Fin stabilisers with curved fins, new from Sleipner, 50-60 foot boat bracket

[Portofino]

Assuming two fitted with the concave surface facing outwards .
On a rolling movement the one with concave ( think of scooping your hand swimming front crawl ) may Benifit from " grabbing "more water but the other - which is now pressing on its convex surface will slip / loose grip cos it's curved the wrong way

One benefits other looses out and they swap roles when the boat attempt to roil the other way .

 

[MapisM]

You're right that the need to work in both directions at anchor, but blimey it's surely obvious that benefit of the curve works in BOTH directions

No it isn't J, and I plainly refuse to believe that you don't see why.
Even assuming that the effect of the different vector angle (as guessed by Nick_H, and also shown in the "detailed info" which you linked), can be relevant - and I accept that the angle remains the same in both direction, though the curvature probably makes that angle slightly variable through the whole excursion - my point is that it doesn't take a cray computer to understand that the vector magnitude is bound to be lower when the fins spin inward.

Besides, if the vector perpendicularity with the rolling axle would be that critical, what pops to my mind is that it should be better to install the fins closer to the keel, whilst experience proves that the opposite is true, in real world.
But I'm saying it without thinking too much about pros and cons (one con obviously being related to physical constraints, like draft or interference with props), but just as a point in principle.

 

[Elessar]

You're right that the need to work in both directions at anchor, but blimey it's surely obvious that benefit of the curve works in BOTH directions

Nope not obvious.
I reckon Nick H is right. After all a fin in the middle of the boat pointing down would instinctively seem efficient (but with downsides) and that would fit Nick's "rule".

 

[cryan]

I may be jumping into the Lion's Den here but I think there is a bit of misunderstanding as to how Stabs work. They work like aeroplane wings as the boat moves forward, providing differing lift on either side of the vessel so as to counter the rolling motion. The curved fin will work similar to the winglet on an air liners wing which stops the flow escaping off the end of the wing thus increasing efficiency. From my experience of fins (granted, all gained aboard ocean going vessels) The most important part of the system is the control module and it's ability to keep up with the vessel's roll. I would doubt there is very much advantage at all whilst at anchor apart from the slapping effect similar to the effect that a kayak paddle gives to a canoeist. I'm sure the salesmen will disagree!
One of the biggest problems facing the motor yacht market is that the vessels are so small that the rolling motion will be quicker which will really challenge the gyroscopic control system and actuators. Using stabilisers in too calm a rolling motion can result in a juddering sensation aboard the vessel.
Certainly if crossing the Channel or blue water cruising then a more efficient fin shape should reduce the need for so much actuation and in turn save power. If in the Solent all the time or the Med I don't think (IMHO)I would bother spending the money.

 

[RobWales]

I may be jumping into the Lion's Den here but I think there is a bit of misunderstanding as to how Stabs work. They work like aeroplane wings as the boat moves forward, providing differing lift on either side of the vessel so as to counter the rolling motion. The curved fin will work similar to the winglet on an air liners wing which stops the flow escaping off the end of the wing thus increasing efficiency. From my experience of fins (granted, all gained aboard ocean going vessels) The most important part of the system is the control module and it's ability to keep up with the vessel's roll. I would doubt there is very much advantage at all whilst at anchor apart from the slapping effect similar to the effect that a kayak paddle gives to a canoeist. I'm sure the salesmen will disagree!
One of the biggest problems facing the motor yacht market is that the vessels are so small that the rolling motion will be quicker which will really challenge the gyroscopic control system and actuators. Using stabilisers in too calm a rolling motion can result in a juddering sensation aboard the vessel.
Certainly if crossing the Channel or blue water cruising then a more efficient fin shape should reduce the need for so much actuation and in turn save power. If in the Solent all the time or the Med I don't think (IMHO)I would bother spending the money.

Lions Den!

Yup this should be a good evening's thread coming up

 

[rustybarge]

Lions Den!

Yup this should be a good evening's thread coming up

We live in the age of technology, 3d printing, quadcopters with go-pro cameras, smart phones that can run your porfolio account as you commute......
£70k does seem like a lot for a electronic digital gyro that's on a standard remote control helicopter, your ipad or games controller, and a few hydaulic rams and a couple of steel fins!!

It should be possible to build a DIY version for a few thousand, not £70k......

 

[RobWales]

"I would doubt there is very much advantage at all whilst at anchor apart from the slapping effect similar to the effect that a kayak paddle gives to a canoeist"

I was infact referring to the statement above!

Especially after coming accross a vid on youtube showing that they do clearly work on the original poster of this threads boat (ship) whilst at anchor!

 

[jfm]

Wow, lots of good questions. I'll have a go at answering them. I'll do it all in one thread, to avoid piecemeal stuff, and in no particular order:

1. @ cryan post 36, you are monumentally missing the point. Sure, fins work as you describe underway but at anchor there is zero similarity with a plane's wing. If we only cared about underway stabilisation, fast boats would have tiny fins because the hull speed would allow huge righting torques from tiny fins. But we don't. We want STAR as well, and that needs big fins (because they "swim", not act as plane wings), and so anything you can do to create more stabilisation from a smaller fin (so you waste less fuel when underway) is a good thing and the name of the game here

2. Just some housekeeping basics, @Portofino. (1) Bart has fitted an electrical system to his Canados NOT to overcome a space issue. He has a huge boat with zero space problems and his stab actuators are in fact waaaay taller than even Sleipner's previous actuators let alone these new smaller ones. The advantage of electric fins for BA was the ease of retrofitting them, and that was a smart decision by BartW for sure. (2) I'll skip any debate on "Curved fins = marketing". Changing the shape of something this big that you drag thru water at 20+ knots will always have a big physical effect. 3. The shareholders in Sleipner ARE the engineers in the company .

3. Nick has it right in #15. The curved fins overcome the "modest deadrise" problem that Nick describes, and trick the boat into behaving like it has a really steep deadrise, so that the fin shaft gets closer to intersecting the roll axis. Putting that more longwindedly, the curve changes the direction of the force vector that is applied to the hull. I'm using the word "vector" in the proper engineering/mathematical sense. Imagine you have a round bilge hull: you would generally mount the fins close to the sides and away from the keel, and the reason is that you get the fin shafts as close to pointing at the roll axis as you can make them. But on a planing boat the hull is basically flat so moving the fin away from or closer to the keel doesn't alter the direction of the fin shaft and therefore doesn't alter the direction of the force vector. The direction of that vector is perpendicular to the surface of the fin, ie parallel to the hull's flat surface (when viewed along the hull's roll axis) and therefore it is whatever the deadrise of the hull dictates it to be. This new curved fin changes the direction of the vector. It is no longer perpendicular to the fin shaft (when viewed along the hull's roll axis) and instead the vector becomes more vertical. If the deadrise of the boat is say 20 degrees then the force vector from these fins will be in a direction (say) 30 degrees to the horizontal, not 20, and that is a big deal as regards the physics. Exactly why does the curve change the vector from 20 to 30 (or whatever)? Because the direction of the vector is the magnitude-weighted average of the directions of the individual microscopic vectors arising from water molecules striking the fin, each of which vectors is always perpendicular to the local surface of the fin. The curvature in the fin therefore changes the direction of the anti roll force vector

All the above relates to the direction of the vector, not its magnitude. See below for magnitude.

4. A separate consequence of the curve is that it changes the distance between the vector and the roll centre of the boat, the "lever arm" you could call it, and thereby changes the torque exerted on the hull around the roll axis by the fins. In the case of any normal typical planning motorboat the distance increases and so the torque increases, which is a good thing. This needs a vector-y diagram to explain perhaps, if debated

5. @MapisM, #22, in a pure textbook case with a V hull it makes zero difference whether you mount the fins close to the chine or close to the keel. The force vector is identical, and as a matter of simple Newtonian physics it just does not and cannot make a difference. Now, in the real world people mount them near the chine for a whole load of good practical reasons and to keep their water disturbance out of the flow of water to the props, but they do not mount them there to improve the direction of the force vector or its magnitude

6. @MapisM I'm confused where you said in #22 "if the vector perpendicularity with the rolling axle would be that critical". I haven't said that is relevant. Maybe I'm misunderstanding what you meant?

7. BJB your Q2 in #7 is a very perceptive question and I thought the same initially, but asked further. A flat fin, set at an angle to its shaft, would indeed work as you say. However the tests and models showed that having an angle near the hull created a negative effect on the outward sweep as the water flow struck the reverse chine that you now get on most mobos. Thus, the fins need to be not-angled close to the hull. The angle only works if it is away from the hull surface. That would make you think of a fin with a crease down the middle, but that doesn't work well for obvious water flow reasons at speed, hence the curve. Note by the way that the winglet is perpendicular relative to the motion of the fin not to the immediately adjacent surface of the fin, which is clever.

8. Finally, the magnitude of the vector, @mapisM #22 and Ellesar #23. We were talking at crossed purposes I think. When I said it is obvious the curve does its thing in both directions, I was referring to it changing the direction of the vector, which is the main benefit of this things as stated in my OP. I think that part is obvious, right? But I see now you were actually referring to the magnitude of the vector not its direction. When the starboard (say) fin rotates CCW, ie outwards, at anchor, there is a "scoop" effect caused by the curve and the magnitude of the vector is greater than with a flat fin, plus it is a better direction, plus it has a longer leaver arm. That is utopia. When it rotates CW, the magnitude of the vector decreases relative to a flat fin, as you correctly thought (and I'm sorry I didn't see your point - we were at crossed purposes). However, Sleipner have measured this carefully. The maths proves that (a) the reduction in magnitude on the CW rotation is less than the increase on the CCW rotation (relative to a flat fin) and (b) the reduction in the vector magnitude is more than compensated mathematically by the increased verticality of the vector's direction, so you are still ahead, overall net net, in both directions of fin rotation, compared with flat fins.

 

[cryan]

Wow, lots of good questions. I'll have a go at answering them. I'll do it all in one thread, to avoid piecemeal stuff, and in no particular order:

1. @ cryan post 36, you are monumentally missing the point. Sure, fins work as you describe underway but at anchor there is zero similarity with a plane's wing. If we only cared about underway stabilisation, fast boats would have tiny fins because the hull speed would allow huge righting torques from tiny fins. But we don't. We want STAR as well, and that needs big fins (because they "swim", not act as plane wings), and so anything you can do to create more stabilisation from a smaller fin (so you waste less fuel when underway) is a good thing and the name of the game here

Just to confirm, I did NOT suggest that the fins act like aero wings when at anchor. I suggested they act more like a canoe paddle being held flat against the water. But I'm still not convinced that the effect at anchor will be that great as you are relying on the passive effect of small fins.

 

[jfm]

Another very important point especially for smaller boats, where people have mentioned above that the main benefit is just the compact actuator. In a small boat, say a 50 footer compared with an 80 footer of the same genre, you generally have a much shorter/faster natural roll. To have effective STAR, you need more stabilisation force relative to the boat's weight than in a big boat

This means that the effect of yaw is greatly increased. Think of the boat's roll axis as a big bearing about which the boat rolls, but then remember that bearing is really sloppy because it is just the boat's inertia, nothing else. As the fins sweep left/right at anchor, the whole boat shudders/jerks sideways. This is almost imperceptible in and 80 footer weighing 55-60 tonnes, but it is a very big deal (as regards comfort) in a 50 footer weighing 16 tonnes

Now, if you had Nick-H's theoretical boat with a cylindrical shape of even density, the fins would produce only perfect anti-roll forces if mounted centrally along the hull's length. The line of the shafts of the fins would intersect the roll axis, so the antiroll vector would be a tangent to the circular section of Nick's boat, which is perfection. But none of our boats are like that, and the direction of the antiroll vector from the fins is not tangential to Nick's cylinder. Hence in the real world you get sway (ie sideways jerks of the hull as the fins sweep), and in addition you get yaw (where the boat's heading alters, ie it turns left right as the fins sweep) if the fins are not perfectly centred in a fore-aft direction, which they won't be. These new curved fins significantly reduce these nasty side effects, which is a big deal in the 50-footer segment. I mean the lightweight 50 segment where boats weigh 15 tonnes or whatever

 

[Robg71]

you make it sound... simples.... You have way too much time on your hands JFM
personally, i related them to aircraft ailerons, in a very simplistic way.... cos I'm.... simples

 

[jfm]

Just to confirm, I did NOT suggest that the fins act like aero wings when at anchor. I suggested they act more like a canoe paddle being held flat against the water. But I'm still not convinced that the effect at anchor will be that great as you are relying on the passive effect of small fins.

OK but you are still missing the point big time. You said "The curved fin will work similar to the winglet on an air liners wing which stops the flow escaping off the end of the wing thus increasing efficiency" and that is wholly wrong. The curve on the new fins has nothing at all to do with the winglet issue. All decent fin stabilsers on boats have had winglets for years, as do these new curved fins. This curvature is an entirely different matter and produces an entirely different physical effect (namely a change in the direction of the "lift" force vector) from winglets.

Likewise, while the scoop effect of the curved fins is similar to a scoop shaped canoe paddle the similarity ends there. When you are canoeing, you can hold the paddle in any direction you want, so you don't have the constraint of the hull's deadrise that stabilisers have, till now, had to live with. These new fins basically bust out of that constraint

As for "But I'm still not convinced that the effect at anchor will be that great as you are relying on the passive effect of small fins", you have obviously never tried them! They're not "passive" of course. In an anchorage where the boat is rolling as badly as at 1:00 in this video, these fins can eliminate it, given perfect control (which, I agree, is an important part of the total installation). (The opening credits contain a typo - it should say "1 sweep per 5 seconds", not "5Hz")

 

[jfm]

you make it sound... simples.... You have way too much time on your hands JFM
personally, i related them to aircraft ailerons, in a very simplistic way.... cos I'm.... simples

Yes but with an aircraft aileron, the designer can mount it at any angle he chooses, and therefore get the effect he chooses.

With a mobo, you can only mount them so they sweep on an axis that is perpendicular to the hull surface. Unless we all build totally custom boats, we cannot choose our hull surface angle, our deadrise. And THAT is the whole point about these curved fins - they allow the owner of a boat with 20 degrees deadrise to "cheat" and have the same stabilisation that he would have with a 30 degrees (say) deadrise hull. THAT is the big deal about these things (and sorry if not explaining it well! )

 

[Robg71]

I find that surprising that there appears to be little or no water movement to give the stabs any chance of moving the boat... thats pretty impressive

 

[benjenbav]

Thanks for all the answers j. I think my first experience of stabs when stationary was on your boat at the Swanwick show in May this year when there must have been 30 or more people on board, all moving about a bit - as people do - and yet the boat felt like as still as something welded to the dock.

 

[rubberduck]

Is there anything like this for the 40-50ft size range yet ?

 

[jimmy_the_builder]

Crumbs. This thread makes me realise just how pitifully small my brain is.

Cheers
Jimmy

 

[jfm]

I find that surprising that there appears to be little or no water movement to give the stabs any chance of moving the boat... thats pretty impressive

You mean in the video? That's cos they were set to 50% of maximum sweep and angriness, because I was worried my flybridge would clout my neighbour's flybridge if I let the boat roll too much. I promise you that on full power, and when working hard in big waves, you get a huge swoosh of water each side of the boat as the things sweep, and on the downstream side they create quite a big hole in the water. BartW and MYAG will testify to this as they have basically the same sized hardware.

 

[cryan]

Sorry in the time it took to write that you have answered, Is it the fins movement that is causing the roll?

Ok Deleting previous post as I misunderstood what was happening in the video.

 

[Robg71]

Hold on a minute! What the h*ll is going on in that video? The boat is not being moved by the water. It's not at anchor, it's alongside in a marina which seems to be flatter than a mill pond on a flat calm day. To make the boat roll like that I suspect a weight (possibly people) was being moved from side to side. Stopping this is what stopped the roll not the fins! I certainly wouldn't take this video as evidence of the effectiveness of fins when stationary. Show an anchorage where every boat is rocking and rolling and the finned boat is as stationary as the Isle of Wight that would be a more effective marketing tool. If the boat in the video is being rolled like that by the sea in those conditions then someone needs to do some serious stability calculations fast!

no, if there is water movement over the fins, maybe even imperceptible, the boat will induce roll, if told to do so....
we had to test the anti heave on a 22000ton DSV in relative calm water. If you can stop roll, you can also induce it just as easily.....

JFM, if thats 50% im really impressed considering its as flat as a witches tit really....