Difference in fuel consumption: Displacement v Planing

[scubaman]

And - yep - a quick blat at the end keeps the glazing at bay - IMO

I too have wondered about the effects of extended "pootling" on the engines. One article on the matter suggested that as long as the engine is driving the boat (ie. on gear), glazing should not be a problem. The same article suggested that glazing should not be a problem on run in engines at all.

Other than that my thinking says that giving a blast periodically (especially in the beginning to properly warm up the engines) to get rid of exhaust deposits, would be good. But what do i know...

A quick googling didn't return any comments from our engine profs, but it would be interesting to hear their view on how the low-rew running effects the engines...

 

[Loggo]

apart from the obvious one that it takes forever to get anywhere

Can't be doing with reading all the replies but surely that is all what it is all about

 

[Hurricane]

Yup, exactly correct imho

jfm
Sorry
I didnt mean to diminish the effect of stabilisers - remember I was on Match in that big beam sea when Sleipners were commisioning - and it was VERY obvious to me that I wouldnt wanted to have been out in that sea with JW whereas Match was comfortably handling it.

I was just pointing out that we wouldn't have gone (indeed didnt go) out in that kind of sea.
And also pointing out that if you dont have stabs, you can still have great pootling cruises in the Med at this time of the year.

And to put it in perspective - the installation of the stabilisers would have cost me as much as the whole of my fuel bill for the last 3 years - and we've travelled further in that time than most boats.

 

[jfm]

jfm
Sorry
I didnt mean to diminish the effect of stabilisers - remember I was on Match in that big beam sea when Sleipners were commisioning - and it was VERY obvious to me that I wouldnt wanted to have been out in that sea with JW whereas Match was comfortably handling it.

I was just pointing out that we wouldn't have gone (indeed didnt go) out in that kind of sea.
And also pointing out that if you dont have stabs, you can still have great pootling cruises in the Med at this time of the year.

And to put it in perspective - the installation of the stabilisers would have cost me as much as the whole of my fuel bill for the last 3 years - and we've travelled further in that time than most boats.

No need to apologise, and I agree the logic of what you say. Perfectly sensible choice. Indeed, I'd confess to a bit of stabzealousnesswhich I'm afflicted with becuase I'm so new to them. You're right, that in certian types of boat usage you absolutely don't need them, as you are proving. I was really just agreeing with Mapism that even in slight seas with a slow roll of +/- 1 degree, the stabs do actually make the boat +/- 0.1 degree. I would accept however that that is being quite fussy

BTW, there is a prin67 retrofitted with stabs. Listed for sale on yachtworld, in thailand iirc

 

[rubberduck]

I drive a medium fast car but don't do 150 everywhere, likewise I think of the boat the same way. If I go out with Nicky & Gizmo we are quite happy at 7 knots, with the boys flat out seems to be the preferred option with loads of completely unnecessary sharp turns. Always amuses me the Sam has to leave any change of course until the last minute & then try & put the boat on its side.
On another note, we have two Volvo D6 330's, would it do them any harm to spend lots of time on tickover ? Well, both the kids are off to Uni etc in the not too distant future.

 

[Assassin]

The issue of pootling on engines is an interesting one as it often depends on the type of engine fitted, its age, and how well it meters its fuel into the engine.

Older diesels do not respond so well to very low engine revs, and they can carbon up due to insufficient working loads or engine revs which means they do not generate sufficient engine combustion temperatures. These older engines need a good periodic blast, and to work within their designed operating range for best engine life.

Newer diesels are much better, this is due to the electronic controls which meter fuel much better, and obtain much better and more efficient combustion, this means they work within a much wider working range efficiently. The only real issue is with soot build up in the exhaust side of the engine, this builds up more readily at low working revs as the exhaust pressures are too low to remove them, so they need a periodic high speed blast to clean them.

This is very general as all engines differ in their working applications.

 

[Deleted User YDKXO]

Not convinced about the roll argument if you pick the weather.
Here's a video that shows a typical "pootle" - no stabilisers.
The other boats in the video are 50 footers.

And - yep - a quick blat at the end keeps the glazing at bay - IMO

I didnt get any sound on the video, Mike? Yes I agree that on many Med days you dont need stabs. We certainly didnt feel any need for them during our recent trip. On that subject, I feel that planing boats still give some hydrodynamic lift to counter rolling even at d speeds; its only really when they're not moving that they roll like pigs in a beam sea

 

[MapisM]

I was really just agreeing with Mapism that even in slight seas with a slow roll of +/- 1 degree, the stabs do actually make the boat +/- 0.1 degree. I would accept however that that is being quite fussy

Yep, agreed about fussyness, if such word makes sense.
Otoh, the whole pleasure boating thing is about being fussy, innit?
And when it comes to slow & stabilised cruising, having developed a taste for that during more than a decade now, I'm a bit spoiled...

 

[MapisM]

I feel that planing boats still give some hydrodynamic lift to counter rolling even at d speeds

I don't think that has to see with dynamic lift, unless you're considering as displacement also two figures speeds (which is incorrect, unless applied to BIIIG boats).
It's rather the shape itself of a P hull which resists to roll more than a full D hull.
But comfort is not just about degrees of rolling, because the roll period affect onboard comfort even more: a slow, predictable roll is much more comfy, for any given roll degree.
And that's where full D hulls are very different from P hulls.

 

[jfm]

a slow, predictable roll is much more comfy, for any given roll degree.
And that's where full D hulls are very different from P hulls.

Agreed, but it's not the fact they are D hulls per se that creates the slow roll. It's the fact they're heavier, and the extra weight is not concentrated at the roll axis, and therefore they have much higher angular inertia. You could add 20 tonnes to a P 65 footer so it weighs the same as a D 65 footer, and it would then have a slow roll despite being a P hull.

 

[MapisM]

You could add 20 tonnes to a P 65 footer so it weighs the same as a D 65 footer, and it would then have a slow roll despite being a P hull.

Yup, all agreed about angular inertia, though also the round bilge and deep keel, typical of D hulls, all affect the roll behaviour - as do bilge keels, when fitted.
I'm just not sure I'd throw a 20T ballast on a planing boat just to smoothen rolling...
Otoh, I must say that I'd be curious to see the behaviour of Blue Angel at D speed. She must be one of the heavier planing 70 footers around, pretty much comparable to many SD and also full D boats - bar steel ones, of course.