Cheap Seakeeper 5 for small boats launched....

[MapisM]

Ballpark 200 amps at 12v DC and 100Amps at 24v DC.

I stand corrected.
In my post #12, I said 300A because for some reason I considered 3kW @ 230V AC, after quoting rustybarge question mentioning 2kW...
And I even pulled his leg about A level electrical class, doh!

 

[rustybarge]

Balmer (very popular with raggies) might be a little cheaper - I'd think you would be looking at a lot less than a grand for 150A alternators based on the prices of their 90A and 110A bolt-on replacements aimed at yachts that want to charge at low rpm preferably. Generally regarded as good quality too. Going back a few years a 110A cost a few hundred at most. Remember that wiring may well need beefing up too if you upgrade alternators.

Wow; these babies put out 310 amps each! http://www.balmar.net/98-series.html

jfm's little 150amp alternators are whimps.

 

[rustybarge]

I stand corrected.
In my post #12, I said 300A because for some reason I considered 3kW @ 230V AC, after quoting rustybarge question mentioning 2kW...
And I even pulled his leg about A level electrical class, doh!

Electrickery is complicated; even more so with those damn chip thingies. Bring back dynamo's and valves I say. :nonchalance:

 

[jfm]

Wow; these babies put out 310 amps each! http://www.balmar.net/98-series.html

jfm's little 150amp alternators are whimps.

Tee hee! The Balmars look very nice but you have to compare like with like to get a sensible answer. 310 amps is at a mere 12v, where AOTBE the tension in the V belt is 50% of the tension at 24v. There is only so much power you can put thru a Vee belt reliably. At 24v and 3800 rpm, the Balmar makes 180 amps whereas my Mastervolts make 150Amps. Not a huge difference, and not enough to make me abandon the electronic integration I get with a combined system all made of mastervolt parts (where, just for example, a temperature sensor on the batteries feeds data to the Mastervolt alternator charge regulator, on a plug and play basis)

 

[grumpy_o_g]

Wow; these babies put out 310 amps each! http://www.balmar.net/98-series.html

jfm's little 150amp alternators are whimps.

Yep, but as jfm said this is real horsepower we're talking, probably 7 or 8 bhp per alternator at full load. Imagine a small diesel engine running at full throttle trying to stop your engine by driving the fanbelt backwards :eek And if you haven't some fancy circuitry to keep it all under control then you'll get the same sort of instant torque from an alternator that electric motors are famous for.

 

[JumbleDuck]

Drawing on my vast knowledge of physics I would say the gyro would would exert more righting force the further it was above the cog...?

It applies a torque, not a force, so where you put it (as long as the orientation doesn't change) makes no difference.

Absolutely not. The gyro is gimballed athwartships and spins on a vertical axis, so it can only exert a "twisting" force on the boat about the boat's fore-aft axis. It therefore has no material effect on steering

It opposes the sea's rolling torque by applying a pitching torque but that shouldn't have much effect on a boat which is much longer than it is broad. It might do interesting things in big pitching seas - try to oppose them with roll - but I presume the control systems on these things stop that happening by freeing the thing up when pitching is detected. How do they cope with quartering seas and a combined pitch/roll motion?

Agree that there won't be much yaw torque, though there will be some as the axis of rotation is allowed to move away from the vertical.

 

[jfm]

It opposes the sea's rolling torque by applying a pitching torque...

Happy to be put right but I don't think that is correct. In the natural state, with no interference with precession, the athwartships gimbal prevents the gyro applying any pitching torque whatsoever, yet it produces an anti rolling torque. Now if you prevent the precession by locking the thing then yes it induces pitch as the boat rolls but the boat is so stiff in the pitching sense that the gyro hardly produces any anti roll torque whatsoever.

It might do interesting things in big pitching seas - try to oppose them with roll ...

Again, in the natural state the thing is free on its athwartships precession axis/ gimbal which makes it immune to pitching. It cannot know the boat is pitching so it cannot do anything in response. Now, if you lock it or limit the precession relative to the hull then yes as the boat pitches it will induce roll. The control systems must have something slight in them to help with this. Not much though- angular acceleration in pitching is a fraction of that in rolling.

Agree that there won't be much yaw torque, though there will be some as the axis of rotation is allowed to move away from the vertical.

Yup. That is precisely why I wrote "materially"

 

[MapisM]

It opposes the sea's rolling torque by applying a pitching torque

Do you mind elaborating further?
Aside from not understanding what you mean in the specific case of gyro stabs, I can't see how that could happen from a physics standpoint... :ambivalence:

PS: ops, I was writing my post while jfm posted his. I fully agree with him, fwiw.

 

[JumbleDuck]

Happy to be put right but I don't think that is correct. In the natural state, with no interference with precession, the athwartships gimbal prevents the gyro applying any pitching torque whatsoever, yet it produces an anti rolling torque. Now if you prevent the precession by locking the thing then yes it induces pitch as the boat rolls but the boat is so stiff in the pitching sense that the gyro hardly produces any anti roll torque whatsoever.

A gyroscope tries (well, Newton's First Law of motion, in an integrated way, tries) to keep its axis of rotation constant. If you have a gyroscope with a vertical axis, its angular momentum is a vertical vector. If you try to tip that axis to one side, by rolling the boat, the new angular momentum vector is made up of the original (vertical) one with a horizontal one added to one side of it. That horizontal vector represents rotation around a horizontal axis, which is why an unrestrained gyroscope which has its axis pushed over to one side will try to tip at right angles to that.

Aside: this is what happens in a toy gyroscope balanced on one end: gravity tries to pull it over and it responds by moving the axis at right angles which, because the bottom is fixed, means that it moves around in a circle. This called nutation, or torque-free precession although that's a misnomer because it's driven by the torque caused by the weight not being above the support point.

So, if you prevent or oppose the pitching motion, you also prevent or oppose the rolling motion and if you allow it to pitch freely it will also roll freely. The simplest way to maintain roll stability is to have a gyro fixed upright in whatever you want to balance. However, this would give a very bumpy ride ("... the boat is so stiff in torque ..." as you say) and, while it can resist roll, has no way of recovering once roll has happened. Instead, you mount the thing on an athwartships/horizontal gimbal and control the rate at which it pitches. In the case of the Seakeeper this is done with a couple of hydraulic rams.

and you can see much the same thing in the Wolseley Gyrocar, though I believe that in that case it was a rather simpler system (you can see the actuating crank beside the gyroscope) which simply damped roll, whereas the Seakeeper system, I think, can actively drive the gyroscope in pitch.

Bottom line: gyroscopes are three dimensional beasts and can do nothing in two dimensions. A vertical gyroscope unrestrained in pitch can't oppose roll, a vertical gyroscope wholly restrained in pitch will rather roughly oppose roll and a vertical gyroscope intelligently driven in pitch can very effectively oppose roll.

I generally detest appeals to authority, but if it helps I taught three dimensional kinematics to engineering students at A Very Famous University for many years. One of the oddities about gyroscopes is that they are remarkably easy to describe and analyse using vectors and matrices and remarkable difficult to describe and analyse in a more scalar way.

Sorry, a bit long.

Edit, making it even longer. All the above is for small displacements only. Once the axis of rotation has moved sideways and therefore forwards, everything gets more complicated and you can get a mixture of pitching and rolling torque out of an unrestrained gyroscope. It all gets a bit horrible though, as anyone who has seen what a standard artificial horizon does during aerobatics knows.

 

[jfm]

All understood. But your post doesn't disagree with anything in the piece you quoted from me at start of your post - I think!(?)

The mix up here possibly is you are talking about pitching of the gyro while everyone else is thinking about pitching of the boat. Can you elaborate on that please? I'm thinking that if you have a gyro on a boat with free unrestrained precession about an athwartships axis, and the boat pitches in a head sea, the gyro won't even be aware (so to speak) of the pitching and will maintain its vertical spin axis, so won't induce any roll. Yet if the boat then also rolls it will still create an anti roll torque as it precesses (ie as the gyro itself pitches). So it creates anti roll torque on the boat by a pitching torque on the gyro but not on the boat. Is that right?

(All the above changes if you restrain the gyro's precession but I wanted to get the basics right!)

 

[kashurst]

I was/am still interested in a seakeeper but didn't really understand how it works. So I bought a cheap toy gyroscope from amazon for a few £ to try and get a better understanding of how it works - its easy to demonstrate on a toy and its behaviour is really quite counterintuitive.

 

[MYAG]

I was/am still interested in a seakeeper but didn't really understand how it works. So I bought a cheap toy gyroscope from amazon for a few £ to try and get a better understanding of how it works - its easy to demonstrate on a toy and its behaviour is really quite counterintuitive.

This video shows (from 1:50-2:07) the counterintuitive behaviour at work.

 

[jfm]

Thanks Myag. That model is good at showing the basic function. That gyro is completely free to precess (ie rock back and forth about the athwartships axis) and you can see that as soon as the boat rolls the gyro precesses. There is pitching of the gyro, but the pitching of the boat is completely unaffected by that gyro because there is a free bearing athwarthships

When you start "braking" the precession, which is what the hydraulics on seakeeper do (very cleverly, using complex algorithms), then you get a pitch effect on the hull but you do not notice it becuase the hull is incredibly stiff in the pitching sense.

By the way Dave Marsh didn't understand this in his recent rock n roll article http://www.mby.com/gear/could-boat-stabilisers-spell-the-end-of-rock-n-roll-45319. He wrote:

Quite differently, a gyro boat stabiliser extracts the precessional energy that it uses to correct heeling by pushing the spinning gyro off its vertical axis.

Once the rams involved have reached the full extent of their travel, they can’t extract any more energy until the boat starts to roll in the other direction and the process is reversed

That is just not correct. The rams could, but do not in a seakeeper or Mitsubishi, push the gyro off its vertical axis as a basic modus operandi. The precession is an entirely natural Newtonian effect and no push is needed to make it happen. The rams generally slow down and damp the precession, not induce it. (OK I appreciate that within the complex algorithms there can be moments of inducing precession but that is a fine detail and not the basic modus operandi). Ypou can see that on that seakeeper video - the precession rocking of the gyro happens all on its own, with no help. This slowing down of precession is critical in big seas because when the gyro has finished precessing you don't want it to precess back the other way in an uncontrolled fashion. Also Dave's "precessional energy" part is a bit off the mark and the problem is not that the rams have reached the full extent of their travel but that in this application no gyro will be any use once it has swung (precessed) to about 45 degrees because it needs a significant component of vertical axis spin. If you install long hydraulic cylinders and push it up so it has a near longitudinal spin axis and let it precess there, then it will function as a rudder killer by creating a pure yaw torque onto the boat, which would obviously be nuts.

This sums up the fundamental issue with gyros: to work on a boat they need a vertical spin axis. But also they need to precess, ie depart from that vertical spin axis. So as they work, by precessing, they move towards a state where they don't work. Thus, they fundamentally have a limitation on how much ant-roll then can create in one direction before needing the boat's roll to go the other way. That's their one dynamic limitation compared with fins: they can't deal with biiiig long waves and big twisty quartering seas, where the wave still has plenty more to do when the gyro runs out of puff. But in most real life leisure boating that isn't a very big deal at all

 

[rustybarge]

Thanks Myag. That model is good at showing the basic function. That gyro is completely free to precess (ie rock back and forth about the athwartships axis) and you can see that as soon as the boat rolls the gyro precesses. There is pitching of the gyro, but the pitching of the boat is completely unaffected by that gyro because there is a free bearing athwarthships

When you start "braking" the precession, which is what the hydraulics on seakeeper do (very cleverly, using complex algorithms), then you get a pitch effect on the hull but you do not notice it becuase the hull is incredibly stiff in the pitching sense.

By the way Dave Marsh didn't understand this in his recent rock n roll article http://www.mby.com/gear/could-boat-stabilisers-spell-the-end-of-rock-n-roll-45319. He wrote:

That is just not correct. The rams could, but do not in a seakeeper or Mitsubishi, push the gyro off its vertical axis as a basic modus operandi. The precession is an entirely natural Newtonian effect and no push is needed to make it happen. The rams generally slow down and damp the precession, not induce it. (OK I appreciate that within the complex algorithms there can be moments of inducing precession but that is a fine detail and not the basic modus operandi). Ypou can see that on that seakeeper video - the precession rocking of the gyro happens all on its own, with no help. This slowing down of precession is critical in big seas because when the gyro has finished precessing you don't want it to precess back the other way in an uncontrolled fashion. Also Dave's "precessional energy" part is a bit off the mark and the problem is not that the rams have reached the full extent of their travel but that in this application no gyro will be any use once it has swung (precessed) to about 45 degrees because it needs a significant component of vertical axis spin. If you install long hydraulic cylinders and push it up so it has a near longitudinal spin axis and let it precess there, then it will function as a rudder killer by creating a pure yaw torque onto the boat, which would obviously be nuts.

This sums up the fundamental issue with gyros: to work on a boat they need a vertical spin axis. But also they need to precess, ie depart from that vertical spin axis. So as they work, by precessing, they move towards a state where they don't work. Thus, they fundamentally have a limitation on how much ant-roll then can create in one direction before needing the boat's roll to go the other way. That's their one dynamic limitation compared with fins: they can't deal with biiiig long waves and big twisty quartering seas, where the wave still has plenty more to do when the gyro runs out of puff. But in most real life leisure boating that isn't a very big deal at all

Exactly.

Every engineering solution comes with a major compromise.

this is how the professional fishermen in Alaska do it, with external folding 'bat wings' all mounted outside the hull; no holes compromising the integrity of the hull.

So how about converting these hinged 'bilge keels ' into active fins?

not so long ago if you wanted ariel shots you had to hire a helio at ££££ an hour; now you can buy a drone with camera for a few hundred quid which does the exactly same job for a fraction of the cost.

so why not use tech from model helicopters?
you can buy a radio control helio/drone gyro for a couple of hundred pounds to send control signals to the external fins.
you can fabricate the fins and hinges/lifting gear for a few hundred in materials.

What sort of hydraulic gear is needed to swivel the fins, and how would you connect the gyro servo's to it?

 

[jfm]

Those things are a world apart RB. Sure they create roll stiffness in a crude way, but they are at an end of the cost vs crudeness compromise that most pleasure boaters don't want to be at.

Now if we take your idea of having fins like those alaska ones but with hydraulics to "flap" them, you need to remember that to get decent stabilisation underway you need to twist them (so to speak) so that they create lift. (That's already been invented - it's called fin stabilisation!). You could twist them by having an athwartships hinge axis anda hydraulic ram to create the twist. This would be in addition to the longitudinal axis hinge needed to fold them away when docking. All doable, if you really want...

 

[rustybarge]

Those things are a world apart RB. Sure they create roll stiffness in a crude way, but they are at an end of the cost vs crudeness compromise that most pleasure boaters don't want to be at.

Now if we take your idea of having fins like those alaska ones but with hydraulics to "flap" them, you need to remember that to get decent stabilisation underway you need to twist them (so to speak) so that they create lift. (That's already been invented - it's called fin stabilisation!). You could twist them by having an athwartships hinge axis anda hydraulic ram to create the twist. This would be in addition to the longitudinal axis hinge needed to fold them away when docking. All doable, if you really want...

Its a few years since I had RC planes, but I do remember an article in a mag about this bloke who built a giant RC copy of a b52 bomber. What he did was install the basic RC gear with servo's, then made up some giant slave servo units with big powerful electric motors and manually connected these via variable resistor to the little servo's so that they copied the position of the standard rig. Electric motors are already used to operate fins like in Bart's boat(?) so they are readily available in our budget

i envision the same sort of set up, and with modern RC equipment you can actually program the servo's to do amazing things automatically with the inbuilt sophisticated programable computers.

everything seems to be there for a home builder, at the fraction of the cost of commercial fins.

guess : £2k

 

[jfm]

Sheesh! I think you'll need over £3k for this project but if you can do it for £2k then chapeau to you

 

[vas]

Sheesh! I think you'll need over £3k for this project but if you can do it for £2k then chapeau to you

Just need the motors with flanges and stuff. Easy to machine the flanges and axles
Then you need a decent android phone for the sensors
Then you need a couple of arduino boards and Hurricane to help with the programming

job done.

OK, it is tongue in cheek but the concept is not really far off...
but as you lot say, the devil is in the details

Maybe a project to consider when I get bored with not having anything serious to tinker with MiToS

cheers

V.


PS. John, I recon 5K euro and a bit of grp work and it's ready

 

[rustybarge]

Just need the motors with flanges and stuff. Easy to machine the flanges and axles
Then you need a decent android phone for the sensors
Then you need a couple of arduino boards and Hurricane to help with the programming

job done.

OK, it is tongue in cheek but the concept is not really far off...
but as you lot say, the devil is in the details

Maybe a project to consider when I get bored with not having anything serious to tinker with MiToS

cheers

V.


PS. John, I recon 5K euro and a bit of grp work and it's ready

congrats on the completion of your project, I have nothing but respect for your rebuild undertaking. I've built in Steel myself, so I can imagine the work involved with ply.

i think technology can 'short circuit ' traditional industries, look what the internet did to recording companies; programming the software for the fins is now home project not the exclusive domain of the fin manufacturers.

The advantage of the bat wings is that all the electrical and mechanical bits and pieces would be located at deck level; Drive would be brought down to the fins by actuation rods. the wings will fold up at planing speeds to save on drag and to facilitate docking and shallow water canal use.

so all round the idea seems to tick lots of boxes, but most important of all the concept is already in daily use by the professional trawlermen, albeit in fixed 'angle of attack' form.

 

[Deleted User YDKXO]

The rams generally slow down and damp the precession, not induce it. (OK I appreciate that within the complex algorithms there can be moments of inducing precession but that is a fine detail and not the basic modus operandi). Ypou can see that on that seakeeper video - the precession rocking of the gyro happens all on its own, with no help. This slowing down of precession is critical in big seas because when the gyro has finished precessing you don't want it to precess back the other way in an uncontrolled fashion. Also Dave's "precessional energy" part is a bit off the mark and the problem is not that the rams have reached the full extent of their travel but that in this application no gyro will be any use once it has swung (precessed) to about 45 degrees because it needs a significant component of vertical axis spin. If you install long hydraulic cylinders and push it up so it has a near longitudinal spin axis and let it precess there, then it will function as a rudder killer by creating a pure yaw torque onto the boat, which would obviously be nuts.

Thanks, jfm, good explanation as usual especially on the limitations of the gyro. Any idea how the algorithms that control the hydraulic cylinders that damp the precessional movement were developed? Are the algorithms specific to each and every boat that a gyro is fitted in?