Stability Monitor

The April 2011 issue of Soundings has an article about a stability monitor for boats which is under development by Hook Marine, a Scottish company. The article says "the device continuously monitors a vessel's rolling motion and gives early warning of developing situations that could lead to a capsize." It has a laptop size display screen which dispays the boat's GM numerically and graphically, and also has a color coded signal. The primary market is fishing vessels, and NIOSH office in Anchorage has acquired two for evaluation. Extimated cost is $4000.

The device is also described on the Hook Marine website http://www.hookmarine.com/seawise.html The description there includes:
"The SeaWise stability monitor monitors the loss of stability through continuous roll period measurement." Also "The roll test to establish a value for the metacentric height (GM) will be familiar to most boat owners. This test, carried out in the calm water of a harbour, uses the period of the vessel's roll to provide the skipper or surveyor with a value for GM. Hook Marine's SeaWise™ Vessel Stability Monitor carries out what is in effect a continuous roll test while the vessel is at sea, utilising any rolling motion of the boat to provide input to the device, while the installed software filters out the less significant rolling motions to which the hull is subjected by wind and waves."

A device such as this could be valuable on a fishing vessel which operates with a wide range of load conditions, particularly one which carries a large deck load at times. For vessels which operate in icing conditions it could detect and signal when ice buildup was causing a signficant deterioration in stability. Usefulness for recreational boats in most situations is less clear.

 

Experienced seamen have one of those things built into the seat of their pants.

The gizmo described probably uses an accelerometer to record and or diagnose the rolling or pitching action. It might be a useful component for solo sailors trying to catch a few winks of sleep or ones who have become very fatigued. An alarm would notify the sailor of impending trouble but there is some question as to whether the alarm would be issued soon enough.

 

My understanding is this device is intended to warn in changes of the stability characteristics of the vessel, in particular the CG height, by analysis of the roll spectrum to identify the inherent roll period/frequency. It doesn't analyze whether the magnitude of the roll is too great or not.

Several large, commercial fishing boats have capsized in recent decades, apparently caused by too high a CG. CG on a commercial fishing boat can change significantly with different loads, particularly deck loads, and ice build-up on the superstructure. Flooding of compartments can also affect stability.

 

Would weight and location of catch, disposal of heavy gear around the decks, etc also be part of the risk ?

 

SMC from Finland developed the SMCfish motion monitor equipment (See at the bottom of: http://etnaftp.org/pdf/SMC.pdf), but they seem to have discontinued it (http://www.shipmotion.se/).
There have been several attempts to develop and market such monitors for commercial fishing vessels, but none of them have been succesful, to my knowledge.

Cheers.

 

The problem with such systems, imho, is the randomness of the hull-motion exciting forces (due to wave and wind action).

A computerized monitoring system could take measurements of boat's accelerations and rolling motions over a long period of time and establish spectrum of frequencies involved. Than, perhaps, it could even find a mathematical correlation which would allow a more or less successful near-future prediction of the evolution of said parameters - if they continue to behave in a statistically coherent and regular way.

But then again, that's just a bunch of hard words to mathematically describe things which an experienced helmsman will do routinely and automatically, even without thinking too much.

What the automatic system doesn't have is:
- eyes to see what's going on in and around the boat, like an unusual formation of sea waves, approaching wind gusts, slightly moving cargo or deck items which are about to get loose and start flying around etc.
- a brain to recognize these visual alarm signals and act BEFORE the events become dangerous or uncontrollable
- an intuition, or a so-called "gut feeling", which sometimes can warn us of unusual situations that a conscious part of our brain didn't elaborate, but our subconscience somehow did - and has recognized a danger.

I believe these are some of the reasons why the previously cited attempts didn't deliver a desired result. And I have a feeling that a lots of water will pass below the hull before they will manage to do so.

Cheers!

 

Three or four years ago I collaborated for some time with SMC to develop their system. They had their share of problems with the maths analysis routines part of the thing and definition of boundaries, as well as dealing with the main problem of rolling motion induced by waves not being the same as the natural roll period of the ship (the one used to compare things), absolutely, but I think the main reason for the lack of success was the inexistent interest of commercial fishermen in such a piece of equipment.

Here is Spain I know of at least two other failed attempts by "spin-offs" of the universities of La Coruña and Madrid.

Cheers.

 

I't just a spectral analysis of the roll data. But yes when you look at the FFT of the data it can be unclear just which peak is due to the natural ship frequency and which to the swell if they are close. When they are well separated it's quite clear. So over time you can get accurate roll period and draft readings from these sorts of instrumentation as the vessels operates on different headings.

 

Yes, the SMC system was intended to work like that, "learning" from its own "experience".

Cheers,

 

In France, the company Sepac (www.gm-meter.com) has developped since quite long time a "GM-Meter" which carries out spectral analysis of roll motion with FFT.

They separate the peaks in the spectrum to get natural rolling one based on experience of the spectrum analysis. Then with gyration radius extract from inclining experiment (when GM is accurately measured) they manage to display correct GM at sea. They claim precision of approx. 10-15% (checked during field tests on a car-ferry).

Have a look to their site (unfornutaly in french only).

Cheers,

 

Roll Constant in still water cannot be used at sea

Correlating roll period to at "sea GM" should not use the stillwater roll constant "K".
This is due to the variable entrained mass and roll damping, that are quite different to values obtained from a stillwater roll test.

 

According to the a.m company, they overcome the problem of variable entrained mass and roll damping through very small roll angle measurement (using a very precise sensor). In that case damping and entrained mass are not significant according to this company.

They claim that they don't need large roll amplitude, and affirm being able to get good or acceptable results with far less than 1° of roll. The secret is in the signal treatment: correct parametring of FFT extractions, etc...

Cheers,
Francois-Xavier Nettersheim

 

I have got the info from the french company about the GM-meter. It looks interesting, although I still do not know the price. It is being used by one of the major shipyards in Spain (Navantia). I have to ask them for their feedback.

 

Yes you ar right, they have made some business with Navantia.

In France, they have sold their equipment to DCNS (more or less the Navantia equivalent in France) and some civil owners (mostly in the ferry activity).

As you know regarding ferry operation, Solas stipulates that the master should verify and document its stability before going to sea at each voyage. Usually this is done through on-board stability computer (using "book keeping" data supplied by the shore). The accuracy is not perfect. French administration has accepted (Solas gives such possibility) the GM-Meter equipment from this company as equivalent. The stability (metacentric height) is measured when vessel leaves the pier in few seconds. Accuracy has been compared during field tests for administration approval : during many weeks the "manual" calculation of GM (computer assisted...) has been compared to GM-Meter indications. Results have been proved acceptable : 10% error maximum, taking the "manual" book keeping metacentric height calculation as a reference.

I am not connected with this company but I am cooperating with them on stability matters and for inclining experiments.

Cheers,
Francois-Xavier Nettersheim

 

Hello François-Xavier,
Do you use the GM-meter to perform official inclining experiments? Do your authorities accept it? Does it worth its price for this use, when comparing it with the humble but effective pendulum? More accurate? Usable in small ships/boats?

I'll greatly appreciate your feedback. Thanks in advance.