Boat in build pics (2013 Fairline Squadron 78)

MapisM said:
Pardon? I don't get what you mean.
If you agree that the a/p is able to keep the boat on a straight line along the tracked route, even with cross current (which is what actually happens), then no matter what algorithms it's using, there's just no way that the rudders stay straight.
Basic physics.
Click to expand...
I don't get what you mean. Let's say boat wants to travel W to E, BTW is 90deg. Skipper wants a COG 090deg. If no current, HDG will be 090deg and COG will be 090deg and boat will track straight to waypoint, with rudders straight.

If there is 3 degrees worth of current pushing the boat southwards, the heading of the boat will be 087 degrees, while the boat will track along a 090deg COG. Rudders will still be straight. This is provided the a/pilot "learns" that 087deg is the equilibrium heading, which logically you'd expect it should (not exactly rocket science to program it that way, though none of us knows for sure the algorithms) becuase if it continued to think 090 HDG was correct it would always be woodpecking its xte limit and steering back northish in a ziz-zag fashion, which we all know doesn't actually happen at least on modernish a/pilots.

I don't see how "basic physics" comes into it; nor how you can say "no matter what algorithms it's using". It depends entirely on how the algorithm in the a/p is written. It seems that wine is good :)
 
Last edited:
JFM, are you saying he rudders stay straight or the rudders stay constant? Surely not the former? They must stay at a position that would steer the boat at 87% (HDG) generating COG of 90%. On they have worked out that 87% HDG gives 90% COG they will not move. If they stayed straight the current would be continually taking them off course.
 
Last edited:
is it a matter of MM talking about what the a/p rudder combo doing in RECEIVING the order to course to wherever under said conditions whereas JFM is arguing on what the a/p-rudders combo will be doing once the ship is at its dynamic equillibrium and is doing its thing properly?
maybe talking blx though...

V.
 
petem said:
JFM, are you saying he rudders stay straight or the rudders stay constant? Surely not the former? They must stay at a position that would steer the boat at 87% (HDG) generating COG of 90%. On they have worked out that 87% HDG gives 90% COG they will not move. If they stayed straight the current would be continually taking them off course.
Click to expand...

Alternatively the a/p could calculate an alternative initial course (e.g 5 degrees) and set the oat in that sirection, then centre the rudder for the remainder of he journey. This would produce a very odd track and could only be done for a short journey.
 
jfm said:
Definitely not. It is a case of the a/p learning what number of degrees to add/substract from BTW (to allow for the cross-current) and using that result as its centre. With the rudders straight. QED :). I still think I'm having that cigar :D :D
Click to expand...
No, if I understand you correctly, the AP definitely does not compute a new BTW when it goes off track and then follow that. That would be potentially dangerous if there was an obstruction slightly off the preset track. In track mode, the AP should attempt to put the boat back on the preset track. How quickly it does that will depend on how it is set up
 
Deleted User said:
No, if I understand you correctly, the AP definitely does not compute a new BTW when it goes off track and then follow that. That would be potentially dangerous if there was an obstruction slightly off the preset track. In track mode, the AP should attempt to put the boat back on the preset track. How quickly it does that will depend on how it is set up
Click to expand...

Surely in a cross current you never actually steer the BTW.
The course to steer would be (in the examples above) 87deg and be constant hence rudders straight.
Once the 3deg current has it's effect (boat effectively crabbing sideways) the COG would equal 90deg = BTW.

So although the boat is pointing at 87deg it is going 90deg, the safe path.

Perhaps, at higher speeds at least, the ap uses COG to steer the boat over the actual direction of the bow.

A lot of assumptions made by me as I've never used an auto pilot..
Apologies if I'm completely wrong!
 
petem said:
JFM, are you saying he rudders stay straight or the rudders stay constant? Surely not the former? They must stay at a position that would steer the boat at 87% (HDG) generating COG of 90%. On they have worked out that 87% HDG gives 90% COG they will not move. If they stayed straight the current would be continually taking them off course.
Click to expand...

The rudders are straight; HDG is 087; BTW and COG are both 090. Your statement "If they stayed straight the current would be continually taking them off course" is profoundly absurd. :eek:

.
 
Deleted User said:
No, if I understand you correctly, the AP definitely does not compute a new BTW when it goes off track and then follow that. That would be potentially dangerous if there was an obstruction slightly off the preset track. In track mode, the AP should attempt to put the boat back on the preset track. How quickly it does that will depend on how it is set up
Click to expand...

All agreed. I didn't say it computed a new BTW. BTW is always 090 and the a/pilot knows that. The job of the a/pilot it to achieve COG 090 and I said it computes (or rather learns by trial and error) the value for HDG that gives the correct COG with minimal zigzagging. It starts off with an 090 vlaue for HDG (what else can it do!) but then quickly revises that to 087 by trail and error, as it reaches equilibrium (to use Vas's word). Thereafter the rudders are basically straight for the rest of the trip

In my post that you quote perhaps it would have been clearer if I said "HDG" not "centre". But if you read that short post in combo with the one above you can see what I'm saying
 
Last edited:
vas said:
is it a matter of MM talking about what the a/p rudder combo doing in RECEIVING the order to course to wherever under said conditions whereas JFM is arguing on what the a/p-rudders combo will be doing once the ship is at its dynamic equillibrium and is doing its thing properly?
maybe talking blx though...

V.
Click to expand...
Possibly so. I'm definitely talking about the equilibrium phase, as well as a little bit about the a/p's learning phase while it gets to that equilibrium phase. TBH, I thought MM was too, but how he thinks a cross current will make the rudders not straight in the equilibrium phase of "track" leaves me very confused :)
 
If its just a steady current pushing you sideways (no wind and waves) its the whole mass the boat sits in that moves sideways. If you then apply rudder to steer say 5 deg against the current and then release rudder to neutral your boat still heads 5 deg off the bearing of the wp but the cog will be straight for the wp.

If its wind that is pushing you sideways you would need a constant rudder input of x- degrees for keeping the cog to the wp.

Test it your self on a smaller scale. Put a model boat in a (big) tub and pull the whole tub sideways. Then let the tub sit still and power up a big fan blowing 90 deg. on the boat and spot the difference regarding rudder input needed to keep the heading straight.

In real world conditions it will normally be a combination of these + the wave factor so you would normally need a bit of constant rudder input.

Just my My 2 NOK
 
jfm said:
I don't see how "basic physics" comes into it; nor how you can say "no matter what algorithms it's using".
Click to expand...

Hang on, we're now going in circles - if I can say so, considering the topic...
In my post #615, when I replied to your statement ...it must somehow "learn a new centre" when the boat is on "track" with a cross current, I assumed that you accepted the principle that the a/p recalculates the rudders centerline according to the conditions (current, or whatever), in order to follow the plotted track.
That's what I understood anyway, and that's the reason why I replied that basic physics dictate the need to keep the boat steered, in order to follow a 90° COG while keeping a 90° heading also in cross current.
Now I see what you actually meant, and I don't have any "basic physics" objections to that. :)

But the more I think about it, the more I'm convinced that the a/p doesn't work as you suggest, for various reasons.

One has already been mentioned by Deleted User: by following the "calculated" 87° heading, the boat would get immediately off-course as soon as the current would change strength and/or direction, and the a/p wouldn't realise that till the plotter tells it what's happening.
Which of course is something the plotter can do, but only with some GPS delay.
And at that point, the a/p should restart a trial and error process, to determine that the new calculated heading is 92 (or whatever) instead of 87 - go figure! As you surely know, in practice the a/p reacts MUCH faster than that to any heading change, however generated (typical example being waves, in rough stuff) - also when in track mode.

Another reason is what I previously called the proof of the pudding.
I know for sure that my a/p recalculates automatically the center rudder necessary to keep the boat on track, even with the mother of all currents - i.e. when the boat is running on one engine alone.
Incidentally, I already mentioned that last summer, in post #56 of this thread.
And since there is no integration at all between the engines/throttles and the navigation instruments, neither the a/p nor the plotter can have any idea of the reason why the boat doesn't follow a straight line when the rudders are centered. All they can do is "sense" that this is happening, and react in some way to bring the boat back on track.
Now, if the a/p would work as per your assumptions, there would be simply no way for it to to correct that, because it should put the boat on a new bearing and bring the rudders back to their REAL center. But as soon as the a/p would do that, the boat would start steering again, because there would be no such thing as an "equilibrium heading".
What happened instead was that:
I left the marina with port engine off, helming manually.
Once cruising at 7 knots steady, I saw that approximately 10 degrees to stbd were required to keep the boat on her course, and I engaged the a/p (I was circumnavigating the island where I'm based, and I have the route stored on the plotter, which allows me to "set it and forget it").
I expected the a/p to re-center the rudders, before steering again to bring the boat back on her route, but to my surprise, the boat just kept going straight, with the rudders slightly moving around the 10 degrees to stbd mark.
I realised afterward that the a/p recalculates automatically the optimal rudders center ("trim" in Furuno jargon) at all time, even when in standby.
In fact, when I checked the "trim" on the display (a keys sequence is required) and found that it had been adjusted to "9.7 S", meaning that the new "neutral" rudders position, which made the boat run straight in those conditions and at that speed, was considered by the a/p as 9.7 degrees to stbd.
Now, the one engine cruising is obviously an extreme example, but I can't see why/how the a/p could work in a different manner when the boat heading is affected by any other factors.

There's also a third, more technical reason why I don't think the a/p logic (in my unit, at least) is the one you envisage, and it's based on what I just re-read on the manual.
Btw, I didn't expect it to explain in detail the algorithms used and their priorities, and in fact it doesn't.
But in the main NMEA sentence which the plotter feeds to the a/p, the so called "APB", there is a constant flow of all the data which are necessary for the a/p to operate according to "my" logic, whilst the inputs which "your" logic would require are not there.
I can give you more details if you're interested, but I'm not insisting on this point, because you might still object that the a/p could elaborate those numbers itself. And even if I'm sure it doesn't, based on the above observations of the actual a/p behaviour, I don't have any "scientific" proof of that.
...yet. One of these days, I'll see if I can find some in an empty bottle of Pinot Noir... :)
 
Last edited:
Yes, agreed, but that is moving the goalposts. The "exam question" set at the beginning which started this debate dealt only with a cross-current
 
sap_2000 said:
If its wind that is pushing you sideways you would need a constant rudder input of x- degrees for keeping the cog to the wp.
Click to expand...
Seconded, and added to my previous reasons.
You must have posted it while I was writing my novel above... :)
Btw, in turn wind also affects the fins centerline, as current does, just to go back to the initial point. :D
 
jfm said:
Yes, agreed, but that is moving the goalposts. The "exam question" set at the beginning which started this debate dealt only with a cross-current
Click to expand...
Yeah, but you started the goalpost movement after all, bringing the discussion from the current to a/p algorithms... ;)
And the point sap_2000 is making is very relevant to the latter.
 
Thanks MM. I've said all along I don't know the a/p algorithms so I'm happy to hear new info there. But that is all very different from the cigar question about whether our boat wishing for 090 COG in a southerly current will have straight or turned rudders. I'm assuming now that you finally agree the rudders will be straight (and HDG 087)?

Yes the algorithms must be complex. As well as finding an equilibrium HDG value in the 090 COG scenario we were discussing, I agree with you that the a/p also finds a rudder trim value. When driving out of the port in Antibes I often run on one engine (to stay < speed limit) and flick "auto" on the pilot, and I can see that the boat runs straight on the demanded heading and the rudder gauge shows ~10deg of rudder. So the a/pilot must have in its brain a value for "trim" as we're calling it

But rudder trim has nothing to do with cross currents. I disagree with you that one engine off is the "mother of all currents". A current, whether mother or daughter type, never affects HDG. It only affects COG. In contrast, running with one engine off rotates the boat and instantly affects HDG. It's obvious then that the a'/pilot needs to find a value for rudder trim to deal with the one-engine scenario. But that has little to do with how the a/pilot deals with a cross current because cross currents and the requirement for rudder trim are utterly different things.

You seem stuck in a world where you think constant rudder (ie "trim") is needed to deal with a cross current. It isn't. When there is cross current you adjust HDG then keep the rudders straight. If the a/pilot's value for rudder trim is say 0.1deg in calm-no-current conditions it will also be 0.1deg in our boat with COG 090 and HDG 087. I think you need to re-think this bit...
 
You must log in or register to reply here.

Other threads that may be of interest

wombat88
Sorry, another what boat thread...
2
Replies
26
Views
763
Scubadoo
S
cyberkryten
We finally bought a boat - Merry Fisher 895s2
2 3
Replies
41
Views
1K
Cathy*
C
G
Shipmate RS 8400
Replies
6
Views
186
glynsue
G
steved
Original boat paperwork
2
Replies
30
Views
970
steved
steved

Share this page

Top