Boat rolling factors

[bitbaltic]

It's also not clear from the OP whether he thinks his boat rolls too far, or too quickly; not sure exactly what the motion he is experiencing is.

 

[oldfatgit]

So does it follow that increased weight aloft increases roll tendency but slows the periodicity

- CORRECT!

whereas less weight aloft decreases the tendency but increases the period?

er no, less weight aloft decreases the tendency and DECREASES the period. Which is what you were driving at wasn't it?

 

[bitbaltic]

- CORRECT!

er no, less weight aloft decreases the tendency and DECREASES the period. Which is what you were driving at wasn't it?

Yes exactly. My bad!

I think I actually understood some actual physics tonight! Well that deserves beer. Or, even, rum.

Cheers!

 

[ELAiNe333]

Trim the sails?

Apologies if i am teaching my grandmother etc...BUT I was taught and always follow the warning not to allow sails to billow too far forward when running downwind as the wind spilling around them will cyclicly amplify the roll.
Ensure you have enough kicker to prevent the head of the main curling ahead of the mast and similarly take the genoa cars forward so the foresail is pulled downwards and does not curl forward too far. ( the joy of rope adjustable cars )
Is this generally accepted or am I alone?

 

[LittleSister]

So does it follow that increased weight aloft increases roll tendency but slows the periodicity, whereas less weight aloft decreases the tendency but increases the period?

Not exactly. The extra weight aloft adds inertia. So it actually reduces the tendency to start rolling, but once in motion it also reduces the tendency of it to stop.

Conversely, with little weight aloft, it tends to more easily begin a roll, but will also more easily stop.

Neither extreme is desirable - you wouldn't want to have nil inertia and following slight every angle of every wave and swell, but neither do you want to keep going over when a big wave hits.

It's a matter of balance - getting a sufficiently great inertia to resist the initial roll, and slow the roll period, without becoming excessively prone to continuing the length of the roll. For different purposes the 'ideal' balance will be different - reactive versus comfort (compare to car suspension, for instance).

It is also balanced with the righting moment from the hull shape and weight of ballast. (Note the latter also affects the roll inertia in the same way, heavier and deeper, more inertia, but has the reverse effect on righting moment.

 

[johnalison]

Apologies if i am teaching my grandmother etc...BUT I was taught and always follow the warning not to allow sails to billow too far forward when running downwind as the wind spilling around them will cyclicly amplify the roll.
Ensure you have enough kicker to prevent the head of the main curling ahead of the mast and similarly take the genoa cars forward so the foresail is pulled downwards and does not curl forward too far. ( the joy of rope adjustable cars )
Is this generally accepted or am I alone?

I was thinking the same and was going to say something similar before I got to your post. A similar situation can arise when under spinnaker, when better sheeting will correct it. Boats and rigs may well be the main factor, as well as keel depth, but sails and even the helmsman can greatly affect comfort.

 

[LittleSister]

the helmsman can greatly affect comfort.

But not as much as a helmswoman, I find.

 

[William_H]

Roll period

i remember is physics class at school being sat on a chair that had been mounted on some sort of vertical axle with ball bearings so it would easily rotate. (I guess a modern office chair might be used to reproduce this today, although such things didnt exixt back then!!) We were given a weight to hold in each hand and told to hold your arms out wide and then someone would spin the chair and you would rotate relatively slowly.

Then when instructed you would bring your arms in to your sides and as you did so the rate of rotation increased - alarmingly.

IIRC it was all to do with second moment of inertia and basically the mre weight you put on the outside of the rotation the slower it spins.

The problem of course in a boat being that if you stick a big weight at the top of the mast the boat will capsize!

You could send someone aloft next time and see if the roll rate reduces

This analogy or explanation is misleading. What we have in your physics class experiment is quite simply that the mass you are holding at arms length has a velocity (rotational of course) When you pull your arms in the mass tends to want to maintain its original velocity but with now a smaller circumference of its path it must increase the RPM. Similarly if you push your arms out again RPM will slow but mass will maintain same velocity.
This is not related to pendulum speed (resonance is another word for it) olewill being picky (sorry)