Stabilizers for Blue Angel, engineering question

[rafiki_]

Huh? I don't think that's fair, V.
Nobody ever said - CMC included - that electric motors don't generate heat at all.
As I understand, they just tried to explain why the answer to the question about whether heat is an issue or not is no.
What lead you to think the opposite?

There are some simple physics equations that show that the magnetic fields and windings generate heat. I think this is what JFM is saying. Forget about the precision engineering in the motor bearings and gearbox, these will produce some small elements, but generally the oil film in shear should reduce losses to near zero. It is actually the conversion of electrical energy into kinetic that will create the heat losses, if I remember my physics.

 

[jfm]

Agreed 100%.
My statement which you quoted was related to jfm example according to which, in some conditions, the electricity goes ENTIRELY into heat.
That just hurts common sense...

MapisM i agree to divide this into the two separate points

On the heat thing, it is absolutely the case that a stalled electric motor's electricity consumption goes entirely to heat. I mean 100% of it. That is exactly the electromagnet crane analogy. The crane driver goes off to lunch with 1 tonne hanging and 50 amps being drawn by the electromagnet coil. ALL the electricity goes to heat because no work is being done by the electromagnet

A stabiliser motor is like a nearly stalled motor. It does very little work. Rather, it creates a large force with very little movement. When the fin is stationary in my "big long wave" analysis ALL the electical energy goes to heat becuase the motor is stalled. Most of the time the fins sweep+/-10 degrees gently but with huuuge force and in that scenario the motros are almost stalled - lots of force but little distnace travelled. In this scenario the electrical energy mostly goes to heat not mechanical work. It is totally wrong and total marketing BS to say that gearbox losses are relevant here to heat generation, other than slightly and incidentally.

This electrical analysis doesn't offend any common sense in my book, and anyone hesistating on that merely needs to think of the electromagnet crane which is self evidently doing no work so ALL its electricity makes heat in the coil. Remember, in the crane we are not even trying to do work. Heck, we could use a hook and that would consume no energy! The electromagnet creates a force as a function of amps and number of coils in the winding and alas we only have copper as a suitable material (at zero kelvins we'd use a superconductor instead, LOL), and when push a current through copper we have heat as an unfortunate by product.

Fin sizeYep I have heard many times that it is better to stop a roll than cure one that has started. Hence fast reaction time matters. All agreed. But we are talking milliseconds not nanoseconds and i really struggle to believe there is a significant difference between geared electric and hydraulic in getting out of the starting blocks. Furthermore, because you have in effect unlimited power with hydrualics you can have a bigger fin. Even if the reaction time of hydraulics is, say, 0.1 seconds worse than electric (which I don't think it is) common sense tells you that 1msq will be better at anchor than something quite a lot smaller. I think there is some marketing BS here also. I'm not saying electric is worse or there is anything wrong with them, but I don't buy the story they are noticeably better than hydraulic on this score.

 

[jfm]

It is actually the conversion of electrical energy into kinetic that will create the heat losses, if I remember my physics.

Rafiki, no that's absolutely not what I'm saying and that statmeent is not correct. What I'm saying is that the work done (kinetic energy) by a fin stab mecahnism is diddly squat. The function of the actuator mechanism is to create huge force (torque) but not to move very far. Every electric motor has an element of the electromagnetic crane in it but in my quadcoptor motors it is about 1% and in the fin stabiliser motor it is more like 99%. Now, when you are creating a force as opposed to doing work, then your motor winding becomes a resistor like an electric fire, and although all you actually WANT is electric current, man unfortunately has no ways at 20 degrees C to make current without heat, because all the materials we have discovered have electrical resistance at normal temps (as distinct from zero kelvins when we have superconductors). Thus in this scenario with the fin moving +/- 5 degrees or whatever and almost no work being done and almost no kinetic energy being created (literally a handful of Joules is all the kinetic energy you're making), almost all the energy consumed goes to overcoming electrical resistance in the winding, ie making heat not doing work.

 

[rafiki_]

Feedback loop control is a really interesting science, and comparing electric motors with hydraulics has caused some consternation. My experience is nearly all automotive, but we are moving from hydraulic powered assistance on steering systems to electric assist to reduce pumping losses, hence fuel consumption with hydraulic pumps. When the leccy systems first came out 10 years or so ago, the steering feel was awful, as we didn't understand sufficiently the hysteresis loops, so we had very synthetic steering feel, and I much preferred to tune hydraulic steering. However the feedback control systems for the Leccy motors is now so much more sophisticated. The leccy steering on my Beemer F30 is fantastic and I would be very surprised if the electric stab systems weren't using automotive controllers and sensors as a consequence, not least because they are reliable and affordable. In conclusion, you should be able to design a very responsive and reliable electric stab system. However, it will still produce heat. Clearly the CMC system is able to deal with the heat generated without reliability problems, from the feedback to Bart.

 

[jfm]

All kinds of engines generate heat (and noise/vibration) because they're not 100% efficient at converting their input energy into output energy. A quick wiki reveals that brushless motors typically have efficiencies of 85-90% which is pretty damned good but still 10-15% of the energy generated is lost, probably mainly in heat generated

Mike, you are missing the point. The data you quote is for motors that are doing work. If you stall a motor or near stall it, ie you put in a position of creating a large force but moving it almost no distance, which is what a fin stab motor does in the "hold the fin at an angle while boat does 20kts" scenario, you are deliberately causing the motor not to do work. Then it isn't 15% of the electricity that makes heat but 95%

 

[rafiki_]

Rafiki, no that's absolutely not what I'm saying and that statmeent is not correct. What I'm saying is that the work done (kinetic energy) by a fin stab mecahnism is diddly squat. The function of the actuator mechanism is to create huge force (torque) but not to move very far. Every electric motor has an element of the electromagnetic crane in it but in my quadcoptor motors it is about 1% and in the fin stabiliser motor it is more like 99%. Now, when you are creating a force as opposed to doing work, then your motor winding becomes a resistor like an electric fire, and although all you actually WANT is electric current, man unfortunately has no ways at 20 degrees C to make current without heat, because all the materials we have discovered have electrical resistance at normal temps (as distinct from zero kelvins when we have superconductors). Thus in this scenario with the fin moving +/- 5 degrees or whatever and almost no work being done and almost no kinetic energy being created (literally a handful of Joules is all the kinetic energy you're making), almost all the energy consumed goes to overcoming electrical resistance in the winding, ie making heat not doing work.

I am violently agreeing with you, without your eloquence. . I think it is the magnetic flux in the windings creating the heat, but it is so long since I did this sort of physics that these things are lost in the mists of time.......

 

[BartW]

Quick update,

The GRP bottom thickness is only 16mm, but this is at its minimum,
A lot of the surface (>70%) is more,
in the corners joining the stringers and the frames thickness varies between 20…25mm

stringers (longitude beams ? ) approx. 100mm x 100mm are spaced at 47cm between them.
frames (L-R beams ?) are 17cm wide, and up till floor level (70cm high above the keel In the master cabin.)
these are spaced 900mm, and some of them are closer,
ao the space we need in the bathroom / dressing room , there is 67cm space between the two frames.

The ultrasound measuring tool didn’t give accurate or reliable measurements on GRP,
don’t have time to find out why, It didn’t work accurate on GRP nowhere,
perhaps with the glass fibress inside, different materials ?
no time for further investigation..
The knitting needles gave a reliable measurement

 

[MapisM]

MapisM i agree to divide this into the two separate points

On the heat thing, it is absolutely the case that a stalled electric motor's electricity consumption goes entirely to heat. I mean 100% of it. That is exactly the electromagnet crane analogy.
The crane driver goes off to lunch with 1 tonne hanging and 50 amps being drawn by the electromagnet coil. ALL the electricity goes to heat because no work is being done by the electromagnet

J, this is EXACTLY the point which doesn't hold water against my "common sense" check - naive as it might be.
Saying that 100% of the electricity is converted into heat implies that no other sort of energy comes into the picture: you supply 50 amps, and all you get is a crane magnet where you could fry your eggs.
But if that would be the case, what on earth keeps your ton of metal from falling to the ground? Surely, you can't fight gravity force with heat alone, can you?
Now, how much of those 50A will be converted in a magnetic field and how much will just be lost in the air in the form of heat, I don't have a clue.
But I just can't believe that what you get in that scenario is EXCLUSIVELY 50A worth of global warming.
That just doesn't make any sense.

Fin size...
...common sense tells you that 1msq will be better at anchor than something quite a lot smaller.

Nope, sorry. My own common sense tells me that the fin surface is the only (well, almost) key factor for stabilizing a CRUISING hull.
When cruising, a large fin can achieve a fantastic stabilization even with slow(ish) and limited fin movements.
But just think of a huge fin, which not only reacts slowly, but also accelerates very slowly when moving, in the zero speed scenario.
That sort of movement is not going to achieve any result, no matter how big the fin is, because it can not count on the forces generated by the water flow.
In fact, what CMC says is not only that their system is more reactive (I agree that a difference of 0.1 second alone can't make a huge difference), but also that the rotation speed and the acceleration is higher.
Now, I did say that the proof can only be in the pudding, but from my viewpoint this argument does hold water on paper, because I can see why trading off a larger surface for a faster movement can make sense at zero speed.

 

[MapisM]

The GRP bottom thickness is only 16mm

B, you must jump in the water again I'm afraid.
I wouldn't believe this number if I had seen it with my own eyes...

 

[rafiki_]

Bart, that does sound very meagre for a boat such as BA. Rafiki's hull is thicker than this, and she is only a 5 tonne boat.

 

[jfm]

J, this is EXACTLY the point which doesn't hold water against my "common sense" check - naive as it might be.
Saying that 100% of the electricity is converted into heat implies that no other sort of energy comes into the picture: you supply 50 amps, and all you get is a crane magnet where you could fry your eggs.
But if that would be the case, what on earth keeps your ton of metal from falling to the ground? Surely, you can't fight gravity force with heat alone, can you?
Now, how much of those 50A will be converted in a magnetic field and how much will just be lost in the air in the form of heat, I don't have a clue.
But I just can't believe that what you get is in that scenario is EXCLUSIVELY 50A worth of global warming.
That just doesn't make any sense.

Erk , I cannot think of any better way to explain this than I already have done. Your common sense meter is out of calibration here. There is no work done by a magnetic field unless it moves something, and in the crane scenario nothing moves. Imagine that you replaced the crane magnet with a traditional steel hook. The hook holds a tonne of steel, yet needs no power supply. Hence, the holding of one tonne of metal suspended above the ground needs no power and no source of energy. It consumes zero energy. It only needs a force, to resist gravity, but you can have a force with no energy supply - that is what a hook does, or a nut and a bolt, or a million other things you can think of like Velcro. Now, an electromagnet needs current to create a force. It does not need energy to create a force so if you went to zero kelvins and used a superconductor that had zero resistance and a large current with almost no potential difference, the the crane would work with almost zero power supply. But up here at 20deg C mankind has no way to make current without forcing the electricity through a resistive conductor. ALL the electrical energy is used to overcome the resistance, just like in a toaster. ALL the electrical energy therefore goes to heat. I remind you that no work whatsoever is done on the tonne of metal hanging on the crane

Nope, sorry. My own common sense tells me that the fin surface is the only (well, almost) key factor for stabilizing a CRUISING hull.
When cruising, a large fin can achieve a fantastic stabilization even with slow(ish) and limited fin movements.
But just think of a huge fin, which not only reacts slowly, but also accelerates very slowly when moving, in the zero speed scenario.
That sort of movement is not going to achieve any result, no matter how big the fin is, because it can not count on the forces generated by the water flow.
In fact, what CMC says is not only that their system is more reactive (I agree that a difference of 0.1 second alone can't make a huge difference), but also that the rotation speed and the acceleration is higher.
Now, I did say that the proof can only be in the pudding, but from my viewpoint this argument does hold water on paper, because I can see why trading off a larger surface for a faster movement can make sense at zero speed.

All agreed, except two things. First, it isn't a given that the big fin accelerates more slowly. When you have the effectively unlimited power of hydraulics you can accelerate/decelerate a big fin fast, all day long. You'd be amazed how fast my fins move. As they move from out past the chine back in to under the hull they can leave a huge dustbin sized conical hole in the water that fills with a whirl pool. It's quite a sight

Second, and this is a biggie, if the small fin's rotation speed is higher than the big fin's, as it must be to create equal stabilisation force, then it moves for less time. That's because in all systems the stroke is limited to say +/-40deg. That means in a certain level of wave it runs out of steam before the wave period has finished, while the slower big fin still has another 15 degrees left to travel. This is a biiiiig deal and the trade off in your last sentence is not a smart thing to do imho . I'll take the bigger-area and slower-moving fin every time

 

[MapisM]

All agreed, except two things. First, it isn't a given that the big fin accelerates more slowly. When you have the effectively unlimited power of hydraulics you can accelerate/decelerate a big fin fast, all day long. You'd be amazed how fast my fins move. As they move from out past the chine back in to under the hull they can leave a huge dustbin sized conical hole in the water that fills with a whirl pool. It's quite a sight

Second, and this is a biggie, if the small fin's rotation speed is higher than the big fin's, as it must be to create equal stabilisation force, then it moves for less time. That's because in all systems the stroke is limited to say +/-40deg. That means in a certain level of wave it runs out of steam before the wave period has finished, while the slower big fin still has another 15 degrees left to travel. This is a biiiiig deal and the trade off in your last sentence is not a smart thing to do imho . I'll take the bigger-area and slower-moving fin every time

Yep, it isn't a given that bigger fins as such can't accelerate faster, but according to Mr.CMC electric motors are faster AOTBE, i.e. also with the same fin size.
They use smaller fins for any given hull because they can get the same results with smaller fins, not the other way round.
That's what he said, anyhow, but we shouldn't forget that they used to build (and still do, upon request) hydraulic stabs.

Can't see why you see your second point as a biggie - not in a zero speed scenario, anyway.
If waves are long and big enough to make the stabs run out of steam while anchored, I'd rather run for cover!

On the matter of heat, I still fail to understand how part of those 50A can't be absorbed by the need to keep the magnetic field active, and I've never seen an electromagnet getting as red hot as a toaster, anyway.
But I'll rather give up, you seem convinced of what you're saying and I know that you wouldn't, unless supported by good reasons...

 

[jfm]

according to Mr.CMC electric motors are faster AOTBE, i.e. also with the same fin size.

Now that really is marketing BS. AOTBE, they are the same speed!

They use smaller fins for any given hull because they can get the same results with smaller fins, not the other way round.

. More marketing BS. Sure they say they get the same result, but you don't actually believe them surely? As I said you can indeed get the same result with 50% of the fin size, but for 50% of the time. Maybe, just maybe, the real reason they use smaller fins because they can't dissipate the heat created by using larger ones

Can't see why you see your second point as a biggie - not in a zero speed scenario, anyway.
If waves are long and big enough to make the stabs run out of steam while anchored, I'd rather run for cover!

It sure is a biggie, at least in my experience and imho. I had plenty of times in Match1 when the anchorage wasn't so awful that I needed to leave, and perhaps didn't want to leave for whatever reason like lunch wasn't finished, but the stabs were using their full angular stroke to keep the boat still. Also when you do decide you want to run for cover, but you have say the tender to recover, you're glad of 100% worth of stab effect not 50%

On the matter of heat, I still fail to understand how part of those 50A can't be absorbed by the need to keep the magnetic field active, and I've never seen an electromagnet getting as red hot as a toaster, anyway.
But I'll rather give up, you seem convinced of what you're saying and I know that you wouldn't, unless supported by good reasons...

I can't explain it any better but if you don't see that no work is being done in a stationary system then you don't see it. And of course electromagnet cranes don't get as hot as toasters: the current density is nothing like that of a toaster element plus have cooling fins or whatever is needed to dump the heat

 

[MapisM]

Now that really is marketing BS. AOTBE, they are the same speed!

C'mon J, 'twas obvious that I (not Mr.CMC) used "AOTBE" in that context just to mean for any given fin size.
Actually, he also mentioned the max rotational speed of their electric system vs. the faster hydraulic system, but I can't remember the numbers.
The difference was relevant anyway, like 30% or so higher, and it didn't depend on the fin size.

More marketing BS. Sure they say they get the same result, but you don't actually believe them surely?
As I said you can indeed get the same result with 50% of the fin size, but for 50% of the time.
Maybe, just maybe, the real reason they use smaller fins because they can't dissipate the heat created by using larger ones

Now, show me when I told that I believe everything they say.
I'm the one who always pointed out that there's only one way to check their claims, and it's to try the pudding.
Though in this respect, as Bart can confirm, I suggested him (even before this thread started) to take more time for such an important and expensive upgrade, postponing it to the end of the season, which would have given him more time to evaluate all the details.
Otoh, it seems to me that you're now constantly raising the BS flag and insisting on the heat issue just for the sake of it, as I'd rather expect from a Sleipner salesman at a boat show.
I see no reason why - in principle - IF a system can keep the rolling at bay more promptly than another one, a shorter timeframe shouldn't be sufficient to achieve a comparable result.
Besides, Bart told us (I didn't discuss that personally with Mr.CMC) that their SMALLER motor is used to drive either a 0.6 or a 0.8 Sq.M fin, but they only install the latter on the SL82.
So, bigger boat, bigger fins, and the same motor.
Do you recon that they could as well get rid of the onboard heating, and use pipes to route the heat from the stab motors throughout the boat?

 

[jfm]

C'mon J, 'twas obvious that I (not Mr.CMC) used "AOTBE" in that context just to mean for any given fin size.
Actually, he also mentioned the max rotational speed of their electric system vs. the faster hydraulic system, but I can't remember the numbers.
The difference was relevant anyway, like 30% or so higher, and it didn't depend on the fin size.


Now, show me when I told that I believe everything they say.
I'm the one who always pointed out that there's only one way to check their claims, and it's to try the pudding.
Though in this respect, as Bart can confirm, I suggested him (even before this thread started) to take more time for such an important and expensive upgrade, postponing it to the end of the season, which would have given him more time to evaluate all the details.
Otoh, it seems to me that you're now constantly raising the BS flag and insisting on the heat issue just for the sake of it, as I'd rather expect from a Sleipner salesman at a boat show.
I see no reason why - in principle - IF a system can keep the rolling at bay more promptly than another one, a shorter timeframe shouldn't be sufficient to achieve a comparable result.
Besides, Bart told us (I didn't discuss that personally with Mr.CMC) that their SMALLER motor is used to drive either a 0.6 or a 0.8 Sq.M fin, but they only install the latter on the SL82.
So, bigger boat, bigger fins, and the same motor.
Do you recon that they could as well get rid of the onboard heating, and use pipes to route the heat from the stab motors throughout the boat?

OK, no worries, let's abandon the heat discussion becuase we've done it to death. And er it is at risk of gettin heated . I'm not being a sleipner salesman (though, since you ask, their equipment is fantastic and I strongly recommend it ) and have agreed with BartW that electric stabs make perfect sense for BA. I just think with an investment like this it is important to cut through marketing BS. The claim that there is no heat because the gearbox is a swiss watch is proper first grade BS, though in view of the fact you are struggling to digest where the energy flows are in an electromagnetic system (which includes a motor) and you're smarter than any stab factory engineer I should perhaps forgive them for that. Furthermore, as everyone has said, heat aint a propblem so long as you can get rid of it

On rotational speed the proof is in the pudding as you say. But when a stab maker goes on about the high speed of his fins that rings a big alarm bell with me that the fins are too small. 30% faster means 30% less time before it hits the stops, and I know from watching my stabs operate that sometimes you're glad of the full stroke (I mea stroke in terms of time, not degrees of rotation). And What makes CMC think hydraulics couldn't be made to go faster? I think, from watchin gthe beasts in action, you don't want them to go faster. Fast reaction time is important I agree, but fast rotation speed is an alarm bell ringing

 

[MapisM]

Agreed, let's refrain from ...heating further the heat debate.
Also because the only person who REALLY knows how much heat those motors generate has already confirmed that it ain't a problem, but he happens to be the one who builds the thing.
Besides, even if I never pretended to be smarter than anyone else, I've already been smart enough to give up on that point a couple of posts ago.

Let me just ask you one thing though, because I've yet to hear your view on this point.
I understand that the SL94 - all 100+ tons of her - has CMC stabs with 1Sq.m fins, i.e. as big as your 78.
Now, of course you are smarter than any SL94 buyer/captain, but why do you think they are that much less smart, to the point of accepting undersized equipment on their brand new toy - expensive as it is, btw?

 

[BartW]

Quick update,

The GRP bottom thickness is only 16mm, but this is at its minimum,
A lot of the surface (>70%) is more,
in the corners joining the stringers and the frames thickness varies between 20…25mm

stringers (longitude beams ? ) approx. 100mm x 100mm are spaced at 47cm between them.
frames (L-R beams ?) are 17cm wide, and up till floor level (70cm high above the keel In the master cabin.)
these are spaced 900mm, and some of them are closer,
ao the space we need in the bathroom / dressing room , there is 67cm space between the two frames.

The ultrasound measuring tool didn’t give accurate or reliable measurements on GRP,
don’t have time to find out why, It didn’t work accurate on GRP nowhere,
perhaps with the glass fibress inside, different materials ?
no time for further investigation..
The knitting needles gave a reliable measurement

did new measurements today, other methods and at different positions,
did found the source of the inaccurate / low figure yesterday,

but still, hull is only 19-20mm thick.
but again this is at less then 30% of the surface,
much of the surface has one or more extra layers of GRP; 5....10mm extra , for laminating the stringers and the frames.
pics will come later.

meeting with yard was good,
cooperative and practical approach,
we discussed different way's for making the hull 80mm thick.

his proposal is either solid 80mm GRP, or:
almost equal strength, sandwitch GRP - wood - GRP
and very practical approach (imho) about how to execute the work.
so let's wait for his quotation.

wants to do all in one go / lift out,
- removing old antifoul (got a much better quote now then initially, 50% less)
- replacing 3 corroded seacocks
- fitting 4 UWL's
- new antifoul
- reinforcing the hull to 80mm in a surface between two frames (67cm wide) and 3 stringers (2 x 57cm)
all nicely laminated togeter (the middle stringer of these 3 will be cut out)
all this could be done in 2 or 3 weeks max.

drawing from the boat with all frames and stringers, and a cross section in the stab area is almost finished,
ready for the engineer(s) for investigation of our plan.

 

[jfm]

Let me just ask you one thing though, because I've yet to hear your view on this point.
I understand that the SL94 - all 100+ tons of her - has CMC stabs with 1Sq.m fins, i.e. as big as your 78.
Now, of course you are smarter than any SL94 buyer/captain, but why do you think they are that much less smart, to the point of accepting undersized equipment on their brand new toy - expensive as it is, btw?

Just because lots of people do something doesn't make it a smart thing to do. Ferretti, a generally fine yachtbuilder, tells its customers (including 94 footers) that its Mitsubishi ARGs are the best thing and recommends folks to buy ARG, and the customers say yes. But fact is ARG is inferior to Seakeeper - out here in the objective world of interim forums we all know that. Also, Ferretti did an inhouse test of brightness of a Lumishore vs their standard fit Xenon, and the chief of whatever department is was concluded the Xenon was brighter. Now, there is no room for subjectivity here becuase counting lumens is a bit like counting Euros, so you have to conclude they decided the answer they wanted before they did the test. Perhaps someone in the foodchain prefers to buy from their current supplier, for some reason.

Fact is, these yards (including SL) have other commercial factors that go into the mix when deciding what they wish to fit to their boats, and my view is that they then say to the customers -with a complete absence of objectivity- that the thing they recommend is a great thing. Customers follow the recommendation , because SL or whoever says it. Twas ever thus, but my point is you get far more objectivity and cut through of marketing BS on this forum than from some commercial suppliers.

Mapis I have used 1 sqm on a smaller boat than SL94 and I know there are times when the things use their full angular stroke. You then have these CMC guys shouting about how fast their fins move. Jeeze, can you not hear the alarm bell that "moving fast" means "moving for less time"??? Also, the size of my fins versus size of boat is fully in line with the fin sizes used by Trac. CMC is the outlyer here

These CMCs could be perfect for BA. BartW wants to understand them though, and I'm adding a valid observation that moving fast could be a compromise feature when at zero speed. That's a factor he needs to consider, even though it may well be a perfectly good trade off for the much easier install of electric fins. But this forum is far too good to accept the view that "SL is doing it so it must be ok".

 

[jfm]

his proposal is either solid 80mm GRP, or:
almost equal strength, sandwitch GRP - wood - GRP
- reinforcing the hull to 80mm in a surface between two frames (67cm wide) and 3 stringers (2 x 57cm)
all nicely laminated togeter (the middle stringer of these 3 will be cut out)
all this could be done in 2 or 3 weeks max.

That sounds a good plan imho. Only thing is that I wouldn't use the wood idea unless threy are sure they can press it onto the wet GRP with enough force to get a great bond. Lots of weights or vacuum bag. And, they must be certain that the wet surface of the GRP is flat which means the must lay the glass in carefully cut patches to "fill in" the 20mm zone and bring it up to the level of the thicker zones near the frames and stringers. If there is any doubt about achieving all this, forget the wood and used solid GRP. Mine are solid GRP and I'm much happier with that than a plywood core

Edit - bedding the plywood down on a layer of Crestomer would also be another good way to eliminate any voids

 

[MapisM]

Just because lots of people do something doesn't make it a smart thing to do. Ferretti, a generally fine yachtbuilder, tells its customers (including 94 footers) that its Mitsubishi ARGs are the best thing and recommends folks to buy ARG, and the customers say yes. But fact is ARG is inferior to Seakeeper - out here in the objective world of interim forums we all know that. Also, Ferretti did an inhouse test of brightness of a Lumishore vs their standard fit Xenon, and the chief of whatever department is was concluded the Xenon was brighter. Now, there is no room for subjectivity here becuase counting lumens is a bit like counting Euros, so you have to conclude they decided the answer they wanted before they did the test. Perhaps someone in the foodchain prefers to buy from their current supplier, for some reason.

Fact is, these yards (including SL) have other commercial factors that go into the mix when deciding what they wish to fit to their boats, and my view is that they then say to the customers -with a complete absence of objectivity- that the thing they recommend is a great thing. Customers follow the recommendation , because SL or whoever says it. Twas ever thus, but my point is you get far more objectivity and cut through of marketing BS on this forum than from some commercial suppliers.

Mapis I have used 1 sqm on a smaller boat than SL94 and I know there are times when the things use their full angular stroke. You then have these CMC guys shouting about how fast their fins move. Jeeze, can you not hear the alarm bell that "moving fast" means "moving for less time"??? Also, the size of my fins versus size of boat is fully in line with the fin sizes used by Trac. CMC is the outlyer here

These CMCs could be perfect for BA. BartW wants to understand them though, and I'm adding a valid observation that moving fast could be a compromise feature when at zero speed. That's a factor he needs to consider, even though it may well be a perfectly good trade off for the much easier install of electric fins. But this forum is far too good to accept the view that "SL is doing it so it must be ok".

Agreed, this is not what all of us expect from this forum. In fact, this is not what I was suggesting, by all means.
And since we all expect "far more objectivity and cut through of marketing BS on this forum than from some commercial suppliers", let me try as objectively as I can to separate the facts from the marketing BS.

1) I asked you why an SL client should accept an undersized equipment, and you answered that they accept CMC stabs because that's the brand SL recommends, for commercial reasons alone and "with a complete absence of objectivity".
You also hinted that bribing can be another reason behind that.
Now, in my books, statements like these either can be supported by factual evidence, or they are just slandering.
Which, as we all know, is yet another form of marketing BS. A cheap and subtle one, but often rather effective.
But that aside, did you RTFQ, J? I didn't ask why SL clients accept CMC, I asked why undersized.
"Their" reason to accept CMC vs. your for choosing Sleipner was already covered by my foreword on the smartness difference.
But why undersized, ffs? CMC could well supply to SL (and to all their other clients, most of which - with all due respect for the Oundle folks - are not exactly in the same league as FL) much bigger fins.
All the way up to FOUR Sq.m, which means the DOUBLE of the bigger Sleipner fins.
I agree that CMC is the outlyer (btw, isn't it outlier?) vs. the hydraulic stabs builders, but if their marketing BS is aimed at selling smaller equipment, then they'd better rethink their marketing entirely, and double quick!
Not to mention that if they would be bribing purchasing managers (or whoever), just to invoice LESS rather than more as a result, now THAT would be indeed weird.

2) Talking of objectivity, let's consider our own positions in this debate.
Myself, I have ZERO interests with ANY stabs builders, and I can summarise my experiences with them in a few lines:
I got in touch with a couple of ABT engineers in the recent past, the first during a sea trial and the second in a few confcalls, to get some help in sorting out a commissioning problem.
I also worked together with a Naiad engineer many years ago for a maintenance check on my own boat, but I'm not even the one who specced those stabs when the boat was built, having bought her used.
Eventually, I spoke with Mr.CMC by phone in this occasion to discuss some basic technicalities of their stuff.
Oh, and I have no less than a dozen of years of first hand experience with stabs behaviour in all sorts of sea conditions, though not with zero speed.
Plus somewhere between 15 and 20 hours overall also with zero speed stabs, with boats other than mine, in more or less decent sea conditions.
And that's about it.
For yourself, obviously I'm not aware of all your connections and interests, if any - up to you to clarify them, if you wish.
Otoh, it's public knowledge that you are a Sleipner client, and afaik their one and only testimonial.
In fact, there's even a "user experiences" section in their online brochure which only reports a statement of yours.
Which btw makes one wonder why they used the plural, in the heading "user experiences"...
In turn, this implies that you have at least some vested interest in Sleipner image, if nothing else because their equipment is part of your boat value, also if and when you would want to sell her.
And let's face it, here we aren't just discussing a technology which clearly has major advantages when retrofitting stabs on a boat which wasn't designed and built for them, as in the case of BA.
We are talking of something which has various advantages vs. hydraulic systems also for any new builds.
And IF on top of that (note that it's a big IF) these things are really more effective, to the point of needing neither bigger fins nor longer actions to achieve a comparable result, that would be enough to make your stabs obsolete even before they'll hit the water.
After all, none of us like having big fins and shafts sticking out of our hulls - particularly on a planing boat.
It's just a means to an end, and in this sense, the smaller the better for any given final result, not the other way round!
Mind, with all the above, I'm not saying that your opinions so far on this matter were purposely disguised, far from that.
But surely, the way you approached my question, as per previous point 1, is one of the least objective I've ever read from yourself.

Oh, and just as an aside:
I'm sorry to hear that Lumishore didn't manage to make any business with Ferretti, because even without any first hand experience on their products, based on all I read around here I'm sure they are indeed good.
That said, I've heard for decades, during my previous job, commercial people arguing that they couldn't sell to this or that client because someone in their foodchain preferred to buy from their current supplier, to use your own polite but not unclear wording.
And you know what? They were always the worse salesmen (or even sales managers) who used that sort of arguments.
I never heard the best ones using such justification, even when they missed their target - which can obviously happen also to the best, occasionally.
No personal inference on whoever in Lumishore managed the negotiation with Ferretti, of course.
Just reporting a very solid first hand experience, simply to try - yet again - to separate facts from mktg BS.