what does 'cruising speed' mean?

Re: Glad to serve!

What do you mean by horribly inefficiently?
I guess it's probably a PITA to cruise at 6kts or so with a boat like that (rolling like hell, engines at or near idle running rough), so surely 20+ kts is a best cruise speed overall.
But talking strictly of MPG, it's hard to believe that hull speed doesn't give the best result.
Possibly it's not much worse at slow planing speed, but better? Naaahhh....
 
Re: what does \'cruising speed\' mean?

[ QUOTE ]
The most fuel efficient speed for any boat of any size, bar none, is the hull speed.

[/ QUOTE ]

That's just not right Mapis, at least if we are defining efficiency as max MPG. During the displacement speed range of a boat, the boat's MPG will increase as the throttle is opened and its speed is taken up to hull speed. MPG is at its highest when the boat is just in gear at idle rpm 650 ish, which is perhaps 5kts or so depending on the boat

I agree though that there is no definition of "crusing speed". It's a loose term, normally something around 70-90% of Vmax but there is no definition
 
Re: Glad to serve!

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

Agreed entirely. (*)
I did make it simpler than it is - alas, apparently I was still too complicated...
What I was pointing out is just that, in spite of what many boaters think (and also Volvo, apparently), the most economic speed of any boat is always the "slower" one.
I'm aware that at 7kts you get a better MPG/range than at 8kts, and an even better one at 6, but that's a debate which is better suited for PassageMaker...
You know, the 'are 5k gal of cheap diesel bought in Venezuela enough to reach French Polinesia?' sort of thing... /forums/images/graemlins/smile.gif

(*) Assuming that by 'the boat's MPG will increase as the throttle is opened...' you actually meant decrease, of course.
 
Re: what does \'cruising speed\' mean?

Cruising speed.

Not wishing to sound pompous but I am amazed about how little you guy's understand about your engines.

There are some real basic terms for pleasure craft engine applications.

Rated RPM; This is the maximum engine rpm where engine is producing max INTERMITTENT power.

Cruise speed; is the CONTINUOUS rating rating for your engine, less desired rpm for maximum efficiency.

For example; Yanmar 6LYA STP 370. Actually 350 proper Hp @ 3,300 Rated. Maximum CONTINUOUS rating is 3100 rpm reflecting 290 Hp.

Therefore cruise rpm is no higher than 3100 rpm and dependent vessel the most efficient hull speed, probably somewhere between 2900 rpm and 3100 rpm as power curve falls big time below 2900.

Remember electronic engines KNOW how they are being treated, so if you are running your Volvo D4/6 everywhere at 3500 rpm pedal to the boards and the critical parts want to exit to atmosphere whilst still within their pathetic 12 month warranty coverage, you ain't got no warranty coverage. Big bang, download ECU, sorry Mr customer, you were not running at 3500 for more than 1 Hr without dropping 300 rpm off the top for six hours. Here is your quote for repairs.
 
Re: what does \'cruising speed\' mean?

The point you are missing is that cruise speed for the engines might well exceed what is comfortable for the hull.

3500rpm's (out of 3900rpm flat out) might be a valid cruise speed as far as Volvo Penta are concerned, but not if SWMBO is glaring at me 'cos the 4 year old is about to bounce off the stern...

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

Yes, much to your suprise, many of us know that. It's a bit like the city at the moment, loads of promises, but actually they are bust.

Same with engines, you can get 350hp out of it, but if you try, it breaks. Cruising speed is the speed it wont break at. Stopped seems the answer for most boats.
 
Re: what does \'cruising speed\' mean?

It's perfectly fine to give your views and say that a post is wrong, but to make a blanket generalist comment that we/I know little about our engines is a bit pompous.

There is no definition of "cruise speed" in the way you say. What you have done is define the maximum continuous duty parameters for the engine, which is fine, but that doesn't amount to a definition of "cruise speed". "Cruise speed" is an altogether looser casual term, and is expressed in knots

Incidentally you mentioned "maximum efficiency". It would be quite a coincidence if an engine happened to reach its peak thermal efficiency at its maximum continuous duty running condition. Could happen, but would be mere coincidence. And there is no sensible engineering meaning that can be given to your expression "most efficient hull speed". Each hull requires more power the more speed you apply, except either side of the hump. The speed at which a hull will be most efficient, ie will require the least amount of work to be done to move it a given distance, is always 0.00000000001 knots.
 
Re: what does \'cruising speed\' mean?

My reply was short but took into account my engine specs.
If you need absolute accuracy

cummins-mercruiser 4.2 L D300
rated300 hp @ 3900 , max revs 3950 governor +- 25

continuous 15% max revs =3400

Max revs for no more then 10% of duty cycle.

port gear box ratio 2.52 stbd 2.54

Port revs to 3975 srbd 3950

I usually cruise p 3350 s 3340

I have also studied my Torque/fuel use curve which is very favourable @ 3400 rpm, cummins-mercruiser have mapped their engine for max fuel economy to peak with high torque at 15% under max rpm
fluke ?, I think not !

That is why in my case the most economical cruise speed is not just on the plane (16 knots) it is 24/25 knots .

All the above is from memory, I know my engine intimately thank you.

I have only once made use of 3800 cruise to out run 50 knot squalls and lightning, this was well with in the 10% duty cycle and there was no sign of increased engine heat.
 
Re: what does \'cruising speed\' mean?

[ QUOTE ]
The speed at which a hull will be most efficient, ie will require the least amount of work to be done to move it a given distance, is always 0.00000000001 knots.

[/ QUOTE ]

Is that true? I heard it was 0.000000000000000000000001 knots.





/forums/images/graemlins/grin.gif/forums/images/graemlins/grin.gif/forums/images/graemlins/grin.gif Hope you're keeping well down there in sunny PV.

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

Tee hee! Yup you're right. Bagsy you tell Magnum, not me.

Yep sunny PV is still lovely. Was sunny last weekend, looks like will be this weekend. I plan to go this friday, unless I decide to jump off a ledge like everyone else here in meltdowns-ville :-S
 
Re: what does \'cruising speed\' mean?

Yeah but you are mixing up maximum thermal efficiency of the engines (3400rpm ish) with max mpg of the boat (which would be more like 16kts, not 24/25, aren't you? Or are you saying your mpg is higher at 24/25 than 16kts?

Why are your gearbox ratios different? Is there some cunning trick there?
 
Re: what does \'cruising speed\' mean?

If you look at the detailed tests on Boattest, you'll see some very high peaks on the graphs they provide. My hull and engine combination is most efficient on the plane, at 28knots, and my own experience backs that up. OK, it will do better at 3knots, but on the plane.....
 
Re: what does \'cruising speed\' mean?

Sounds like you are heading long and healthy engine life.

Although not strictly a CMD engine, a V.M with all the performance stuff was done by the guy's from Detroit Diesel.

I always thought the High Output V.M 4.2's had a rated speed of 3,800 with high idle at just over 4,200, however no matter you are running them real easy.

It was just that certain comments here appeared to be missing the point. The the vital being that the propellers drive the boat not the engines.

Properly set up your engine HAS to be allowed to reach the rated rpm plus a percentage of the governor droop to allow for bottom fouling. As a properly designed propeller is only efficient at rated rpm if you look at the propeller law curve you can see despite all the good things that the engine is doing as you come down the curve the propeller is doing a less than brilliant job at transmitting the power.
 
Re: what does \'cruising speed\' mean?

I'm not sure about which guys you think don't understand engines, also because you seemed to reply to jfm and myself, when we were discussing something which had nothing to see with the points you are raising.
But I'm always interested to learn something new of course.
For example, could you post any link to an engine manufacturer who classifies the ratings in terms of RPM, as you suggest, rather than in terms of load factor, which was my understanding so far?
 
Re: what does \'cruising speed\' mean?

Hi jfm,

I couldn't reply last night as I needed to find the link to my engine curve.

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.

You need calibrated rev counters to actually see it ( I have calibrated mine).

Now back to my boat efficiency

I have true electronic controlled engines, same as a modern car diesel, car manufactures used to move the timing to get max mpg at 56 mph, now not sure what they do to get under 160 co2, but in effect they can move the torque curve and power and HP. ( chipped or mapped)
People who have their chips changed will loose somewhere on the map.

CM have had control over the VM engine and they have mapped for max efficiency @ 3400 rpm.

My boat will get further on 20 gals of diesel at 25 knots than it will at 16 knots.

loads of power to get to 16, not much extra to get another 10knts.

3200 rpm uses same( almost) as 3400 rpm

graph here to download

My first reply post was short and aimed at 95% of forum readers, I realise you are interested in detail but most couldn't be bothered to try to understand this detail.

Lateboater, I was just trying to show you that although my reply was brief, it was based on sound knowledge of my engines.
You should also realise that I wouldn't stuff up to the tune of 200 rpm.

All the above is accurate , 3950 max, governor -/+ 25 rpm
I shortened 3925-3975 rpm to 4000 as that is a lot easier to read.


slight hickup

in error I said max torque at best fuel efficiency.Should have read high HP
ie/ 275 hp is available at 3400 which has a flat spot on fuel use.
Torque is aimed to get the boat over the hump.

50 hp extra for no extra fuel
 
Re: Glad to serve!

[ QUOTE ]
What do you mean by horribly inefficiently?
I guess it's probably a PITA to cruise at 6kts or so with a boat like that (rolling like hell, engines at or near idle running rough), so surely 20+ kts is a best cruise speed overall.
But talking strictly of MPG, it's hard to believe that hull speed doesn't give the best result.
Possibly it's not much worse at slow planing speed, but better? Naaahhh....

[/ QUOTE ]

Easy, 0-Hull speed gives in the region of 0.5Nm/Ltr (at 1100rpm approx)and 22kts gives 0.7Nm/Ltr at approx 3200rpm. Remember that petrols have hopelessly low effective compression at light throttle whereas diesels still have full compression so the engine efficiency holds up at low speed and is way ahead of a petrol. The engine is injected so no rough running at all...

Our 5.6m RIB (2-stroke 90hp) is even worse, 0.45Nm/Ltr off-plane and 0.75-0.8Nm/Ltr when up on plane, again it'll be the engine efficiency (or not) killing the low speed economy.
 
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