Fuel consumption difference between two engines

[jfm]

Why, of course it knows where the (physical) zero is.

OK, if it does then fine and I would eat all of my words. But how does it know? Who has calibrated it and set up the true physical zero? How did they do it?

(When I argued about this after your post #11, we were not talking about your boat. You were advising others to use a trick, so we were talking about boats generally, not yours. But I do not mind whether we discuss this by reference to your boat or to boats in general)

I seriously struggle to understand how you can think that this could happen with one prop longer than the other

We may be at crossed purposes. Sorry if not explaining it well. I meant that if you sync your engines by ear to within 15rpm (or by a reference grade tacho), and drive in a perfect straight line, and then you set that as your zero on the a/pilot, you would be all wrong if, unbeknown to you, one prop happened to be longer than the other. On older boats (pre 3D prop scanners) props can easily be say 3% different length. They're only sand cast. You know yours are perfectly matched? How? Scanned?

 

[Pinnacle]

......
But when we went at sea together, we rather discussed which type of aperitif to have before dinner while overnighting at anchor, selecting the best swimming spot for the wind conditions, and other similarly trivial things, you know...

Blimey MapisM, you do have to endure some pretty tough and unpleasant tasks don't you...........and all for the benefit of the readers of the Forum. Still, I suppose someone has to do it!

 

[MapisM]

LOL, naaah, both myself and S did enjoy the company a lot. And we do appreciate also decent drinks and food, you know!
Not to mention when we have also an excuse to catch up all together with the local friends, as we will have tomorrow when IT will play against Uruguay.
I can already see a week of diet at the horizon, to recover after tomorrow night dinner....!

Btw, P, the open invitation for yourself and S is always valid, of course.
We will have other guests from mid of next week up to Jul 22nd, but we are available Jul 23rd up to Aug 5th, if any dates in between suit your agenda.
From Aug 6th onward we will not be onboard, probably up to the 20th or so, but after coming back we will stay for most of Sep and Oct.
Just drop me an email if you will be able to make it!

 

[MapisM]

But how does it know? Who has calibrated it and set up the true physical zero? How did they do it?

I never asked them, but my educated guess is that they used some long straight sticks to set the rudder blades perfectly parallel with shafts first, adding afterwards a little bit of toe-in based on experience, and eventually tightening the tie bar.
Done that, setting to physical zero the on the a/p rudder sensor is just a matter of leave the rudders centered, find the sensor position which shows zero on the display, and tighten a couple of bolts on the sensor - which I suppose was done by a Furuno engineer upon commissioning.
Anyway, I did also check the alignment myself, just after I purchased the boat, because I was unhappy with some a/p settings, and while I was at reprogramming it, I also thought to have a look at the rudder indicator. And I can assure you that if you stay far enough from the stern, aligned with the keel(with the boat on the hard of course), mk1 eyeball is enough to spot a 2 degrees misalignment, either side.

On older boats (pre 3D prop scanners) props can easily be say 3% different length. They're only sand cast. You know yours are perfectly matched? How? Scanned?

Well, leaving aside the fact that a 3% unbalance couldn't explain the fuel burn difference the OP is experiencing, my train of thoughts (summarised in the last paragraph of post #18) was actually different:
1) I'm aware that the a/p is consistent in automatically adjusting its trim, and that when it's close enough to zero, the engines are synced;
2) This implies that in such condition the thrust is balanced;
3) Since the thrust is balanced, with the engines synced and the same gear ratio, by definition the props must be perfectly matched. Well, close enough to perfectly, anyway.

A couple of further comments:
a) if your last question above was meant to ask how props (not just mine, but in general) were balanced in the pre-3d scanners days, afaik they used three methods: testing, testing and testing.
Mind, I would still want to do a fair bit of that also today, if I were signing the acceptance papers of a new boat, and paying the balance accordingly.
Believe it or not, I'm aware of pre-scanned props which required some adjustments after the sea trials.
b) I accept that the previous point 1 is a critical assumption, when suggesting to use this method to anyone else, as I did with the OP, and that not only the a/p rudder sensor could be badly adjusted, but an a/p could even not have a rudder sensor at all. I also accept that having just mentioned "a decent a/p" in my initial caveat was not specific enough, but by now the OP has either understood what the requirements are, or decided to forget my suggestion altogether - either ways, that's fair enough.

 

[jfm]

OK, happy to leave it at that. The long straight sticks thing (maybe you dreamt it!) made me giggle - I assume the Furuno guy came down straight away to install the rudder position sensor before anyone moved the rudders, right? LOL!

Your 1,2,3 still rely on the whole system knowing that "it's close enough to zero". The system doesn't know that in a standard a/p install, and can only know it if someone has gone to loads of effort to create a zero

I never said 3% explained OP's problem and I already gave my answers on his problem. 3% is merely a sensible guess about the error band of the error in your "trick" assuming you do it very carefully. A bog-standard a/p install would have none of the care you applied and therefore a bigger error unless the installer was lucky. My point was that it is engineeringly flawed to use the a/p computer's number for the rudder position as a system to measure engine work done, that's all.

But I'm happy to agree we have done this to death!

By the way, I have never come across an a/p with no rudder indicator. Would seem quite crude. I mean the computer could fe see that it needs to steer the boat to port, and open the hydraulic valve to push the tiller, or turn on a motor if it is a small-boat electric system, but it would never know how much the rudder moved so the impulse would be controlled just by a timer I guess. That would be very crude indeed. The little potentiometer that the position indicator consists of is perhaps 1/50th the cost of the whole system. No worries, and just thinking aloud here...

 

[MapisM]

The long straight sticks thing (maybe you dreamt it!) made me giggle - I assume the Furuno guy came down straight away to install the rudder position sensor before anyone moved the rudders, right? LOL!

Not a dream by any means. I did call it an educated guess and that's what it was, no less, no more - just meant to try answering your question.
They might as well do it differently, with a reference mask, or whatever.
Along the same lines, I suppose that the yard could have completed the rudders/tie bar installation right before (or at the same time) the installer was fitting the electronic bits.
Why should that be so difficult to organize?

A bog-standard a/p install would have none of the care you applied and therefore a bigger error unless the installer was lucky. My point was that it is engineeringly flawed to use the a/p computer's number for the rudder position as a system to measure engine work done, that's all.

Sorry, but I couldn't disagree more. Of course it's an approximate measurement, but why flawed?
Given all the previous assumptions, it can and does work. Sure, the accuracy is nowhere near the engine load displayed on electronically controlled engines - and it says nothing about the absolute values, just about the engines balance - but it's still much better than nothing.
OTOH, my previous Q still stands, if you can suggest any other zero cost alternative, no matter how accurate, I for one am interested to hear it.

Re. bog standard a/p installations, I just rechecked my Furuno manual, because you made me curious.
It clearly explains how to install and adjust the component that they call "rudder reference unit" (aka FAP-6101, though the current models are surely different), and it isn't such a big deal. In fact, I would feel pretty confident in doing that myself.
If you know for sure that at Fairline (or any other yard for that matter) they rely on the installer luck rather than on a 10 minutes job, well, THAT is an engineeringly flawed approach, in my books - even if it wouldn't affect the a/p operation in practice.
After all, if the a/p components, when properly adjusted, can also give a very accurate indication of the rudder position, why accept anything less just because the yard skimps on its installation?!?

 

[jfm]

We really have done this to death but just to answer a few questions you put:

but why flawed?

Because unless you know the thing has been calibrated to a very good zero, you're doing maths on bum information, that's all. You were recommending tweaking the throttle levers until the instrument shows nearly zero trim, but you could easily have 3 or 4 degrees trim on the rudders while the gauge is saying zero. A few degrees is a much bigger error than a 15rpm delta so you're mixing good date with (relatively) a wild guess and calling that science. Fact is, most boats do not have a reasonably good zero (because they don't need it). Yours might have a good zero, but that's an exception not a rule. For everyone else, you're advising them to measure the length of a table without having the zero cm starting end of the tape measure hooked nicely over the edge. I've done that point to death though so please let me stop!

Given all the previous assumptions,

None of which were in your post #11 when you recommended this technique to everyone! Fact is, on most boats the assumptions are NOT met. Because they don't need to be, so there's no reason for a builder to bother

OTOH, my previous Q still stands, if you can suggest any other zero cost alternative, no matter how accurate, I for one am interested to hear it.

On principle, I totally refuse to play the game "You can't criticise my idea unless you propose a better one". Some things are just flawed, and if they are I'll say so. As it happens, at the time of writing (!) I cannot think of a zero-cost alternative

Not a explains how to install and adjust the component that they call "rudder reference unit" (aka FAP-6101, though the current models are surely different), and it isn't such a big deal. In fact, I would feel pretty confident in doing that myself.

Sure you could get it to within a few degrees, but no better without serious effort. A few degrees is a much bigger error than a 15rpm delta. As I say, builders don't bother searching for a perfect zero because they don't need to. The a/pilot computer doesn't need to know where zero is to drive the boat in heading hold or tracking mode. So long as the rudder gauge on the dash is roughly 6 o'clock when the boat is driving straight, an aesthetic point, the builder/installer's job is done.

If you know for sure that at Fairline (or any other yard for that matter) they rely on the installer luck rather than on a 10 minutes job, well, THAT is an engineeringly flawed approach, in my books - even if it wouldn't affect the a/p operation in practice.

Sort of losing the will to live here because you're trying a potshot at Fairline because you're wound up. It is a ten minute job to get a rudder zero that is within 3 or 5 degrees ish, using mk 1 eyeball. It is a massive job to do better than that, unless you have electronic engines, which everyone does these days including Fairline. With electronic engines it is 5 minutes job to do it perfectly on the first sea trial, if you really want to. Whether a builder puts in the effort to find perfect zero or deliver the boat with a 3 degree error makes no difference to the function of the a/pilot so it just aint worth the candle

why accept anything less

Because it brings no benefit with electronic engines. Absolutely none whatsoever. Just about everything else, including the colour of the flowers the builder gives you as he hands you the boat, is much more important

 

[MapisM]

Sort of losing the will to live here because you're trying a potshot at Fairline because you're wound up. It is a ten minute job to get a rudder zero that is within 3 or 5 degrees ish, using mk 1 eyeball. It is a massive job to do better than that, unless you have electronic engines

Yeah right. YOU are loosing the will to live, and I AM wound up.

I never recommended this technique "to everyone". I just gave my 2c to someone who said that he has 20yo engines, as I do.
You thought to jump in and dismiss my suggestion as if I were trying to sell snake oil at one thousand quids a bottle.
You are now pretending that no yard on earth can install a rudder position sensor with an accuracy of more than 3 to 5 degrees, which is ridiculous, and makes me understand that you NEVER looked at rudders steered 3 degrees either side, with the boat on the hard.

I'll tell you what.
You're happy with the flowers you got on handover, I'm happy with the accuracy of my rudder indicator.
That's fine. If we were all the same, it would be boring.

@penpal: it would be interesting if you will let us know about your findings, all the very best for your troubleshooting.