Semi displacement / planing, fuel prices

[MapisM]

I certainly reckon my SD would travel a shorter distance A to B in broken sea if it didn't roll so much.

Yep, that's the point.
It has almost nothing to see with drag, but rather with the improved capability to keep a straight route.
This is definitely true at D/SD speed, whilst at 20+ kts, any boat keeps a straight route "automatically", so to speak.
Otoh, at 20+ kts, drag does matter. A lot. In fact, drag correlation with speed is exponential.
I heard many years ago from the chief mechanic of Buzzi that on a test boat they were able to improve top speed by two knots just by relocating a bolt (used for the support of the surface transmissions) whose head was interfering with the water flow, go figure!
Mind, 'twas a 100+ kts boat...

 

[Piers]

I've heard that one too Piers. It's one of those stories oft-repeated by some sales types or boatbuilder staff who have a loose grasp of engineering. AKA bollox! Fin stabs cost about a knot in top speed and add a litre per mile at cruise speed.

Aha! Right. I'll soon sort that then. The person who told me is quite senior....

 

[jfm]

Are your figures based solely on planing boats as opposed to semi or maybe even disp ones,

Yes, becuase Piers's post to which I was reply mentioned planing boats

I agree with you that it is different at D speeds. In fact Sleipner have a sound theory that if you have separate a/p and stab systems they fight each other and create more drag. They want ultimately to have a single computer controlling the rudders and the fin stabs, to minimise drag. Makes sense

 

[stillwaters]

Thats an interesting idea. Planing boats have a very high degree of hydrodynamic stability at planing speed due to the flat surfaces aft designed to give the hull lift so they roll less in a beam sea and maintain an optimum attitude which minimises hydrodynamic drag. SD boats on the other hand develop less lift and less hydrodynamic stability which means that they roll more at planing speed and because of that, hydrodynamic drag is increased as the optimum attitude of the hull cannot be maintained. Hence stabilizers help to increase speed in a SD by maintaining a more level planing attitude? Is this the theory?

What I've found with my SD is that both the upside and the downside of an SD are quite well demonstrated by making way through big seas,the type where you no longer sit on top of at least a couple of waves at a time but,due to their height and distance apart,you drive up one side and then cascade down the other. This is where the SD generally has the drop on a P boat which would by now be slamming into everything unless it could remain on the plane. The upside is that it can handle it no problem behaving,as it does,a little bit like a cork,floating perfectly with its hull effortlessly ploughing along through the waves. The downside,though is also that it floats like a cork and the resultant passage is akin to a corkscrew as it rolls one way going up each wave only to roll the opposite way whilst racing down the other side. The result of all of this is that although A to B is covered in safety,the ground is covered in a zig-zag line. Don't get me wrong,I'm under no illusion that in such conditions stabs would produce a straight-line passage,nor would that necessarily be desirable but,having seen the amount of course deviation caused by excessive roll I can't help feeling that a reduction in roll would also reduce the degree of deviation (damn sight more comfortable,too).
I'm certainly no expert on stabs but I rather think their recent improvements,possibly the faster speeds at which they now react compared to the earlier hydraulic controls,could well produce better economy on most SDs.
Incidentally,I find trimtabs rather limited for much of the time on my SD,they're often just too slow to have a positive effect compared to using them at higher speed on a planing boat,say. Similar principal maybe to the stabs now being more effective due to having faster reaction times.

 

[Deleted User YDKXO]

The downside,though is also that it floats like a cork and the resultant passage is akin to a corkscrew as it rolls one way going up each wave only to roll the opposite way whilst racing down the other side. The result of all of this is that although A to B is covered in safety,the ground is covered in a zig-zag line. Don't get me wrong,I'm under no illusion that in such conditions stabs would produce a straight-line passage,nor would that necessarily be desirable but,having seen the amount of course deviation caused by excessive roll I can't help feeling that a reduction in roll would also reduce the degree of deviation (damn sight more comfortable,too).

Again going off on a tangent, this Kadey Krogen video http://www.youtube.com/watch?v=FtbXfOzD-_k&feature=player_embedded#! tries to justify the superior course keeping capability of their 'wineglass' shaped D hull against typical hard chine type D hulls. Not sure I agree with what they're saying but tends to confirm the zig zag motion you talk about

 

[stillwaters]

Yep, that's the point.
It has almost nothing to see with drag, but rather with the improved capability to keep a straight route.
This is definitely true at D/SD speed, whilst at 20+ kts, any boat keeps a straight route "automatically", so to speak.
Otoh, at 20+ kts, drag does matter. A lot. In fact, drag correlation with speed is exponential.
I heard many years ago from the chief mechanic of Buzzi that on a test boat they were able to improve top speed by two knots just by relocating a bolt (used for the support of the surface transmissions) whose head was interfering with the water flow, go figure!
Mind, 'twas a 100+ kts boat...

I find that both,not just drag or course deviation,have an effect on speed & economy. Even planing on flat water,an SD is slower and thirstier than a P boat using the same amount of thrust - it's a basic principle of the fundamental difference between what is a hull shape designed to skim the surface as much as possible compared to one that is designed to both plug through heavy seas as a D boat whilst also offering some sort of lift to get on the plane,sort of. That's why SDs are also referred to by some as semi-planing or,the best or the worst of all types.
Also,lets not overlook that if a boat is long enough (maybe it ceases to be a boat?) such as many disp super yachts and above,it will be capable of doing 20+ knots but,as it will be in disp mode it will still be subject to the laws of physics.So bye bye to the automatic right to travel in a straight line. Did you mean any planing boat,maybe?

 

[stillwaters]

Again going off on a tangent, this Kadey Krogen video http://www.youtube.com/watch?v=FtbXfOzD-_k&feature=player_embedded#! tries to justify the superior course keeping capability of their 'wineglass' shaped D hull against typical hard chine type D hulls. Not sure I agree with what they're saying but tends to confirm the zig zag motion you talk about

We've been off on a tangent for ages. I reckon one of us needs to start a new thread asking for suggestions for SD boats in the 55-65 ft range.
Great video,certainly demonstrates the principal even if you don't agree with the claims for their solution. To me their principle makes sense but I can think of at least a couple of things to consider. First,does their hull provide sufficient of the solution in all sea conditions - the video shows medium swell,how about long wave at-anchor swell or really serious waves? In my experience,round chines can suddenly start to roll even more than flatter chines in big stuff and the change can be quite sudden. Second. What effect does the hull shape have on internal accomodation/space in engine room etc.?
Not saying they don't have the answer,just suggesting don't necessarily believe what they want us to believe without question. I'm sure I wasn't this suspicious before I bought a boat.

On another point,K-K are an illustration of the greater number of SDs available in the US than here,mostly made in China/Taiwan,of course,I wonder how much is involved in bringing one across and getting a CE on it? Or maybe getting one direct?

 

[MapisM]

I find that both,not just drag or course deviation,have an effect on speed & economy. Even planing on flat water,an SD is slower and thirstier than a P boat using the same amount of thrust...

Oh yes, of course. A SD boat is also typically heavier than a P boat, so it's bound to burn more fuel at high(ish) speed.
But as I understood it, the point was whether the reason why at D speed the stabs can reduce the fuel burn is that, by reducing roll, they reduce drag OR because they reduce route deviations.
In my empirical/common sense experience/opinion, the reason is definitely the latter.
After all, just think about it: how much do you think the total wet surface can increase, when the boat rolls?
I suppose it's difficult to measure that, but if it would actually be measured, I wouldn't be surprised if the wet surface actually increases when the boat is kept horizontal by the stabs, against the oncoming wave, rather than the other way round.
Otoh, if as a result of this the trip from A to B becomes shorter (both in distance and time), the efficiency increase is pretty obvious.

Did you mean any planing boat,maybe?

Absolutely. In the context of this debate, I assumed we were talking of boats for which, as a rule of thumb, ...
<10 kts = D speed
~15 kts = SD speed
>20 kts = P speed
I'm aware that the QM2 doesn't jump on the plane when she cruises at 26kts!

 

[MapisM]

I agree with you that it is different at D speeds. In fact Sleipner have a sound theory that if you have separate a/p and stab systems they fight each other and create more drag. They want ultimately to have a single computer controlling the rudders and the fin stabs, to minimise drag. Makes sense

Mmm... does it, from an engineering (rather than a marketing... ) viewpoint?
I mean, if the fins are correctly placed on the hull, they shouldn't introduce any correction to the boat COG (which is the rudders' job).

It's only as a secondary consequence of the reduced roll, that the boat becomes less subject to COG deviations introduced by the waves effect.
In fact, the a/p works less when stabs are on, not more! Or, if you prefer, leaving the a/p off and keeping the rudders centered, the boat keeps a straighter COG with stabs on, than with stabs off.
All tried and tested - strictly at D speed, of course, but as I understand that's what you're also talking about now.

Now, I'm not 100% sure of what by "fight each other" Sleipner actually means.
But if I take it literally, I would understand that the fins action contrasts the rudders, which as a result are bound to work more, AOTBE.
And if that's what they mean, I'm afraid that's plain...

 

[stillwaters]

Fully aware of that one, Trev! Anything a bit closer to home and in the 58/63 size?

Here's a thought if you can't find a suitable charter.
Elburg Yachting in Holland have had a Hershine 61 (Taiwanesse GRP as made for Jefferson in US) for sale for a while,I believe it's actually located in Croatia (I think your neck of the woods?). Maybe they might be willing to let you have a play if they think you might be interested in taking it off their hands (well they only have to think it,don't they?). This is a more than useful SD complete with good-sized Cummins 330s with 650hours and TRAC stabs. Genny's a bit small at 7kw,though. Asking 795,000 euros but that's not too important,I guess.
www.elburgyachting.nl/pdfs/Y113208.PDF

 

[DTIM]

Generally I like the trawler yacht look but I think that Hershine 61 fell out of the ugly tree and hit every branch on the way down

 

[stillwaters]

Fully aware of that one, Trev! Anything a bit closer to home and in the 58/63 size?

Another thought,the Swift 52 at 55ft is nearly that size.Just in case you don't know there appears to be one or more for charter in Croatia in at least Sibenik/Split area.Googling "swift 52 charter croatia" htrows up a few hits.

 

[BartW]

Now, I'm not 100% sure of what by "fight each other" Sleipner actually means.
But if I take it literally, I would understand that the fins action contrasts the rudders, which as a result are bound to work more, AOTBE.
And if that's what they mean, I'm afraid that's plain...

well I very much like this flag pole MapisM, but I tend to disagree with your conclusion;

when travelling in perfectly flat sea with BA on a/p,
the boat does a small zigzag oscillation, with a consequently slight oscillating roll.
I thought it was a setting of the a/p, but how hard we tried changing the settings, we couldn't improve it, Alfonso confirmed that he alway's had this on BA. Perhaps this doesn't occur on more modern a/p's, better controll algorithm,....
but I could perfectly imagine that stabs would try to stabilise this roll, and consequently influence again the behavious of the a/p,
yes and perhaps even "fight each other"

in more heavy sea we don't have that zig zag effect, but also there I could imagine that certain actions of the stabs, in combination with a beam sea, could "fight" with the action of the a/p,

so therefor I completely understand Sleipners statement that a/p and stabs better have the same controller.

apoligies if I'm talking bollox

 

[jfm]

Mmm... does it, from an engineering (rather than a marketing... ) viewpoint?

I love the flag MapisM but I agree with Bart. A/pilots inevitably operate the rudders left/right the whole time. I'm sure you agree that operating the rudders induces roll. The stabs react to that and correct it. But if a single computer drove both, it could correct the roll before it occurs, by operating the rudders and the stabs at the same time, and that would (intuitively) reduce drag so improve mpg., as well as improving ride comfort (slightly). I say "reduce drag" because it takes less fin deflection to counteract a rolling force predictively before the roll starts, than to negate it after it has started

(Overall I suspect the overall effect is slight. But, hey, BMW is fitting bigger alternators that clutch in as the car brakes and overruns, and de-clutch when the engne is working to drive the car, all to save 1 teaspoon of fuel every 100km or whatever )

I think. Feel free to wave the flag again if necessary

 

[MapisM]

I think. Feel free to wave the flag again if necessary

LOL, I'm tempted...

Hang on a minute folks, let's understand what we're talking about, first.
D or P speed?
And "perfectly flat sea", as Bart said, or any sort of sea where stabs are actually useful?

Because these alternatives create a sort of 4 quadrants matrix, whilst I had in mind just one of them with my previous post - namely, the "D speed/rough sea" combination.
In fact, I did specify that I was talking of D speed. I didn't mention the sea state because I assumed that we were talking of conditions where the stabs matter, so I didn't consider the dead flat sea behaviour, but I'll come to that later.
And otoh, I also didn't consider the "P speed" quadrants (neither with flat nor rough sea state), because I don't have enough experience to talk about those conditions, though I suspect that the "fight each other" thing hardly applies to them, too.

Anyway, if we talk of D speed and rough sea, I know exactly how my boat behaves (I've actually tried also a few others, but tbh I never made the same tests I made with mine).
And the results of these tests are those I previously explained:
1) with stabs on, the A/P works significantly less than with the fins center locked (which is still better than no stabs at all, btw);
2) with the A/P off and the rudders centered, the boat keeps a more straight COG with stabs on, compared again to center locked.
Now, call me naive if you wish, but to my simple mind this is enough to prove that the stabs work concurrently with (rather than against) the A/P, even when the two equipments are not integrated at all.
And aside from having tried this onboard, I wouldn't predict a different result also from a theorical standpoint, because in comparison with the waves forces which the stabs are meant to compensate, the rudder induced listing is ridiculous. Not only that, but it's totally unpredictable whether such effect, in any given moment, will work with or against the waves.

Talking instead of "perfectly flat sea", well, I accept that the rudders induced motion can be perceived, occasionally. BUT!
First of all, in dead calm sea, the A/P should work veeeery little.
I accept that at D speed it has to work a tad more with P boats, 'cause they don't have the deep keel and inherent stability of D hulls, but such difference can't be huge.
If it works constantly, to the point of introducing a constant roll by mean of rudders "hunting" as Bart describes, there's definitely something wrong with it.
By heart, I would say that the dead band is set too narrow, to start with. But it's impossible to tell without knowing exactly the a/p features. Bart, do you have a PDF manual which you could send me?
One thing is for sure: with old A/Ps, setting them accurately is a nightmare. And I mean it: when I bought my boat, she had already cruised for 4 years with her previous owner, and according to him, the Furuno technician spent a couple of hours during the commissioning just to calibrate the A/P.
In spite of that, after I bought her, I wasn't fully satisfied with such calibration, and after studying carefully all features, I tried countless different settings. Overall, it took me days, not hours. But the result exceeded my most optimistic expectations.
Btw, the calibration of a modern A/P is ridiculous in comparison: you put it in "learn" mode while in open calm waters, it does its stuff for a few minutes, job done. That's cheating!

Anyway, back to the point: it's a sort of catch 22, really.
If and when the sea is rough, the stabs have much bigger fish to fry than the rudder induced motion.
If and when the sea state is flat enough to make the rudder induced motion perceivable, the A/P (a correctly calibrated one) should work so little that such motion is practically non existant.

Last but not least, even accepting that on some boats the rudder induced motion can be slightly perceivable in dead calm waters, I have a suggestion for Sleipner: they'd better recommend their clients to keep the fins locked in such condition. The onboard comfort won't suffer in practice (particularly on big boats like Match or BA), but at least some wear and tear will be saved. Not to mention maybe a couple of fuel teaspoons every 100km...
...Surely more than what could be saved by integrating the A/P and stabs control, anyway!

 

[BartW]

let's understand what we're talking about, first.

First of all, comment to myself; get your facts together (as Jfm would say )

Secondly, I have very limited experience with a/p’s , only on one planning boat, and none with stabs,
but I like to dig in to this and think about it thoroughly.

agree with you that my setting could be / should be improved, I’m hoping we could manage that together one day . I’ll e-mail you the manual, so you can start study’ing from now
(I believe its very similar the yours)

Now, from some of your remarks, my conclusion still remains that a/p and Stabs should better be controlled from one computer;

like in my case, it would be stupid to cure a non correct setting of the a/p (oscillating roll at flat water) with the stabs,

You suggest that in flat sea you better switch off the Stabs, well actually if all from one computer, this could be done automatically, or integrated in the algorithm

I don’t read the “fighting with each other “ as a permanent battle between the two systems,
Just that occasionally the systems might not react in a optimum way together, (fe if one of them is not set accurately) and occasionally create contradicting reactions.

I would be very glad if I could keep my old a/p at the moment whenever I might be able to afford and decide to retrofit stabs on BA. But I’m convinced that the stabs would NOT work optimal with the today’s setting of the a/p’s

I think your argument comes from your experience, with a boat and a/p and stab system that you perfectly know, and which are set very accurate and correct, and at D speed only,
While Sleibners statement is a more general rule for whatever unexperieced shipper for whatever size of system in whatever boattype either D or P in whatever sea state.
Similar to the modern cheating a/p learning that you refer at.
But then again this is just my perception, no real experience AT ALL with stabs.

Back to my rolling effect from a/p, at flat sea this IS noticeable by the guests, (at the edge of being annoying) so this should be improved. Glad to hear your suggestions, unfortunately no trial possible now in Marsh when you're there (boat under a cover),
And second problem, the display is very badly faded by the sunshine.
Reading and changing the settings is difficult, but possible from the inside helm position (only)
Whenever I buy stabs, I better replace the a/p display’s anyway

 

[MapisM]

I’ll e-mail you the manual, so you can start study’ing from now
(I believe its very similar the yours)

Yep, please do, you know my email already. Is it a Furuno?

Back to my rolling effect from a/p, at flat sea this IS noticeable by the guests, (at the edge of being annoying) so this should be improved.
Glad to hear your suggestions, unfortunately no trial possible now in Marsh when you're there (boat under a cover),
And second problem, the display is very badly faded by the sunshine.
Reading and changing the settings is difficult, but possible from the inside helm position (only)

Ok, but again, at which speed is the effect you're talking about so noticeable in flat sea?
It should take quite some rudder angle to make BA listing at D speed, whilst on a flat sea just a few degrees corrections should be more than enough.
Otoh, at higher speed I suppose that even small rudder angles, particularly if regularly alternated on either sides (which is what is normally called "rudder hunting"), could do the trick of constantly oscillating the boat.
But that's strictly a matter of having a properly working A/P - nothing to see with stabs.
Mind, even the most simple stabs would flatten that rolling perfectly, but the A/P would still continue to work "behind the scenes". Which is wrong anyway, also for the wear of pump, leverages, etc.

I don't think the faded display should be a big problem, though.
Most of the settings which are critical for the A/P behaviour should only be calibrated once, and after that, you typically use just one button to select between a few pre-defined sea states.
Glad to have a look at all that when we'll meet in Rome.
Of course any possible calibration will have to be tested at sea afterwards, but the logic is pretty much something that should be checked/understood on paper first.

 

[BartW]

at which speed is the effect you're talking about so noticeable in flat sea?


at higher speed I suppose that even small rudder angle, particularly if regularly alternated on either sides (which is what is normally called "rudder hunting"), could do the trick of constantly oscillating the boat.

alway's P speed MapisM, we only use D speed in Marina's
seriously though, have never used a/p at D-speed (yet)

“Rudder hunting” that sounds like it;
doing a straight course at flat sea, the boat is slightly going off- course, the a/p is giving a small correction to the rudder, it takes quite a while (wild guess, 2..3 sec) for the boat to react, then boat going the other direction off -course, again quite a while before reaction is noticeable,…

It seems that the reaction time between the command and the movement of the rudder is too long.
Hydraulics too slow? or "play" in the system ?

This picture is the only picture I’ve got,
that shows a bit from the effect when looking at the wake

Apologies to Henryf for highjacking this thread

 

[henryf]

Carry on. It served its purpose for me some pages ago.

Henry

 

[vas]

“Rudder hunting” that sounds like it;
doing a straight course at flat sea, the boat is slightly going off- course, the a/p is giving a small correction to the rudder, it takes quite a while (wild guess, 2..3 sec) for the boat to react, then boat going the other direction off -course, again quite a while before reaction is noticeable,…

It seems that the reaction time between the command and the movement of the rudder is too long.
Hydraulics too slow? or "play" in the system ?

since Henry doesn't mind, I think we are simply talking about A/P gain control adjustment Bart. Had the same issue on my Cetrek a/p because the setup menu is so retarded and unintuitive that I accidentaly managed to max out the value on this. Result was much worse than what you describe, but trying to fix it I passed from this stage you're facing and eventually minimised it (and I'm talking D speeds since my one turbo was stuck...)
There's another complimentary value that has to be tuned as well, sure MM will come and correct me soon

V.