Stabilizers for Blue Angel, engineering question

Piers said:
Hi Bart,

Apologies if this has been covered, but I was once part of an interesting discussion concerning the positioning of stabs which included the fact that an incorrect position would casue the autopilot to fight stab corrections since stab movement would alter boat heading. Depending upon the sea state, it could become a nasty form of what I'd call Dutch Roll (flying term), or corkscrewing. Getting an architect to nail his colours on the correct position wojuld be something I'd do.

Again, apologies if this has already been mentioned.

Piers
Click to expand...

This is very valid. I had trim tabs with auto function on my previous boat and the autopilot was unusable when engaging the auto tab function. The systems would have needed to be integrated to work properly together.
 
BartW said:
fighting stab controll soft and autopilot, this topic has been well covered or discussed by Jfm and MapisM in another thread some months back, and iirc there was no formal agreement on that.
Click to expand...
Actually, there was an agreement, to some extent.
But the discussion was about whether having a joint control of the stabs and the rudders would have brought meaningful benefits.
My view was that such benefits, if any, could only be significant wherever the stabs are NOT actually doing a lot to stabilise the roll.
And in such conditions, they could as well be kept free floating, hence letting the fins self-center themselves along the ideal position at any speed, reducing in turn the system wear and also getting rid of the power absorption.
Incidentally, in the case of electric stabs, it's even more obvious that in a flat sea it's better to shut the whole thing down, in order to avoid using the genset.
IIRC, also jfm agreed that an integrated rudders+fins control can only be relevant when the fins are not working a lot.

What Piers says is a bit different.
In the debate with jfm, we always assumed that the fins were placed correctly, to start with.
I would agree with Piers that a wrong fins placement (i.e. too far from the boat CoG) could also affect the hull direction, rather than just its roll.
Otoh, I would think also that in order to introduce such unwanted consequence, the fins position must be AWFULLY wrong.
I mean, going back to our previous assumptions, if for instance the "ideal" placement would be right below the tanks, I would not be worried one iota in moving them half a meter in either direction, for practical reasons.
On a 70 footer, I would even think that up to one meter in either direction, the steering effect of the fins can't be too bad...

But this is yet again armchair engineering. Of course having a pro doing some math is not a bad idea!

PS: Re. the thickness bets, I'm aware that my value is pretty high, but it's based on stories which I heard from various yards about "overbuilding" during the first years of GRP, coming from wood construction background.
Some of them even told me that they consider the GRP hulls made in those years among the stronger plastic hulls ever built.
Besides, BA is pretty heavy for her size...
...We'll see, I'm indeed curious. But I'd be very surprised if BA bottom would be less than 40mm thick.
 
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baylabayla3288 said:
This is very valid. I had trim tabs with auto function on my previous boat and the autopilot was unusable when engaging the auto tab function. The systems would have needed to be integrated to work properly together.
Click to expand...
Yep, agreed, but that's in a totally different league compared to fin stabs.
Trim tabs are placed in the worst possible place in terms of steering neutrality, to the point that with fast and small(ish) planing boats you can actually steer with them.
That's because they are meant to control the boat AoA to start with, rather than its listing/rolling.
 
MapisM said:
LOL, how very true... And you've not yet heard of converting every alu parts to brand new shining chromed ones...! :p

Anyway, now that the measurement method is sorted, shall we make this thread really interesting with some bets?
My guess is a couple of inches.
Click to expand...

Put me in for 40mm +/- 10mm. Now where is that fence that I was sitting on?
 
rafiki_ said:
The only person who can award Match is JFM, and I'm not sure he will want to hand over his new toy just yet ;)
Click to expand...
didn't mean THAT match rafiki!
anyway, I couldn't afford the mooring and/or fuel, so i'd politely decline and ask for a tour onboard at some time ;)

cheers

V.
 
rafiki_ said:
Scuse, non ho capito...................:)
Click to expand...
LOL, I just meant that ranges make the guess too easy.
The rule has go to be that the nearest guess wins - I take it that yours is 40mm anyway... :)
But Vas is right, we need an award.
Ideas, anyone? Other than the old virtual mars bar, that is...!
 
MapisM said:
Actually, there was an agreement, to some extent.
But the discussion was about whether having a joint control of the stabs and the rudders would have brought meaningful benefits.
My view was that such benefits, if any, could only be significant wherever the stabs are NOT actually doing a lot to stabilise the roll.
And in such conditions, they could as well be kept free floating, hence letting the fins self-center themselves along the ideal position at any speed, reducing in turn the system wear and also getting rid of the power absorption.
Incidentally, in the case of electric stabs, it's even more obvious that in a flat sea it's better to shut the whole thing down, in order to avoid using the genset.
IIRC, also jfm agreed that an integrated rudders+fins control can only be relevant when the fins are not working a lot.

What Piers says is a bit different.
In the debate with jfm, we always assumed that the fins were placed correctly, to start with.
I would agree with Piers that a wrong fins placement (i.e. too far from the boat CoG) could also affect the hull direction, rather than just its roll.
Otoh, I would think also that in order to introduce such unwanted consequence, the fins position must be AWFULLY wrong.
I mean, going back to our previous assumptions, if for instance the "ideal" placement would be right below the tanks, I would not be worried one iota in moving them half a meter in either direction, for practical reasons.
On a 70 footer, I would even think that up to one meter in either direction, the steering effect of the fins can't be too bad...

But this is yet again armchair engineering. Of course having a pro doing some math is not a bad idea!

PS: Re. the thickness bets, I'm aware that my value is pretty high, but it's based on stories which I heard from various yards about "overbuilding" during the first years of GRP, coming from wood construction background.
Some of them even told me that they consider the GRP hulls made in those years among the stronger plastic hulls ever built.
Besides, BA is pretty heavy for her size...
...We'll see, I'm indeed curious. But I'd be very surprised if BA bottom would be less than 40mm thick.
Click to expand...

Yup to all that. Some further thoughts:

1. I cannot see how CMC claim a floating centre (like Sleipner's) unless they are powered up. Without electric current, they are free to flap, unless they have a locking pin which (must have a fixed, not floating, position), or a clutch (which i doubt). In contrast, Sleipner have a floating centre with no power draw, just by shutting the valves. BartW, do you know about the locking arrangements on CMC? You would expect they are built to be safe and allow the boat to go astern with a failed genset, which surely means a locking pin?

2. Sort of to state the obvious, the CMC stabs need the genset all the time. you cannot run long passages without a genset and just use the engine PTOs to power the stabs. That's a pity, but I guess worth living with to make up for the easier installation of electric stabs compared with hydrualic, at least in a retrofit case. As we all know, boats have lots of compromises. The Italian market (Sanlorenzo) wont care about this becuase they build boats that always need 230v when underway. Other markets will care. Now a 230v alternator running off a main engine would be interesting here so you can stop the genset.

3. Reference armchair engineering, remember Princess fitted 1m sq fins to their current 72 hull back in 2010, under the master cabin becuase that was easier, against the advice of the engineers and architects. At sea, the bow of the boat lifted. They had to move the fins back and they put them under the wing fuel tanks, urgh. To do this they lifted out a pair of Cat C32 and the fuel tanks, on a finished boat, would you believe. And patched up the holes. This proves you can get it wrong because they wouldn't have done all that engines-out work if it wasn't necessary

I'm sticking with my 30mm guess, assuming we are on a flat hull panel with no extra thickness caused by the GRP strips that attach the frames and stringers, which should be the case with a seacock :)
 
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MapisM said:
But Vas is right, we need an award.
Ideas, anyone? Other than the old virtual mars bar, that is...!
Click to expand...
How about the plug that gets cut out for the fitting of the stabs! You just know this project will happen so the prize should not take too long to deliver!:)
 
I believe you had some good fun on here while I was trying to earn money for paying a stab system :)

jfm said:
1. I cannot see how CMC claim a floating centre (like Sleipner's) unless they are powered up. Without electric current, they are free to flap, unless they have a locking pin which (must have a fixed, not floating, position), or a clutch (which i doubt). In contrast, Sleipner have a floating centre with no power draw, just by shutting the valves. BartW, do you know about the locking arrangements on CMC? You would expect they are built to be safe and allow the boat to go astern with a failed genset, which surely means a locking pin?
Click to expand...

I haven’t discussed such technical details yet, first find an agreement on cooperation and traject to follow,

but I bet that the stabs need to be powered, to stay in the rest position.
good question for them: how to lock them fe in case of a system or genny failure

anyway, these are the main operational modes / buttons on the control panel:

1) START/STOP : command to the hydraulic power unit.
2) FINS at center: the fins are controlled to the corresponding mechanical centre position, this
function is useful during ship maneuvering
3) Navigation: the electronic control is active on underway conditions , with this function the
operator can set some parameters to the regulator , in particular LIST and SENSE command.
4) Anchor mode : the electronic control is active on anchor conditions , with this function the
operator can set some parameters for speed and fin sensitivity
5) ADAPTIVE MODE: with this function the electronic regulator is working in adaptive mode
which means that the regulator parameters are automatically settled according to the modified
operational conditions.


jfm said:
2. Sort of to state the obvious, the CMC stabs need the genset all the time. you cannot run long passages without a genset and just use the engine PTOs to power the stabs. That's a pity, but I guess worth living with to make up for the easier installation of electric stabs compared with hydrualic, at least in a retrofit case. As we all know, boats have lots of compromises. The Italian market (Sanlorenzo) wont care about this becuase they build boats that always need 230v when underway. Other markets will care. Now a 230v alternator running off a main engine would be interesting here so you can stop the genset.
Click to expand...

I repeat these specs posted in #78

Absorbed power both fins
(underway mode)
Average power 350 W
Peaks power 750 W

Absorbed power both fins
(anchor mode)
Average power 2.5 kW
Peaks power 6 kW ( for t < 0.2 s)

So it seems that the stabs can run in underway mode, on 1 (or 2) 24V 3000W invertor.
The alternators on the MAN engines are 120A each, which is 2 x 2880W (probably not continuous)
So these can easily handle the 350W average power and even the 750W peak power underway,
and I have the battery’s to cope with the peaks.

Somewhere In my warehouse I have a 230V 3000W travelpower add-on generator
A leftover from testings with a electrically driven scubatanc compressor, but the travelpower couldn’t cope with the peak start current from the lectric motor.
When we tested with 2 x Victron 12V 3000W invertor in cascade, this DID work,
but as we needed to run both Volvo diesel engines (with 110Amp alternator each) at min 2000RPM, we skipped the electric project and bought a petrol engine instead for running the compressor.
But my conclusion is that these Victron invertors can cope with some peak power.

But anyway, I wouldn’t mind running the Genny’s during navigation,
I will rather be more annoyed running the genny’s at anchor when we want to have a swim,
But hey, we know there will be compromises.



jfm said:
3. Reference armchair engineering, remember Princess fitted 1m sq fins to their current 72 hull back in 2010, under the master cabin becuase that was easier, against the advice of the engineers and architects. At sea, the bow of the boat lifted. They had to move the fins back and they put them under the wing fuel tanks, urgh. To do this they lifted out a pair of Cat C32 and the fuel tanks, on a finished boat, would you believe. And patched up the holes. This proves you can get it wrong because they wouldn't have done all that engines-out work if it wasn't necessary
Click to expand...

CMC confirmed today in a mail that our proposed position is good, (just in front the cog)



important remark about the stab systems in San Lorenzo's,
such boats are almost alway's helmed by a captain, not the owner,
I guess that these Captains are not involved, or are not in a position to have a "opinion" about the stab system,
they are not qualified don't have the knowledge to have a "objective observation" about the engineering quality's of such a system (I think)
 
DTIM said:
How about the plug that gets cut out for the fitting of the stabs! You just know this project will happen so the prize should not take too long to deliver!:)
Click to expand...

Excellent suggestion. I think it should be presented to the winner in person by Mrs BartW. :D:D
 
Ah, ok, thanks, I stand corrected and it is nice that you can run the stabs off the inverter via engine alternators when underway. Excellent!

I'm still a bit worried about heat and would want some technical analysis on that, but I might be over-worrying. I guess i am concerned that the fin area has been restricted to reduce power and heat generation when at anchor, because 1m sq feels right to me (and doesn't even look especially big on an 80 foot boat)

Yes I agree, many 24m boat captains can be a bit lightweight technically though of course there are some very smart guys out there too whose experiences and opinions would be useful to you. All depends who the captain of the SL72 happens to be :)

Are you thinking of going ahead with this idea before the start of 2013 season BartW?
 
jfm said:
Sleipner have a floating centre with no power draw, just by shutting the valves.
Click to expand...
What valves? My Naiads are free floating whenever turned off, with no need to do anything.
I can see why that would require a clutch with electric stabs, anyway.
And I also don't think they use one, though I actually can't tell for sure.
 
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