Passive fin stabilisation of fast catamaran

I'd like to get a design consideration study going for passive fin stabilisation of fast catamaran project that is currently in build.

Things I'd like to discuss about implementing this include,
1. Engineering - required strength calculations, damage survivability, surrounding structure design.
2. Ship Dynamic stability calculation, fin sizes and locations, number of fins etc
3. Any other relevant considerations.

So to get things started, how would you go about initial design layout and defining the initial problem. The design goal is simple- to minimise pitch heave and roll of fast catamaran using passive fins whilst underway.

The first embodiment envisaged, is horizontal fins protruding inwards into the tunnel area, from the lowest practical part of the forward hull area. The fins would look similar to a horizontal rudder and attached by way of a weak link in case of impact so that they break away instead of tearing a hole in the hull.

Some things I'd like help with; how to determine optimal fin area and aspect ratio based on initial ship stability parameters.
Any links to similar concepts would also be appreciated.

 

Groper


This is an admirable task. But sadly it is also fraught with much frustration too. Since to affect the motions (simplistically, to get you started) you have to do several things:-

1) Change the weight of the vessel
2) Change the waterplane area
3) Appendages.

1 and 2 is clearly out. Thus using appendages what can you get? Firstly the actual periods of motion of your vessel would need to be determined, to ascertain the amount of change you could expect. Without model testing not so easy; but you can use simplified formulae with some known constants to arrive at a “best guess”.

Below is some model testing we have done. As can be seen appendages around the bow do not make a significant effect on the motions. The reasons for this is that these were optimised for a higher speed vessel, i.e minimise drag. Hence the appendages are small. Otherwise the drag is so large the power required to achieve service speed becomes a little silly.



Does this mean it can’t be done. No of course it can. But you need to understand the effects. Since all you can do is dampen the motions. But this requires large plan form areas to counter the very stiff restoring moment on a catamaran. Yup…large = drag. Since appendages, and its size effects the added mass…which ties into #1 above.

So Firstly are you willing to accept a reduction in speed for the dampening?

There are several good paper that may assist in your quest, one here and the other is “Ocean catamaran seakeeping design, based on the experiences of USNS Hayes” by Hadler et al, SNAME 1974.

 

Groper

Here is some simple analysis to get an understanding.

Take rolling, a simplified equation for period of roll is thus:


T = 2π.kxx
√(g.GMT)


The GM(T) is proportional to the BM

BM = I/V

I = waterplane inertia
V = volume

Take a 30m cat of displacement say 90 tonne and has a nominal BM = 10.0m and a typical roll period of 2.7 seconds (this is from actual data). To reduce the BM, the I (WPA) must be decreased or V (displacement) must be increased.

If the displacement is doubled, to 180 tonne, an increase in V, what is the BM? BM = 6.1m. The GMT is 4.0m the resulting change in roll period is only from 2.7sec to 3.7sec.

If the I is halved, the BM = 5.0m and GMT is 2.53m & T = 4.7sec. So changing the I, or WPA, has a major effect.

The kxx term is the mass + added mass moment of inertia. In general terms the added mass is around 80% of the vessel. So if we increase that…well, to change the rolling period from 2.7s to say 4.7s (to match the decrease in the I), means a change of 174% in the Kxx term. In other words, increasing the value of the added mass to be almost double the actual mass for the vessel. This would equate to very very large plan form area to create such an effect. The simple way to do this, as has been done (and we have also before) is to have a foil that spans between the two hulls.

This is the only way to significantly increase the added mass. But a foil that spans between the hulls needs to be

1) structurally sound
2) minimum drag
3) located corrected to ensure no synchronous pitch-heave coupling resonance.

Thus more food for thought.

 

Is this with your build thread boat ? Surely you want to trial it without these appendages first ?

 

Adhoc - thanks for the input. Any other considerations as far as active stabilization? I was under the impression that the benefits of going with active control were not all that significant compared to passive on high speed vessels?

To clarify - yes im willing to sacrifice some speed for the dampening, i did not expect a free lunch in terms of resistance and having extra appendages in the water. Also, the primary effect sought is a reduction in pitching - not so much roll - as the effects would be small on such a beamy vessel.

Also, materials for this foil will be carbon fiber composite. The shape of said foils can be "ideal" and not restricted by building methods associated with metal etc. So the foils can be tapered in both thickness and plan form area for optimal span wise loading and minimum drag. Ideally, implementing a full tunnel span foil may not be practically achievable for this particular catamaran as the separation between hulls is proportionately larger than for a typical power catamaran. ie its quite a beamy boat, and results in a very long foil - thus the twin or quad cantilevered configuration is envisaged.

Thanks for the data - seems it made little difference in that particular configuration. Can you advise what the plan form area and aspect ratio of the foils VS displacement for that vessel were?

Mr E - these foils will definitely be removable and not permanent in case they are more of a hindrance than a help...

 

Quite the opposite is the case..which is why so many high-speed catamarans have active motion damping systems. The difference in results achieved comparing active versus passive solutions is huge.

Any passive solution (foil/fin..bilge keels) provides damping only when the motion you want to dampen is actually occurring to a significant degree. A passive bow foil is not going to resist pitch rate unless a lot off pitch rate is occurring. In addition, the passive appendages must be large to produce any measureable difference.

Active fins/foils/trim tabs/interceptors, on the other hand, produce appreciable damping forces opposing undesirable motion, almost before that motion even begins to occur. The feedback that drives the devices is based on vessel angular velocities and accelerations, which are sensed at very low threshold values by modern motion control systems, but the force produced by the active appendages is largely dependent only on vessel speed.

Are passive solutions better than nothing for application on lightly damped hulls that tend to exhibit resonant motions?...they can be, yes. But a compact and appropriately selected active solution will produce results that are an order of magnitude better.

 

The cost ( active stabilization ) would be prohibitive, would it not ?

 

Ok thanks bill. Perhaps i should start a new thread then if not continue to continue it here?

I had originally planned an active system, but i got thrown off the trail in reading about roll stabilizers for recreational cruiser type vessels where the passive fins performance were considered quite good provided the speeds were high enough.

So in considering an active system primarily for pitch damping, is the added complexity and cost involved with 2 pairs of controlled appendages, fore and aft, that much better than what can be achieved with a single pair of appropriately size forward appendages?

I guess what im getting at is - trying to define the appropriate configuration of foils for this vessel so as to achieve maximum pitch damping performance for minimum cost.

 

Aaahh...isn't everyone??

For an active type..Bill's your man to sort you out

 

??? What then explains the fact that nearly all fast passenger catamaran over ~ 35m in length and operating on routes with other than routinely calm conditions are equipped with at least some active motion damping equipment? On many, the suite of motion damping equipment runs in to the millions of $US. No owners or operators I've ever encountered are keen to install expensive kit that they do not need or that does not work well and contribute to the bottom line.

The simple fact of the matter is that, lacking effective active motion damping systems from Sea State (Austal), Naiad Dynamics (ex MDI), Island Engineering, etc, the large(er) fast passenger catamaran would never have survived it's early gestation period as a viable transportation vehicle.

 

The approx. size and speed and speed of your proposed design is important to know before I could say anything quantitative. Resonant pitch is almost always the problematic response of catamarans, yes, although any active system designed primarily to dampen the obnoxious pitch motions will dampen roll motions as well.

Active kits have included forward foils only (as on the very fast cats Ad Hoc designed and built some years ago for Hong Kong), aft trim tabs or interceptors only (the lowest cost solution ...there are more cats out there running around with only active trim tabs or interceptor systems than any other combination) and, for top notch performance, the wiggly bits on both ends. Most of the Austal, Incat and other "big cats" in the 70-90m range (including JHSV, TSV, HSV, USN X-craft) have the full monty, and in some cases also include appendages for active yaw damping too.

 

I mean in this particular instance, of course, this boat is only 10 or 11 metres. A toy compared to the boats you are referring to.

 

Oh. Quite true. And I read through the thread again and didn't see the length identified.

We've found that the typical threshold for the current range of active solutions is roughly 30m. There are some exceptions, of course; we have systems installed on yacht cats as small as 22-m.

 

OK, apologies for assuming you had noticed his build thread.

 

Let's not get ahead of ourselves here. Smaller boats have much larger motions, so I'd argue that there is a greater need for such a system on small boats! Cost being the only prohibitory reason- well that's not a concern here...

Mr E- I'd kindly appreciate it if you'd mind your own business in this matter. You have no idea where I'm going with this, the information I seek doesn't affect you so what business do you have here?

Bill, at this point I should inform you that I have my own design for an active system which I would like to test on the 11m composite cat I'm building. In this day and age, the solid state gyros, accelerometers, PD controllers, servo motors and actuators are so cheap, it's an absolute joke on how low cost I can implement this system. Hell, even the processing power of my new mobile phone exceeds that of the PC I do my 3D design work on!

Despite me having a design which I'm confident will work, I do not know enough about this field to optimise it - it could no doubt be made better with some help from more knowledgable folks like you.

More about what I've already got...
The servo is actuated by processor which is given feedback from both a solid state gyro and accelerometer. I get a PWM signal output to the servo motors based on an algorithm derived from both angular rate from the gyro and also accelerometer g's which seems to have good progressive response. I've set limits based on papers I've read for the forward foils of +12deg -8deg which seems realistic. They can operate independently as the algorithm is setup for both pitch and roll, and the response times are extremely quick.

The physical design I've envisaged thus far is quite simple and based on successful rudder designs I've seen in other boats. The foil which is similar to a rudder assembly is mounted horizontally down near the keel. The foil has an internal shaft which runs inside a bearing tube inside the hull. The bearing tube is glassed to a bulkhead, bulkhead glassed into the surrounding hull. The bearing tube is a wet tube, water is allowed to lubricate it, and the tiller head on the end of the foil shaft protrudes upwards vertically inside an fore/aft elongated cassette which rises above the waterline internally. The top of this cassette is open inside the boat and allows the tiller head a range of fore/aft movement. Here the connection is made to the actuator, perhaps a linear actuator inside the hull, neatly above the waterline with easy access to everything inside the boat. In the event of severe impact, the composite shaft breaks off flush with the hull- I've verified this works in real world testing, the shaft is strong enough to take the design foil loadings, but not strong enough to survive impact. The surrounding hull structure is much stronger again.

I would also like to incorporate interceptors on the transoms, however I'm not sure it's possible as I would like to run outboard motors. The flow from the interceptors down would likely cause cavitation problems for the motors unless they were deliberately mounted say 2 inches lower, I'm still not sure there would be no problems however? Failing this, duplicating the forward foils in the aft part of the vessel is also a possibility.