Stabilizers for Blue Angel, engineering question

[MapisM]

the middle stringer of these 3 will be cut out

Bart, is that necessary for space/placement requirements, or what?
Based on your latest numbers, I would have thought that it's better to reinforce only one rectangular area (each side, obviously) between the frames and stringers, and place the fins inside it, as centered as possible.
I would avoid cutting any structural part if at all possible, particularly considering the unbelievably thin bottom, which proves that the builder relied more on stringers and frames (which are massive in comparison, and rather close each other for the hull size) for the overall hull integrity.
It's just a thought, anyway. It sounds like at that yard they know their job, and I don't want to teach them to suck eggs, of course.
In the meantime, have you been able to get some indications from Canados, through your contact in Rome?

 

[vas]

Bart, is that necessary for space/placement requirements, or what?
Based on your latest numbers, I would have thought that it's better to reinforce only one rectangular area (each side, obviously) between the frames and stringers, and place the fins inside it, as centered as possible.
I would avoid cutting any structural part if at all possible, particularly considering the unbelievably thin bottom, which proves that the builder relied more on stringers and frames (which are massive in comparison, and rather close each other for the hull size) for the overall hull integrity.
It's just a thought, anyway. It sounds like at that yard they know their job, and I don't want to teach them to suck eggs, of course.
In the meantime, have you been able to get some indications from Canados, through your contact in Rome?

+1000

Bart, 20mm grp and closely placed stringers/frame members whatever you call them, is a clear indication that these frame members DO a lot of the stress relieving in the whole structure.
ie, taking bending/flexing whatever forces and evenly distributing them to the rest of the hull.
Obviously your hull construction is before FEA complex calcs and "borrows" some of the traditional timber construction logic (not my field, just IMHO!)
I'd go as far as claiming that you should mainly support the stabs on the frame members and forget the grp skin, rather than strengthening the grp (in a flawed way) and supporting the fins there!

cheers

V.

 

[rafiki_]

+1000

Bart, 20mm grp and closely placed stringers/frame members whatever you call them, is a clear indication that these frame members DO a lot of the stress relieving in the whole structure.
ie, taking bending/flexing whatever forces and evenly distributing them to the rest of the hull.
Obviously your hull construction is before FEA complex calcs and "borrows" some of the traditional timber construction logic (not my field, just IMHO!)
I'd go as far as claiming that you should mainly support the stabs on the frame members and forget the grp skin, rather than strengthening the grp (in a flawed way) and supporting the fins there!

cheers

V.

+another 1

 

[MapisM]

Obviously your hull construction is before FEA complex calcs and "borrows" some of the traditional timber construction logic (not my field, just IMHO!)

That's an excellent point indeed, V.
I didn't look at that from this angle, but what you're saying makes a lot of sense, and I couldn't agree more - based also on quite a few wooden boats which I've seen.
Not to mention that IIRC, back in those days, Canados started building the 70' in timber, not many years before they jumped into GRP construction for the hull.
BA still has a wooden superstructure, in fact.
Bart, I still think that the yard you're talking with sounds a pretty decent and experienced one, but it's maybe worth mentioning these thoughts to them, and hear what they think about them?

 

[Erik C]

That's an excellent point indeed, V.
I didn't look at that from this angle, but what you're saying makes a lot of sense, and I couldn't agree more - based also on quite a few wooden boats which I've seen.
Not to mention that IIRC, back in those days, Canados started building the 70' in timber, not many years before they jumped into GRP construction for the hull.
BA still has a wooden superstructure, in fact.
Bart, I still think that the yard you're talking with sounds a pretty decent and experienced one, but it's maybe worth mentioning these thoughts to them, and hear what they think about them?

I think he should talk to Canados, they have no vested interest and should know more about the boat and it's construction.

 

[MapisM]

Agreed, but I suppose it might be a bit difficult to reach whoever was really in the know at that time (hence my question to B, at the end of my post 241).

 

[Erik C]

Agreed, but I suppose it might be a bit difficult to reach whoever was really in the know at that time (hence my question to B, at the end of my post 241).

Even if the people are no longer there, and I find that hard to believe, they should still have the original drawings and calculations. These would go a long way eliminating any guesswork.

 

[BartW]

While sitting on the fence, (well actually in my car on the way home from SOF)
I just want to add a small comment to the discussion;

I’m sure that there might be wrong wording, or wrong interpretation from my side, in my posts;
with my limited knowledge of the subject, and me nor mr CMC are native English speaking.
Or that small important details are lost somewhere (the devil is in the detail )

f.e. in the discussions above, is many times referred and contested that the speed of the electric fins is a advantage,.
So now I have to say, that when I referred to the faster reaction of the fins to Mr. CMC as being an advantage,
he answered “its not really the speed of the fins, but more important, the usage of the very high Q accelerometers (3pcs) and the processing power behind which is the power in the system. “ (detecting variations or acceleration in the roll movement)
Again, I don’t want to go in discussion about these details, just because I don’t know much about brushless electric motors nor stabilizers, but I don’t think it makes sense to discuss the quality’s of CMC without knowing accurate details.

Nevertheless, I know for sure that the moving speed of the electric fins is higher then their hydraulic fins. I don’t remember where, but I have read accurate rotating speed figures from the CMC electric fins, in comparison with their hydraulic fins. (something like 25% faster iirc)
But again, I can’t explain if or why this could be a advantage, and don’t want to.


About the heat issue, my observation is very simple:
CMC states there is absolutely no problem, in none of their systems,
I will be using the smallest system with the smallest fins (they have two bigger sizes of fins available for the same motor)
and most systems / motors are installed in a watertight box, not in BA,
(I can’t even see heat sink ribs on these DC motors, like the ones you know from average AC motors.)
So my conclusion is that I don’t expect issue’s with heat.

And finally; I’m absolutely not interested in a score nor a qualification of the CMC system compared to any other brand,
The CMC system has some advantages that suit me very well;
Compact, relatively easy to install, can run without using the genny’s during navigation, and they are aimed to stabilize a boat during navigation and at anchor. They have a few good references as well.
I don’t care if there is another system that does one or the other slightly better, when missing all these other important (to me) aspects.
As there is no alternative for CMC, (in my case) I’m about to order a system.

 

[BartW]

Bart, is that necessary for space/placement requirements, or what?
Based on your latest numbers, I would have thought that it's better to reinforce only one rectangular area (each side, obviously) between the frames and stringers, and place the fins inside it, as centered as possible.
I would avoid cutting any structural part if at all possible, particularly considering the unbelievably thin bottom, which proves that the builder relied more on stringers and frames (which are massive in comparison, and rather close each other for the hull size) for the overall hull integrity.
It's just a thought, anyway. It sounds like at that yard they know their job, and I don't want to teach them to suck eggs, of course.
In the meantime, have you been able to get some indications from Canados, through your contact in Rome?

MapisM, I believe that if you could see it with your own eyes,
you would agree that taking out this stringer (hight x Width is 100 x 100mm) ,
and replace this with a solid (or sandwitched) flat new surface, that is correctly bonded with the adjacent stringers, and frames, that this would be the strongest solution. Yes the bonding has to be done correctly, and the yard had some good idea's about that.
(hight x Width of the frames in this position is approx 300mm x 100mm.)

Do you know that these stringers and frames are just a beam of foam, covered with layers of GRP (10mm total)
would you want the flange of the stabs be pressed on a sanwich with foam inside ?


yes the framing looks indeed similar to old wooden boats,
what kind of planks are used in your CNA ? more then 20mm thick ?

will make some drawings and post pics later

 

[BartW]

I think he should talk to Canados, they have no vested interest and should know more about the boat and it's construction.

hopefully coming week on wednesday, tbc.

 

[rafiki_]

While sitting on the fence, (well actually in my car on the way home from SOF)
I just want to add a small comment to the discussion;

I’m sure that there might be wrong wording, or wrong interpretation from my side, in my posts;
with my limited knowledge of the subject, and me nor mr CMC are native English speaking.
Or that small important details are lost somewhere (the devil is in the detail )

f.e. in the discussions above, is many times referred and contested that the speed of the electric fins is a advantage,.
So now I have to say, that when I referred to the faster reaction of the fins to Mr. CMC as being an advantage,
he answered “its not really the speed of the fins, but more important, the usage of the very high Q accelerometers (3pcs) and the processing power behind which is the power in the system. “ (detecting variations or acceleration in the roll movement)
Again, I don’t want to go in discussion about these details, just because I don’t know much about brushless electric motors nor stabilizers, but I don’t think it makes sense to discuss the quality’s of CMC without knowing accurate details.

Nevertheless, I know for sure that the moving speed of the electric fins is higher then their hydraulic fins. I don’t remember where, but I have read accurate rotating speed figures from the CMC electric fins, in comparison with their hydraulic fins. (something like 25% faster iirc)
But again, I can’t explain if or why this could be a advantage, and don’t want to.


About the heat issue, my observation is very simple:
CMC states there is absolutely no problem, in none of their systems,
I will be using the smallest system with the smallest fins (they have two bigger sizes of fins available for the same motor)
and most systems / motors are installed in a watertight box, not in BA,
(I can’t even see heat sink ribs on these DC motors, like the ones you know from average AC motors.)
So my conclusion is that I don’t expect issue’s with heat.

As there is no alternative for CMC, (in my case) I’m about to order a system.

Bart, this is a really interesting thread, and you are getting all sorts of opinions, which if like mine, are quite uninformed. Some, like JFM and MM have good experience with other systems, and some of us have an engineering background (JFM qualifies on both counts).
However, it really makes sense to me, that if the system has good sensing and control properties, then it should work better than without. However, I can't believe an electric motor system is fundamentally more responsive than a hydraulic system, as there is more inertia in the electric motor and gearbox. I still struggle with the info you have from CMC that the motor does not generate heat. The magnetic flux, and electrical energy in the electric motor will always generate heat, brushless or not. As JFM has tried to explain, holding the motor in position against a restoring force will generate heat through the resistance in the windings. My analogy is to consider you are trying to stop your car from rolling down a hill, you are backs to the wall, holding the car in place. You are going to feel mighty hot after a while, even though the is no motion or kinetic energy.
Anyway, the bottom line is that you are going ahead, and I look forward to hearing about the results in due course.

 

[jfm]

MapisM, I believe that if you could see it with your own eyes,
you would agree that taking out this stringer (hight x Width is 100 x 100mm) ,
and replace this with a solid (or sandwitched) flat new surface, that is correctly bonded with the adjacent stringers, and frames, that this would be the strongest solution. Yes the bonding has to be done correctly, and the yard had some good idea's about that.
(hight x Width of the frames in this position is approx 300mm x 100mm.)

BartW you are absolutely right to cut out the stringer in my book, as I have said from the beginning. To leave in a 100x100 length of foam inside an otherwise nice thickened block of 80mm hull is engineeringly not smart imho. It would be different in a wooden boat where continuity of the wood fibres matters, but this is GRP where the maximum continuity is mostly 2 inch lengths of chopped glass strands plus some longer rovings, and your shipyard will no doubt bond and fair the new 80mm thick hull section into the exisiting frames and stringers using continuous rovings and woven rovings. All with much careful surface preparation of course. You could ask Canados if you want, but this is such a no-brainer that I wouldn't even bother tbh (and if they said to leave the block of foam in there I think I'd ignore them )

 

[vas]

To leave in a 100x100 length of foam inside an otherwise nice thickened block of 80mm hull is engineeringly not smart imho.

JFM, you absolutely sure its foam in there though?

 

[MapisM]

Do you know that these stringers and frames are just a beam of foam, covered with layers of GRP (10mm total)
would you want the flange of the stabs be pressed on a sanwich with foam inside ?

yes the framing looks indeed similar to old wooden boats,
what kind of planks are used in your CNA ? more then 20mm thick ?

Well, as you surely know, an empty 100x100mm square tube made of 10mm thick steel is much stronger than a solid bar made with the same quantity/weight of steel.
But no, I wasn't actually aware that BA stringers and frames aren't made of solid GRP.
And I'm also surprised to hear that - not because there's anything wrong, but I wouldn't have expected it in a Canados of that vintage.
Heck, where does her massive displacement come from?
The hull skin is thinner than all of us expected, frames and stringers are cored...
...are you sure that there isn't some lead ballast hidden somewhere?!

Anyway, with the strengthening I was envisaging, i.e. "filling" one rectangle included between frames and stringers (but without cutting them), the stabs flanges should NOT press any stringer or frame.
In fact, these structural parts would only "enclose" the "filled" part of the hull, and that's the area where the flanges should work.
Otoh, I did ask in the first post of this subject whether there are also space requirements behind the choice of cutting one stringer.
Since now you mention that the flange would work against the cored stringer, I suppose that for some space constraint the shaft+motor could NOT fit inside one of those 67x57 cm rectangles, right?
If this is the case, the whole debate becomes academic, because I don't think there's any practical alternative to cutting the stringer and filling two of those rectangles.

Re. your last question, my boat has 40mm iroko planks and 100x100mm oak frames.
Besides, the frames are coupled together (hence becoming actually 200mm wide and 100mm thick), and the distance between them is slightly LESS (!) than the width of each coupled frame - i.e. 15 cm or so, I never checked that exactly.
But the typical wood construction for a planing boat of similar size is MUCH lighter.
My boat is overbuilt, so to speak, just because with full D hulls the weight is the last of your concerns.
In fact, the strengthening where the fins are placed is nothing special - practically just enough to fill the area where the flanges are placed.
My suggestion for BA was not based on the same assumptions, obviously.

 

[jfm]

JFM, you absolutely sure its foam in there though?

No, and BartW can make sure, but nothing else is plausible. They'd be crazy to use 80x80 timber. How would you bend it at the bow? Sure you can do bend wood like that with a wooden hull where you pull the timber onto frames and a stempost, but not in a GRP hull where you'd have to push a piece of timber mid span to bend it like that. Also BA substantially post dates the introduction of Airex/Divinylcell style closed cell rigid foam products for boat hull coring. They won't be solid GRP because there is no easy way to apply GRP that way and it would be an absurd waste of material and labour to make them solid GRP - less material wrapped around a bigger foam core is stiffer than solid, as you know

 

[MapisM]

They'd be crazy to use 80x80 timber. How would you bend it at the bow?

Not that it really matters in this context, but I'm not sure to see what the problem you're talking about would be...?

 

[BartW]

JFM, you absolutely sure its foam in there though?

I'm 200% sure there is foam inside,
as I drilled a hole in a stringer,
and we had to cut a hole in a frame.

 

[Deleted User YDKXO]

Well, as you surely know, an empty 100x100mm square tube made of 10mm thick steel is much stronger than a solid bar made with the same quantity/weight of steel.
But no, I wasn't actually aware that BA stringers and frames aren't made of solid GRP.
And I'm also surprised to hear that - not because there's anything wrong, but I wouldn't have expected it in a Canados of that vintage.
Heck, where does her massive displacement come from?
The hull skin is thinner than all of us expected, frames and stringers are cored...

IMHO foam filled stringers are quite normal in the boat building industry. In fact isn't foam filling sold as an advantage over timber filling due to it's 'rot free' characteristics? Surely the weight of the boat is mainly due to how dense (closely spaced) the grid of stringers is and if the grid is dense than the thickness of the hull can be a bit less. If the grid is less dense then the hull has to be thicker. In the case of BartW's Canados, I wouldn't read anything at all into the thickness of the hull and the stringer construction; it seems the designer decided on a dense stringer grid and the fact that Bart's boat has been happily floating around in the Med for 30yrs or whatever suggests that the designer got it right

 

[MapisM]

All agreed, M.
Actually, I'm pretty sure to have seen old(ish) GRP hulls with stringers and frames made also with solid GRP, but coming to think of it they must have been older than BA.
And as you say, with BA the pudding has been tried for decades (though actually a couple rather than three, but still quite enough anyway... ), and the recipe is still excellent.
Having been onboard, I can confirm that the last worry you would feel is about the boat integrity!
You must be right also re. the overall displacement, it must have more to see with the density of stringers and frames than anything else.

 

[jfm]

IMHO foam filled stringers are quite normal in the boat building industry. In fact isn't foam filling sold as an advantage over timber filling due to it's 'rot free' characteristics? Surely the weight of the boat is mainly due to how dense (closely spaced) the grid of stringers is and if the grid is dense than the thickness of the hull can be a bit less. If the grid is less dense then the hull has to be thicker. In the case of BartW's Canados, I wouldn't read anything at all into the thickness of the hull and the stringer construction; it seems the designer decided on a dense stringer grid and the fact that Bart's boat has been happily floating around in the Med for 30yrs or whatever suggests that the designer got it right

Agreed - it must be ok based on the obvious evidence of 20 years successful navigation
I'd bet the displacement is a result of the superstructure rather than unusual dense hull scantlings. Wood decks and a ply superstructure, of the large scale of BA's, plus the ~2x thickness of glass 20 yrs ago, will add several tonnes compared with a cored-GRP-on-aluminium-spaceframe superstructure