hobby-horsing

Does anybody know if small active fins near the bow of displacement yachts has been used to dampen hobby -horsing? The typical aft trim tabs are used for running attitude and I am not aware of units that can activate fast enough for this purpose. While canard type fins could change their orientation rapidly and be controlled by chips and attitude sensors. I also wounder if this would work best with a sharp wave piercing type bow.

 

Though I have not heard about using them for this (which really doesn't mean much, I'm no expert, just stating a fact) I have read about one gent experimenting, using scale models, with controlled bow planes in order to modify/control the form of the bow wave as part of his investigation into bow waves etc.

http://www.ivorbittle.co.uk/Article...he bulbous bow for my site This one again.htm

While what you bring up was not his concern, looking at figures 44-46 it does in fact seem like he was also lifting or dropping the bow at the same time as well as modifying the bow wave.

I would guess that what you are after will require some form of, um, "swim by wire" system to manage the fine tuning without human input.

 

With computer controlled actuators, you should be able to "keep up" with trim states, though this is an expensive way to handle a problem.

It might be easier if you explain what you are trying to achieve, on what boat, etc.

 

Par no particular boat in mind. I was just thinking actuated fins have been fairly successful on curbing roll so why not actuated fins for stabilizing fore an aft motion and trim. Then I thought using actuated trim tabs would be mechanically difficult but not so horizontal fins somewhere near the bow. No doubt there are other design aspects to handle the problem such as weight distribution and bow bulbs.

 

Why does it have to be active ?

 

US navy successfully retrofitted huge horizontal stabilizers up the bow to reduce pitching on their large catamarans way back in the 70`s. With Only passive foils, the results are impressive. Approx 30% reduction in pitching and damped the vessels seakeaping so it behaved more like a vessel of much larger displacement. If you google "Ocean Catamaran Seakeeping Design, Based on the Experiences of USNS Hayes" by Hadler, Lee, Birmingham & Jones 1974, you will find sound evidence.

So yes, it works, and better than buoyancy, but no your not the first to think of it, missed the boat by about 40 years in fact...

 

Has to be weighed against additional drag I suppose. I hope your cat doesn't need the retro-fit. Rear lifting foils would have the same effect ?

 

Again, instead of engineering a solution for a potential or presumably perceived problem that may not exist, wouldn't it be wise to address the issues at hand? Simply put, the design can handle porpoising, as can the skipper, as can trim adjustments, so why invent a new solution, where several already exist?

Large vessels already can address this, though I'm not sure how appropriate these would be on a yacht. Without an idea of what you're looking to get in the performance package, it's difficult to suggest what approach might be effective.

 

Pitching is usually due to sea state and relative course. Many small vessel skippers deliberately drive into waves diagonally rather than bow on.
But as to automatic dampening, the ancient Chinese cured excessive pitching on their junks by making the bow compartment free flooding. An additional design feature, while the 1st bulkhead was easy to make watertight, because the sides compressed against it's edges, the pram type bow they used, was hard to seal. A nearly right angle junction with sides. So why try?
This first compartment between pramson and 1st bulkhead, had holes in the bottom and as the junk rose to a wave, the bow buoyancy was slowly lost as compartment flooded. So it didn't rise too quickly or too much, and as the crest receded farther aft the weight of ballast decreased in bow, free draining also, and thus effectively reduced violent pitching. How did they calculate how many holes and what size to cut in the bottom to tune a new junk? Don't know. Maybe a hand sent in the compartment with a brace and augur bit while underway in heavy weather.

 

My cat is in building hiatus again - we have bought another catamaran, which is already in the water and in need of renovation so all attention is on it for now...

And i suspect rear foils would be less effective than up near the bows, because the bows generally see higher accelerations than does the stern. Higher accelerations = more angle of attack = higher damping forces.

Additional drag would for the most part be virtually immeasurable as the reduction in pitching decreases the drag of the hull itself in a seaway. Its a difficult problem to model in real world conditions. But most commercial active fin stabilization systems, report negligible drag increase. Its also worth noting that a very high percentage of new commercial vessels utilize active fin stabilization these days because it works so well. I ran some calculations a while back, and the size of the foils can be quite small for a very significant restoring force.

The fastest ocean racing multihulls all utilize lifting foils these days - not just for reduction in drag, but reduces pitching. This enables the rig to operate more efficiently, delivers more thrust, and allows them to push harder in rough conditions. The speed and passage times of these modern designs utilizing foils increased dramatically when this passive technology was introduced. The next logical step becomes active control of the lifting dagger boards in a seaway controlled via gyro feedback- the technology already exists to do it, im sure its just around the corner before the next record breaking yacht uses it.

I see the main disadvantage being the vulnerability of protruding appendages, and their reduction in performance with marine growth. These are very real, real world problems - which i suspect is where PAR`s comments are leading to. ie - problems can be solved in other ways, but first need to understand how the problem occurs before deciding on the most appropriate solution.

 

The wand used on Moth foils works fast and reliably, providing it does not encounter very short wavelengths when the wand is in air and the foil in water

Stationary lightvessels used to have antiroll watertanks which were fine tuned to the prevailing waves. Thus making a positive use of free-surface effects

Richard Woods

 

I would think the active bow fins would not need to be very large and could be semi delta with aft sweep. The location would probably allow for brush cleaning from the deck with a purpose made brush. A spring loaded impact device could help with vulnerability issues. The blades could be replaceable in water with little effort. I suppose the blades could even be rotated and folded against the hull when not needed for anti pitching duty. I think most of the arguments against bow fins also apply to the now common roll fin stabilizers and the market has spoken on that issue.

 

The evidence is sound ... Most High speed ferry catamarans are using active stabilization these days... Both pitch and yaw...

Passive stabilization on recreational yachts never seemed to catch on for some reason ... but most cruising sailors are largely quite conservative by nature which explains alot. The vulnerability issue is not easily solved. They need to be significantly strong to shirk the restoring forces to the hull and damp the motion, these forces get very large as speed increases, not so bad at low speeds.

You can't put them too far forward on a recreational boat either as they will regularly become airborne and ineffective without a sufficiently deep forefoot to keep them immersed. It's not as simple as it might initially sound, but is certainly feasible with proper consideration.

 

I dont know where the fear of expense comes from today. An iphone or ipad easily has the compute power, sensors, and I/O to do the job, and simple one axis control is probably the first homework assignment in an undergrad's control class. Structurally, its a rudder with an autopilot, again, almost in the noise for any vessel that would desire such a thing.

Its 2014. Needs a computer? Cheapest, easiest kind of problem to solve.

Google around more, there are many vessels that use this technology.

 

Just wondering if this would be possible with conventional zero speed stabilisers which cost up to £70k and more; surely it can't be that difficult to fabricate a couple of delta wings with a hydraulic ram, and connect to a pressure pump using an iPhone or consul games controller to detect the rolling motion.

Interesting project for someone?