Roger Long
Senior Member
Rather than try to individually correct some misconceptions in the two stability / rolling threads, I’ll offer this explanation:
The period of a vessel’s roll is a function of two factors, GM and how weight is distributed out from the center of gravity. Think of a vessel with some lead ballast in the bilge. If the ballast is moved up and stacked in the middle of the deck, the vessel’s weight and trim will not change. Therefore, the “M” in GM, which is a function of the hull’s underwater shape will not change. The “G”, the center of gravity will be raised however, reducing GM. The vessel will roll slower and heel farther due to side force of wind. It will also have a shorter range of stability.
Now, take that pile of ballast and move it into two piles along the port and starboard rails. The center of gravity will not be raised or lowered nor the hull shape changed so GM will be the same but mass will be distributed farther from the center of gravity and the roll period will increase, slowing the speed of the roll.
The force you feel in rolling, how hard you need to grab that stanchion, is a function of two different factors. One is the roll period and the other is the roll angle. If you go farther in a fixed amount of time and stop to reverse, you will be moving faster and the force will be greater. The angle through which a vessel rolls is largely a function of hull damping which is the resistance of water flowing transversely over the surface of the hull as it rolls. Add bilge keels and the vessel’s roll period will not change but the angle through which it rolls will be reduced along with the forces experienced by the crew. A wide, low deadrise angle, chine hull will have more damping than a rounded traditional shape. Given the same GM and mass distribution (Radius of Gyration), the two hulls will have the same roll period but the chine hull will roll through less of an angle.
The hull damping that reduces roll angle, but does not change roll period, is achieved basically by forcing some water to move with the vessel as it rolls. This increases the resistance because the characteristics or appendages that resist the transvers water flow have to be pulled through it. The damping effects also disrupt water flow. There is a reason why traditional, low power, fuel efficient vessels tend to roll more. Our Gulf Star is one of those. My co-owner hates rolling and knows that we roll more than many trawlers with boxier hulls. I tell her that, yes, we could add bilge keels or quarter fins (not often used but I have found them even more effective than bilge keels). These would reduce the angle of the roll but not the roll period. Still, the boat would be more comfortable. However, we would be paying in fuel every hour that we ran the boat. Since we are primarily protected waterway, river, and canal cruisers and these modifications would cost some thousands of dollars, I don’t feel it worthwhile.
I would feel reducing our roll angle worthwhile if I knew it would make my co-owner happy on the occasions we have to cross open water. However, I built my career on producing research vessels that were better working platforms for their size than previously thought possible. They are legendary for their minimal rolling motions. However, I know that she wouldn’t be happy on one of those boats either. They roll less but they still roll some.
If you cruise the east coast, you have probably seen my designs.
https://www.cruisingonstrider.us/RVdesigns.htm
The formula for those research vessel designs is this:
High GM and short roll period combined with high roll damping. Vessels roll most when the period of the waves matches the natural roll period of the hull. There is a relationship between wave period and wave size. The short roll period of these boats are in tune with the waves generally created by winds in the 10 mph range when the waves tend to be small. When the larger waves created by stronger winds are encountered, these vessels are no longer in tune with them.
These vessels are damped by virtue of hull shape and wide chine flats or quarter fins. They are so highly damped that it is hard to do roll tests on them because they tend just flop back and stop after being heeled. You have to work to get them rolling. This high initial stability is also advantageous for handling heavy loads which they all do. The high damping keeps the angle of rolling low but their fuel consumption is also on the high side for their size and weight.
The center of gravity of these vessels is quite high, nearly to the main deck level. The high GM to achieve the short roll period is obtained by beam which is also nice for handling a lot of gear on deck. Vessels roll about a point located between the center of gravity and the water line. The high center of gravity of my research vessels put this rolling center closer to the deck. This makes the rolling more of the deck changing angle under your feet than being move a larger distance from side to side. Not only is this comfortable for the people but objects set on the deck tend to stay in place more before being tied down and secured. This is good for handling expensive and delicate gear.
The downside to this hull design concept is a jerkier motion that is more responsive to waves. It is a less predictable motion but seldom extreme. It's not so much about comfort for the people as for handling expensive, well over half a million dollars sometimes, and delicate gear being deployed and retrieved while hanging from the A-frames and cranes.
The period of a vessel’s roll is a function of two factors, GM and how weight is distributed out from the center of gravity. Think of a vessel with some lead ballast in the bilge. If the ballast is moved up and stacked in the middle of the deck, the vessel’s weight and trim will not change. Therefore, the “M” in GM, which is a function of the hull’s underwater shape will not change. The “G”, the center of gravity will be raised however, reducing GM. The vessel will roll slower and heel farther due to side force of wind. It will also have a shorter range of stability.
Now, take that pile of ballast and move it into two piles along the port and starboard rails. The center of gravity will not be raised or lowered nor the hull shape changed so GM will be the same but mass will be distributed farther from the center of gravity and the roll period will increase, slowing the speed of the roll.
The force you feel in rolling, how hard you need to grab that stanchion, is a function of two different factors. One is the roll period and the other is the roll angle. If you go farther in a fixed amount of time and stop to reverse, you will be moving faster and the force will be greater. The angle through which a vessel rolls is largely a function of hull damping which is the resistance of water flowing transversely over the surface of the hull as it rolls. Add bilge keels and the vessel’s roll period will not change but the angle through which it rolls will be reduced along with the forces experienced by the crew. A wide, low deadrise angle, chine hull will have more damping than a rounded traditional shape. Given the same GM and mass distribution (Radius of Gyration), the two hulls will have the same roll period but the chine hull will roll through less of an angle.
The hull damping that reduces roll angle, but does not change roll period, is achieved basically by forcing some water to move with the vessel as it rolls. This increases the resistance because the characteristics or appendages that resist the transvers water flow have to be pulled through it. The damping effects also disrupt water flow. There is a reason why traditional, low power, fuel efficient vessels tend to roll more. Our Gulf Star is one of those. My co-owner hates rolling and knows that we roll more than many trawlers with boxier hulls. I tell her that, yes, we could add bilge keels or quarter fins (not often used but I have found them even more effective than bilge keels). These would reduce the angle of the roll but not the roll period. Still, the boat would be more comfortable. However, we would be paying in fuel every hour that we ran the boat. Since we are primarily protected waterway, river, and canal cruisers and these modifications would cost some thousands of dollars, I don’t feel it worthwhile.
I would feel reducing our roll angle worthwhile if I knew it would make my co-owner happy on the occasions we have to cross open water. However, I built my career on producing research vessels that were better working platforms for their size than previously thought possible. They are legendary for their minimal rolling motions. However, I know that she wouldn’t be happy on one of those boats either. They roll less but they still roll some.
If you cruise the east coast, you have probably seen my designs.
https://www.cruisingonstrider.us/RVdesigns.htm
The formula for those research vessel designs is this:
High GM and short roll period combined with high roll damping. Vessels roll most when the period of the waves matches the natural roll period of the hull. There is a relationship between wave period and wave size. The short roll period of these boats are in tune with the waves generally created by winds in the 10 mph range when the waves tend to be small. When the larger waves created by stronger winds are encountered, these vessels are no longer in tune with them.
These vessels are damped by virtue of hull shape and wide chine flats or quarter fins. They are so highly damped that it is hard to do roll tests on them because they tend just flop back and stop after being heeled. You have to work to get them rolling. This high initial stability is also advantageous for handling heavy loads which they all do. The high damping keeps the angle of rolling low but their fuel consumption is also on the high side for their size and weight.
The center of gravity of these vessels is quite high, nearly to the main deck level. The high GM to achieve the short roll period is obtained by beam which is also nice for handling a lot of gear on deck. Vessels roll about a point located between the center of gravity and the water line. The high center of gravity of my research vessels put this rolling center closer to the deck. This makes the rolling more of the deck changing angle under your feet than being move a larger distance from side to side. Not only is this comfortable for the people but objects set on the deck tend to stay in place more before being tied down and secured. This is good for handling expensive and delicate gear.
The downside to this hull design concept is a jerkier motion that is more responsive to waves. It is a less predictable motion but seldom extreme. It's not so much about comfort for the people as for handling expensive, well over half a million dollars sometimes, and delicate gear being deployed and retrieved while hanging from the A-frames and cranes.
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