boats and fuel consumption

Not sure I'm following you. I agree that tickover is best, for mpg, and it's what I'd do if caught out in rough weather and wanted best mpg. However. Best mpg for me is 28-30 knts. The graph I've shown displays clearly that best mpg is at about 37mph.
 
Not sure I'm following you. I agree that tickover is best, for mpg, and it's what I'd do if caught out in rough weather and wanted best mpg. However. Best mpg for me is 28-30 knts. The graph I've shown displays clearly that best mpg is at about 20mph.
 
I dont follow. Which bit of the graph do you mean when you say "the rise in the graph". The basic zig zag shape of that graph applies to all planing hulls, not just stepped,. The MPG effect of a stepped hull compared with unstepped is pretty subtle on mpg and doesn't at all account for the basic zigzag shape.

I'm not sure we're disagreeing though? Heck i might have lost sight of the question :-) IF the question is "where is MPG and range the highest, assuming you dont want to plod everywhere at displ speed (ie you want to plane)?" the answer is always just past the hump ie 2650rpm on your redline graph - I guess that's not in dispute. Your boat's max planing mpg and range is not at 28kts, it's at something much less (a bit less than 14kts in fact), stepped hull or not, right?
 
I've grabbed the wrong graph, the one I had for similar hull had best mpg at 30knts ish. Which is very similar to my own experience of it. I'll have a root around. I know I have a hard print somewhere, so will have to scan it if I can find it.

This is probably more like it, but mine has best mpg at 30 knts when cruising.

Regal2200.jpg


The old graph with mercruiser 4.3 engine and alpha leg seems to have gone, though hull size about the same and probaly a bit lighter than mine as a cuddy, but different engine and leg. The hull and engine combination that used to be on boattest was very similar to my own experience.
 
Scrathching my head here brendan on that second graph, the Regal 2200. I understnad the first graph, Regal 1900

The second graph, Regal 2200, looks fine from 1250 onwards. It has the classic zig zag, same as your first (1900) graph. But the 500-1250rpm section is wierd - MPG increases as speed increases, in displacement mode. How come? Could be a funny prop slip effect, but looks duff data to me. No matter, we dont care aobut this 500-1250 bit

Other than that both the 22oo and 1900 graphs show the classic zig zag, as you'd expect.

I think there is a glitch in the graphs though. Try this for an thought. As you slowly open your throttle and the boat pushes its bow wave hard your speed climbs and the motor works hard, and just before it pops over the hump let's say the speed is 12 knots (the actual numbers dont matter, just stick with my train of thought). In other words I'm saying you are just not planing at 12 knots, and you are pushing a big wall of water and burning lots of fuel. Then you pop over the hump and plane fast. Now, slowly close the throttle and bring the boat back to exactly 12 knots. It's still planing right? It might even stay planing down to 10 knots, right? So there are 2 modes in which this boat can do 10-12 knots - planing and not planing. The planing mode is much better mpg - the throttle butterfly is more shut. The Regal graphs dont allow for this and I think they are plots of data as a boat is accelerated. If you had a decelerating set of curves you'd get a differnet shape and max planing MPG at less than the speeds shown on these graphs. Note, I could be talking complete bollox in this para so feel free to flame me :-) - whaddya reckon?
 
agree, when you get over hump, things get easier, and you can ease off on throttle to stay on plane.

Where a multi stepped hull differs, is that the more you open throttle, the more the steps play a part, as you go through each step in sequence? So you go over multiple humps? I'm not sure it works that way, as you don't feel multiple humps, as it rises to plane very flat. No idea how it works in theory, I only know what it's like to drive one.

I know that trying to stay at 20-24 knt speed that most boats that length reside, the boat is not as happy as being at higher speed.

The sequential part is not felt, just that you get higher planing speeds for little application of throttle.

After years of throwing it around and cruising a hundred plus miles in one go, 28 knts gets best mpg, while 20knts is less fuel efficient.
 
Yes, most of us know, that the slower we go, the less fuel use.

So six. or eight knots ok'ish. But ten to twelve knots is bad news for fuel burn per mile. But once on the plane, umm 13/14 knots, the giraffe sort of levels out, then gets a bit sick again, towards max. So for most boats, best fuel economy is about six to eight knots, Or maybe even slower. Failing that, it's about sixteen, yes you can do fourteen, but the waves might slow you down, causing you to add more revs. Or cause you to to go back into the Hump position. Smaller boats will slow down quicker than bigger boats, so we can treat them as about the same. Best economy is as JFM says, tick over. But not sure that is the answer folks are looking for. For aplaning boat, I'd say, whatever speed is just above planing speed, dependant on weather, so on average 16 knots.
 
I read a different theory on the benefits of steps but taxi outside and I gotta go, and was a bit complex. Will post later

Leaving aside the step effect, best mpg must be at minimum planing speed, decelrating having first accelrated past the hump. so those Regal grpahs are bollox (you agree I think?) becuase they dont plot the deceleration curve separate from the acceleration curve?

Cheers, gotta run
 
Depends on if you after best mpg, which is slow speed, or best cruising speed.

If you are not going everywhere at low speed, in which place get a displacment hull, then best cruising speed is important, and in which case 28 is mine.
 
Hmm - good stuff Gavin, especially as my 8800 is a close relative of your Ghibli. I had long reckoned that the speed range of 25-35kts on mine didn't make much difference on fuel consumption, but had held at 25kts for the sake of the crew.

Is your Ghibli a single engine? The 8800 was made in two variants FC (fun cab) and CC (custom cab) the FC being twin engine which is mine.

My 8800 was re-engined in 1999 taking out two AQ171's and fitting two V6 Vortecs with a gas conversions at the same time, so the consumption table you show, doesn't (unfortunately) tally with my experience! My experience being 12 gallons or 54L per hour at 25kts (and maybe up to 35kts).

What it does show is a) the efficiency of diesel and b) the future benefit of a single engine, if indeed yours is single.

At 45ppl, I am still OK with mine for now!
 
Just comfortable cruising speed and I don't want to perpetually press the old girl! Longevity is also a consideration and cruising at sub 3000rpm on engines that are capable of 4400-4800 seems to make sense to me - so err yes - a bit of gut instinct, or maybe "mechanical empathy"
 
How does that equate to fuel economy though. Are you running at best speed for mpg, or keeping revs back so not to run the engine at what you think is too high revs?
 
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dont know much about the efficiencies of diesels, In other words, for a given journey, it doesnt matter what speed I drive at in terms of fuel consumption.

Thats not what I expected.

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Hey up!
For a "Given Journey"
That,s the crux of it.
So You are based in Hythe.
The Windy can belt along at phenonimal speeds in rough conditions I am led to believe.
Must be true cos Snelsons got one.
Sometimes He has to get over the Bar to get to the Bar rapid like.
That,s a critical, nay ,serious situation for Him on occasions.
However back to Hythe and stuff.
I would imagine it,s a busy nautical place down there.
Once out of the Marina and Your in a situation You can get planing, the IOW , Frog Land etc etc are pretty Prox?
Like in 3 hours You can cover 90 nautical at 30 kts?
Question is, how often do You get chance to maintain such velocity, what with weather and other circumstances?
My experience with planing hulls?
Yer gets up over the "Hump" and backs off till comfy.
Comfy?--- Yep, just zipping along without banging and the engine sounds happy and the boat seems to be trimmed ok and the Sea is flat and all seems hunkey dory.
= X GALLS AN HOUR.
= An excellent Day out.
How many times do We get such conditions?
Not enough unfortunately.
That is best Scenario though int it.
So the "Given Journey" is the Criteria.
Pootling about cos it,s busy or the weather conditions aren,t right etc etc.
Or we are off on a cruise to the IOW or France or somewhere.
Stuff the Graphs and theory.
Depends more on what usage One puts the boat to.
Optimum consuption with a planing hull has to be just on the plane.
So in Your case I would imagine that,s about 25/30 as Mr S says.
At 8 knts gonna take You 3.1/2 hours longer to get where You are going so yer burning fuel for longer anyway.
Flat out only a bit quicker but She will sup a lot more for maybe a few knts more.
Anyway who gives SH%T?
Boats are a daft economical equasion, full stop!
So don,t worry about Your fuel stop. /forums/images/graemlins/grin.gif
 
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fuel.jpg




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Hi Thanks for the chart, this was the data I meant.
I know we are always going to get the highest MPG at tickover but with a planing hull I recalled that the fuel efficiency improves after we get over the hump giving a most economical planing speed as seen in the red line on the second graph.
I have been running at 1900-2000 so was in the right area with a clean bum I get about 18-22 knts.
 
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I read a different theory on the benefits of steps but taxi outside and I gotta go, and was a bit complex. Will post later

Leaving aside the step effect, best mpg must be at minimum planing speed, decelrating having first accelrated past the hump. so those Regal grpahs are bollox (you agree I think?) becuase they dont plot the deceleration curve separate from the acceleration curve?

Cheers, gotta run

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2 points

1. re acceleraion / deacceleration curves - agree in practive but theoretically not true (bumble bee). pilots have been doing it for years by climbing overheight and dropping onto the cruise heigth / revs and obtaining a higher cruise/revs than simply leveling off
2. the regal curves are for petrol engines which do not deliver a linear power/revs in the same way as diesels. this is also why a larger petrol engine can be more economical (MPG) than a smaller one in the same hull and also why the curves can be so different (although I agree the underlying overlay of displacement / hump / plane will create the same basic shape)

Bringing petrols into this thread is not going to be hugely helpfull to the OP (I know you didn't!)

finally most testers admit the bit of the graph between 500 - 800 rpm is bolloks ; mainly because the engine doesn't even run at that speed in most instances!
 
What on earth were we doing at 02:40?? Anyway, I generally run the boat with both engines at 3000rpm or a tad below. This is an easy and unstressed operational speed for them - and probably fine with the 290DP's as well. Broadly speaking CI shifts 12gph on gas and I have never tried cruising more slowly (other than in rough weather) or more quickly coz I have never needed to. Its just what suits me I guess?
 
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