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Stabilizer Trim control
brettski
Posted: Sunday, July 29, 2012 2:04 AM
Joined: 27/07/2010
Posts: 2


I looking for answers to questions I have about yacht stabilizer 'trim control' functions. Are there proven benifts to not using the 'trim control' function when a yacht is listing during heavy weather ? Is it better or worse to trim a yacht by ballasting fuel over to the windward side and not using the trim control? I have some personal opinions however I would like to know if anybody has actual facts and or data to support these facts. I'm wondering if a large weight offset due to fuel ballasting can effect the seakeeping abilitys of the stabilizers due to the imblance that the boat will have as she rolls. I have been told that trim control contrubutes to increased fuel consumptions (which i could agree with in calm seas but once the yacht is rolling I am having my doubts) Anybody out there have ideas ?
Philwhittaker
Posted: Wednesday, August 1, 2012 2:26 PM
Joined: 18/02/2011
Posts: 2


Normally when one refers to trim you refer to fore and aft angle of the yacht as she moves though the water, I am assuming that in this case you are talking about list or an heal on the boat from outside forces or from an in-balance of fuel or water on board.
A stabilizer system is designed to keep a boat from rolling, in other words keep a boat flat in the water. If there is a list then the system will work to correct that by creating a force on the hull that rolls the yacht back to horizontal.Unfortunately this force does not come for free, it has a side effect of creating drag and we all know that increased drag means increased fuel consumption. This of course is the theory in flat water, however when you put the yacht in rougher seas the drag is already being created so there is really no lost eficency as far as increasing the drag on the hull. When there is a loss is in the performance of the system. The system typically is programed to react to movement of the yacht at a set speed. This speed is controlled in the initial set up of the system and afterwards by the gain control of the system. The higher the gain the faster the fin reacts to a roll bringing the boat back to horizontal. Now when you have a list on the yacht, the fin is already working to keep the boat flat and will react more or less to keep the boat from moving. This additional movement creates a jerky effect on the boat making her feel almost punchy. Is there a benifit to adjusting the system to allow for this list, not really. You have stabilizers to keep the boat flat, why would you want to allow the boat to lean? Is there a benifit to movign fuel side to side, no. Stabilization is based on the Metacentric height of the boat, this is the distance between the centre of gravity and the metacenter (the point that the boat rotates around during a roll). The metacenter is a fixed point that is determined byt he design of the boat, a wide flat boat has a high metacenter while a round boat has a low metacenter. We have the ability to move the center of gravity around by adding tenders, moving fuel etc. Moving fuel side to side does not change the metacentric height or GM adding fuel does. Ironically the higher the center of gravity the smaller the stabilizer is that you need. So when you add fuel to the boat in low tanks your stabilizers have to work harder. When you add a tender to an upper deck your stabilizers have to work less.

So in summery, no there are no benefits to trim control (list) in heavy weather, yes it would be best to ballast fuel over allowing the stabilizers to work more effectively. This would make the ride a little more comfortable allowing you to get out of the nasty weather quicker.

Fore and aft trim is a whole different argument, there are most definitely benefits to making sure the yacht is not tail or bow heavy.

brettski
Posted: Wednesday, August 8, 2012 2:58 AM
Joined: 27/07/2010
Posts: 2


Thanks Phil for a very intellegent and through answer. By the way yes you were correct and I did mean list not trim control, slip of the keyboard !!