you go, the more you drink

kts lph lpm
16 27 1.69
21 32 1.52
26 43 1.65
31 57 1.84
34 74 2.18
38 93 2.45
43 116 2.70

 

[ebbtide]

"I shall try harder to be more considerate to others with different boats"

Please adapt your theory to semi-displacement hulls. For a start I'm not sure when the boat actually planes! But the wake starts to leave the transom at about 7 knots, with another 5 in reserve. So??

 

[AdeOlly]

Indeed so. I've worked my fuel consumption ( 2 x AD31 on outdrives) out using two way GPS readings and the quoted full load fuel burn figures to calculate my optimal cruising speed, and once on the plane the faster I go the better it is up to ~95% max rpm. This exactly matches Volvo's recomendation to cruise at max rpm-200 for my engines. I know using the fuel burn curves does not give completely accurate figures, but good enough I reckon. My real burn figures are about 15% better than theoretical and I largely put that down to the 7nm in/out of Chich each trip at 6 knots.

I actually cruise at WOT -350 rpm as the engines seem happier at that speed and the fuel burn is only about 0.15 mpg worse, plus I have some mechanical sympathy /forums/images/graemlins/wink.gif

 

[DAKA]

Obviously flat out costs an arm and a leg, no argument.

I am just trying to point out that just on the plane is cheapest per hour but not cheapest per mile.

Does anyone that actually cruises long distance ever manage to agree boat mag stats ?

I am not suggesting they are not accurate for my boats, just haven't seen any that look very sensible to me.
If they are using fuel flow meters and log over short distances then the figures are unreliable.