# roll period

Discussion in 'Stability' started by kelp, Oct 1, 2009.

1. Joined: Feb 2009
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### kelpJunior Member

The roll period of my 34' steel St.Pierre Dory is 3.2. Using the designers formula I came up with a maximum period of 2.4 for offshore cruising. The Bay of Bengal Safety Guide for small offshore fishing vessels includes a chart for calculating maximum period roll for a given size boat,where 2.5 is the maximum roll for my boat. We are basically coastal wimps but we are planning a cruise next summer from Bath Maine to Newfoundland which includes a couple of 70 mile jumps accross the Bay of Fundy and The Cabot Strait. So offshore capability becomes a concern. I am not mathematically inclined enough to do elaborate stability calculations but I would feel comfortable if I could get to 2.5. I assume ballast is easiest way to do this. Is there a simple way to calculate the ballast needed, assuming that is the way to go? The boat is a power dory designed by Tom Colvin,which I built over the last 20 months. Nothing was changed in the hull. I did modify the superstructure but in no way did I increase tophamper or change the center of gravity. Would like to get this settled, not looking forward to turning turtle in the Bay of Fundy and getting sucked down in one of their famous tidal whirlpools. Any info appreciated. Dave

2. ### Paul KotzebuePrevious Member

Roll period is a poor way to evaluate stability. I suggest you consider having the stability evaluated against the ISO standard for power boats (ISO 12217-1) for the appropriate design category.

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### GuillermoIngeniero Naval

Yeap!
Roll period tells you something about the initial stability (GMo), but nothing about ultimate stability, which is of utmost importancy when going blue water cruising. Checking against ISO 12217-1 is a sensible idea.

Cheers.

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### gonzoSenior Member

A longer period can make the motion more confortable. A sailboat without the mast on has a shorter period and a rather violent rolling.

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