Boat in build pics (2013 Fairline Squadron 78)

[jfm]

Yup we're agreed on the principles, and only disagreeing on the relevance of dynamic centreline in a scenario when stabs are working but not working hard. Neither of us has data so let's agree to disagree, unless I can be bothered to collect the data over some say 50nm trips on M2...

BTW and FWIW I did leave M1's stabs on all the time no matter what the weather, as a high tech alternative to shouting at the crew

Hmm, we can debate the stats of sleipners smaller number of installs but you can also make judgements from the engineering. Look at Steve Dashew's pic below of the busted Naiad on Windhorse - that's just craap engineering and you dont need data to prove it. The sleipner metal inside the (one piece) fin is much bigger than the rather pathetic Naiad construction so the sleipners are fundamentally better on this score

Regarding...

Re. current, your objection makes me think that you must be in SoF, where wines are even better than in AU...
Scenario: cruising from A to B, true route 90°, stored on the plotter, a/p on track mode.
Question: does the optimal fins centerline remains the same with a) no current; b) current flowing E or W; c) current flowing N or S?

you are kidding me aren't you? I can't see for sure whether you're being serious. Just in case you are, the answer is yes it stays the same in a/b/c. You know it does.

If you disagree then here are 2 questions for you and that Australian red:
1. If you had some $5000 Nike trainers that let you run faster than your old trainers, could they fail to make you run faster if you were running on the airport travelator? And
2. In general when cruising from A to B, true route 90°, stored on the plotter, a/p on track mode, what is the effect of us travelling at 1000mph relative to the centre of the earth and accelerating towards the centre of the earth at about 9.8m/s/sec2? And what is the effect of us travelling along our orbit relative to the sun at approx 100,000km/h?

 

[John100156]

The answer to all above is 42......

 

[Elessar]

Thread drift ahoy:

Whilst on the subject of interesting phenomena: here's an observation from real-life. The circumstances behind the discovery are not important and, whilst perhaps this should be in the Lounge, the only people who will read it there won't be interested in the observation per se; only in calling each other nazis for obscure reasons.

So, I'm short-sighted (actually more complex, but that'll do for the moment). I can't, for example, drive my boat or a car easily without corrective lenses.

But, if I look in the rear view mirror, which is in the sweet-spot of my optical abilities, I can see all sorts of things which are in focus because the mirror is in focus.

So, I lose my specs. No problem to drive home. Just get in the car and reverse. Or mount a mirror in the back, turn round and drive forwards.

How good is that?

Must be a complex condition because the mirror doesn't change the focal distance, just bends it. You don't focus on the mirror.......
And yes it is thread drift!

 

[Elessar]

No? Surely the mirror is seen in two dimensions only; with a fixed focal distance.

nope.......

this should explain it - if the focal distance is the mirror, the angle is more acute and therefore the focal plane moves closer to the lens. (the red line....)

 

[matm]

Great thread as usual JFM - followed M1 with interest. I now know more about the physics of stabs then I will ever need to know as I'll never have a boat with any! More information to cram into a full head Will probably have to download my childhood soon to make room. Cheers and thanks for posting.

 

[Elessar]

Yup we're agreed on the principles, and only disagreeing on the relevance of dynamic centreline in a scenario when stabs are working but not working hard. Neither of us has data so let's agree to disagree, unless I can be bothered to collect the data over some say 50nm trips on M2...

BTW and FWIW I did leave M1's stabs on all the time no matter what the weather, as a high tech alternative to shouting at the crew

Hmm, we can debate the stats of sleipners smaller number of installs but you can also make judgements from the engineering. Look at Steve Dashew's pic below of the busted Naiad on Windhorse - that's just craap engineering and you dont need data to prove it. The sleipner metal inside the (one piece) fin is much bigger than the rather pathetic Naiad construction so the sleipners are fundamentally better on this score

on the price of stabs why would you skimp on that reinforcement?? Bonkers.

 

[jfm]

Must be a complex condition because the mirror doesn't change the focal distance, just bends it. You don't focus on the mirror.......
And yes it is thread drift!

Yup BJB, eee iz 100% correct. Putting it another way, if you look in your rear view mirror at a lorry that is 100m behind you, the thing you are looking at, namely the image of the lorry, is 100m in front of you. The rv mirror might be only 0.5m from your eyes, but as far as your eyes know they are looking at a thing that is 100m away.

 

[benjenbav]

Yup BJB, eee iz 100% correct. Putting it another way, if you look in your rear view mirror at a lorry that is 100m behind you, the thing you are looking at, namely the image of the lorry, is 100m in front of you. The rv mirror might be only 0.5m from your eyes, but as far as your eyes know they are looking at a thing that is 100m away.

Heyho. The only time I have actually broken both my regular glasses and all of the various spare pairs I keep lying around was when I was on a charter boat racing round the cans in the Solent. I had to get my boat home from Cowes and then drive home. But I don't think I'd have fancied reversing all the way. I actually solved that particular problem by taping the broken lenses to my head.

Per curiam: the last time I saw you I was probably wearing contact lenses. I think it was Easter 2012. Soon after that I noticed an unsightly patch of blood in one eye and haven't really worn them since.

 

[MapisM]

you are kidding me aren't you? I can't see for sure whether you're being serious.
Just in case you are, the answer is yes it stays the same in a/b/c. You know it does.

No cigar, I'm afraid.
Just think about it, and you'll easily understand why between (a) and (c) there is indeed a difference.
If you have a current flowing perpendicularly to your tracked route, you're not letting it bring the boat to another destination, are you?
The a/p surely isn't, anyway. Not if set on track mode, as per above assumptions.
So, what it does is keep the boat constantly slightly steered to contrast the current, keeping the XTE within the pre-defined limits.
Which in turn affects the fins centerline. Obviously, I hasten to add - sure as I am that you don't need any further explanations on that.

Now, this was actually the easy part of my my previous question.
The tricky one was in the difference between (a) and (b).
Let's say that you're willing to keep 20kts steady along all that route, and you have a current flowing exactly with or against you.
Does that affect the optimal fins cenerline vs. no current?
Big hint: the answer is "it depends", but it depends on what?

Oh, and the answers to your questions are NO and NONE respectively.
Unless of course you would consider tides (hence currents) as a side effect of your second point...

 

[MapisM]

on the price of stabs why would you skimp on that reinforcement?? Bonkers.

Actually, I would think that they must have changed the design at some point in time, because on my boat the shafts are almost as long as the fins.
And coming to think of it, I suppose the rationale might be a reduction of the "strong" exposed parts, which could damage the hull in the event of a grounding, or when hitting something anyway. Not a bad idea, in my books. I'd rather change a fin than sink the boat...

 

[petem]

Actually, I would think that they must have changed the design at some point in time, because on my boat the shafts are almost as long as the fins.
And coming to think of it, I suppose the rationale might be a reduction of the "strong" exposed parts, which could damage the hull in the event of a grounding, or when hitting something anyway. Not a bad idea, in my books. I'd rather change a fin than sink the boat...

I didn't understand JFM's point either. The whole point of being sacrificial is to reduce the core part to minimise damage in case of an impact.

 

[jfm]

No cigar, I'm afraid.
Just think about it, and you'll easily understand why between (a) and (c) there is indeed a difference.
If you have a current flowing perpendicularly to your tracked route, you're not letting it bring the boat to another destination, are you?
The a/p surely isn't, anyway. Not if set on track mode, as per above assumptions.
So, what it does is keep the boat constantly slightly steered to contrast the current, keeping the XTE within the pre-defined limits.
Which in turn affects the fins centerline. Obviously, I hasten to add - sure as I am that you don't need any further explanations on that.

Now, this was actually the easy part of my my previous question.
The tricky one was in the difference between (a) and (b).
Let's say that you're willing to keep 20kts steady along all that route, and you have a current flowing exactly with or against you.
Does that affect the optimal fins cenerline vs. no current?
Big hint: the answer is "it depends", but it depends on what?

Oh, and the answers to your questions are NO and NONE respectively.
Unless of course you would consider tides (hence currents) as a side effect of your second point...

Well I'll stand corrected if electronics gurus know better but you are assuming MapisM the machine tries to follow its compass heading rather than just minimise XTE. I think it just tries to minimise XTE and ignores heading (but will stand corrected as i say if the algorithm-knowledgeable know better) in which case I do get the cigar

 

[Deleted User YDKXO]

Yup, all agreed. Scuse my ignorance Deleted User, but what temp will the oil typically be at when the thing is running in steady state? There is a seawater oil cooler in the system, pic below

Obviously don't know your system but hydraulic systems tend to run between 20degC and 80degC with about 50degC being average operating temp. I suspect that a seawater cooler will probably be more efficient than the fan air coolers fitted in construction machines so I expect the operating temp in your system may be lower

 

[rafiki_]

Can we call a truce on this please? The techie side of stabs, great, the blue versus red corners, not so great

 

[Deleted User YDKXO]

Hi Lozzer
Pressures are in the order of 200bar. That is huge compared to most things, but run-of-the-mill in the world of hydrualics.

Yup, 200bar is potentially v dangerous but most hydraulic machines run in the 250-350bar range. You do not want to be standing next to a hydraulic hose when it fails at that sort of pressure. Thankfully that kind of catastrophic failure is rare

 

[jfm]

Can we call a truce on this please? The techie side of stabs, great, the blue versus red corners, not so great

We're not fighting; we're pursuing knowledge. We've discovered 2 different understandings on how a/pilot algorithms work and I'd like to get to bottom of that. If it is coming across to you are "red v blue corner" then I apologise for that but it is intended as a pursuit of knowledge exercise not a fight and I'd like to know the answer

 

[benjenbav]

We're not fighting; we're pursuing knowledge. We've discovered 2 different understandings on how a/pilot algorithms work and I'd like to get to bottom of that. If it is coming across to you are "red v blue corner" then I apologise for that but it is intended as a pursuit of knowledge exercise not a fight and I'd like to know the answer

I was talking to a group of Italian in-house lawyers the other day and, steeped in an inquisitorial system, they found it diffficult to understand our anglo-saxon, adversarial ways of beating each other with entrenched viewpoints in order to reach the truth.

It's the best way. Oh no it isn't.

etc

 

[vas]

We're not fighting; we're pursuing knowledge. We've discovered 2 different understandings on how a/pilot algorithms work and I'd like to get to bottom of that. If it is coming across to you are "red v blue corner" then I apologise for that but it is intended as a pursuit of knowledge exercise not a fight and I'd like to know the answer

+1

fascinating discussion, no handbags around so plz do go on (just stick a few pics in it everynow and again plz )

V.

 

[Alan_C]

We're not fighting; we're pursuing knowledge. We've discovered 2 different understandings on how a/pilot algorithms work and I'd like to get to bottom of that. If it is coming across to you are "red v blue corner" then I apologise for that but it is intended as a pursuit of knowledge exercise not a fight and I'd like to know the answer

Me too! When I first saw the stabs on M1, they blew my mind. Still all way beyond my level of understanding, but both explanations seem perfectly reasonable. I'm sure the M1/M2 threads have done much to further everyone's knowledge. Thanks for sharing.

 

[Deleted User YDKXO]

We're not fighting; we're pursuing knowledge. We've discovered 2 different understandings on how a/pilot algorithms work and I'd like to get to bottom of that. If it is coming across to you are "red v blue corner" then I apologise for that but it is intended as a pursuit of knowledge exercise not a fight and I'd like to know the answer

Yup it interests me too because I always thought that an AP in track mode works as you describe ie keeping the boat within XTE limits either side of the desired track. I always assumed that was why the AP menu normally gives you the option to set XTE limits. I'm sure the algorithm controlling the AP is more sophisticated than just activating the rudders when the boat bumps it's XTE limit but I thought that was the basis of the system. If you think about it, what other way can an AP work in track mode because the boat can be pushed off track by tide or wind without it's heading being altered?