what does 'cruising speed' mean?

[Its_Only_Money]

Re: Glad to serve!

[ QUOTE ]
Hull speed of a Fairline Sprint is about 5.3knts, hull speed is the square root of the waterline length in ft x 1.2(depending on hull type). I used to have a similar petrol boat and it did 5mpg at hull speed, not exactly horribly inefficent . Your probably running at 6-7knots and that is way above hull speed so will be massively inefficent. The longer the boat the better the hull speed, thats why the QM2 etc... have a hull speed of circa 30kts or so.

[/ QUOTE ]

I know how to calc hull speed. You may have done but how similar is similar (Sprints are renowned for being heavy for their length). No I'm not (within Hamble speed and wash limits so sub 6kts and very low wash) and yes I know. Thanks

 

[Chris_d]

Re: Glad to serve!

Didn't mean to sound condesending, my appolgises. I was just making the point that very short boats have a very low hull speed. My experience was with a 22ft 220hp engined Holiday 224, using it on river and sea I got to know it low speed fuel consumption very well. My suggestion was that if you are using more than 1gph at low speed then you above your hull speed and that maybe by only 0.5kt. Certainly I used to see 5mpg at about 5knts or 1gph, but at 6knts conusmption was 2gph+ or <3mpg. /forums/images/graemlins/smile.gif

 

[Its_Only_Money]

Re: Glad to serve!

No worries, my reply reads a bit brusque!

Just shows that literally mileage may vary, I'll admit to running on the paddlewheel for the low speed consumption figure so there could be some error there - although not as much as when tide flow affects a gps reading.

Overall my consumption equates to 2.9Nmpg which (just) backs up the view that my low speed seems worse than cruise...accept your point that small changes on a small speed can be a big difference but I do tend to be guided by wash rather than a speedo readout so don't believe I am fast enough to be pushing the hump - instantaneous consumption readout goes down to 0.3Nm/ltr during the transition to planing. And I check the calibration of the fuel flow gauge whenever I fill up....

 

[oceanfroggie]

Re: what does \'cruising speed\' mean?

Our stats are as follows:

7.5kt @ 1600 rpm fuel burn 2.2 gph range 650 miles (displacement mode)
16kt @ 3000 rpm fuel burn 10 gph range 260 miles (planning mode)
20kt @ 3400 rpm fuel burn 15 gph range 200 miles
26kt @ 4000 rpm fuel burn 25 gph range 160 miles

We usually cruise at 7.5kt, 16kt or 20kt, never between 7.5 and 16kt. At 16kt due to SD hull form she is well up on the plane and a very happy quite bunny and economic too at only 10gph. Depends on sea state and loading too.

 

[Searush]

Re: what does \'cruising speed\' mean?

For me its 2.5-3k revs, 3-4.5 kts. Very relaxing, quiet, steady speed, time to put the kettle on & get a sarnie sorted. Nothing happens too quickly, nothing rattling/ vibrating, no smoke hardly any noisier than sailing.

 

[jfm]

Re: what does \'cruising speed\' mean?

[ QUOTE ]
All twin engined boats rev different P & S, ( well normal gear boxes, I know Piers has some special type that this may not apply to ) but for most of us here with shafts in order to get counter rating props one gear box has to be in reverse.

We all know how low our cars reverse gear is.
A good gear box for a boat needs to be similar but can not be the same or the gear cogs would bang into each other, hence they are always different.


[/ QUOTE ]

That's just all wrong Daka . Your specific gearboxes might have different ratio fwd/reverse but in general marine gearboxes dont. There is no intrinsic reason why marine gearboxes on shaft drive boats must have different fwd/rev ratios ("gear cogs banging into each other" is just incorrect). Marine gearboxes are invarialby designed with identical ratios precisely so that they can be used on dual engine boats with contra rotating props (one gearbox in reverse, as you say)

 

[DAKA]

Re: what does \'cruising speed\' mean?

I stand by my comments in general.
Again my posts are kept short in the interests of well interest.
I did add a bit to say some bigger boats may have different gear boxes and I was talking about my boat to start with just to show I knew about my engines. I dont know about others engines, aart from when I have helped freinds out.

Now after a quick search to check I was not too odd


Volvo Penta D4 series engines use ZF63 boxes
Volvo Penta D6 series engines use ZF 63 boxes

That is a fair representation of shaft drive boats who normally show an interest in this forum but I expect there is only you and I left on this thread /forums/images/graemlins/grin.gif


ZF 63 gearbox note A=for wards B=Reverse

Please dont ask me to link every Volvo Penta engine and gearbox.

I think the Volvo Penta 63L used the 630 series gear box that also shared the same ratios as the 63.

Happy to agree your engines are different and use a different gear box, perhaps not even hurth zf.

Also happy to agree that many boats on shafts dont have Hurth 630, of ZF63 and happy to concede I have not checked every Hurth ratio and not even looked at other fringe companies of which there could be loads.

Perhaps the Old style of box that clonked when it went into gear had the same ratio which is why it clonked ?
New ZF 63 do not clonk.

edit

just reread my old post
I did say all gear boxes have different f/r ratios that could well be a very misleading phrase, sorry.

 

[Latestarter1]

Re: what does \'cruising speed\' mean?

Daka

In my previous post I said ‘propellers move boats not engines’.

Showing a marketing data sheet without the associated propeller law curves tells you nothing about what your engine is doing. You need to understand the relationship between engine power and propeller with a fixed pitch prop.

Look at the CMD link below to the 270 rating Hp and fuel consumption curves. I know this is not your rating, but it shows a proper engineering data sheet and is just to illustrate the point.

Look at the propeller law curves of power and consumption.

If you look at 3400 rpm on the power cure it gives you 258 Hp interpolate down to the prop curve and you get just under 200.

Where did the missing 58 Hp power go? The answer is that the engine is not generating it. The Hp curve shows the engines POTENTIAL not ACTUAL power output. When you adjust the throttle of a marine engine you are not directly adjusting the fuel flow. Instead you are adjusting the governor which regulates fuel flow to maintain constant rpm, not unlike cruise control on a car.

Since the propeller is only asking for 200 Hp at 3400 the governor limits fuel flow to the engine at this rpm and not incidentally the fuel consumption, therefore you look at the propeller consumption curve you get a consequently lower figure.


[image]http://marine.cummins.com/attachments/public/marine/Products/Recreational%20Inboard/4.2/BC%209120.pdf[/image]

The Mercruiser engines of your generation have electronic features which is due to the Bosch VP37 fuel pump. This is a dumb mechanical pump with a smart electronic governor which picks up a signal from a needle lift sensor on the injector.

There is some limited timing control which makes for nice clean start ups. But this is not a full authority electronic engine in the true sense which can employ multiple injection events and injection rate shaping for quantity/timing control such as the later QSD versions of the engine.

 

[DAKA]

Re: what does \'cruising speed\' mean?

[ QUOTE ]
The Mercruiser engines of your generation have electronic features which is due to the Bosch VP37 fuel pump. This is a dumb mechanical pump with a smart electronic governor which picks up a signal from a needle lift sensor on the injector.

There is some limited timing control which makes for nice clean start ups. But this is not a full authority electronic engine in the true sense which can employ multiple injection events and injection rate shaping for quantity/timing control such as the later QSD versions of the engine.

[/ QUOTE ]

Not what the experts say, my engines are 2004, they had 10 years head start on Volvo Penta at that time.

quote from my engine spec sheet

The 4.2 EI 300 combines the quiet, virtually
smoke-free, smooth performance of a traditional gasoline engine with the sheer power and
durability of a diesel.
<span style="color:red"> These high-revving powerhouses feature an
advanced microprocessor to monitor all engine settings </span> For pleasure or light commercial applications, Cummins MerCruiser diesels
are a reliable alternative to power a new boat or an excellent choice for repower.
TURBOCHARGED, WASTEGATED,
AND AFTERCOOLED
FOR QUICK RESPONSE AND
SUPERIOR PERFORMANCE
NEW 2-STAGE INJECTORS PROVIDE
LOWER EMISSIONS AND NOISE LEVELS
(MEET SAV1 REQUIREMENTS)
NEW DESIGN OF THE SPIRAL-SHAPED
INTAKE PORT FOR HIGHER SWIRL AND
BETTER COMBUSTION
EXCELLENT LOW-SPEED
TORQUE
<span style="color:red">ADVANCED ELECTRONIC
CONTROL SYSTEM FOR
OPTIMUM PERFORMANCE
AND SAFETY
</span>
VIRTUALLY SMOKE-FREE
OPERATION
Worldwide Recreational Marine

Your link shows an interesting falll in fuel use at 3000/3200 , the same fall occurs at 3200-3400 on my curves

4.2 L 300 HP

 

[jfm]

Re: what does \'cruising speed\' mean?

"Where did the missing 58 Hp power go? The answer is that the engine is not generating it. The Hp curve shows the engines POTENTIAL not ACTUAL power output. When you adjust the throttle of a marine engine you are not directly adjusting the fuel flow. Instead you are adjusting the governor which regulates fuel flow to maintain constant rpm, not unlike cruise control on a car"

Well said, great explanation of why using the manuf curves can give very misleading results (It's what I said to you in the PM exchange earlier today Daka, ref the lack of usefulness of those curves)

 

[MedDreamer]

Re: what does \'cruising speed\' mean?

The point immediately before SWMBO complains about me going too quicky and the ride getting too bumpy

 

[MapisM]

Re: what does \'cruising speed\' mean?

[ QUOTE ]
using the manuf curves can give very misleading results

[/ QUOTE ]I don't disagree in general, but in my and other fellow boaters experience the prop demand curves are reasonably accurate.
Particularly on displacement boats, for rather obvious reasons, but also on planing boats.
In which case, I guess that the rpm range of the hull transition between displacement and planing is where also the prop demand curves are totally unreliable, but nobody would want to cruise at that speed range anyway, with a planing boat.

 

[jfm]

Re: what does \'cruising speed\' mean?

Ah yes, I agree, if you are using a <u>prop</u> curve and the prop/conditions are similar to your boat then you will get accurate results. What I meant was that if you use the max engine rating curve (which is what DAKA ws linking to on the Cummins Data sheet) you will get substantially inaccurate results. Lateboater's post above gives you the order of magnitute of the error - 200 vs 258.

 

[DAKA]

Re: what does \'cruising speed\' mean?

I dont have issue with what is being said, I actually showed and used the graph to demonstrate fuel use in relation to revs and in particular the idea that a modern engine with electronic control can manipulate the curves to cause a distortion around a favourable range.

By Fluke or design you will note

huge torque to get over the hump.

a flat spot of fuel use around the 3350 which is 15% under my max revs.

I am a little baffled as to why we have needed to analyse this in such detail on a thread regarding cruising speed and I have my doubts that it has any value to the forum, my boat is a one off having 2004 engines, most Princess have older Volvo 63L or 63P so our analysis of P360s isnt really much use to anyone else.

As to my cruising habits, I have had the boat long enough and covered enough miles in varied conditions to be able to know my boat, I have had the props re pitched to benefit from maximum cruise speed and economy BUT far the most import factor of my cruising speed is being able to pop across the channel to France in two and a half hours which keeps my family interested.

If anyone is seriously thinking of taking their family to France/CI at 16 knots in an attempt to save a few gallons I respectfully suggest they get mates to help with a delivery trip and send the family on the seacat.

With the weather patterns we have had this year looking for a weather window of 3 hours + safety margin is a lot easier than finding a 6 hour gap as any delay will have the tide turning on you.

Dont forget while your boat is at 16 knots , the props are pointing down 20 degrees as you are not fully level, that is wasted propulsion.

Speed gets you level and props more efficient.

 

[Latestarter1]

Re: what does \'cruising speed\' mean?

DAKA

Were your 'experts' salesmen by any chance?

4.2 EI in all ratings were replaced by 4.2 QSD Quantum engines in 2006. Your engine did not meet EPA Tier II emissions and no longer legal in Tier II regulated markets.

QSD 4.2 all of the 4.2 VP37 engines but far more. Bosch have now wound up production of VP33/37/44 family of fuel systems, however product will be backed for next 10 years realistically far longer.

More efficient direct injection.

Bosch CP3 common rail with multiple injection events. Pilot injection resulting in 5 dBa noise reduction at idle.

Totally smoke free with ratings up to 350 Hpm.

Serious comment that it is nice to hear how enthusiastic you are about your motors. However it is a shame your 'experts' did not tell you about the pending launch of far more advanced QSD 4.2 which was certainly being openly discussed 2004 due to lack of compliance of indirect injection engines.

 

[MapisM]

Re: what does \'cruising speed\' mean?

[ QUOTE ]
it is a shame your 'experts' did not tell you about the pending launch of far more advanced QSD 4.2 which was certainly being openly discussed 2004 due to lack of compliance of indirect injection engines.

[/ QUOTE ]Yes, of course they should have told him, in 2004, that the EI engine would not have been compliant with the regulations for vessels to be built from 2011 onward.
And obviously they should have warned him that even the QSD might have had the same problem, if installed in a new boat after 2016.
/forums/images/graemlins/laugh.gif
C'mon, get real.