Roll period

I have read Dave Gerr's Masthead publications and used the formulas therein to calculate the roll period for the Diesel Duck 38. Roll period was 4.61 secs, roll period/beam was 1.19 under 1 is uncomfortble and excessively long rolls are dangerous but what does the roll period mean.

example. If a ship in a beam sea rolls 30 degrees to port, does it mean it will take 4.61 secs to return to vertical. The same ship rolling to 35 degrees could also return to vertical in 4.61 secs because the greater heel creates a larger righting moment.

Am I right or wrong also in terms of rolling how slow is dangerous.

 

For any vessel, there is a natural period of roll. What this means, is that in a flat clam water, if the vessel is disturb, by a passing wave, or a gust of wind, for example, and occurs just once, rather like pulling the vessel from its mast and then letting go, then the period of motion where the vessel rolls from side to side, with no further external load, the period of motion is its natural period, and relates to the 4.6 seconds you quoted above.

Just like if you take a ruler, place it on the edge of a table, pull it down and then release it, it shall vibrate up and down. With no further ‘pulling’ down, it shall simply vibrate away, the ruler shall go up and then down and back up again. It does this with the same period, or time taken, for each one complete cycle. The same is true for the boat.

However, if there exists waves which are heeling the vessel to 30 degrees in your example, this complicates matters considerably. The wave is an exciting function, and can vary in magnitude and period itself. This can have a significant effect on the motion of the boat.

If the period of the exciting function is equal to the natural period of the vessel, then you can get what is called resonance, where the amplitude, or distance moved from side to side is increased. So instead of the say 30 degrees you cite, it could be suddenly increase to 50 or 60 degrees. To complicate matters further, a vessel that is rolling has restoring forces to counter the exciting function. These restoring forces are affected by the geometry of the vessel as well as its KG.

So in a nut shell, if there is just one single wave on an otherwise flat clam sea, yes. Otherwise, no.

 

If the period of the wave is equal to the the period of the vessel it can increase the motion until the vessel capsizes. It is like if you are pushing someone on a swing. Each time the swing goes forward, you give it a push and it accumulates energy going higher each time.

 

Hi Mick,
This can give you a deepest insight in the matter:
http://en.wikipedia.org/wiki/Metacentric_height

 

And here you can find an interesting and lively debate:

http://www.boatdesign.net/forums/stability/gerrs-roll-period-recommendation-38540.html

 

known rule to inspectors, designers and users-
1. a roll period is the time elapsed that the ship needs to roll from one side to the other and back again.
2. The roll time is commonly- the number of seconds should not exceed the width of the ship expressed in meters , i.e beam 4.5 m and the OK rolling period is 4.5 secs or less. Easy!

It doesn't matter from what angle. It will be the same.

Carl