Rolling, rolling.

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Roger Long

Senior Member
Joined
Jul 14, 2015
Messages
451
Location
Albany
Vessel Name
Gypsy Star
Vessel Make
Gulf Star 43
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.
 
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Roger

Our DF 48 rolled when at the dock and tides were changing. Surface disturbance none at this point. I thought maybe this movement was due to different layers of water moving around hull at different rates. Your thoughts?

BTW, TF is fortunate to have you onboard.
 
Thank you Roger, I could see this in my head but could not seem to put it into words. When that happens I normally sense what is correct but don't fully understand all the interactions to be able to explain something in words. You filled in a few blanks with
 
Roger.

Our DF 48 rolled when at the dock and tides were changing. Surface disturbance none at this point. I thought maybe this movement was due to different layers of water moving around hull at different rates. Your thoughts?

BTW, TF is fortunate to have you onboard.


Hard to say but it takes very little excitement to produce a good roll if the period exactly matches the vessel's natural roll period. I doubt underwater currents would have a consistent pulsing period. If a cross current, it could be that the particular hull shed a vortex off the keel that got into sync with the rolling. Rolling could trigger the vortex shedding to create a feedback loop. Any current should have put a damping friction on fenders or lines so I'm mystified.



I remember that an H28 in Boothbay Harbor often seemed to be rolling on calm days when every other boat and the water surface looked still. Something about the land configuration produced invisible waves from the ocean swells that exactly matched its rolling period.
 
Roger-
That is very interesting. Thank you.
We bought an older Grand Banks 42 that I am slowly working on and when investigating all the various cubbies and bilge spaces I found lead ingots placed under the aft stateroom berths outboard, outside the centerline of the vessel.
As a former sailor who is used to deep keel offshore sailboats I thought that ballast should be consolidated into a lower centered deep bilge area rather than placed outboard like it is now.
Other projects have side-tracked my dealing with that ballast but according to your advice it is better to have that ballast outboard to soften the roll of the boat and in fact placing it low and centered would be the wrong move. Is that correct?
Please advise.
Thank you
 
Yes. Leave that ballast right where it is.
 
The lower the ballast is, the more it will help ultimate stability (AVS) by lowering the center of gravity. Having ballast on the centerline may matter for AVS once your AVS is approaching 90* or more. If AVS is lower than that, centered vs outboard shouldn't matter if the ballast is at the same height.

Having weight further outboard will tend to make rolling motions slower and more gentle, as the boat will have more momentum in roll.

So the best place for ballast will depend on how the boat behaves, how much stability it already has, how the rest of the weight in the boat is distributed, etc. A boat with fuel carried in saddle tanks out against the hull sides will naturally have significantly more weight outboard than a boat that carries fuel close to centerline, so differences like that will impact optimal ballast placement to get a comfortable motion.
 
Thanks for this detailed explanation, it was near exactly the move of our Long-Cours 62.
"Big" GM (think remember 1.62m), + 2.3 T of lead ballast (unfortunately near the center line...not on the side)...the result was a very short roll period ( at the beginning little more than...2" !) After added...solar panel, mast and boom, bilge keel (not big one 4 m x0.28m) It change a lot the comportment of the boat.
The little weight of the mast, boom/sail and rigging ( 72kg,30kg, 60kg )
change a lot something in relation with "inertia" who work in cube.
 
Thanks for that helpful explanation, Roger. Helps me understand what has always seemed true through observation and instinct, which is that beamer hulls with firmer bilge sections or harder chines are initially more stiff, and that their rolling, when it occurs, is snappier and more energetic than narrower hulls with rounded sections (such as your 43' Gulfstar). Also helps explain why some boats seem to roll noticeably in a sheltered anchorage more than the boats around them. It's not always just about the narrowness of their beam, or even the shape of their bottom.
 
The rather large rolling chocks installed on my boat made no difference to fuel consumption, which was carefully measured just before and just after installation.

I'd be interested to see a lines drawing of one of the research vessels, having a hard time visualizing the "quarter fins".

One thing you mention in passing, that goes unmentioned a lot in these discussions, is the position of the crew WRT the roll center. Something like the Norhavn's high pilothouse or anything piloted from the flybridge puts the crew way above the roll center, exaggerating motion. Some have pilothouses well forward so pitching is experienced as heave. An aft cockpit sailboat puts the crew near the least motion on the boat.
 
Roger, Thanks very much for a very informative post! Your explanation puts a complex subject in layman's terms.

Last summer I added bilge keels to my Bayliner 4788 because of it's tendancy to synchronize it's rolling with seemingly small waves making for a very uncomfortable motion.

The 12" X 24' Bilge keels had the result of dramatically reducing the perceived motion you feel in beam sea conditions. It seems like the worse the conditions the more they help, if my explanation makes any sense.

Thanks again for the great write up!
 
Rolling chocks with minimal drag

Thanks again, Roger. Very clear visual images.

Rolling chocks can be made with reduced drag; here's one from a local company who fits these kinds of stabilisers to commercial vessels here routinely:
 

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