# Passive fin stabilisation of fast catamaran

Discussion in 'Stability' started by groper, Jul 22, 2015.

1. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

2 and 3.

I didn't use tan 5 deg * 10.6...

I used tan 5 * 6.3 which is the the distance from FP to LCF. This equates to drawing 3. Tan5 * 6.3m = 550mm bow displacement.

Then in the Other equation for the moment, trim arm, and MCT, I used double this value which is 1100mm. - drawing number 2.

2. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

The trim of 5 degrees over the Lwl assumes a total trim of the vessel. That is from its lowest position to its highest. Hence the first image. However does the vessel trim about the stern, 4th image…no, about the LCF…hence the second image, and third image.

So the total trim, at 5 degrees can be seen in the first image over the vessel’s length, but then corrected for the centre of trimming/rotation about the LCF. Thus the fwd trim is circa 550mm.

This is a pitch up of 5 degrees.

By similar triangles,

The total trim, for 5 degrees is tan 5x10.6= 927mm.

Trim(fwd)/6.36 = Trim(total)/10.6 ==> since the trim about the LCF and the LCF is 4.24m fwd of the transom.

So the trim fwd = 556mm.

Therefore trim fwd = 556 and trim aft = 927 – 556 = 371mm.

Your TOTAL trim is the “trim” result, in the 92.7cm = w.d/MCT

Thus you can either view this total trim as a pitch up and down of ± 2.5 degrees at the bow, of one cycle…or a single ½ amplitude pitch up of 5 degrees.

3. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

ah yes - as the LCF is not dead midship, that is where the difference in 1100mm to your 927mm - yours being correct of course

Now thats all cleared up - back to the pressing matters...
So if a vessel will pitch +-5degrees, more in a syncronous sea state condition, still what is a reasonable set of design constraints?

From a practical view point - implementing 4 foils of 0.2sqm and the associated loads seems quite feasible. The associated forces delivered @ 23kts will produce a longitudinal vessel trim change of approx +-3 degrees. To me this sounds like a very favourable outcome and something worth achieving, especially if it can be done myself for a very low cost. The cost vs benefit to the vessel seakeeping looks great. Clearly this is a very different scenario if i had to pay big dollars to a commercial outfit which was supplying the stabilization system to me - id likely choose to keep my pennies in the bank and put up with the pitching, or simply run the vessel at a different heading and or slower speed.

I guess next up i will look at the drag from the foils and get an idea of the increased resistance...

4. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

Well, that’s for you to decide, it’s your boat!

BUT..before you get carried away…as I noted previously, these are very rough crude simplistic numbers to give you an idea. Now you have the idea..what else is there to consider..lots…as Bill will tell you.

Firstly…since the centre of pitch is aft, the lever is much less than up fwd. So, to maximise your restoring moment, it is better to have your fin area up fwd. You don’t see many fins aft, for pitch control!

Secondly.. if you were to place a wave profile..the max wave that your vessel is likely to experience…on top your hull what will occur? Well, if you draw the wave profile how much buoyancy does the hull provide?...since this relates to the exciting moment produced by the waves. (Which relates to the resonance aspect). The amount of buoyancy that the hull can provide for a section of hull up fwd, you’ll discover is not much, especially with such a fine angle of entry bow/hull.

Thus, whilst your fins may provide a given lift force….as the simple calculations have shown…the hull can only react to so much. So does this mean you don’t need as much fin area?....nope….this is where it gets complicated. Since the hull can only react to so much of the wave profile..what else occurs….yup…heave. The vessel shall heave as well as pitch. In other words, as a wave passes your vessel, it shall pitch and heave. And looking at your hydro’s..there is a couple of roughly 0.3x displacement. And this moment needs to be taken care of too…but to make matters worse…there is still the heave to consider. You have only focused on pitch control…but your vessel will heave, whether you want it to or not.

So you shall also require heave control as well as the coupled pitch-heave…..and since the amount of buoyancy a fwd section of the hull can only provide is so much..the vessel shall heave an appreciable vertical displacement as the hull follows the wave profile. This is where the resonance and wave excitation comes in for each axis of translation and rotation.

And this is where it begins to get more complicated… as I’m sure Bill will roll his eyes here

5. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

yes i know... the last paper i read is this one - View attachment 2012063001.pdf

It talks about the pitch and heave couple and equations to deal with it in a way that is subtractive rather than additive (decoupling) as far as i understand it.

Some of the equations they show do not make sense to me as the symbols they use are unfamiliar to me. Dont suppose you can help me make sense of it?

Regardless - in the stabilization algorythms running in the CPU, the accelerometers should sense the heave and take care of it. The algorythms run by the processor have simultaneous PID control for 3DOF - pitch, roll and heave. It computes the 4channel output based on this and can move all 4 appendages independantly. I could add yaw but i dont really see the need at this point...

6. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

There are 2 ways.

1) See top left of page 3, all you need is there.
2) If that doesn't satisfy, then to get a better grasp you'll need to do the same as most NAs, and study the hydrodynamics of seakeeping which can be anything from 6 months to 1 year of study. It is very complex.

I gave a rough intro to it here.

You are basically trying to establish the hydrodynamic coefficients numerically. Viz:

Hope your maths is as good as Leo's....i could do analysis this 30 years ago when at Uni..not any more. I struggle with 2 + 2

7. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

Stuff that, at this point I think my time is better Directed at model testing...

The computer algorithms are already setup for 6dof control. We only need to utilise 3 of those and they should work reasonably well provided I can position the foils so that each foil generates equal moments about the LCF. I can tweak the propotionate integral and derivative gains on each channel until I get the best possible corrective outputs.

In other words, trial and error

8. Joined: Mar 2007
Posts: 967
Likes: 45, Points: 28, Legacy Rep: 361
Location: Maryland

### BMcFSenior Member

The cost of the control electronics side has become much lower over the years, certainly. Within a 1.5 million USD motion stabilization package for, say, a 75m cat, the entire electronics package might be \$20K total.

I've never attempted active interceptors with outboards directly behind them. The variation in local water surface height aft of the interceptor could indeed be problematic for an outboard engine. Same would be the case for trim tabs.

We've successfully built and tested a number of small all-electric servo motion stabilization systems for models, manned and, mostly, unmanned. We never tried to turn one of those in to a commercial product though. Your approach overall seems sound and well planned out. Good luck with it.

9. Joined: Dec 2015
Posts: 13
Likes: 0, Points: 1, Legacy Rep: 10
Location: Western Australia

### GivitagoJunior Member

Ive been searching for a topic just of this nature "active pitch control assist foils"for hi speed catamarans.
Great reading this thread but when the formulas start flying my eyes glaze over. I'm a trail an error man myself.

I too have a concept design to affect the same results, using a different simplified process. But i have no means of developing said concept. I would like to share while retaining some sort of intellectual rights if you are interested. I also need input from people like yourself s to prove any validity in the concept.

10. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

You will need to have some degree of maths ability and back ground in hydrodynamics to understand better how to manipulate and effect the motions.

11. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

The concept as i envisaged it, is nothing new and with no intellectual rights to protect - therefore i have no issue sharing the idea on the internet.

The idea is simple, 4 foils, basically L foils as seen on americas cup foiling sailing catamarans, moved via high torque DC motor actuators, controlled via a very simple feedback loop based around PID algorithms. The feedback is received from accelerometers (rate of change) and solid state gyros (actual trim), and compared with a set point which is equal to the running trim of the vessel in calm water. Any deviation from the set point provides a rate of change and a magnitude direction direction in 3 axis - roll, pitch and heave. This is used in the PID controller to calculate the appropriate magnitude of control actuation in order to correct the difference between the actual trim and the neutral running trim.The PID algorythm gains are tuned whilst at sea until a satisfactory response is achieved. This is the same methodology that a multitude of automated industrial processes use to control everything from brewing beer, distilling whiskey, mineral extraction, etc the list is almost endless. The PID algorthym simply compares a set point to actual live data points, and applies the changes which need to be made in a way that restores the set point quickly, and without overshoot or oscillations when the gains are tuned well.

https://en.wikipedia.org/wiki/PID_controller

12. Joined: Mar 2007
Posts: 967
Likes: 45, Points: 28, Legacy Rep: 361
Location: Maryland

### BMcFSenior Member

What size and speed range of catamarans are you proposing this system for?

13. Joined: Jun 2011
Posts: 2,467
Likes: 123, Points: 73, Legacy Rep: 693
Location: australia

### groperSenior Member

Not sure if you are referring to me bill? But if so, it's just my little recreational craft of 11m and 4 tonnes max design displacement. Design cruise speed is 20-25kts...

14. Joined: Dec 2015
Posts: 13
Likes: 0, Points: 1, Legacy Rep: 10
Location: Western Australia

### GivitagoJunior Member

Groper ive seen your 10m build looks amazing nice work.
I do understand the electronics involved and my system relies on the same. I work in the subsea industry so have a good understanding of these systems and hydrodynamics."good understanding meaning i am operator not developer." and yes my eyes still glaze at the in-depth mathematics involved.

What i would like to protect is the mechanics of the concept where metal meets water so to speak.
Ad hoc from reading this is your area of specialist. a great painter may not know how the chemistry to make paint but use it very well.

I apologize for being so coy about the concept but i need a great mind to prove the idea. this i need help with without losing any possible rights if to be a successful project. this is the pickle im in.

What i can say, it is a modular design built into a removed section of hull, removable fail-safe. hydraulically driven, insusceptible to weed or foreign objects. can be mounted fwd alone or fwd & aft depending on hull design and size. my thoughts originally were for trail-able outboard powered vessels but i belive can be up scaled to any size.

15. Joined: Mar 2007
Posts: 967
Likes: 45, Points: 28, Legacy Rep: 361
Location: Maryland

### BMcFSenior Member

An interesting market "niche", that. I'm not aware of anyone yet introducing a ride control system for that small of a vessel. I'm sure it's not far away though..as I noted in earlier post, we've built some systems for even smaller vessels, but they were "one off" packages for scale models; either tow tank or free-running. The technology is well in hand and the costs for the control hardware and electronics keeps coming down.

Last edited: Dec 11, 2015
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.