Boat rolling factors

wklein

wklein

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Because my boat is quite narrow aft and rolls quite sickeningly downwind (or quarter sea. My boat also has haystacks which make it appreciably heavier.

If i were to remove the haystacks which would make the boom lighter would it increase or decrease the roll.
 
wklein said:
Because my boat is quite narrow aft and rolls quite sickeningly downwind (or quarter sea. My boat also has haystacks which make it appreciably heavier.

If i were to remove the haystacks which would make the boom lighter would it increase or decrease the roll.
Click to expand...

I am not too sure what you mean by 'haystacks' but in general, the heavier the mast and bits and bobs, the slower the roll will be due to the inertia. An extreme example is the fact that a yacht without any mast will roll very quickly and feel very strange. In my case I added a mast to my motorboat to reduce the period of the roll and has been very successful even without the sail being set.

Running downwind with the sails set goose winged will cause most yacht to roll so I do not think you are alone with this issue. Perhaps running with the wind on the stern quarter with both sails set on the same side may make you boat more comfortable.
 
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Rolling boat

I can't say I know much about the subject. Indeed I don't know why a boat with narrow stern would roll more in a following or quartering sea. I also don't know what a hay stack is. (not deterred by ignorance however...) As nobody else has responded...
I imagine the rolling you are concerned about is from a lack of stiffness. ie resistance to being heeled by wind. This stiffness comes both from the pendulum effect of the keel and the buoyancy of the hull at the sides. (a catamaran being the extreme case of this buoyancy at the sides).
The pendulum effect does not come into real play until you go beyond about 30 degrees or more. ie good for self righting from a knock down but not good to stop rolling.
However the mass of the mast especially up high will add to the tendency to keep rolling from side to side once it is started by a wave. So yes any reduction in upper mass must reduce the tendency to roll.
On the other hand a sail boat without a mast can be quite a wicked roller. Perhaps it is the inertia and mass of the keel or perhaps the lack of sail to provide a sideways damper.
So like I said I don't know but these comments even if wrong may get a discussion going. Good luck olewill
 
Just to carry on from the post by William H, my understanding is that the greater the mass above the deck, the greater the angle of roll will be but this will make the boat more comfortable. Although the roll angle is greater, the period of oscillation is longer, hence less fatiguing on the crew.
 
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I think I read an article where someone was building a no expense spared yacht, and ISTR that a roll period of about 13 times per minute was the most comfortable.

On a metronome, when the weight is high up on the lever, the beat is slower. Without knowing much about the physics of it, this might suggest that weight up the mast will slow the roll period, and vice versa.
 
wklein said:
Because my boat is quite narrow aft and rolls quite sickeningly downwind (or quarter sea. My boat also has haystacks which make it appreciably heavier.

If i were to remove the haystacks which would make the boom lighter would it increase or decrease the roll.
Click to expand...
I can only assume you mean a 'stackpack' type sail cover on the boom. It will have an insignificant effect on the degree of rolling of a yacht the size and weight of an Excalibur 36 - a very well respected 1960s cruiser-racer. All monohull yachts roll downwind.

Lots of Thames sailing barges actually used to carry hay from Kent and Essex to London, to feed the horses that drew carts and hansom cabs. See http://www.flickr.com/photos/mynameismisty/5700524114/ for a photo of one surprisingly recent.
 
Perhaps if when you removed the haystack you parked the combined harvester in the farmyard instead of on the aft deck
That would make the rolls smaller just like having a lighter mast. The rolls might be quicker but not such a big angle
Personally I would rather have the lighter mast & maintain stability.
Besides , once the harvest is in the combine just gets in the way
 
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 :D
 
Haystacks are rails on the side of the boom which allow a flat platform for the sail to be flaked onto, much the same as the park avenue booms on many hugh end yachts.

They also make a good handhold when travelling to the mast and provide a greater gap between lazyjacks to prevent snagging.

I guess i probably over though the original post. What i should have said is:

"If you are broad reaching with a quarter sea, what difference does boom weight make to roll?"
 
wklein said:
Haystacks are rails on the side of the boom which allow a flat platform for the sail to be flaked onto, much the same as the park avenue booms on many hugh end yachts.

They also make a good handhold when travelling to the mast and provide a greater gap between lazyjacks to prevent snagging.

I guess i probably over though the original post. What i should have said is:

"If you are broad reaching with a quarter sea, what difference does boom weight make to roll?"
Click to expand...

I think it would have to be pretty heavy to make more than a percent or two difference to the moment of inertia of the yacht. Most yacht booms are quite heavy to start with.
The top half of the mast is heavier and much further from the roll centre.
Increasing the roll moment will make it roll slower, but probably further.

The best analysis of this I've seen is in Larrson and Eliasson's book on yacht design.

Are we talking about a very broad reach?
Are you using enough kicker to stop the top of the main driving the masthead to windward?
 
wklein said:
Haystacks are rails on the side of the boom which allow a flat platform for the sail to be flaked onto, much the same as the park avenue booms on many hugh end yachts.

They also make a good handhold when travelling to the mast and provide a greater gap between lazyjacks to prevent snagging.

I guess i probably over though the original post. What i should have said is:

"If you are broad reaching with a quarter sea, what difference does boom weight make to roll?"
Click to expand...
Learn something every day! I know Park Avenue booms, but Haystacks are new to me. The idea of a handrail along each side of the boom though is a good one. Still don't think the weight there will really have any very perceptible effect on rolling. It will however definitely slow (ie lengthen) the natural roll period a bit. However that natural roll period may be different by that imposed by waves - a boat in a seaway is not just a simple pendulum.
 
Off the top of my head, I'm guessing that resistance to roll might have something to do with the boat's righting lever. So if the centre of gravity has been moved closer to the centre of buoyancy than the designer intended, the boat might roll more, because the righting lever is weaker and the boat less stiff?

Weight aloft would affect this but perhaps the boom is not really the problem. Are there lots of heavy things strapped to the coach roof and in the saloon lockers?

Just a thought and happy to be corrected.

Cheers
 
Delving into the turgid and inaccurate depths of my A Level Physics, I think we are talking about the Moment of Inertia here. I=mr2 where I is the Moment of Inertia, m is the mass and r the radius of the centre of mass from the axis. On a boat the axis is its Metacentre (sometimes called the Centre of Buoyancy). Therefore, the greater the mass for a given r the greater the moment of inertia. The moment of inertia is analogous to inertia with regard to linear mechanics and resists changes in momentum, ie the system will continue to move in a rotational manner or remain stationary unless acted on by an external force (c.f. Newton's 2nd law). So increasing the moment of inertia by raising the weight on a metronome will mean that with a fixed spring operating in a consistent way the period of the oscillation is increased.

However, a boat does not include a fixed spring but is righted by the effect of the centre of gravity moving to the side of and beneath the metacentre and developing a righting moment which varies in accordance with its stability curve. Increasing the mass of the mast or any other part above the centre of buoyancy will raise the centre of gravity and reduce the extent of the stability curve. So by increasing mass up the mast you reduce the righting moment for any particular amount of heel. This means that you increase the moment of inertia and reduce the forces which have to overcome them to right the boat.

So when running where the sails do not tend to hold the boat at a fixed angle the boat is only righted by the stability curve. By carrying mass at height you increase the moment of inertia for the boat and decrease the righting forces. This means that the period of the oscillations increase, however, if you take it to the limit the boat will not right as the moment of inertia keeps it rolling with little or no righting moment to bring it upright. However, the important bit is the effect of r2, which means that the moment of inertia increases as the square whereas the righting moments reduce linearly. So it is entirely possible to tune a design by increasing the height of the rig so as to reduce the periodicity of oscillations to something more comfortable and keep it with a safe stability curve.

So, for the OP, remove your haystacks and add the equivalent mass at a suitable point to the mast further up. Better still don't change a thing as you boat is probably fine as it is having been designed by a Naval Architect in the first place.

I think this is about right but as my will to live has begun to fade I draw your attention to http://en.wikipedia.org/wiki/Righting_moment from where I draw the following:

"The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. Metacentric height also has implication on the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently high but not excessively high metacentric height is considered ideal for passenger ships."

Or to turn it around, reduce the Metacentric height by carrying too much weight aloft and you will increase the periods of roll to more comfortable levels, but don't go too far.
 
Good grief I think that is more or less what I said but more so. 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? That's what I take from it anyhow. Just like the metronome (which is easier to start the higher the weight, but quicker once going the lower the weight).

I'd be inclined to try and make sure the centers of gravity and buoyancy are as close to the designers intentions and leave it to the rum after that.
 
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