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

[jfm]

Not sure I buy the curved fins concept despite JFMs very thorough and initially convincing explanation. I can believe the side effects bit, but not the getting the better axis bit.

One way I like to test any theories I have is to extrapolate and see where that takes me. In this case I would extrapolate the curvature to 90 degrees and in that case the end of the fin would sweep through the water longitudinally and therefore provide no benefit at all. From that I would reason that even if you could argue that the angle of the vector is better, it would lose its intensity at a similar rate therefore undermining that overall benefit. I suspect that the net effect would therefore considerably less than you might expect with the logic that I think I understand from JFM.

I like your thinking in your second para, the extrapolation approach. (I'm not sure what to say about your first paragraph, as this is Newtonian physics not religion, so "believing" doesn't come into it! )

So if you extrapolate as you say, to the point where you have a flat fin whose face is angled 90deg to the shaft, then yes you have zero effective surface area, and the things would indeed be useless as you say. 100% agreed.

But your words following "From that" do not follow from the words that precede them. The reduction in effective surface area of the fin caused by angling it follows a cosine curve (Cosine of 0 degrees is 1; cosine of 90 degrees is zero, but the points in between are not a straight line). At a modest angle you have made a big change to the direction of the antiroll vector (because that IS a linear relationship) but you've made only a small reduction in the effective surface area of the fin (because it is in the first half of the cosine curve). In other words, I'm very much disagreeing with your "at a similar rate" , but agreeing everything else that you wrote.

Once you jettison the flawed (imho) "at a similar rate", you can easily see, by intuition or maths, that the interaction between (a) the positive thing namely the improvement in the direction of the anti roll vector and (b) the negative thing namely the effectively reduced surface area of the fin, results in a humped graph. As you move from the current world of fins that are parallel to their shaft, to our imagined useless fins that are at 90 degrees to their shaft, antiroll torque rises, then peaks, then falls to zero, AOTBE. These new Sleipner fins are of course close to that peak

That is I believe firmly based on your approach to how to think about this (though feel free to disagree if I've misunderstood you) and it is exactly what happens in practice.

 

[rwoofer]

I like your thinking in your second para, the extrapolation approach. (I'm not sure what to say about your first paragraph, as this is Newtonian physics not religion, so "believing" doesn't come into it! )

So if you extrapolate as you say, to the point where you have a flat fin whose face is angled 90deg to the shaft, then yes you have zero effective surface area, and the things would indeed be useless as you say. 100% agreed.

But your words following "From that" do not follow from the words that precede them. The reduction in effective surface area of the fin caused by angling it follows a cosine curve (Cosine of 0 degrees is 1; cosine of 90 degrees is zero, but the points in between are not a straight line). At a modest angle you have made a big change to the direction of the antiroll vector (because that IS a linear relationship) but you've made only a small reduction in the effective surface area of the fin (because it is in the first half of the cosine curve). In other words, I'm very much disagreeing with your "at a similar rate" , but agreeing everything else that you wrote.

Once you jettison the flawed (imho) "at a similar rate", you can easily see, by intuition or maths, that the interaction between (a) the positive thing namely the improvement in the direction of the anti roll vector and (b) the negative thing namely the effectively reduced surface area of the fin, results in a humped graph. As you move from the current world of fins that are parallel to their shaft, to our imagined useless fins that are at 90 degrees to their shaft, antiroll torque rises, then peaks, then falls to zero, AOTBE. These new Sleipner fins are of course close to that peak

That is I believe firmly based on your approach to how to think about this (though feel free to disagree if I've misunderstood you) and it is exactly what happens in practice.

Yes you did understand correctly and took it further in doing the maths thinking (the cosine bit). I only got round to the intuitive bit cause my maths brain definitely switches off after a bottle of wine. So i agree with your conclusion, that the rate of change of the two factors being considered as the angle changes (at the relatively small angles we are talking about) is different and hence there is indeed a net benefit.

 

[MapisM]

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

Last edited by jfm; Yesterday at 21:07. Reason: added last sentence in case MapisM makes objection!!

LOL, makes me wonder if I should feel honoured by your consideration for my objections, or read that as an attempt to hush them...
Either ways, I like your style, J!

Anyway, I would have never objected to the principle that the lighter the boat, the more sensible it is to "nasty effects".
Quite the opposite, in fact. I've always argued against the idea that P boats offer the "best of both worlds" just because by fitting stabs and slowing down you can use them also at D speed.
They will always be less comfortable than proper D hulls anyhow. The fact is, lighter boats are more sensible to sea motion, period.
Fins-induced motion, either sway or jaw, is ridiculous in comparison to the sea motion, and in this sense vector fins are a solution to a problem that doesn't exist, other than in an academic debate like this.

In fact, coming to think of it, if and when the sea is calm enough to make those effects measurable (let alone perceivable), I have some good news for all the poor chaps (yourself included) who can't enjoy the "new and improved" formula: just shut the whole thing off, and kill two birds with one stone: ZERO nasty effects, and ZERO engine power wasted.

Oh, and apropos of "new and improved": that reminded me of a great strip, which I've been able to find by googling around a bit.
Do you feel a bit like the fat cat by now, J...?

Re. your other comments in post #27, at point 3 you say 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.
Now, while I agree with the first sentence, the second is a non-sequitur.
Of course the vector direction remains the same as the deadrise in a P hull, but the ideal target remains having the fin shafts pointing to the roll axis (btw that's what I meant by "vector perpendicularity", which I accept that could have been a misleading expression).
So, why on earth should it make zero difference whether you mount the fins close to the chine or close to the keel (aside from turbulences etc., of course)?

On the other hand, let's try to consider numbers, rather than perceptions.
Not that we've got many, and they are as generic as any marketing hype can be, but let's try to work with what we've got.

1) Max roll angle at anchor with no stabs, 9.4°. With vector fins, 2.6°.
This means a 72% roll reduction, which as I understand is what Sleipner considers a measure of the system efficiency.
As an aside, it's pretty obvious that the only meaningful number is the one which is missing, i.e. a comparison between vector fins and straight fins, rather than between vector fins and no stabs at all. But as we all know, marketing folks will never learn that someone can actually turn the brain on, before reading their claims...

2) We are also told that vector fins are 50% more efficient than straight fins at anchor.

On this basis, we can deduct that with straight fins the max roll angle should be 4.9° instead of 2.6° (i.e., 48% roll reduction, before the 50% improvement which raise this number to 72%, courtesy of the vector fins).
Now, if anyone believes that curved fins alone can make such difference, I'm interested in offering them a pretty sweet deal.
You know, I've got these 50m USD stuck in a Nigerian account, and I am willing to offer a 10% commission for just a small help...

Actually, there are also other - possibly even more important - inconsistencies which popped to my mind after reading the leaflet and the latest comments on this thread.
But today I'm a bit busy travelling from Toulon back home, by train first (which is where I'm writing from) and by car later.
So, I won't try to explain all of them (thanks God, I hear you saying...?! ).

Otoh, after a glorious end of the boating season onbard BA for the FDC 2013, I can already see some interesting winter debates at the horizon...

 

[Groot T]

hmmm... interesting replies here. At the risk of heavily over-simplifying this or being flamed for completely getting this wrong, I'm going to chip in to see if I have this right.

This is what I believe is the main reason for the fin being created, everything else, I would say, is a 'side benefit'.

In this Summary I will be avoiding the subjects of 'increased surface area for a given physical distanced protrusion from the hull' etc.

Perceived Problem
For lighter vessels the current fin solution produces a jerky feel which is, depending upon user experience, more unpleasant than natural roll experienced otherwise.

Why does this occur
To produce the necessary counteracting forces, the amount of energy require and the size of fin needed is too great for this size vessel.
Also, the rate of response isn't quick enough as the platform is it trying to stabilise is very twitchy due to its lighter weight.

Theoretical cures
The best theoretical cures are;
1) Create a force which is 90 degrees to the surface of the water
2) Increase the distance at which the force is exerted

Application of the above
1) Either create a recess on the hull which angles the fin is closer to parrellell to the surface of the water OR use the existing mount and curve the fin
2) Use an extender pole

Resulting practical solution
Curved fin


So, what have I missed?

Toby
currently at my desk, procrastinating like true young professional...


*** EDIT *** - By curving the fin, it looks as if the distance on the moment is increased due to the shift in vector direction. (Any physicists are about to cry over my wording here, much like you grammer nazis shouting at me for posting hear..)

 

[vas]

J,

I got the point of the curved fins re axle of boat movement and all that. Fine!
got a quick Q though:
how important is to have the fins as close as possible to the hull?
Reason is that a 20deg deadrise could easily hold a 40deg axled stab. Only obviously as the stab moves about it has to miss the hull so got to be 100-200mm from the hull at the free end of it. Shorter and deeper or trapezium sided fins could be a solution.
If you wish, my argument is that engineeringwise I'd go for either an articulated fin, or a flat fin properly mounted with the axle not vertical to the hull but the boat axle of movement...

I have to declare that the above were written whilst sober

cheers

V.

 

[grumpy_o_g]

So, taking this to it's logical conclusion, the optimum design would have a round bilge (as form stability would work against you at anchor in a beam sea) and a high aspect fin or two on the roll axis, ideally with the ballast as low as possible? If that wasn't sufficient you could always have some kind of stabiliser above the waterline, though it would obviously need to be lightweight and as large an area as possible. Something like this..?










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[Deleted User YDKXO]

As an aside, it's pretty obvious that the only meaningful number is the one which is missing, i.e. a comparison between vector fins and straight fins, rather than between vector fins and no stabs at all. But as we all know, marketing folks will never learn that someone can actually turn the brain on, before reading their claims...

Yeah tend to agree with that. Surely the efficiency improvement versus straight fins depends on the deadrise angle of the hull and the height of the roll axis so it is impossible to state any definitive improvement %. Yup, it would have been far more informative to see a comparison against a straight fin of the same area.
jfm, maybe I've missed it in previous posts but can you give any information as to why this precise curvature shape was chosen and whether it's the same shape across the range of vector fins?

 

[jfm]

Sorry slow reply. Been busy.

@MapisM I gotta say I found your "Do you feel a bit like the fat cat by now, J...?" a bit off. I have a boat with version N of some kit then the manufacturer announces version N+1. This happens all the time really, eg iPhones or whatever. You are poking fun, or discrediting my comments on N+1, or something, just because I have version N? I don't get it. Are you not getting the ball so going for the man? For the record, I'm very happy for there to be new gear that improves on my current gear, and I'll buy the new stuff as soon as possible. This is normal isn't it? I'm not off the mark in finding your er joke a bit weak - I have several unprompted messages/emails from readers who thought the same

The forces and vectors are really hard to describe in a forum post. I cut lots of corners in my explanation above. I'll try to explain a bit better/differently and respond to some of the points, but this really needs to be done in a pub with a sketchpad, so bear with me (or skip all this!)

1. Let's imagine a boat with a perfect roll axis bearing. Imagine the boat has a long steel bar running through it, through the transom, through the cabins, and sticking out at the bow. Along the roll axis. Imagine each end of the steel bar is set into rock with a ball bearing. Such a boat cannot sway or yaw or lift or fall or pitch; it can only rotate (=roll). Such a boat should have its stabiliser fins sized to reflect their radius from the roll axis and the boat's roll characteristics, and the fins should be mounted so their shafts point perpendicularly towards the roll axis (or, strictly speaking the fins can be mounted so their shafts are parallel to a radius drawn from the roll centre and at the same tangential distance; it's the same thing). With such a "perfect bearing" set up, flat fins can make a pure anti roll torque and there is no need for vector fins. All nice and easy

2. Now return to the reality of a real boat, where there is no steel bar set in rock at each end. The roll axis, the "bearing", is only the boat's inertia and the sea's viscosity, which is quite mushy (at least, in the horizontal sense - in the vertical sense the bearing is gravity+buoyancy, which is somewhat stiffer than the boat's inertia). Think of Nick H's cylindrical hull again. If you mounted the stab fins at 6 o'clock, on the keel, they produce a sideways vector which only works as a torque to the extent there exists a roll axis. If the boat were infinitely light and were in zero viscosity water, there would be no rotational "bearing" and the fins would move the boat sideways like a bowthruster/stern thruster combo. But a real boat has mass, and water is viscous, so a bearing exists (albeit mushy) and therefore the fins' force vector makes an antiroll torque. Not perfectly, because the bearing is mushy so some of the fins' energy is used to move the boat sideways rather than rotate it, but reasonably well.

3. So on Nick-H's cylindrical boat, the ideal place to mount the fins would be 3 o'clock and 9 o'clock. (or 4 o'clock and 10 o'clock, if still underwater). Then the forces that would shove the boat off its mushy roll axis bearing cancel each other out, leaving only a torque created by the fins about the roll centre. The 3 and 9 o'clock fins ( or 4 and 10 o'clock) make no attempt to shift the boat off its roll axis, and therefore the inertia of the boat and the viscosity of the water isn't needed to create a roll axis

4. But in the real world on planing hulls we have to mount our fins at sort of 5 o'clock and 7 o'clock, ie with the their shafts more vertical than horizontal. For practical reasons we have no choice - the deadrise is what it is. On a round bilge boat we might be able to mount them a bit more horizontal, eg at 4 o'clock and 8 o'clock, due to hull shape, but on a planing boat we don't have any choice. Therefore the horizontal component of the fin's force vector creates an anti roll torque only to the extent that the boat's inertia functions as a rotational bearing, and shoves the boat sideways to the extent the bearing is mushy and allows the boat to slide (and, by the way, both both fins act in unison in the horizontal direction, ie no cancelling-out effect). Some of the fin energy is wasted shoving the boat (yaw and sway), rather than rotating it.

5. The curved fins change the director of the force vector that the fins apply to the hull. With flat fins, the vector is parallel to the hull's deadrise. With a curved fin it isn't; it's more vertical. Therefore the boat is tricked into thinking the fins are mounted at 4 and 8 o'clock, when in reality they are mounted at 5 and 7 o'clock. If you resolve the forces, less of the fin work is done shoving the boat sideways, and more is done vertically where one fin lifts the hull and the other pulls it down, thus creating the rotational torque. So for same fin size, by effectively mounting them at 4 and 8 o'clock, you get more rotational torque AOTBE. This is basically the benefit of the new vector direction that you get with the curved fins as opposed to striahgt fins - and is a different way of explaining it in the limited confines of an internet forums. Sorry if it's still confusing and badly explained

6. You may prefer just to look at Sleipner's vector diagram, rather than the words above. Physicists and engineers will see it easily, but I think everyone can see intuitively that a change in direction of the anti-roll vector does indeed occur with the curved fins, and that the horizontal component of that vector (which is wasted energy, in yaw and sway, to the extent the roll axis slides sideways) is made smaller (ie the right-hand red arrow below is meaningfully shorter than the left hand one) while the vertical component (which doesn't get wasted in roll axis slide due to force cancellation plus buoyancy of the boat and gravity) is made larger

7. Separately from all the vector analysis above, there is the "scoop" effect thing. The "scoop" effect that the fin curvature produces is not the main event but it is interesting. The CFD models (run by CFD computational firms for Sleipner) show that the scoop effect improves the fin push by about 23% on the outward rotation, and the "convex" effect worsens it by about 8% in the other direction. But even on the convex stroke, the direction is improved - you can see intuitively that the fin pushes the water downwards somewhat. Because of this downwards push, the anti roll effect is still improved in net terms

8. MapisM and Deleted User, sure it would be better to compare curved fins with flat fins in a test. No-one disagrees with you on that, but the absence of such a test isn't a reason to bash Sleipner. Fact is, as of now there does not exist a pair of AOTBE boats one with flat and one with curved fins, so they can't do the test. No doubt one day it can be done. But CFD isn't exactly rubbish, and real at-sea data has been measured (by Princess and their test crew, not Sleipner; David King was on board supervising) on the Princess 56 that has the curved fins

9. @MapisM, and this is a sideshow discussion of course, but as a matter of Newtonian physics it is a fact I'm afraid that it makes no difference on a planing hull whether you mount the flat fins near the keel or near the chine (ignoring grounding risk, disturbed water flow to the props, etc, which makes you want them near the chine). Regardless of where you place the fin, the direction and magnitude of its force vector doesn't alter, and on a planing boat the vector's perpendicular distance from the roll axis remains the same. If you do not change at least one of direction, magnitude or lever arm, you haven't done anything, period. Ergo, it cannot make a difference where you mount it. In contrast, and interestingly, with a curved fin you do need want it near the chine. The force vector of the curved fin is not parallel to the deadrise (see diagram above) so as you move the fin away from the keel towards the chine you lengthen the lever arm - the complete opposite to what happens when the vector is parallel to the hull deadrise

10 @MapisM, the "% improvement" figures quoted by Sleipner are the anti roll work figures derived from the CFD modelling, not the degrees of roll. But no big deal on this point I think, unless I'm missing something? I have the CFD data for Predator 80, Sunseeker 28m and Princess 56 but the maths is h-e-a-v-y

11. @Vas, yes you could have straight/flat fins with their shafts not perpendicular to the hull surface, and avoid them clunking the hull by mounting them on long shafts. But the vortex losses at the top, or the drag of the winglet to reduce those losses, plus the drag of the shaft itself, would far outweigh the benefits. At high speeds say 20kts and above, the drag of a bare round shaft is generally higher than the drag when the fin is installed on the shaft; a round cross section is a very bad thing to drag through the sea!

In my view these curved fins are a really big deal. No extra hardware to be bought or dragged thru the water, no change to the complexity/failure risk of the hardware, yet a meaningful % improvement in performance, based on good old Newton. Feel free though to come up with/argue for different answers from the above if I have gone wrong. But keep it straightforward and objective MapisM, eh? .

 

[jfm]

Surely the efficiency improvement versus straight fins depends on the deadrise angle of the hull and the height of the roll axis so it is impossible to state any definitive improvement %.

Planing boats are kinda similar with hull sections (in the deadrise part) more horizontal than vertical. But yes, the exact % improvement (however you choose to quantify or define that) will be a bit different for each boat. I have looked at the CFD maths for 3 different boats and the % numbers are different, but all are a meaningful improvement.

jfm, maybe I've missed it in previous posts but can you give any information as to why this precise curvature shape was chosen and whether it's the same shape across the range of vector fins?

I don't have that data. The fin is parallel to the shaft at the top for the reasons already given. It is a smooth curve because edges/creases cause drag. The thing generally is a compromise between wanting more angle to alter the force vector direction more, but not so much angle that the effective surface area of the fin (measured looking along the direction of its movement) is reduced too much. I believe the shape of the curve is basically the same for all fin sizes but I don't have hard data on that

 

[jfm]

hmmm... interesting replies here. At the risk of heavily over-simplifying this or being flamed for completely getting this wrong, I'm going to chip in to see if I have this right.

This is what I believe is the main reason for the fin being created, everything else, I would say, is a 'side benefit'.

In this Summary I will be avoiding the subjects of 'increased surface area for a given physical distanced protrusion from the hull' etc.

Perceived Problem
For lighter vessels the current fin solution produces a jerky feel which is, depending upon user experience, more unpleasant than natural roll experienced otherwise. Yup. You feel it more in a lighter vessel because the anti roll force per tonne of boat is higher than in a big boat, but you still waste antiroll force even in a bigger boat, even if you feel it less
Why does this occur
To produce the necessary counteracting forces, the amount of energy require and the size of fin needed is too great for this size vessel.
Also, the rate of response isn't quick enough as the platform is it trying to stabilise is very twitchy due to its lighter weight. Well, rate of response is currently as good as it gets. 80-90 degrees per second is achieved with CMC electric fins plus the better brands of hydraulic. But the problem with the light twitchy boat is that as you apply faster/harder stabilisation, the thing twitches more, and having even faster fin actuation isn't going to help much there. What these curved fins do is change the direction of the force, reducing the horizontal component that creates the twitchiness, and that is a big deal
Theoretical cures
The best theoretical cures are;
1) Create a force which is as close as possible to 90 degrees to the surface of the water Yes! You got it. Even light twitchy boats have a lot of "roll axis bearing stiffness" in the vertical direction!2) Increase the distance at which the force is exerted

Application of the above
1) Either create a recess on the hull which angles the fin is closer to parrellell to the surface of the water OR use the existing mount and curve the fin Good theory, but not practical!
2) Use an extender pole Good theoretical concept but doesn't work for reasons given above (drag)

Resulting practical solution
Curved fin Yes. It's one of those very simple but brilliant ideas


So, what have I missed?

Toby
currently at my desk, procrastinating like true young professional...

*** EDIT *** - By curving the fin, it looks as if the distance on the moment is increased due to the shift in vector direction. (Any physicists are about to cry over my wording here, much like you grammer nazis shouting at me for posting hear..) Yes, exactly

Makes good sense Toby. I added a few thoughts in colour...

 

[Deleted User YDKXO]

jfm, I wasn't questioning the physics which I think is easy to understand and I accept all of what you say above. It was the 50% improvement figure I was questioning because this does depend on the deadrise and the height of the roll axis and I don't think its right that Sleipner quote this figure as a general statement. And thats why I said it would have been helpful to have an actual comparison against flat fins. Overall though I agree with you, the vector fins should offer a significant improvement over flat fins. I'm still intrigued though why Sleipner chose the exact curvature of the vector fins that they have

 

[Deleted User YDKXO]

jfm, I wasn't questioning the physics which I think is easy to understand and I accept all of what you say above. It was the 50% improvement figure I was questioning because this does depend on the deadrise and the height of the roll axis and I don't think its right that Sleipner quote this figure as a general statement. And thats why I said it would have been helpful to have an actual comparison against flat fins. Overall though I agree with you, the vector fins should offer a significant improvement over flat fins. I'm still intrigued though why Sleipner chose the exact curvature of the vector fins that they have

Ooops sorry we were both typing at the same time and you clicked post first. Thanks for your answers above

 

[jfm]

Ooops sorry we were both typing at the same time and you clicked post first. Thanks for your answers above

Hi Deleted User
The "50% improvement" numbers are the roll impulse. You can debate what is the perfect measure, but at least this one is scientifically sound, avoids subjectivity, and it does broadly correlate to how stabilised the boat feels (AOTBE, including effectiveness of the timing of the stabilisation impulse relative to the wave rolling the boat)

When I say roll impulse, I mean, dimensionally, Newton Metres Seconds. It is the anti roll torque at each instant (in newton metres) multiplied by the time period (in seconds) for which the torque is exerted. It is therefore a quantity that is dimensionally similar to spin energy, as distinct from spin force or spin power, if you get my drift. (I'm being a bit loose in my physics just in that last sentence; physicists please don't flame me)

As you say, each boat is different. If you put a single spec/set up of stab onto lots of different boats you'd get some lousy results of course. But if you adjust the stabiliser sizing, power, and settings to deal with the boat differences, you do get quite similar results. Now, as you say, deadrise is important and I think the real-world bots for which Sleipner had had CFD done happen to have similar deadrise. But they were 2 sunseekers plus a Princess and I think (but will stand corrected) that there aren't many mainstream boats with steeper deadrise are there? There are plenty of flattie hull boats with less deadrise, but they will see MORE than 50% improvement by this measure, not less. So I reckon the "50%" isn't to much wide of the mark.

If you can stand some more numbers, from the CFD modelling (which is b-i-g stuff these days - the computers run all night) the % improvement from curved fins over straight fins is for the Princess 56 computed at 53%. Now, what exactly is that 53%? It is assumes the following parameters, which or course you can vary but the ones chosen seem sensible. The parameters chosen are that you stroke a pair of 1ms fins 2x 37 degrees (ie a 74 degree sweep about the centre) in one second, which is quite active by the way, rather similar BartW's fins in the FDC video posted today, though with Sleipner fins you would need a much rollier anchorage than the one in the video to make them flap as hard as the small CMCs were flapping. The rotational speed of the fins followed a cosine curve. The CFD results show about 4000NmS of impulse from flat fins, and 6000NmS for curved. I've rounded the numbers in case they are sensitive to Sleipner, but the delta is 53%. If you apply the same CFD to 28m and 80 pred, you get 47% and 49% in NmS difference. Sure you'd get different % values for each different model of boat, and you'd get lower % values in a boat with steeper deadrise than the 3 I mentioned, but I don't think many mainstream boats have steeper deadrise than these. So Sleipner's claims of "some 50%" and "40-50%" are fair claims, I'd suggest, unless you disagree strongly with the impulse in NmS as a sensible thing to compute

Of course people might prefer other things than "energy impulse" in NmS as the right measure. Feel free to suggest others. Roll angle would be ok maybe, though obviously it can only be measured as it depends on the actual waves; it isn't sensibly computable unless you assume a wave form. It will obviously be will be directionally the same as NmS. You could use torgue applied to the boat, but that varies across the 1-1.5 seconds of fin sweep and so you might have to compare max torque, but that would get distorted if the torque curves are different shapes. FWIW the CFD says the max torque extorted by the curved fin pair 1msq is also about 50% higher than the same size straight fins, though of course at the end of the stroke when all fins work AGAINST you (ie encourage the boat to roll) they work more against you too. This is why the impulse, ie torque x time, including the negative part at the end of the stroke, seems a better measure

Sorry for long answer. Boring you to death maybe!

 

[jfm]

I'm still intrigued though why Sleipner chose the exact curvature of the vector fins that they have

I believe that lots of factors went into the mix, even once you decide to have a curve and a perpendicular part closest to the hull. They basically blended all the following, with much calculating rather than seat-of-pants guesswork:

1.As much vector effect as possible, of course
2.Avoiding cavitation/hole in the water when angled out hard underway and fast, "scooping" the water
3. Avoiding so much curvature that you get foil riding at speed especially on boats with steep deadrise

 

[Deleted User YDKXO]

Thanks jfm, thats a very comprehensive explanation. If you fit these new vector fins on Match 2 it will be interesting to see if subjectively you feel that extra 50%. Btw, do these curved fins cause any extra drag compared to flat ones?

 

[jfm]

Thanks jfm, thats a very comprehensive explanation. If you fit these new vector fins on Match 2 it will be interesting to see if subjectively you feel that extra 50%. Btw, do these curved fins cause any extra drag compared to flat ones?

Yup. There are some tech issues in retrofitting them, so I don't know yet if that will happen. Sleipner are thinking about it. If I did do it, it would be nice somehow to get comparative data but of course you wont have the same waves one week apart so how can you get true comparative data, except by having two boats anchored side by side Tech issue in retrofit is whether I need to change the actuator too, or whether a kind of adaptor can be made to fit curved fins to my existing actuators. Something I haven't mentioned yet (because this thread is already technically heavy!) is that the new curved fins attach to the boat not by the usual method of a nut tightening the thing onto a tapered shaft, but by pumping hydraulic pressure into the centre of the shaft then sealing it off. It is well established big ship/oil rig etc technology

Overall there is an intended drag reduction in that you can have say 25% more NmS of stab impulse (rather than the full 50%) with a smaller fin, say a 0.8 rather than a 1.0 m sq. But if you choose to stick with identical size fin I do not have data but would expect almost identical drag, as the frontal area is same and the bend ought not to do anything significant. That's just an intuitive guess though

 

[vas]

Overall there is an intended drag reduction in that you can have say 25% more NmS of stab impulse (rather than the full 50%) with a smaller fin, say a 0.8 rather than a 1.0 m sq. But if you choose to stick with identical size fin I do not have data but would expect almost identical drag, as the frontal area is same and the bend ought not to do anything significant. That's just an intuitive guess though

J,

did a quick read of all the v.interesting info you're pumping us with, quick convoluted/complex Q:

assuming M2 is performing OK with current stabs, new improved curved jobbies will help in:

• even stabler boat
• reduced linear speed of stabs as they move
• reduced size of fins ==less drag on planning speeds
• reduced size of all anchilaries == less fuel to burn when running them
• less noise (possibly)

would you be as kind as adding %ages next to each bullet Yes I understand that some of them can be either/or but you get the point...

trying to understand a bit more the practicalities of them as it stands.

Further, I feel that on D hulls these curved ones would feel more at home (less drag on fwd movement, curved hull vs curved stabs, etc)

cheers

V.

 

[MapisM]

@MapisM I gotta say I found your "Do you feel a bit like the fat cat by now, J...?" a bit off. I have a boat with version N of some kit then the manufacturer announces version N+1. This happens all the time really, eg iPhones or whatever. You are poking fun, or discrediting my comments on N+1, or something, just because I have version N? I don't get it. Are you not getting the ball so going for the man? For the record, I'm very happy for there to be new gear that improves on my current gear, and I'll buy the new stuff as soon as possible. This is normal isn't it? I'm not off the mark in finding your er joke a bit weak - I have several unprompted messages/emails from readers who thought the same

Well, maybe those readers don't remember when you tagged as "marketing BS" the CMC claims on fins size and motors heat.
Claims that by now some of us have had the opportunity to try and test. And you weren't even joking.
OTOH, I note that by now you feel obliged to specify to Robg71 that Sleipner "will have hydraulic actuation only for foreseeable future".
I take it as a subtle way to actually admit the electric stabs superiority for small boats installations...
Superiority which is so obvious that it's not even worth explaining it, not just for retrofits, but also for new builds.
Besides, you and those readers will forgive me if I couldn't help thinking about the Garfield strip, when I opened the document which you pointed to us as "Detailed info", and found this ad in the first page:

All that said, a joke it very obviously was, nothing else - as you also recognised.
Weak as it might have been, of course (but is that a pro or a con, in a joke?), though I actually find that strip very representative of the world we live in, where words often matter more than facts.
Anyway, if eventually the joke came through as rude I apologise, but you know perfectly that it wasn't my intention, for at least a couple of reasons:

1) I never had any doubt about the effectiveness of your N version with straight fins, and if there's anything I'm poking fun at, it's the claim that a further 50% improvement can be achieved at all - let alone by just bending the fins.
Just think about it: you always praised the effectiveness of your stabs in cancelling just about any perceivable rolling motion.
I never questioned that, because I had already experienced the same with other boats.
In fact, I can now confirm that this is 100% true also with the (supposedly undersized) equipment installed on BA.
So, by all means I never considered Match "old and inferior" in this respect.
I'm instead absolutely convinced of the opposite, because it beggars belief that such already fantastic results can be further improved, let alone by 50%, and no matter how.
Unless we consider putting the boat on the hard, as the next step of seasickness cure...

2) aside from the joke, in all my other comments I tried to be as factual as I could, based on the information available.

In fact, TBH I see more reasons to find your recommendation below a bit off, rather than my previous joke...
I'd be curious to hear on which point(s) exactly you're accusing me to have not been objective, in fact.

free though to come up with/argue for different answers from the above if I have gone wrong. But keep it straightforward and objective MapisM, eh?

Anyway, since you asked, here's a few other comments:

Your points 1 to 6: no objections on the principles.
It's the relevancy of the "side effects" which simply isn't reflected at all in my practical experience.
Under no circumstances, either at anchor or under way, I ever had the faintest perception of fins-driven sway and jaw effects.
Now, let's forget jaw. IIRC we already discussed that, and I maintain that in a proper installation it's totally negligible even in theory, not only in practice.
Apropos, didn't Sleipner also pretend at some stage that stabs could be more effective if integrated with rudders control? Maybe they're keeping that for yet another "new and improved" release?
See? I just can't resist a temptation for a weak joke, sorry 'bout that...
For sway, while I accept that in principle the effect could be there, I never felt it while onboard a stabbed boat. Not one iota.
In fact, just think of how much power it takes, running both bow and stern thruster together, to start a sideways movement of the whole boat.
And then compare that with the stabs power, considering also that as a mean of transferring power to the water for a sideway movement, fins are obviously less efficient than thrusters.
Even if the fins would be placed vertically along the keel, one at the bow and the other astern, the sideways movement of the whole boat that they could generate with a single sweep is just a tiny fraction of what the thrusters can create in the same timeframe.
And I very much doubt that in such short timeframe, even the thrusters could actually begin moving the hull sideways.
In fact, the only occasion where I've ever been able to perceive some sort of sway effect is while playing (a bit too hard) with the joystick on an IPS boat.
But that's understandable, considering the power involved.
Bottom line: in which planet and with which boat did Sleipner practically experience the side effects that vector fins are supposed to cure?!?
Can you honestly tell to have ever noticed anything like that with Match, in the even slightest way?

Point 7:
Obviously I can only take your words for the results of CFD models, but it's hard to think of any logic reason why the scoop effect shouldn't be symmetrical.
And also the 23% improvement on the "positive" side sounds like a helluva lot, also because the curvature is along the height, not the length of the fin.
Makes me wonder why oars blades (which btw would exploit only the "positive" side) are not made with the same curvature...

Point 8:
"the absence of such a test isn't a reason to bash Sleipner"
Well, of course it is. I mean, not the test absence as such. But if they don't have two AOTBE boats to make the test, they should refrain from making claims on improvement of curved vs. straight, at least till they will have such tests.

Point 9:
I already acknowledged that the vector is unaffected by the distance from the keel, on a P hull. No need to ask Newton an opinion on that.
But ideally, the fins shafts should still be pointing as much as possible towards the roll axis, shouldn't they? You already acknowledged that in your post #28, as I understood.
And by moving the fins towards the keel, there must be a point where the shafts will point exactly towards the roll axis, obviously.
Otoh, just bring this to the extreme: don't you agree that if the fins would be placed along the keel, straight vertical fins would be more effective than any curved fins?

Point 10:
the "% improvement" figures quoted by Sleipner are the anti roll work figures derived from the CFD modelling, not the degrees of roll
Well, I'm afraid this is a confirmation of what I'm saying, i.e that these numbers, as they are used in the context of the leaflet, are nothing else than a prime example of marketing hype, being rather aimed at shouting "new and improved", than at informing the reader about something he/she can actually understand...

 

[rafiki_]

At the risk of extending the debate, which I have observed with interest, the best way to prove out improvements is to take objective measurements of yaw, pitch and roll. This needs a rate gyro platform which Sleipner might have to tune their devices? Comfort is measured in Vibration Dosage Value (VDV), expressed in m/s(-3). Any takers?

 

[Groot T]

If I did do it, it would be nice somehow to get comparative data but of course you wont have the same waves one week apart so how can you get true comparative data, except by having two boats anchored side by side

Crazy thought, if you did swop to the curved, would it be worth getting in touch with the owners of Match 1?
Or a less Crazy thought, just put Match 3 next to Match 2...