Faster cruising speeds can save fuel

[Sundays_Child]

Totally agree.

This is the (US gal) fuel usage on my 3208TAs (each)

RPM GPH
1600 4.5
1800 5.2
2000 6.9
2200 9.5
2400 12.1
2600 16.0
2800 21.1

We're on the plane at 2100, and cruise between 2200 and 2300.

I had thought that 2100 would be most economical, but there's a useful increase in speed with only the extra 1 to 200 rpms. Without fuel meters, I estimate this is best for us.

At the same time, we love slowing down on our way home to extend the day.

 

[Wiggo]

Our mpg figures are pretty close to what MBM said when they tested the F37 way back when. Around 1.2 mpg, depressingly (though I'm sure there are plenty here who'd give their eye teeth for that economy...)

 

[DAKA]

I assume you dont pansy around at 17- 18 knots /forums/images/graemlins/crazy.gif


Sealines usually have an excellent cruising speed, 1.2 @ 25 knots is not at all bad for a boat your size /forums/images/graemlins/smile.gif

I suspect it would fall to *1 @ 17 knts

Trouble is many dont go far enough to have a clue and include 7 miles out and 7 miles in of harbour travel.

 

[AdeOlly]

Oi, my figures were worked out in open water /forums/images/graemlins/grin.gif I have do to the 3.5 miles out then in or I get wet feet getting back to the car /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

 

[Wiggo]

No, interestingly enough we found our mpg worsened if we cruised at 18-20 rather than our normal 22-24. Doubtless the gph figure was better, but it took longer to get anywhere so we burned more fuel overall.

We can get 1.4mpg if we leave all the cruising crap at home...

 

[DAKA]

sorry my typo
/forums/images/graemlins/blush.gif
I have amended, I meant to say the slower you go the more fuel per mile you use..

 

[Wiggo]

Yes, but there seems to be a flat spot between 20 and 25 knots where it really doesn't make any difference. Above and below that, the mpg worsens.

 

[DAKA]

[ QUOTE ]
Oi, my figures were worked out in open water /forums/images/graemlins/grin.gif I have do to the 3.5 miles out then in or I get wet feet getting back to the car /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

[/ QUOTE ]

I could clearly see you had gone to a fair amount of trouble to ascertain your economic cruise speed and I didn't mean to include you in the clueless idiots category ,










........but if the cap fits /forums/images/graemlins/grin.gif

 

[MapisM]

[ QUOTE ]
I have argued a few times that 'just on the plane' is not the next best economic speed....
...Most economic cruising speed is 'Level' planning speed.

[/ QUOTE ]Hmmm... What's your definition of 'just on the plane' vs. 'level' planning speed?
In my books, below the level planning speed a planing boat is rather 'almost' than 'just' on the plane.

 

[DAKA]

Level to me means

Zero tabs
bow settled down
faster speed does not alter trajectory

I have seen many boats this year on the plane with wake in a nice V shape from the stern but bow too far up.

As per photo above, boat is settled nicely in the water using most of the hull length.

 

[blueglass]

what's boring about spending ALL day at sea!

 

[Renegade_Master]

Indeed the transition point between dis speed and planning i.e. getting over the hump, is uneconomical, as the boat is known to be "climbing the hill" or bow wave.

For instance my Princess 560 gets on the plane with no perceptable "dropping" over the hump like my little old F36 Sealine. And she will still be on the plan down to 14 knots. However the one speed I do not maintain is the in between bit about 12 knots cos its expensive.

If I want to go at slow dis speed I do so at about 8-9 knots, no higher

 

[Sneds]

I to have twin AD31s on a 28ft sports cruiser (Mustang 2800)
I gues -ti -mate the usage at around 40 litres per hour (total for both engines).
After Nov I guess i will be looking for the most fuel efficient speed!
About 3000 - 3200 RPM seems to suit the boat, what do others think?
Do VP produce fuel usage figures?
I know they would be under lab conditions and all things are variable but they would be interesting to read.

 

[[2068]]

I don't have the curves for the AD31, but I do have the KAD32 ones, which aren't vastly different, apart from that the supercharger gives an extra "bump" to the torque curve.

 

[jfm]

Umm, I'm with houghn, ie trying to work out exactly what your theorem is...

Are you saying that a boat does better mpg when properly on the plane than when struggling over the hump? Well yes, everyone would agree.

But are you also saying that once past the hump and "fully" planing, you get mpg increase by going faster? That would be bollox

All this has relatively little to do with the downward pointing of props and much more to do with drag

 

[[2068]]

For a heavy flybridge boat, I suspect you are correct (don't have the figures).
For a sportscruiser, potentially not bollox at all.

Using a Doral Prestancia as an example, a typical sportscruiser on sterndrives. Planing nicely at 21kts. Accelerate to 27kts and you are using less juice! (per mile).

 

[DAKA]

My boat is no where near the size of yours as you are aware but my boat and (Wiggos F37 flybridge see above) are more economic at 22-24 than 18.

I dont want to get bogged down with hump, 10 knots, etc

I am talking about cruise speed which starts at 17 knots as a comfortable plane speed and really ends at 28 knts for most due to max revs or sea state.

at 17 although the boat is on the plane, most boats are still on tabs and struggling to get the bow down.

Once at 22 most boats have the bow settled down and hence the prop thrust is back wards and not toward the seabed.

I find I get better mpg at 25 knots .

I would expect your boat to react in the same way although your plane speed will be even higher as your hull length will allow an increased displacement speed, perhaps you could get away with 14 knots as displacement ?? but once at 17 much of your thrust will be wasted as it heads to the seabed, you should be level at 20-24 where the propulsion starts to propel you the right way.......forwards not upwards.


I exhibit as proof

I took a Bayliner to Le Harve 110 nm @ 20 mph and ran out of fuel on the way there including the reserve tank I fitted.

On the way back I increased speed to 30 mph and got back with full unused reserve tank.

Two years ago I went across the channel with friends in a slower boat, I went across @ 17 knots, it felt like for ever but I thought at least I would use less fuel..........unfortunately not the case, it took longer and I used more /forums/images/graemlins/frown.gif

If by going faster your boat lifts up higher then some drag will be lost, even prop drag will b less as they are running in shallower water as the boat pivots bow down.

This is my theory I tried to put across the last time we discussed this but I didn't have the diagrams which took me ages to sort out.

I think I am sure about this /forums/images/graemlins/smile.gif

 

[Deleted User YDKXO]

I'm not even sure that every boat is less economical at hump speed either. I've been recording the speed of my boat over recent months at various speeds and various load and fouling conditions and comparing them against the engine manufacturers stated fuel comsumption figures to get mpg figures for various rpm. Basically I've been trying to find the most economical planing speed for my boat and I don't see any decrease in mpg at hump speed which on my boat is at around 12kts. What is important on my boat at lowish planing speeds ie 12-18kts is to maximise the speed using the trim tabs but, basically, I've found that the faster I go, the lower the mpg. There is no 'economical' planing speed, as such
I'm not sure what DAKA is trying to say either. Yup I would agree that most planing boats adopt a more level attitude at higher speeds and, hence, a more efficient drive angle but, as you say, any gain is more than outweighed by increased drag

 

[MapisM]

[ QUOTE ]
I exhibit as proof
I took a Bayliner...

[/ QUOTE ]Not a proof of your theory, really.
Just demonstrates how badly a planing boat can be designed. /forums/images/graemlins/wink.gif

 

[Hurricane]

Actually, I find myself partially agreeing with DAKA here.

Having done a long run with the Princess at 22/23 knots all our calcs for every leg always came out at around 13 litres per mile. These calcs were done using actual fuel input into the tanks. The boat is fitted with MTU's fuel consumption system and it was almost always indicating 150litres per hour per engine. This again proved the actual consumption i.e. 13.3333 litres per mile.

However, I believe, (and I havent yet proved it) that she will do around 27/28 knots at 180litres per hour per engine. This equates to 13.09 litres per mile - a very small saving at the higher speed.

The reason that I say that "I believe this to be true" is that I have very rarely seen the displays go above 180litres per hour per engine ane we've often pushed her up to 28 knots.

So my conclusion is this. If you are going to plane then it is more economical to plane at a reasonable fast speed. There is a limit though - WOT really burns the fuel. If this is true then on a long passage you could save a small amount of fuel and get there quicker.

So to jfm's question WHY.

It must be that the engines are running at the top of their performance curves.

I read DAKAs post as nothing to do with the hump - most of us leave the hump behind at speeds around 15 knots. I understood his post to mean faster planing speeds are more economical than slower planing speeds (22 knots vs 26 knots)

And I think that there's some merit in his statement but the reason is because the engines are operating more effeciently at the higher speeds than the lower ones.